Every moving part on a machine benefits from timely and effective lubrication to help reduce wear, minimize lubricant consumption and maximize efficiency. These benefits can be more fully realized by introducing centralized lubrication technology to deliver the right lubricant at the right time in the right quantity to the right point of use.
All types of standard and specialized machines can run with centralized lubrication systems. Applications encompass equipment used in a wide range of industries, including automotive, machine tool, metals, printing, paper, food and beverage, mining, chemical, plastics, hydrocarbon processing, refinery and wind energy, among many others. (Commercial vehicles, off-road equipment and rail systems represent viable candidates, too.)
In all cases, centralized lubrication feeds lubricant from a central source to the points on a machine or machining system where friction occurs. The goal is to reduce friction and dissipate some of the heat generated by friction. With centralized lubrication, every bearing receives the proper lubricant in an exact amount to minimize wear and promote longer service life. The problems associated with excessive lubrication can vanish, lubricant consumption can fall over time (in some applications by as much as 50% compared with inexact manual methods) and maintenance time, energy and costs can diminish. The only requirements: Refill the lubrication reservoir and occasionally inspect the connected lubrication points.
The potentially staggering number of on-site (and sometimes hard-to-access) lubrication points makes perhaps the most compelling case for implementing centralized (vs. manual) lubrication technology. A customer census, for example, has identified 7500 individual lubrication points for a paper mill; 5500 for an automotive assembly plant; 4000 for a steel mill; 3500 for a refinery; 2000 for a cement mill; 1500 for a plastics plant; and 1000 for a frozen foods facility. Regardless of the number, centralized lubrication systems foster opportunities to improve productivity and profitability by increasing machinery uptime and keeping maintenance issues in check.
Centralized lubrication technology generally falls under two broad “umbrella” categories: total-loss and circulating-oil systems.
In total-loss systems friction points are always supplied with fresh lubricant (oil, fluid grease or grease) at specific intervals (time or machine-cycle dependent) during the lubricating cycle (such as pump run time). The lubricant is furnished in the proper quantity at friction points to allow for buildup of an adequate film of lubricant during the subsequent idle period. Over time, the forces of aging, evaporation, bleeding and leaks will contribute to partial depletion of the lubricant at the friction point.
Circulating-oil lubrication systems provide for the lubricant to flow back into the lubricant reservoir for reuse after passing through the friction points. In this way, the lubricant carries even more benefits as it transfers forces and damps vibrations; removes abrasion particles from friction points; stabilizes the temperature (cooling or heating) of friction points; prevents corrosion; and removes condensate and process water.
Within the total-loss and circulating-oil categories, primary types of installations include single-line, dual-line, progressive feeder and minimal-quantity lubrication systems. Their profiles are as follows:
Single-line. . .
These total-loss lubrication systems supply machinery lubrication points with relatively small amounts of lubricant (oil or fluid grease up to NLGI grade 2) to cover precisely the amount consumed. As such, they operate intermittently as required. Lubricant can be delivered by manually, mechanically, hydraulically or pneumatically operated piston pumps or by electrically driven gear pumps. In single-line systems, lubricant is metered out by piston distributors installed in the tubing system. Exchangeable metering nipples on the distributors make it possible to supply every lube point with the requisite amount of lubricant per stroke or pump work cycle. Metered quantities can range from .01 to 1.5 ccm per lubrication pulse and lube point. The amount of lubricant to be fed to the lube points can also be influenced by the number of lubrication pulses.
The standard layout of a single-line total-loss lubrication system incorporates a pump and spring-loaded piston distributor; main line (connecting to pump and distributor) and secondary line (connecting to distributor and lube point). Performing as a total-loss lubrication system, an oil return line from the lube point to the oil reservoir is unnecessary.
Dual-line. . .
These systems can deliver oil or grease (up to NLGI grade 2) to as many as 1000 lube points (and distribution points can be easily added or removed). They can be configured to run either as total-loss or circulating-oil versions.
Dual-line layouts consist of two main lines with their respective secondary lines and fittings; an electrically driven pump with reservoir; dual-line feeders; reversing valve and control unit.
All the distributors of a dual-line system are pressurized at the same time— resulting in low pressure losses—and the “reset” of the delivery piston is simultaneously the second delivery stroke, which takes place at full pump pressure. This is what makes dual-line versions especially suitable for extended systems and more viscous types of grease. Assemblies with or without compressive seals can be specified to accommodate light and heavy-duty operating conditions.
Whether functioning as a total-loss or circulating-oil system, progressive feeder systems are intended for intermittent delivery of lubricant (grease up to NLGI grade 2) and are capable of handling up to several hundred lube points. They also offer the ability to provide central monitoring of all feeder outlets, if desired, at relatively low cost.
Progressive feeder installations use pneumatically or manually operated or electrically driven piston pumps. Metered quantities of lubricant are fed progressively in predetermined ratios from master feeders to the lube points, either directly or via a secondary downstream feeder. The lubricant does not leave the respective feeder until the preceding one has discharged its volume. If a lube point does not accept any lubricant, regardless of the reason, or if a secondary feeder is blocked, the entire lubrication cycle is interrupted, which can be used to emit a signal to alert operators to the problem.
These specialized types of total-loss metering systems have been variously engineered for the lubrication of tools and chains, oiling of joined parts and converting from “wet” to “dry” machining operations, where only a minimal amount of lubricant (10ml to 50ml per hour) is required to prevent premature tool wear and/or a poor work piece surface finish.
Minimal-quantity lubrication (MQL) replaces traditional “flood” coolant lubrication by enabling lubricant to be fed to the exact friction point between the tools and work piece externally or from the inside through the tool. Combined systems have been developed to accomplish both.
In an external volumetric MQL system, both lubricant and air are supplied to a spray nozzle or mixing point via coaxial feed lines. The lubricant is then atomized using compressed air and applied to the work piece or tool. In an external, continually dispensing system, oil mist is generated in the supply unit and a feed line supplies the aerosol to the tool or work piece. Using internal MQL, the tool applies the aerosol directly to the lubrication point.
By converting from conventional “flood” lubrication to minimal-quantity lubrication for some equipment, shorter production times can be achieved. Cost savings from this method can result from, among other things, cooling lubricants becoming redundant and elimination of entire machine tool components (such as lubricant filters and conditioning installations) and the expense associated with the disposal of chippings and cooling lubricants.
Decision-making for the most appropriate system will depend, in general, on the application and, in particular, on a range of other parameters, such as the operating conditions (variations in the operating temperature and lubricant viscosity); accuracy requirements for lubricant quantities system geometry (size, dimensions, and symmetry); and monitoring demands, among others. When planning and subsequently installing a centralized lubrication system the following guidelines can help advance the process:
- Determine the number of lube points.
- Factor in the amounts of oil required per lube point and the total amount of oil required per stroke (with piston pumps) or work cycle (with gear pumps).
- Select the distributors in accordance with the metering range and available space. A distinction must be made between oil-only distributors and those that are also suitable for fluid grease.
- Choose pumps consistent with the type of actuation and capacity of the system.
- Determine the type of control for the automatic system (timeor load-dependent) and any monitoring system that may be required.
- When installing a system, lay out all the main lines and distributors to enable air in the system to escape on its own via the lube points.
- Check the resistance in the main line, particularly regarding the relief process, when especially large and widely branched systems are involved and when high-viscosity oils are used.
When centralized lubrication systems are properly designed and implemented, advantages will flow. Users can expect reliable lubricant coverage (especially important for machines with dozens or more lubrication points); optimal lubrication intervals and dynamic lubrication; enhanced oversight (supported by available integrated control units and fill-level monitoring); and lubricant consumption-specific setup and adjustment of maintenance intervals via different sizes of pumps and lubricant reservoirs.
It is important to take care during the installation, startup and maintenance of any centralized lubrication system. The designated system should receive the same attention as all other sophisticated equipment on a machine. Partnering early in the process with an experienced, knowledgeable expert can help fulfill the promise these systems can deliver.
Jerry McLain is business development manager, Lubrication, for SKF USA Inc. His experience includes assisting in the development and implementation of customized machinery and equipment lubrication programs for industry. Telephone: (513) 248-4335; e-mail: email@example.com