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8:38 pm
March 21, 2012
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Prevent Premature Failure In Power Transmission Belts: Troubleshooting Problems

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Belts can have issues. Understanding them is key to keeping many of your critical processes up and running.

By Eric Bjork, Gates Corporation

When power transmission belts fail prematurely, productivity plummets and replacement costs add up. Knowing how to troubleshoot belt failure and take corrective and preventive actions saves valuable time and, ultimately, money.

Understanding V-belt failure issues
When a V-belt reaches the end of its service life, it will break or separate due to normal wear over an extended period of time. A properly installed and tensioned V-belt will operate at 93%-97% efficiency and last up to three to five years. Used in harsh conditions, its service life can drop to a year or less. If, however, a belt needs to be replaced more than twice a year, it may indicate a more serious problem.

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  • Improper maintenance. Neglecting to monitor alignment, tensioning and signs of abnormal wear may result in the incorrect amount of tension, worn sheaves and misalignment, all of which can cause failure. Forty-two percent of belt failures can be attributed to inadequate maintenance.
  • Poor design. If a drive is not designed properly or if a user makes changes to the speed or load of equipment, belts can encounter several problems. A properly designed drive takes into account desired belt type, motor rpm and horsepower, service factor, desired pulley rpm or speed ratio and center distance between shafts.
  • Improper installation. The wrong tension level can cause the belt to slip under peak loads, while misalignment between the belt and sheave can cause abnormal wear and belt instability.
  • Environment factors. If a V-belt is hot to the touch, it is operating above its ideal temperature range and may not be suitable for the application. V-belts operating in dusty environments where debris is present are subject to contamination between the belt and sheave, which can cause excessive wear.
  • Incorrect handling and defective components. V-belts should be stored on a flat surface. Hanging them can cause crimping and shorten service life. When it is time to mount the belt, check the sheaves for sharp edges and wear. Also, make sure to avoid prying or rolling it onto the sheave. This can damage tensile cords.

Identifying and solving V-belt problems…
Knowing how to prevent future occurrences is beneficial, but how does one identify current V-belt problems?

Pay attention to the V-belt drive and note any abnormalities. Unusual sounds, excessive vibration and a hot belt are all signs that one should inspect a drive for issues. Examine the exterior of the belt for any signs of even wear, cracking, frayed covers, burned spots, swelling or hardening. For detailed troubleshooting, shut down the drive and inspect not only the belt, but the sheaves, belt guard, bearings and shafts.

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Failure due to normal synchronous belt fatigue

When inspecting a V-belt, look for excessive wear in the following locations:

  • Top surface. This could be caused by the belt rubbing against the guard or by an idler malfunction. Check both and repair or replace the offending part.
  • Top corners. The belt may be too small for the groove in the sheaves. Make sure to match the belt to the correct sheave.
  • Belt sidewalls. Excessive wear could be from incorrect tension, which can be corrected by retensioning the drive, from sheave misalignment, which requires realigning and sheave replacement, or the belt may be the wrong size for the drive and should be replaced.
    If the sidewalls are burned or hardened, it may be a sign of a slipping belt, worn sheaves, an under-designed drive or shaft movement. Retension a slipping belt and replace a worn sheave. If the drive is under-designed and cannot carry the load, redesign it to the manufacturer’s recommendations. Shaft movement could be caused by changes in the center distance between the sheaves, and should be checked and adjusted.
  • Bottom corners. The sheaves may be worn or fit between the belt and sheave may be incorrect. Check sheaves for wear and replace or find the correct belt/sheave match.
  • Belt surface. If the bottom surface is worn, the sheaves may be worn or have debris in them. Clean or replace them. The belt may also be bottoming out against the sheaves because of an incorrect match between the belt and sheave. Find the proper match to correct the problem. Pay close attention to how the belt’s surface feels. If the belt is hard or stiff, it may be due to either a hot environment or belt slip. Provide more ventilation to the drive or adjust belt tension to correct the problem. If the surface is flaking, sticky or swollen, it may have been contaminated by chemicals. Eliminate the source of contamination.
  • Undercord. Environmental conditions, improper storage, belt slip or a sheave that is too small can crack the undercord. Address this by controlling the belt drive environment, storing the belt properly, retensioning to manufacturer’s recommendations and replacing a small sheave with a larger one. Also, check the size of the backside idler. If its diameter is too small, increase the size.

Identifying and solving banded V-belt problems…

When banded V-belts show evidence of tie-band separation or damage, come off the sheave repeatedly or have one or more belt ribs running outside the sheave, it is time to examine the drive.

Here’s what to look for in regard to such problems:

  • Tie-band separation. Check for sheave misalignment, and realign the drive if necessary.
  • Top of tie-band damage. Adjust the guard if it is interfering, or clean and possibly replace the sheaves.
  • Repeatedly jumping off the sheave. Align the drive to correct any misalignment, clean out the sheaves or try single belts.
  • One or more ribs running outside the sheaves. Check the manufacturer’s specifications and retension the belt or realign the drive.

Understanding synchronous-belt failure issues

Similar to V-belts, synchronous belts can last for years if properly maintained. Synchronous belt failure from tensile cord fatigue after a running time of two to three years is normal. While belt life can vary significantly between applications due to numerous factors, including the transmitted power level, the environment, belt installation tension, shaft/sprocket alignment, sprocket condition and even how the belt was handled prior to and during installation, replacing synchronous belts more than once per year may be a symptom of a larger issue.

The most common causes of premature synchronous belt failure are:

  • Belt crimping. A sharp bend may cause individual fibers within the tensile members to buckle or crimp, reducing the overall ultimate tensile strength of the belt. This can occur when a belt is mishandled, has incorrect installation tension, sub-minimal sprocket diameters or when a foreign object enters the drive.
  • Shock load. When intermittent cyclic torque loads are higher than normal, belt stress levels increase, which may lead to failure. Severe shock loads can cause belt tensile breaks that can be identified by a ragged and uneven appearance. If the drive shock loads can’t be eliminated, increase the belt tensile strength. Or you can replace the synchronous belt drive with a more forgiving V-belt drive system that’s capable of inter-mittent slip.
  • High belt installation tension. Applying too much tension to a synchronous belt may result in root cracks, which can cause the belt teeth to fall off, or excessive wear, which exposes individual tensile cords. Prevent these issues by determining and setting proper tension levels.
  • Low belt installation tension. Not applying enough tension can
  • cause belt tooth rotation or tooth wear (i.e., hook wear). Increase belt installation tension levels to prevent these problems. If doing
  • so doesn’t take care of the issue, the drive structure might not be rigid enough to prevent deflection and may need structural support. If increasing belt-installation tension levels isn’t possible, try to increase the sprocket diameters.
  • Sprocket condition. Sprockets can also cause premature belt failure, yet they’re rarely inspected when a belt fails. Sprocket wear may occur from belts that have been installed with excessive installation tension or ones that have had the tooth facing or jacket completely worn away. When a ridge along the tip of the sprocket teeth becomes visible, the sprocket should be replaced.
  • Environment. Belts operating in abrasive atmospheres on applications such as phosphate mining conveyors or foundry shakers can fail prematurely. Extend the life of sprockets and belts that are exposed to abrasive dust and contaminants by installing a sealed guard pressurized with clean air.

Similarly, high temperatures and chemical exposure can degrade belt material, causing cracking and crimping. Consider replacing standard synchronous belts that are operating in temperatures above recommended limits, or are exposed to high ozone levels, with ones constructed to withstand extreme heat and chemicals. MT

Eric Bjork’s responsibilities with Gates include providing engineering support for MRO customers and equipment designers via the PA Hotline and leading Gates’ sponsorship of the FIRST® Robotics Competition. He received his Bachelor’s degree in Mechanical Engineering from the University of Colorado at Boulder.

 

1111feattwo4Ignore Belt Issues At Your Peril

Ignoring underlying causes of belt damage leads to increased downtime, decreased pro-ductivity and higher replacement costs. On the other hand, when they’re properly maintained, V-belt and synchronous belt drives can be trouble-free through the course of their service lives.

 For a wealth of free resources, including a poster to use as visual reference in identifying specific belt issues, click here.

For more info, enter 01 at www.MT-freeinfo.com

 

 

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