Archive | 2007

5363

6:00 am
October 1, 2007
Print Friendly

The Fundamentals: How To Write A Standard Maintenance Procedure

Shortcuts won’t work when it comes to developing the type of clear, easy-to-understand instructions that everyone can follow.

A Standard Maintenance Procedure, or SMP, is a written set of instructions that specifies how a maintenance procedure is to be performed. It should be specific and detailed enough so that a qualified maintenance technician who has never before performed the task can do so successfully by reading and following the instructions contained in it.

Encompassing standards, measurements and specific techniques, the central idea behind an SMP is that there is only one right way to perform any task. Let me say that again: There is only one safest, most efficient and most effective way to perform any given task. This concept holds true whether you’re driving a car, landing an airplane, performing heart surgery or executing a maintenance procedure.

Unpleasant surprises
Keep in mind that it takes only one deviation from that “right” way of doing things to produce a failure. For example, you can lubricate a bearing correctly 99 times in a row, but if you deviate from the SMP during the one-hundredth lube, you run the risk of ruining the bearing and invalidating all of your previous good work. Don’t believe it? Try the following experiment.

Select five of your best maintenance technicians, and send each of them to lubricate a bearing somewhere at your site. If possible, direct them all to the same type of bearing. Then, check the results. How many different types of grease were used? Were the zerks wiped with a clean rag? How many shots of grease did each bearing receive? Were the grease guns cleaned and calibrated? Was the grease compatible with the lubricant that was already in the bearings? Did the technicians leave their work areas clean when they finished?

You will be surprised at how much variability you discover surrounding this relatively simple procedure. And, to be blunt, you will be unpleasantly surprised when you discover how many times this procedure was performed incorrectly out of the five opportunities presented. How much more variability and error do you suppose there is in your more complex procedures? That, in a nutshell, is why you take the time to write an SMP.

There are two types of maintenance procedures that must be placed under the SMP umbrella. They will differ somewhat in scope.

Developing routine SMPs
Routine maintenance procedures—such as a bearing lubrication—are performed over and over again on a regular basis. The need for a Standard Maintenance Procedure with this type of task is obvious. Remember, if we take care of our smaller details correctly and consistently, our larger functions and processes will for the most part take care of themselves.

To write an SMP for your more routine procedures, you must first decide which of them will provide you with the greatest initial benefit. In the beginning, you should be targeting the most frequently performed tasks in your plant. Good examples of this type of procedure include bearing lubrication, gearbox lubrication, drive belt tensioning, alignments, bearing installation, drive chain replacement, hydraulic hose construction and replacement, wear component replacement and clean oilhandling techniques.

Once you have determined your most common routine tasks, you must decide the standard to which you want them performed. If you have a reliability engineer, he/she will be invaluable at this point. By consulting OEM specs, maintenance technicians and machine operators, the reliability engineer can develop a set of standards and instructions for those procedures that must be consistently performed to the same level of excellence each time—no matter who is doing the work.

The “routine” SMP should include safety concerns, tool lists, parts lists and step-by-step instructions about how the procedure should be performed. These instructions must include torque specifications, measurements, readings and oil and grease types. Additionally, in this age of digital cameras, it can be relatively easy (and very helpful) to augment the text with photographs.

As important as an SMP is, it has no value if it is not being used. As your routine SMPs are developed, it is critical that they be communicated to the maintenance technicians. We’re not talking about handing out a sheaf of these documents at the weekly meeting with the suggestion that “You guys need to read these.”

Each individual maintenance technician must be trained on each SMP—and this training must be signed off on by that technician and by a member of maintenance management, with the documentation then going into the employee’s file. Once you have verified the skill and documented the verification, you have accountability.

Periodically, a supervisor must observe a technician in the field to be certain that the standards set forth in the SMP are still being achieved. In other words, if the SMP details hydraulic hose construction, then each member of the maintenance department must construct a hose to specification in the presence of a supervisor or manager— and must do this on the bench, as well as later on out in the field. Should technicians be incapable of doing this, they must be retrained until they can accomplish the task. If you do not intend to verify the knowledge that you are documenting and communicating with these SMPs—and to document that verification—there is not much point to the exercise.

Developing larger, more complicated SMPs
The second type of procedure that falls under the SMP umbrella is the larger, more complicated procedure. Every plant has these—overhauls, pump replacements, motor changes, bushing replacements—and they are generally performed by the technicians who have always done them. Over time, the best way to execute a specific “complicated” task simply will have “evolved.” Provided that the interval has not been too long, a technician usually can remember at least some of how that job was done last time. The problem here, of course, is that your expert may have retired or moved on. Or it has been too long since the task was last carried out and the team can’t remember some of the facts. Or some field engineering occurred that no one remembered to document. Consequently, instead of a well-planned operation with no surprises, what actually results can only be described as another badly planned, poorly executed job that takes twice as long as it should while costing three times as much as it ought to. The SMP is the road out of this cycle.

The method of development for the SMP on a larger, more complicated task has several steps—but the principles of concise documentation and absolute accountability are the same as with those of the routine SMP. The following steps are important when it comes to developing SMPs for larger jobs:

  • Have a pre-plan. Before you begin the large job, have the maintenance planner sit down with all of the personnel who were members of the work team the last time the job was done, or at least as many of them as are available. The planner should write out the steps the way they are remembered. This plan—sketchy though it may be—will form the outline of the upcoming job.
  • Photograph the job. The importance of this step is paramount. Even a well-written job plan can be misread or misunderstood. A photograph, however, speaks for itself. If you have the equipment and personnel, videotaping the procedure is even better. The supervisor is not a good choice for this role, because he/she needs to be supervising. If the reliability engineer is not available, perhaps the scheduler or the clerk can pitch in. Another idea is to enlist the aid of an employee who has been assigned to restricted duty.
  • Write it down. The maintenance planner should be the one to write down the action step-by-step, beginning with the safe lockout of the machine. This individual should assume that he/she is writing the procedure for someone who is a total stranger to the plant and the machines—and that the written procedure will ensure that this imaginary person can successfully complete the job. The planner should be looking not only at what currently is being done, but also for ways to improve on the procedure (including ways the job can go more smoothly in the future). He/she also should be sure to record the number of man hours associated with each step, from kitting the job right on down to cleanup.
  • Write out a complete parts list. This list should be as comprehensive as possible, down to the numbers and grades of the nuts, bolts and washers that are needed. Lead times for special-order or fabricated parts should be noted.
  • Write out a complete supplies, tools and experts list. If special jigs or stands are made for the job, they should be noted on the SMP, including where they are stored. Have there been shortages of special welding rods or bottled gas? Jacks, cranes and special tools also should be noted. What about consultants or factory reps? If they were present last time, chances are they will be needed next time. Does an operator need to be present? Will it be desirable to have predictive maintenance personnel available to take readings for baselines after the job is completed?
  • Include drawings and diagrams. Any tool, document or image that can help the technician as he/she is performing the job should be available.

Once all of the foregoing information has been compiled, it’s time to write the SMP. This should be done with the entire work team present—while memories are still fresh. Whether you decide to write it directly into the CMMS or as a Word document, the important thing is to be as complete as possible. Each step, as it is recorded, should represent the consensus of the work team.

Write the work steps in order from start to finish. Try to make the language as friendly as possible. Present the parts list first, followed by the supplies and tools. Next, specify the lockout and any safety concerns. If anyone has ever been injured performing the task, present this information as a side bar. Finally, move into the job steps themselves.

As you write each step, be thorough, accurate and concise. Incorporate the photos you have taken into the plan. If your particular CMMS does not allow for the inclusion of photos and drawings, make a note in the SMP that these items are available and where they can be found.

Finally, as you write the steps from your notes, the evolutionary nature of the large-job SMP requires you to ask the maintenance technicians if the way the job was performed in the past is the way it should be done from now on. Every time a job is performed, someone on the work team probably will have an idea on how to do some portion of it better. It is critical for these improvements to be captured and incorporated into the SMP for next time.

Remember, we said the central idea behind a Standard Maintenance Procedure is that there is only one right way to perform any task. By treating the large-job SMP as a living document, you will guarantee that over time it becomes the one right way to do a job.

Ray Atkins, CPMM, CMRP, is a veteran maintenance professional with 14 years experience in the lumber industry. He is based in Rome, GA, where he spent the last five years as maintenance superintendent at Temple- Inland’s Rome Lumber facility. He can be reached at raymondlatkins@aol.com

Continue Reading →

2215

6:00 am
October 1, 2007
Print Friendly

The Fundamentals: Selecting an industrial air compressor…It’s Air Time

Spend a little time to analyze your needs before buying an air compressor. Good decisions up front will pay off well for your shop later on.

At first, selecting an air compressor seems so simple. How tough could it be? All you have to figure out is how much air you need and decide how much you want to spend. Right?

Then, when you start poking into it, you discover all sorts of things you have to decide on. At that point, it seems as though you can’t decide on anything without first deciding on something else, which in turn depends on yet another decision, which—sure enough—goes back to the first decision you had to make, and you couldn’t have made that without knowing all the rest. It’s a real cat’s cradle.

In fact, choosing an industrial air compressor really comes down to just two main topics: size and features. With regard to size, compressor capacity and operating pressure take the forefront. Features—and the quality of each—also should be closely evaluated prior to a purchase. Let’s break this down a bit more and make a complicated decision simple again.

Decisions. Decisions.
They can be maddening when it comes to selecting an industrial air compressor. Use the Decision Wheel to help clear your thinking. You can start at any point on the wheel and move around it in any direction.

Deciding on the size
Look at the compressor selection process in a series of logical steps to make the process easier. To begin with, calculate the amount of CFM (cubic feet per minute) required in a shop to determine the necessary capacity of a compressor. Begin by adding the CFM requirements for all the tools that may be used simultaneously, then add another 30% of that to allow for unknown or uncommon compressor usage. The CFM demand will be on the tool itself or in the owner’s manual. Capacity or volume—in other words, CFM—can be figured three ways:

1. Displaced CFM (DCFM) is simply a mathematical calculation of the bore, stroke and rpm. It does not take into account any of the important variables, like temperature, atmospheric pressure, humidity, friction or heat dissipation, therefore it means almost nothing in the real world.

2. Standard CFM (SCFM) is a better measure of reality. SCFM is the flow of free air in a standardized environment— such as 14.5 psi atmospheric pressure (the pressure at sea level), 68 F and 0% humidity. Since this is a standardized metric, it’s the best figure to use in comparing air compressors across the board, much like apples to apples. Specific needs will affect this, however. Someone working in mile-high Denver will have different requirements than someone working at sea level in Louisiana.

3. Actual CFM (ACFM) is the number most needed because it figures in the variables that apply to a specific situation. It will give the output of the pump for the actual working conditions. But, ACFM is a hard figure to get precisely because it does require site-specific data and calculations that may be best left to an engineer.

For selection purposes, your best bet is to compare air compressors based on the SCFM ratings. Note, too, that CFM often is shown at various pressures. These numbers can be very useful in determining if a compressor produces enough volume for the application, but they can be confusing when you try to compare different compressors or compressors rated at different pressures. Again, SCFM is best. It levels the playing field.

Remember to add in that extra 30% of CFM to provide a reasonable buffer against the unforeseen, but don’t exceed it. There’s no point in buying more capacity than you’ll ever need. You just don’t want to buy less.

CFM also will be important to know when considering a single-stage compressor versus a two-stage model. Begin by listing the minimum operating pressure requirements for the tools you’re going to use, which will indicate whether a single-stage compressor or a two-stage compressor will be needed.

Single-stage compressors are fine up to 150 psi. Higher pressures will require a two-stage unit. A single-stage compressor typically will have a higher CFM rating because the cylinder draws in air and compresses it with every rotation. A two-stage model compresses the air up to an intermediate pressure in one or more cylinder(s) and then passes it on to another cylinder to finish the job. Because the air is typically passed through an intercooler between stages, a two-stage compressor is more efficient at higher pressures.

One final thing to keep in mind regarding size is the peak air demand requirements. Plants tend to use constant air for most applications, such as blow-down guns and air grinders. At some point, the plant may be using more air, but the tank will level that out. Still, it’s a good idea to have a backup compressor in the event that pressure drops.

Deciding on the features
Durability is a key term to remember when evaluating compressors. Durability means longevity, and longevity means cost-efficiency over time. Invest in quality up front and it will pay long-term rewards. That means you’ll want to consider long-term features, such as a cast-iron cylinder, a heat-dissipating head, an efficient cooling system, structural protection for critical components and fittings, a heavy-duty steel frame and powder-coat or electrostatically applied paint to resist chipping and wear. Check on the service life expectancy of various models before making a purchase, as well.

Keep hose diameter in mind when surveying a compressor’s features. Don’t skimp on ¼” hose if 3/8” is needed to handle a load of more tools or longer runs of hose. Be realistic about the actual needs, too. Make sure the larger hose will justify its extra weight and cost.

Beyond features, ask questions about the supplier company along with the reliability of its parts and service support. This can indicate quality features—or the lack thereof—that are not readily visible. Are air compressors the primary or sole business of the supplier? Does the company make its own products or source them from a third party? How long has the company been in the air compressor business? (All we know about the future is what we know about the past, so look at the company’s history in this market.) Can the supplier answer questions clearly and explain the subtleties that only an expert would know? What about the availability of technical help, parts and service and the distribution network?

Before finalizing the decision, take a moment to calculate not just the initial price, but the long-term costs of the purchase. When it’s all added up, what will the compressor really cost over time? Consider how it will be used, how often and how long. The big question in the selection decision is this: What would it cost to be wrong?

Overall, the most critical issue to keep in mind from the beginning is the job analysis. Every job application has its own requirements and, consequently, its own set of questions. Be sure that you create a realistic checklist for the specific work situation before making a final decision.

Deciding on the solution
Remember that it makes little sense to buy more capacity than you need, but it makes no sense at all to buy less. Ultimately, you’re not just buying an air compressor—you’re really buying a solution to a problem. You can try to get by with just one aspirin, but you’ll still have the headache. It’s important to thoroughly analyze the decision points early on in order to find the best solution for the present and future.

Dan Leiss is president of Jenny Products, Inc., in Somerset, PA. Telephone: (814) 445-3400; e-mail: dleiss@steamjenny.com

Continue Reading →

231

6:00 am
July 1, 2007
Print Friendly

The Fundamentals: Effects Of Doubling Up On Hearing Protection

0407_fund_personal_1

Most end users think OEMs take particular pains to design things that last. That’s true in most cases, but not all.

Will doubling up or wearing dual protection-an earmuff in addition to earplugs– provide added protection against extreme noise levels? The answer is yes, according to a recently released Sound Source™ bulletin from the Bacou-Dalloz Hearing Safety Group-but maybe not as much as you thought.

The new bulletin, Sound Source #11a, “Dual Protection,” is authored by audiologist Brad Witt, the Audiology and Regulatory Affairs Manager for the Hearing Safety Group. He notes that dual protection is not required by OSHA regulations for general industry in the U.S., but is required for mining operations governed by the Mine Safety & Health Administration (MSHA) for noise exposures over 105 dBA (8-hour time-weighted average). Similarly, NIOSH recommends dual protection for any exposures over 100 dBA, and some companies require it for employees with progressive noise-induced hearing loss despite normal protective measures.

There are, however, risks associated with dual protection. “Using earplugs and earmuffs concurrently seriously isolates the wearer,” Witt writes, “so it is warranted only in extreme noise levels.” He also suggests that dual protection may be overused. “When a high attenuation earplug or earmuff is properly fitted and the user is motivated to use it correctly, some hearing professionals say the need for dual protection is rare.”

Obtaining maximum benefit
So how much protection will doubling up provide? “That depends on the fit,” says Witt, “but, it is not simply the combined ratings of the earplug and earmuff. There is a ceiling effect that limits the amount of combined protection. Even if wearing a perfectly fitted earplug and earmuff with ideal attenuation, we would still hear sound transmitted through our bodies and bones to the inner ear.”

The maximum amount of attenuation that can be attained by most people is 35-50 dB, depending on the frequency of the sound.

As for a rule of thumb for estimating the effects of dual protection, OSHA recommends adding 5 dB to the NRR of the higher rated device. “But this,” says Witt, “sacrifices some accuracy. An earmuff typically adds about 4 dB to the NRR of a well-fitted foam earplug, and about 7 dB to a well-fitted pre-molded earplug.” He also says that an earmuff with moderate attenuation provides the same effect as a high-attenuation earmuff when either is worn over a well-fitted earplug.

According to Witt, the key to obtaining maximum benefit from dual protection is proper fit-especially the fit of the earplug. When a poorly fitted earplug is worn with an earmuff, the resulting dual protection is little more than the earmuff alone.

About Bacou-Dalloz
Sound Source, a free periodic publication of the Bacou- Dalloz Hearing Safety Group, addresses questions and topics relating to hearing conservation and hearing protection. Bacou-Dalloz manufactures and markets a comprehensive range of safety products designed to protect people from hazards in the workplace. The Group specializes in head protection equipment (eye and face, respiratory and hearing protection), body protection equipment (clothing, gloves and footwear) and fall protection equipment. These products are sold through a worldwide network of distributor partners for use in all sectors (construction, manufacturing, telecommunications, homeland security, petrochemicals, medical, public services, etc.)

Bacou-Dalloz Hearing Safety Group
San Diego, CA

Continue Reading →

1654

6:00 am
July 1, 2007
Print Friendly

The Fundamentals: Shaft Seals On Rotary Valves & Airlocks

When it comes to sealing solutions for your bulk handling systems, you may have more options than you thought.

Rotary valves and airlocks are fundamental components in bulk material handling systems. They serve to accurately meter product from storage into a process, as well as to isolate pressure differentials between storage and conveying systems.

The rotary valve is essentially a revolving door, on a horizontal instead of a vertical axis. Situated in the discharge opening of a silo, day bin or other vessel, the upward-facing, wedge-shaped “door” section is filled with process material by gravity. As the section rotates into the circular housing, excess material is scraped away, so that the wedge contains a controlled amount of material.

When the filled wedge section rotates to the opening at the bottom of the valve, a controlled amount of process material will drop into the receiving vessel (which could be a screw feeder or pneumatic conveying or packaging system). Varying the rotational speed of the valve, therefore, alters the rate of flow of material into the downstream system.

A rotary valve consistently delivers a specific amount of material at its discharge.

An airlock is a rotary valve whose vanes fit closely enough to its circular housing to maintain an airtight seal. The airlock then can maintain one pressure on its inlet side and a different pressure or vacuum on its discharge side. Thus, for example, pressure from a pneumatic conveying system on the inlet side of the airlock does not affect a loss-in-weight bagging system on the discharge side. Maintaining pressure differential and a precise quantity of material in each of a rotary valve’s wedge-shaped sections depends upon a tight, accurate fit between the vanes of the valve and the circular housing, and upon properlyfunctioning shaft seals. Both factors exert even greater influence over performance of an airlock.

Sealing options
Several different sealing options are available as standard equipment from different rotary valve and airlock OEMS. Some OEMS take the conventional, 3000-year-old packing and gland follower approach. Some use mid-20th century lip seals. Others incorporate latter-20th century quad seals. Several OEMs are beginning to investigate and use contemporary, contacting face seals. The selected sealing option is fitted into a shallow stuffing box, cast into the endplate of the valve, along with a bearing support.

Packing…
0707_fund_seals1Where packing is used, a simple gland follower is employed to compress the packing. Many different packing styles from many manufacturers are available. Such options include: ptfebased braiding for food grade and chemical environments; ex-foliated graphite yarns for high temperatures; various synthetic fibers braided with ptfe-yarns for abrasion resistance; and graphite braided with various synthetic fibers and or metal filaments to resist corrosion, extrusion and thermal degradation. Packing seals (see Fig. 1) can fail if not regularly attended, and the out-of-the-way locations of airlocks often make packing adjustment diffi- cult or impossible. Once dry product begins leaking between packing and the shaft, abrasive damage can quickly occur, eventually necessitating overhaul of the airlock. By the time leakage is noted, significant damage may have occurred, and any pressure differential across the airlock may have been compromised for a considerable time, sometimes allowing undesired variations in package fill levels.

Lip and quad seals…
0707_fund_seals2Lip seals (see Fig. 2) can lead to the same problems as packing. Lip seals and quad seals use the backup approach for maximizing runtime. By stacking several quad or lip seals in layers, leakage is delayed until the last row is compromised. The components function because the profile of the seal against a rotating shaft generates a vacuum condition at the internal seal location (process side) and a positive pressure on the external side of the lip. The vacuum keeps the process material in.

As the lip wears from contacting the shaft and interfacing with the process material, the vacuum condition degrades, requiring the next ring to seal. This continues through each lip or quad seal until the last seal point fails. All materials and lip configurations are not created equal and some degrade more quickly than others.

0707_fund_seals3Abrasive process material, fine particles and high temperatures are particularly challenging for both lip and quad seal types. The quad seal provides twice the number of contact points per ring (see Fig. 3). This also requires less compressive force against the shaft, resulting in less friction and better wear life. Also, various other lip seal profiles are available with different sealing features.

Sometimes air or inert gas is introduced in an attempt to blow process material away from the seal to prolong service life.

Mechanical seals…0707_fund_seals4

Specialized mechanical seals (see Fig. 4) are available for retrofit in airlocks. These compact, unbalanced, double-faced components can be fitted into the airlock’s packing area and adjusted with the gland follower. They are typically purged with air or inert gas as a barrier fluid for closing seal faces. Careful monitoring of the air/gas pressures can predict product leakage, allowing maintenance personnel with an opportunity to adjust the seals before leakage (and possible shaft damage) can occur.

Unlike packing and lip seals, mechanical seals do not contact the rotating shaft with a non-rotating sealing element. The rotating seal faces are fixed to the shaft, and they seal on a plane that is 90° opposed to the shaft’s axis. This effectively eliminates abrasive wear to the shaft—and in so doing, eliminates the need to replace or resurface a prime overhaul component, saving labor, time and materials during periodic overhauls.

At the same time, this configuration also eliminates abrasive wear of packing or lip seals, and the associated risk of product contamination by seal face detritus. Product purity is more easily maintained. Mechanical seals often require more radial and axial space for installation than lip seals and small-section packings. Consequently, the stuffing box space incorporated in the airlock frame may not be large enough to retrofit a mechanical seal. There are several steps you can take if your stuffing box space proves insufficient.

  • Most airlock end plates are easily removed. If there is sufficient packing box depth, the plate can be chucked and centered in a lathe and the packing radius enlarged.
  • Where there is insufficient packing box depth, the face of the existing box can be faced off in the lathe or milling machine, an extension ring can be centered and welded in place atop the existing box, and the ring and stuffing box i.d. can be re-cut to provide a smooth, consistent bore.
  • Where needed, a larger-diameter gland follower can easily be fabricated from a piece of pipe welded to a steel plate flange, then drilled and machined to suit.

In this way, most rotary valves and airlocks can be retrofitted with mechanical seals in your own plant’s maintenance shop (or in a local machine shop) with little effort.

The true cost of maintenance
The sort of proactive maintenance offered by air-purged contacting face seals can extend the operating life of airlocks between overhauls, minimize product waste and can also aid in predicting maintenance shutdowns.

Airlocks are periodically taken out of service to resurface and machine the vane tips. Some configurations have replaceable vane tips as an integral part of the design. Others are repaired by building up the vanes with weld and re-grinding the tip surface. Replaceable vane tips often are used in highly abrasive wear applications. In non-abrasive applications, seal replacement is the primary maintenance performed between overhauls. Vane tip service and seal and bearing replacement are the main reasons for overhauls which necessitate equipment shutdowns.

To avoid unnecessary shutdowns, the minimum goal for any airlock seal should be to last at least as long as the tips of the vanes. This provides the optimum meantime- to-repair. Ideally, shaft seals should be rebuilt or replaced when the vane tips and body are overhauled. Since the overhaul invariably takes place in the workshop, all maintenance can be performed in a clean, controlled environment, with tools and equipment ready at hand.

Trying to perform seal replacements and other maintenance in the field between overhauls is hard on personnel and, in turn, makes quality workmanship difficult to maintain. This makes mean-time-to-rebuild unpredictable.

The true cost of a single production line shutdown includes the cost of workers idled by the shutdown, product lost before the problem was identified, product not manufactured and product lost while bringing the line back to grade on restart, as well as the actual parts and labor costs associated in the maintenance operation. Taking these factors into consideration, an organization may look on mechanical seals not as a more expensive sealing solution, but as a thrifty investment in reliability. More importantly, when properly maintained and monitored, a mechanical seal’s performance is predictable.

The best sealing option
Any of the sealing options discussed in this article will work reasonably well when sealing non-abrasive materials. Difficulties arise when abrasive materials and very fine materials pass through the rotary valve or airlock.

Materials found in the home building products and mining industries, salts and sugars in the food industry and additives such as TiO2 or starch are all extremely abrasive. Such materials will embed into packing and act as a grinding mechanism, cutting into the shaft and making the rotary valve or airlock unreliable.

Lip and quad seal elastomers also can become abraded, lose their sealing characteristics and begin to cut into the shaft. Face seals can stop shaft damage, redirect wear to a sacrificial component and provide a predictive maintenance mechanism—but at a higher price.

What’s best for your application can best be determined by balancing the purchase cost of any given seal type against the maintenance and downtime costs associated with it.

Paul Wehrle is chief engineer with the MECO Seals Division of Woodex Telephone: (207) 371-2210; e-mail: bird@mecoseal.com

Continue Reading →

2154

6:00 am
July 1, 2007
Print Friendly

The Fundamentals: How To Write An Effective PM Procedure

Follow these steps in developing the type of world-class document and process that provides real value throughout your operations.

If you want to change your current maintenance reality, you must begin with the basics. One of the cornerstones of a successful maintenance effort is the precise execution of thoughtful, well-written preventive maintenance (PM) procedures. There are several steps that must be followed to ensure that these procedures are as effective as possible.

Step #1: Assessment The first step in the development of an effective PM procedure is to determine the condition of the machine or machine center. You have to know where you are before you can decide where you want to go and how you intend to get there. If the machine is newer equipment, this evaluation should be fairly straightforward. If, however, you are dealing with equipment that has been in service for a period of time or has a history of unreliability, this assessment could be quite a lengthy undertaking. Still, it’s time to invest if you want to leave reactive maintenance behind.

The assessment portion of the project should begin with a thorough cleaning of the machine. Having a clean machine will make the remainder of the assessment process easier to complete. Just as importantly, the machine should be evaluated as it is being cleaned to determine if there are any cleanliness issues that may be leading to or masking failures. Examples of these conditions include build-up or residue on electric motors, excess grease on bearings or other moving parts, oil residue on or below components, damp hoses and accumulated residues that might be indicating or hiding deeper problems. Any discoveries of this nature should be noted so they aren’t overlooked when the PM is written. Machine maintenance is a field that revolves around surprisingly few basics—one of these is that a clean machine runs better and longer.

The work team for the assessment portion of the project should be made up of both Maintenance and Production personnel. These are the people who operate and maintain the machine. They are your experts, and that fact alone should be motivation enough to involve them in the PM development process. The issue of “ownership” is another important reason to include these people. If your hourly professionals are involved in the development of the PM at every step of the process, they will have a vested interest in the success of the finished product. After the cleaning is completed, the next step of the process is to conduct a comprehensive mechanical and electrical assessment. In this portion of the exercise, you are looking for what is going right, as well as for what is going wrong.

  • Fasteners should be checked for torque.
  • Drive belts, chains, sheaves and sprockets should be inspected for wear and alignment.
  • Hydraulic components should be observed for signs of leakage.
  • Pneumatic components should be assessed with ultrasonic equipment if you have it and for audible air leaks if you do not.
  • Bearings should be inspected for signs of lubrication issues.
  • Moving parts should be analyzed for wear. Overall machine alignment should be checked to the extent that you are able.

During this mechanical assessment, notations should be made of any condition that is found to be out of spec—whether you intend to repair it or not. This is also the time to update your bill of material for the machine. Think of these variances as messages from the machine about potential trouble areas. The machine is telling you where your maintenance procedures are adequate and where they are not.

Step #2: Documentation and analysis Once the assessment is complete, you will again need both Maintenance and Production personnel to proceed to the next step. You also will need the maintenance records of the machine, including any documentation on breakdowns or failures. If formal documentation of prior reliability issues is not available, you may need to rely on anecdotal evidence or employee memory. Additionally, you will need all owner’s manuals, drawings and installation documentation.

Once you have gathered the necessary team members and documentation, the task before you is to make a written list of every known machine failure that has occurred in the past, as well as every possible failure that the team can envision occurring in the future. Do not forget to include the potential failures that you discovered during the mechanical and electrical assessments of the machine. If these conditions had gone undiscovered, would they have eventually caused machine failure? This exercise is the first phase of a Failure Modes and Effects Analysis (FMEA)—and, perhaps, the most crucial part of the PM development process.

Conducting the FMEA can be a daunting task. Don’t let it scare you. Just remember that the idea is to try to document what has gone or can go wrong with a machine so that you can put a procedure in place to prevent it from happening again. (A good website to visit on the subject of FMEA development is http://www.isixsigma. com/tt/fmea/) There also are some common-sense tips that can help.

  • First, follow the flow of the work that the machine was designed to do. If you are dealing with a hydraulic system, follow the fluid. If you are looking at a widgetmaker, follow the widget.
  • Do not try to conduct the FMEA in small installments over time. You will lose your train of thought, as well as the brainstorming continuity that is necessary to successfully complete the task.
  • Once you have convened your team, arrange the work schedules so that this group can meet eight hours per day, every day, until the FMEA is finished and the PMs are written.
  • Finally, follow the “likelihoods” when you are listing causes. As an example, a bearing failure could have a cracked machine footing as a root cause, but unless you have seen some indication of structural issues during your machine assessment, there is a low likelihood that this condition is causing your machine to fail.

Incidentally, a failure should be defined as any time that a machine: (1) ceases to do whatever it was designed to do; (2) when you want it to do it; (3) at the rate you desire; (4) at the quality specification you require. This is a very important concept. If a machine has been designed to stamp 500 holes per hour and it can only manage 480 holes per hour, the machine is in failure mode—despite the fact that it is still running and producing product. Likewise, if the machine is managing to stamp 500 holes per hour, but 100 of them are in the wrong place, it is exhibiting a sign of failure.

Step #3: Writing your procedures
Once your FMEA is completed, it’s time for the PM procedures to be written. A good place to begin is by reviewing the owner’s manual and the supporting documents that were provided by the manufacturer when the machine was purchased. You want familiarize yourself with the functions the manufacturer suggests conducting and when. This is especially true if the equipment is a new and just being brought online. If that’s the case, following the manufacturer’s suggested procedures should keep you out of trouble until you develop some machine history of your own to evaluate. Keep in mind that the PM procedures and intervals suggested by an OEM are not in and of themselves the road to machine reliability. Each machine and machine installation is unique—and, manufacturers typically have not operated their products in real-world plant environments before supplying them to you. Most importantly, they have not operated them in your plant, with your personnel, at your rates of production. Thus, your reality will differ greatly from the manual. Through your evaluations and research, you have identified many potential failures that must be guarded against and many repetitive tasks that must be performed. Now you must decide the most effective schedule to complete the tasks.

In most cases, there will be a daily perfunctory or runtime inspection, a weekly mid-level inspection, a monthly major PM, as well as a series of regularly-scheduled procedures that will deal with overhauls, major replacements of wear parts, mandated inspections and the like. Regardless of the CMMS system that you have—or even if you do not have one—remember the following points when writing your PM procedures:

  • Keep it simple and short. You do not want to cut-and-paste your entire FMEA into your PM and print it out once a week with “Perform These Tasks” written across the top. Your millwright will be overwhelmed, and nothing will get done. Rather, the work should be divided into a series of shorter operations, optimally of no more than two hours in length. The millwright will experience a sense of accomplishment and an accompanying morale boost every time he completes one of these shorter pieces of work.
  • Keep it safe. The first language that the millwright encounters as he reads his PM document must refer to the safe performance of the task. Lockout/ tagout and PPE should be specified at this point.
  • Keep it logical. If you are checking bearings, check all of them. If you are checking drive belts, check all of them. Give the millwright the benefit of intuition by grouping similar functions and objects.
  • Solicit input. Construct your PM document so the information that must be recorded on it could only be derived by the actual completion of the PM. If your PM contains a series of check boxes, you will get a series of checks. How, though, would you know if the work were actually performed? If you ask for readings, temperatures and measurements, it will be much more likely that the work you have requested will be performed.
  • Build accountability into the document. To put it simply, when the millwright signs the document, he is signing that the work actually was done…that it was done correctly…that it was done according to specification. A good way to ensure conformity is to randomly assign a supervisor to view the work as it is performed or immediately thereafter. This is the standard you must hold, and your millwrights must understand that this is the level of accountability to which they are being held.

Getting where you want to go If you follow these steps detailed here, you will be on your way to a betterperforming and less-reactive process. Remember, however, that a written PM procedure is a living document. It will change over time based on the machine’s performance and the millwright’s inputs. While you may not get it exactly right the on the first cut, over time, a well-written PM procedure can evolve into a world-class document and process—one that will have transferability and application to the other machines in your operations.

Ray Atkins, CPMM, CMRP, is a veteran maintenance professional with 14 years experience in the lumber industry. He is based in Rome, GA, where he spent the last five years as maintenance superintendent at Temple-Inland’s Rome Lumber facility. He can be reached at raymondlatkins@aol.com

Continue Reading →

179

6:00 am
April 1, 2007
Print Friendly

The Fundamentals: Safe Work Practices For Workplace Disasters

0407_fund_work_1

Be careful during “wrench thrown into the works” situations. A Maintenance pro is an investment your company can’t afford to lose.

Despite well-established policies, procedures and recordkeeping, unexpected obstacles or snags often cause setbacks during scheduled or routine maintenance. In most cases, we’ve allotted adequate time to overcome these problems, allowing for safe and thorough completion of the task within a timeframe that doesn’t hurt the up rate of the process. On the other hand, what about unplanned and unscheduled maintenance that cannot be predicted or prevented-and the safety concerns that these situations may raise?

A view from the trenches
A malfunction or miss operation can result in a pretty messy breakdown. Some people refer to this type of incident as “the wrench thrown into the works.” It usually calls for the repair of equipment under a more stressful environment than usual-meaning truly unfavorable working conditions. More often than not, these breakdowns seem to occur at a particularly untimely, unsuitable, inconvenient hour of the day or night, typically when production management is demanding that the impossible be done yesterday.

Most of us working in the “trenches” of the industrial battlefield find ourselves in these situations from time to time. In dealing with unplanned maintenance, it is vitally important for Maintenance teams to adhere to all relevant safety guidelines and procedures supplied to them by their respective companies. The following reminders and strategies are offered simply as suggestions to help teams address future unplanned events.

Protect thyself. . . Don’t become a casualty! This is priority one. All too often, when unplanned maintenance pressures surround us, we tend to react before we assess. Not good. Instead, we need to more deeply assess an unplanned and/or catastrophic situation before we begin repair.

Protect yourself. Don your personal protective equipment (PPE). Don’t rush in-don’t rush the job.

Control the scene. . .
Power down the equipment and isolate all other energy sources (electrical, steam, water, hydraulics, etc.). Lock out/Tag out! Take a look around to ensure that no other troubles have occurred in the area as a result of the original failed equipment.

Once all is secure, begin assessing the trouble spot and the damage. Bring in as much of your Maintenance team as you can. The old saying that “too many cooks spoil the pot” doesn’t apply to the Maintenance field. The more experienced, watchful eyes looking at the problem area, the better our understanding of the failure will be. Moreover, this approach also means there are more eyes to survey for any unsafe conditions that might still remain.

Identify and fix safely and quickly. . .
In most cases, after establishing a safe, secure and confident environment, a skilled Maintenance group can identify the failure quickly. And, because you have brought as many experienced Maintenance personnel on scene as you can, the fix can be evaluated and the repair time estimated at an accelerated rate.

Remember, the more minds the better. There is power in numbers. There is safety in numbers. Now, disperse the team. Some should go get parts. Some should go get tools.

Most importantly, some should start cleaning. The area has to be clean before the work begins. This will eliminate risk of injury and remove any hindrances that could delay the repair.

When the work begins, the Maintenance team needs to keep talking to each other. Give a play-by-play analysis of what’s going on. This type of continuous communication informs everyone on the work that is being done-and the progression of that work. Ongoing communication also can help eliminate errors that might inadvertently (and silently) occur. In the end, the job is completed safely and in a timely manner.

Don’t overlook post-repair steps. . .

  • Once the repair work is finished, clean the area well and reinstall all guards.
  • Bring on power and energy sources slowly with everyone’s knowledge of the steps taking place.
  • Then, as standard practice would have it, all involved should begin to look, listen and feel.

The “Safe” team-oriented approach outlined here, coupled with continuous, quality communication, can help produce a timely and successful repair for your unplanned maintenance situations.

Final notes
Money is the bottom line for all businesses. A conscientious employer knows the value of a Maintenance professional. In the grand scheme of profit and losses, it is not cost-effective for a company to lose a highly qualified Maintenance team member because of an injury resulting from hasty reactions to a chaotic situation.

Take for example a Maintenance worker with 10 years experience. He/she gets hurt. What if the company loses that individual for a single day? Doesn’t this hurt productivity-especially when equipment is down? A losttime injury, though, could last for weeks. Consider what your company could lose in job knowledge and familiarity with the process and equipment while a Maintenance team member recovers from a lost-time injury. This calculation doesn’t even begin to take into account the time and money invested in training and educating that experienced technician over the past 10 years. It’s gone.

0407_fund_work_2

Most employers understand the value of their Maintenance professionals. Some employees within a company may not. Don’t let one person’s ignorance coerce you into taking unnecessary risks.

Do not succumb to outside pressures when it involves your own safety or the safety of another employee. Protect yourself, your fellow workers and your company. Taking a “Safe,” calm approach can help prevent casualties.

Glenn Anderson is maintenance supervisor at Toray Plastics (America), Inc., an ISO 9001 and ISO 14001 certified company, in North Kingstown, RI. Anderson began his career with Toray 15 years ago. For the past 12 years he’s been responsible for the company’s preventive maintenance, repairs and up rate of equipment.

Continue Reading →

806

6:00 am
April 1, 2007
Print Friendly

The Fundamentals: Extending Drive Belt Life

Most end users think OEMs take particular pains to design things that last. That’s true in most cases, but not all.

OEMs are in business to sell products. Consequently, countless OEM drive belts are designed with neither the concept of long-term cost savings in mind, nor with the idea that a Maintenance person might need to replace components because of wear. In fact, many belt products simply are selected (and sold) on the basis of lowest price-and the hope that they will facilitate the widest possible range of field adjustments and operate long enough to make it through the warranty period. After that, it’s up to the end user to make changes. Or, to be more precise, it’s generally left to the Maintenance organization to devise a way to make a belt last longer and cost less to operate.

Rules of thumb
The application with the greatest potential cost savings is a drive that operates 24/7-365 and is larger than 1 HP. It will provide the quickest payback and net the Maintenance department the greatest credibility for the changes.

Expected life with a 24/7-365 operation is commonly three years minimum. In the view of this long-time Maintenance professional, any “100% duty cycle” drive that does not last that long desperately needs revision.

The key is to operate a drive near the “Maximum Belt Velocity” or “Rim Speed.” That maximum is 6500 FPM for cast sheaves, 8000 FPM for Ductile iron and 10,000 FPM for steel. At FPMs higher than these, centrifugal forces will exceed the tensile strength of the material, risking the sheaves flying apart. The safest approach, however, is to use only the 6500 FPM limit. That’s because sometime in the future, someone in your company may try to save money by using the same pitch diameter (P.D.) and install cheaper cast sheaves-without considering the dangerous consequences associated with that decision…

Why start by looking at Belt or Rim Velocity? Think of it this way: Would you use a really short tack hammer to pry out a 4″ spike? No! You would grab the longest wrecking bar you could find because it would let you apply the greatest force with the least effort. The same is true with sheaves-the larger the sheave diameter, the greater the length of the lever is and the easier it is to transmit power. The only limits are the centrifugal force that is generated and the ability of the sheave material to handle it.

With the foregoing in mind, we should expect an 1800 RPM motor to spin a maximum of a 13.7″ P.D. sheave and a 3600 RPM motor to spin a maximum of a 6.9″ P.D. When you look at a drive, if the motor sheave isn’t close to 12″ in diameter (or 6″ for higher-speed motors), it is not designed for long service life. This represents a potential drive-improvement opportunity.

For other shaft RPMs, use the formula for Maximum Sheave P.D.

P.D. max = 6500/(0.2618 x RPM)

The ideal pitch will safely operate just under this maximum.

For example…
An existing drive (roughly 6″/ 8″sheaves) has a drive ratio of 1.33:1. From the sheave catalogue we find the following:

0407_fund_belt1

Note that a 6.2 / 8.2 P.D. sheave set is the largest listed for a 1.33 ratio, yet the belt velocity is calculated and found to be 2840. That is not very close to the previously discussed 6500 FPM maximum. Thus, there is lots of room to move closer to a 6500 FPM belt velocity.

Notice in the HP/Belt column, the same “A” profile belt can handle much more HP as the sheave sets become larger in pitch. Just think what even larger sheaves would do to the values in the HP/RPM and Belt columns?

0407_fund_belt2Next, refer to the Stock Sheave listing for the full pitch range that is available. Stock sheaves in the catalogue are listed from 1.9 P.D. to 37.5 P.D.

Again using a 1800 RPM motor, the closest sheave sizes under the 13.7 P.D. maximum are 12.0, 13.0 and 13.2. These will become the new candidates for the driver sheave on the 1800 RPM motor.

The Drive Ratio we want to keep is 1.33, so the machinery performs the same as it did before we make changes. Thus, we multiply the 12.0, 13.0 and 13.2 pitch diameters by the ratio 1.33.

 

This gives the following results:

12.0 x 1.33 = 15.96 P.D.
13.0 x 1.33 = 17.29 P.D.
13.2 x 1.33 = 17.556 P.D.

Now, which one is close to a standard stock sheave?

15.4, 16.0 and 18.4 are listed as standard stock pitch diameters. The 15.96 calculated P.D. and 16.0 listed stock P.D. are the closest match.

Our drive is now shaping up: We have a 12.0 P.D. driver with a 16.0 P.D. driven sheave

According to the nameplate, our nominal 1800 RPM motor runs at 1725 at full load. So, let’s recalculate the new drive parameters.

Recalculating the driver…
12.0 P.D. @ 1725 RPM driver speed Belt velocity = P.D. x .6218 x RPM = 12.0 x 0.2618 x 1725 = 5419.26 FPM (Safely under the 6500 FPM limit)

Recalculating the overall drive…
16.0 P.D. driven/12.0 P.D. driver = 1.3333 drive ratio (Excellent)

Recalculating the driven…
16.0 P.D. @ 5419.26 FPM Driven RPM = (driver P.D. /driven P.D.) x driver RPM = (12.0/16.0) x 1725 = 1293.75 RPM (Very close to the original measured RPM)

Dynamic pull comparison…
Now that we have the drive specifics, let’s calculate dynamic belt pull. That’s the actual pull that transfers power from the motor to the machine.

Using the formula:

Dynamic Pull = ((HP x 126,000)/(RPM x in. Sheave P.D.)) x 1.5 standard service factor. A 10 HP motor with the 6.2/8.2 sheave set will pull 176.72 lbs. on the tight side of the belt.

A 10 HP motor with the 12.0/16.0 sheave set will pull 91.3 lbs. on the tight side. (That is a drop of 85.42 lbs. to transmit exactly the same 10 HP to the machine-almost half the effort.)

Static pull vs. dynamic pull…
Static pull is the amount of tension a mechanic puts on the belt when the drive is installed. This tension is equal on both halves of the belt.

Let’s say the mechanic puts 175 lbs. of static pull on a drive. With the 6.2/8.2 sheave set, we see a 176.72 lb. pull. How, though, do these conditions affect the static pull? We can calculate it as follows: There is a pull of 175 lbs. of static pull plus 176.72 lbs. of dynamic pull on the tight side. That adds up to 351.42 lbs. of pull-quite a hefty force.

On the slack side, the static pull is reduced by 176.72 lbs. Thus, 176.72 lbs. of dynamic pull is subtracted from the 175 lb. static pull. That leaves us with a a deficit of almost 2 lbs.. The belt’s slack side flops around loosely, the belt slips and squeals. What, then, does the mechanic do? Tighten the belt, of course. This brings the static pull much higher than the dynamic pull to keep the drive running quietly.

With the larger 12.0/16.0 sheave set, the dynamic pull is 91.3 lbs. plus the static pull of 175, which only leaves a tight side pull of (91.3 + 175) or 266.3 pounds and, conversely, a slack side pull of 83.7 lbs. There is, accordingly, no need to readjust the drive.

Keep in mind that all of these examples relate to “running condition” and ignore the starting pull on the drive. That short-duration force can be at least three times the dynamic pull.

The finer points Every rubber drive belt is essentially an elastic drive medium that, because of Dynamic Belt Pull, will stretch longer on the tight side than on the slack side. By making the sheaves larger than the original drive, the belt pull is reduced and so is the amount of belt stretch.

When a belt stretches every revolution under load, driven RPM is reduced. This difference-or “allowed belt slippage”-must be kept below 2%. Above 2%, the belt returns to its slack-side length part-way around the driver sheave, and stretches to its tight-side length partway around the driven sheave. This movement, when in contact with the sheaves, causes destructive wear and heat. Under 2%, the rubber distorts, yet maintains its grip on the sheave, absorbing the movement and releasing heat to the air between sheaves. This results in a long-lasting, cooler running drive.

To see how this works, consider this: A driven sheave’s 1293.75 RPM, calculated in the foregoing manner with a 2% slippage, is actually 98% of the calculated RPM (slower than designed), or:

1293.75 x .98 = 1267.875 RPM

If the driver is actually 1725 RPM, then the measured driven RPM should be between 1268 and 1293 RPM to minimize any destructive effects of slippage.

Belt wrap on the smaller sheave also is a concern. For example, if the small sheave wrap is about 170 degrees on the original, with the 12.0 P.D. sheave and the same shaft center distance and drive ratio, there will be the same angle of contact or belt wrap.

The small sheave circumference, times the amount of wrap will indicate the length of belt gripping the sheave.

The formula for the length of belt wrap is:

= (P.D. x 3.1415) x (degrees of wrap/360)

The improved design example of a 12.0/16.0 drive is:

= (12.0 x 3.1415) x (170/360) = 37.698 x .4722 = 17.8″ of belt contacting the sheave Using the original example of the 6.2/8.2 drive for comparison:

= (6.2 x 3.1415) x (170/360) = 19.48 x .4722 = 9.197″ of belt in contact

An increase from 9.197″ to 17.8″of belt in contact will be almost double-a definite improvement in grip.

Installing the new, larger drive is a bit more demanding with regard to parallel and angular alignment in both the vertical and horizontal planes. For instance, with a motor and a fan (both of which have horizontal shafts), an alignment is usually set closely across the hub area and parallel and angular misalignment are removed. When checking vertical alignment between the motor and fan sheave, one sees that the top of the motor sheave tilts toward the motor and the bottom away from the motor. This drive twist- usually ignorable on smaller drives-becomes a real issue on larger drives, and requires adjusting out to avoid premature wear. When attempting to extend drive life two or three times the current life, remember that small details like this can adversely affect the desired long-term deliverable life.

If you had a chance to watch both the original and new drives under load, you would have noticed that the smaller unit usually exhibited a noticeable slack-side flop, while the tight side would remain stable. The new, larger drive, however, runs with the tight-side straight and stable, and the slack-side stable with a slight outward bow, when it is eyed down the length.

If the original small drive were started up, you would hear a definite thud or pound, as if the unit were hit by a huge rubber hammer. On the other hand, the new, larger drive’s start-up thud will be almost silent-as if a small rubber hammer is being used. Generally, the new drive will run more smoothly, all the way around.

It is important to note that heat is the greatest single destructive element to which rubber can be subjected. Heat makes rubber harder and less flexible-and more susceptible to fracture and breakage. According to the Rubber Manufacturer’s Association, V-belts will operate acceptably at temperature from -30 to 140 F. An internal temperature rise of 18 degrees F in this type of belt will decrease its service life by 50%. With our previously discussed drive modification, the heat generated in repeated stretch-and-relax cycles of the slack and tight sides has been reduced. The slippage heat generated in gripping the sheaves has been cut. The heat generated in bending or flexing around the smaller-diameter sheaves drops significantly.

  • A higher horsepower per belt capability, which, in some cases, reduces the number of belts, or drops the belt profile to a smaller one
  • Increased belt-wrap length for more gripping surface
  • Less belt-bending occurs around a larger sheave set, equating to less flex-generated heat
  • Less slippage or stretch-generated heat
  • Higher belt velocity and sheave rim speed that enhances drive air-cooling
  • New belts for the drive on the same center distance that are longer, resulting in any generated heat being dissipated over a much longer length of belt
  • Much cooler operating belts and sheaves, equating to belts that last exponentially longer
  • Much lower belt pull, which lowers drive-end bearing loads and start-up shock
  • A more efficient belt drive that leads to definite energy savings, given continuous 24/7 power demands
  • With regard to PMs, the elimination or revision of monthly, quarterly or semi-annual inspections-down to only one annual inspection. (Even then, you may not have to readjust belt tension in the first or second year, if the initial installation includes a tension check after the first two to 48 hours of operation, then again, after one-month’s running, to take up the initial stretch and set the new belts.)

Doing it right
What can you really expect when you increase the velocity of your drive belts?

After five years of 24/7-365 operation, such belts will be riding lower in the sheave and the sheave edges will be visible above the crown of the belt, but not running on the bottom of the groove. When the belts are removed, you will find them to be surprisingly supple and flexible, with no segmented cracks anywhere. When the sheaves are inspected, there will be very little indication of the Vgroove wearing to a U-shape. Sheave grooves will appear highly polished, almost like chrome plating-ready for another five years of continuous operation with a new set of belts.

According to industry sources, well designed and carefully installed drives typically will generate very little heat. Furthermore, the cooling effect of the belt through the air will tend to cool the whole drive to a temperature very close to ambient. Such drives are expected to run 24/7-365 for three to five years.

Calculate, specify and install drive belts correctly and they will require very little maintenance for years.

Gary Burger worked himself up through the ranks of Canadian Occidental Petroleum, Durez Plastics Division, to become maintenance supervisor and chief engineer. He then joined the Stevenson Memorial Hospital maintenance team in Alliston, ON, Canada, as chief engineer. Over the past 10 years, he has helped lower this facility’s energy consumption by over 64%, while keeping it all within budget. E-mail: burgergary@hotmail.com

Continue Reading →

2354

6:00 am
January 1, 2007
Print Friendly

The Fundamentals: The Basics Of Torque Measurement

Brushing up on the available methods and tools for measuring torque will help you improve your accuracy, as well as protect your wallet.

Torques can be divided into two major categories: static or dynamic. The methods used to measure torque can be further divided into two more categories: reaction or in-line. Understanding the type of torque to be measured, as well as the different types of torque sensors that are available, will have a profound impact on the accuracy of the resulting data, as well as the cost of the measurement.

Static vs. dynamic
In a discussion of static vs. dynamic torque, it is often easiest to start with an understanding of the difference between a static and a dynamic force. To put it simply, a dynamic force involves acceleration, while a static force does not.

0107_fund_torque_1The relationship between dynamic force and acceleration is described by Newton’s second law; F=ma (force equals mass times acceleration). The force required to stop your car with its substantial mass would be a dynamic force, as the car must be decelerated. The force exerted by the brake caliper in order to stop that car would be a static force, because there is no acceleration of the brake pads involved.

Torque is just a rotational force–or a force through a distance. From the previous discussion, torque is considered static if it has no angular acceleration. The torque exerted by a clock spring would be a static torque, since there is no rotation and, hence, no angular acceleration.

The torque transmitted through a car’s drive axle as it cruises down the highway (at a constant speed) would be an example of a rotating static torque. In such a case, even though there is rotation, at a constant speed there is no acceleration. The torque produced by the car’s engine will be both static and dynamic, depending on where it is measured. If the torque is measured in the crankshaft, there will be large dynamic torque fluctuations as each cylinder fires and its piston rotates the crankshaft. If the torque is measured in the drive shaft, it will be nearly static since the rotational inertia of the flywheel and transmission will dampen the dynamic torque produced by the engine.

The torque required to crank up the windows in a car (remember those?) would be an example of a static torque, even though there is a rotational acceleration involved, because both the acceleration and rotational inertia of the crank are very small and the resulting dynamic torque (Torque = rotational inertia x rotational acceleration) will be negligible when compared to the frictional forces involved in the window movement. This last example illustrates the fact that for most measurement applications, both static and dynamic torques will be involved to some degree. If dynamic torque is a major component of the overall torque or is the torque of interest, special considerations must be made when determining how best to measure it.

Reaction vs. inline
Inline torque measurements are made by inserting a torque sensor between torque carrying components, much like inserting an extension between a socket and a socket wrench. The torque required to turn the socket will be carried directly by the socket extension. This method allows the torque sensor to be placed as close as possible to the torque of interest, preventing possible errors in the measurement such as parasitic torques (bearings, etc.), extraneous loads and components that have large rotational inertias that would dampen any dynamic torques.

According to the above example, the dynamic torque produced by an engine would be measured by placing an inline torque sensor between the crankshaft and the flywheel, thus avoiding the rotational inertia of the flywheel and any losses from the transmission. To measure the nearly static, steadystate torque that drives the wheels, an inline torque sensor could be placed between the rim and the hub of the vehicle, or in the drive shaft. Because of the rotational inertia of a typical torque drive line and other related components, inline measurements are often the only way to properly measure dynamic torque.

A reaction torque sensor takes advantage of Newton’s third law that “for every action there is an equal and opposite reaction.” To measure the torque produced by a motor, we could measure it inline, as described above, or we could measure how much torque is required to prevent the motor from turning,which is commonly called the reaction torque.Measuring the reaction torque helps us avoid the obvious problem of making the electrical connection to the sensor in a rotating application (discussed later). This method, however, comes with its own set of drawbacks. 0107_fund_torque_2

A reaction torque sensor often is required to carry significant extraneous loads, such as the weight of a motor or, at least, some of the drive line. These loads can lead to crosstalk errors (a sensor’s response to loads other than those that are intended to be measured), and sometimes to reduced sensitivity, as the sensor has to be oversized to carry the extraneous loads. Both of these methods–inline and reaction–will yield identical results for static torque measurements.

Making inline measurements in a rotating application will nearly always present the user with the challenge of connecting the sensor from the rotating world to the stationary world. There are a number of options available to accomplish this, each with its own advantages and disadvantages. They are:

Slip ring…
The most commonly used method to make the connection between rotating sensors and stationary electronics is the slip ring. It consists of a set of conductive rings that rotate with
the sensor and a series of brushes that contact the rings and transmit the sensors’ signals.

Slip rings are straightforward and economical solutions that perform well (with only a few minor drawbacks) in a wide variety of applications. The brushes, and to a lesser extent the rings, are wear items with limited lives that don’t lend themselves to long-term tests or to applications that are not easy to service on a regular basis. At low- to moderatespeeds, the electrical connection between the rings and brushes are relatively noise-free. At higher speeds, though, noise will severely degrade their performance.

The maximum rotational speed (rpm) for a slip ring is determined by the surface speed at the brush/ring interface. As a result, the maximum operating speed will be lower for larger, typically higher torque-capacity sensors by virtue of the fact that the slip rings will have to be larger in diameter, and therefore have a higher surface speed at a given rpm. Typical max speeds will be in the 5,000 rpm range for a medium-capacity torque sensor.

Finally, be aware that the brush ring’s interface can be a source of drag torque. This can be a problem, especially for very low-capacity measurements or applications where the driving torque will have trouble overcoming the brush drag.

Rotary transformer…
Rotary transformer systems were devised in an effort to overcome some shortcomings of the slip ring. They use a rotary transformer coupling to transmit power to a rotating sensor. 0107_fund_torque_3An external instrument provides an AC excitation voltage to the strain gage bridge via the excitation transformer. The sensor’s strain gage bridge then drives a second rotary transformer coil to get the torque signal off the rotating sensor. By eliminating the slip ring’s brushes and rings,wear is gone, making the rotary transformer system suitable for long-term testing applications. Parasitic drag torque from brushes in a slip ring assembly also is eliminated. But, the need for bearings and the fragility of transformer cores still limit maximum rpm to levels only slightly better than the slip ring.

This system also is susceptible to noise and errors induced by the alignment of the transformer primary-to-secondary coils. Because of the special requirements imposed by rotary transformers, specialized signal conditioning also is required in order to produce a signal acceptable for most data acquisition systems, further adding to the system’s cost–which is already higher than a typical slip ring assembly.

Infrared (IR.)…
Like the rotary transformer, the infrared (IR) torque sensor utilizes a contact-less method of getting the torque signal from a rotating sensor back to the stationary world. Similarly using a rotary transformer coupling, power is transmitted to the rotating sensor.0107_fund_torque_4 Instead of being used to directly excite the strain gage bridge, this is used to power a circuit on the rotating sensor. The circuit provides excitation voltage to the sensor’s strain gage bridge and digitizes the sensor’s output signal. This digital output signal is then transmitted, via infrared light, to stationary receiver diodes, where another circuit checks the digital signal for errors and converts it back to an analog voltage. Since the sensor’s output signal is digital, it is much less susceptible to noise from such sources as electric motors and magnetic fields.Unlike the rotary transformer system, an infrared transducer can be configured either with or without bearings, for a true maintenance-free, no-wear, no-drag sensor.

While it is more expensive than a simple slip ring, the infrared torque sensor offers several benefits.When configured without bearings, as a true non-contact measurement system, the wear items are eliminated, making this sensor ideally suited for long-term testing rigs.More importantly, with the elimination of the bearings, operating speed (rpm) goes up dramatically–to 25,000 rpm and higher, even for highcapacity units. For high-speed applications, this often is the best solution for a rotating torque transmission method.

FM transmitter…
Another approach to making the connection between a rotating sensor and the stationary world utilizes an FM transmitter. These devices are used to connect any sensor, whether force or torque, to a remote data-acquisition system, by converting the sensor’s signal to a digital form and transmitting it to an FM receiver–where it is converted back to an analog voltage. For torque applications these receivers are typically used for specialty, one-of-a-kind sensors–such as when strain gages are applied directly to a component in a drive line. This application, for example, could be a drive shaft or half shaft from a vehicle.

The FM transmitter can be easily installed on the component, as it is typically just clamped to the gaged shaft. It also is re-usable for multiple custom sensors. Drawbacks include needing a source of power on the rotating sensor, typically a 9V battery, which makes it impractical for longterm testing.

Conclusion
Understanding the nature of the torque to be measured, as well as the factors that can alter this torque in the effort to measure it, will have a signficant impact on the reliability of the data collected.

In applications that require the measurement of dynamic torque, special care must be taken to measure in the correct location–and to not affect the torque by dampening it with the measurement system.

Knowing the options available to make the connection to the rotating torque sensor can greatly affect the price of the sensor package. Slip rings are an economical solution, but have their limitations.More technically advanced solutions are available for more demanding applications, but they usually will be more expensive.

 0107_fund_torque_5

By thinking through the requirements and conditions of a particular application, you can choose the right torque measurement system for the application the first time- and every time. TF

David Schrand is engineering manager with Sensor Developments, Inc. (SDI), of Orion, MI. SDI was established in 1976 as an engineering consulting firm specializing in the science of force measurement and sensor design. Today, the company produces solutions for many industries and applications, including automotive, aerospace, OEM, medical, nuclear and textile operations. For more information on the technologies referenced in this article, log on to www.sendev.com

Continue Reading →

Navigation