Tim Taylor is a field application engineer with Gates Corporation, where he has worked for the past 11 years. He holds a bachelor’s degree in mechanical engineering from Colorado School of Mines. For more information about belt failures, or to order Gates’ free Belt Failure Analysis Poster, visit www.gates.com/ups.
Like cats and dogs, V-belts have a language of their own. The problem is that end users may not understand what their belt is trying to tell them.
Suddenly, something goes wrong. The drive is shut down, and the belt is replaced or retensioned. The belt may not cost much, but the time needed to fix the problem can cost a bundle, especially if the belt is offshore or miles out in the field.
What if the user could speak the belt’s language? It could let the user know how to prevent premature failure, and that could translate into longer belt life, improved drive efficiency, reduced maintenance and downtime and improved productivity.
Isn’t that worth a little lesson in belt talk?
Any mechanical component fails eventually. So, what’s normal failure for a V-belt?
The key factors that influence the frequency of belt replacement, regardless of the application, are speed, load and hours of operation. Normal life for some V-belt drives may be three to five years. Others may have a normal service life of less than one year.
When a V-belt fails under normal usage, it is due to fatigue, so the belt eventually pulls apart and breaks. Long before that happens, however, your belt could be speaking to you about any number of problems, all of which can be fixed.
Recognizing Obvious Problems
Obvious symptoms of a problem drive include excessive noise, vibration, and heat; so a preliminary examination involves looking, listening, and touching. A belt that squeals or chirps, makes a slapping, rubbing or grinding sound or even an unusually loud drive is a sign of trouble. Unusual or excessive vibration, a belt that is hot to the touch or a belt flopping in the sheave also signals trouble.
Any sign of unusual belt wear points to a potential problem with the drive. To conduct a thorough inspection, shut down and lock out the drive and examine all components carefully:
• Belt guard
On the belt, look for uneven wear patterns, cracking, frayed covers, burned spots, swelling, and hardening. The following sections will examine what these clues tell end users about the drive.
When a V-belt fails catastrophically, it breaks and pulls apart. Catastrophic failure is similar to an end-of-life failure. How do you tell the difference? Time. If the belt is designed with a service life of two years and fails after two months, the drive has a problem.
A clean break (Figure 1) could be caused by a drive that is subjected to extreme shock load, by an object falling into the drive or by damaged tensile cords.
Figure 1. Broken belt
In the case of extreme shock load, the drive should be redesigned to handle the shock load. Installing a proper belt guard should prevent objects from falling into the drive. Tensile members are often damaged when operators pry or roll the belt onto the drive during installation instead of installing it correctly. Avoid this problem by ensuring that the motor is moved enough so that the belts are slack when a new belt is installed. Never pry on a belt.
Other symptoms of premature belt failure include edge cord failure (Figure 2) and a belt that simply can’t carry the load.
Figure 2. Edge cord failure
Edge cord failure is typically caused by damaged tensile members resulting from improper installation, damage from contact with a foreign object, misalignment or worn sheaves. To correct the problem, proper installation techniques should be used. Also, obstructions should be found and removed. The drive should be aligned, and worn sheaves replaced.
When a belt cannot carry the load, the drive may be under-designed. Check the manufacturer’s recommendations to fix the problem. Other causes may include worn sheave grooves (replace the sheaves), center distance movement (re-check the center distance and adjust as needed) or low tension (re-tighten the belts).
Severe or Abnormal Wear
A V-belt does not have to fail prematurely to be under duress. Examination may uncover signs that something is not right with the drive.
When inspecting a V-belt, look at these locations and for these conditions:
• Top surface
• Top corners
• Belt sidewalls
• Bottom corners
• Bottom surface
• Undercord cracking
• Sidewall burning or hardening
• Belt surface (hard or stiff)
• Belt surface (flaking, sticky or swollen)
Top Surface Wear
Top surface wear might be caused by the belt rubbing against the guard or by a malfunction of the idler. Check these locations, and repair or replace the guard and/or idler to correct the problem.
Wear on the top corners (See Figure 3) of the belt may indicate that the belt is too small for the groove in the sheaves. Matching the belt to the correct sheave will solve the problem.
Figure 3. Wear on top corners
Excessive wear along the belt sidewalls (See Figure 4) could be caused by several factors. The belt could be slipping due to incorrect tension. If so, retension the drive until the slipping stops. Another potential problem is sheave misalignment, which requires realigning the drive. Worn sheaves may be the culprit, in which case, replace the sheaves. If these are not the cause, the belt may simply be the incorrect size and should be replaced with the correct size.
Figure 4. Wear on the belt sidewalls
Wear on the bottom corners of the belt (See Figure 5) could be due to worn sheaves or an incorrect fit between the belt and sheave. Check the sheaves for wear, and replace them if necessary, or find the correct belt/sheave match.
Figure 5. Wear on the bottom corners
Bottom surface belt wear (See Figure 6) could be caused by debris in the sheaves, sheave wear, or the belt bottoming out against the sheave grooves. Bottoming out is caused by an incorrect match between the belt and sheave and can be corrected by finding the proper match. If the sheaves are worn, replace them. If the problem is debris, clean the sheaves.
Figure 6. Bottom surface wear
Undercord cracking (See Figure 7) may be to the result of a number of factors. Environmental conditions (excessive heat or cold) or improper storage could be to blame. Solutions involve controlling the belt drive environment and following proper storage and handling procedures.
Another cause might be belt slip, which can be corrected by retensioning the belt to the manufacturer’s recommendations. A sheave that is too small for the belt section, causing the belt to wrap too tightly around the sheave, could crack the undercord. Replacing the small sheave with a larger one could correct the problem. Similarly, a backside idler with too small a diameter could be the problem. Fix it by increasing the size of the backside idler.
Figure 7. Undercord cracking
Sidewall Burning or Hardening
Sidewall burning or hardening (See Figure 8) is a sign of belt slip, worn sheaves, an under-designed drive or shaft movement. A slipping belt should be retensioned to the manufacturer’s recommendations. A worn sheave should be replaced.
If the drive is under-designed and cannot carry the load, redesign it to the manufacturer’s recommendations. Shaft movement might be caused by changes in the center distance between the sheaves and should be checked and adjusted.
Figure 8. Sidewall burning or hardening
Hard or Stiff Belt Surface
If the belt surface is hard or stiff (See Figure 9), it might be due to an excessively hot environment or to belt slip. Correct the problem by providing more ventilation to the drive or adjusting the belt tension.
Figure 9. Hard/stiff belt surface
Flaking, Sticky or Swollen Belt Surface
A belt surface that is flaking, sticky or swollen (See Figure 10) may have become contaminated by oil or chemicals. Eliminate the source of the contamination, and never use belt dressing.
Figure 10. Flaking, sticky or swollen belt surface
Banded (Joined) V-Belt Problems
Banded V-belts (multiple belts with a common cover that serves as a tie-band) may exhibit signs that point to a drive problem. The following symptoms call for investigation:
• Tie-band separation
• Top of tie-band frayed, worn or damaged
• Banded belt comes off sheaves repeatedly
• One or more belt ribs run outside the sheave
Tie-band separation (See Figure 11) might be the result of improper groove spacing. Check the sheaves to ensure that they have been manufactured to industry specifications. Another cause might be worn or incorrect sheaves, which requires sheave replacement.
Also check for sheave misalignment. This could force a separation of the tie-bands. Realign the drive to correct the problem.
Figure 11. Tie-band separation
Damaged Top of Tie-Band
If the top of the tie-band is frayed, worn or damaged (See Figure 12), determine if the belt is interfering with the guard and adjust the guard as needed. Another possible cause is worn or incorrect sheaves. Replace the sheaves to fix the problem. Debris in the sheaves might also damage the tie-band, so clean the sheaves if needed.
Belt Repeatedly Comes Off Sheave
When a banded belt repeatedly jumps off the sheaves, two possibilities might be to blame. Either debris has gotten into the sheaves, or the sheaves are misaligned. Align the drive to correct any misalignment problems. If debris is a problem due to the type of application, clean out the sheaves and use single belts rather than a banded belt.
Figure 12. Damage to the top of the tie-band
One or more Ribs Running Outside the Sheaves
A belt that has one or more ribs running outside the sheaves (See Figure 13) could be undertensioned. Check the manufacturer’s specifications and retension the belt. Another possible cause is sheave misalignment. Realign the drive to correct the problem.
Figure 13. Belt rib runs outside the sheave
Broken or damaged sheaves and severe sheave groove wear are also problems that impact V-belt life. It seems improbable that a rubber V-belt could wear out a metal sheave, but it is a fact. Many users replace V-belts several times without bothering to check the sheaves for wear.
Signs of sheave wear include groove sidewall cupping and/or a polished groove sidewall with ridges. Use a sheave gauge to detect excessive sheave groove wear, and replace sheaves immediately when worn.
A broken or damaged sheave also decreases belt life. Sheave damage could result from incorrect installation, such as over tightening the bushing bolts, or the belt may have been pried onto the sheave, causing the damage. Another probable cause of sheave damage is debris falling into the drive. Install a drive guard to avoid debris in the drive.
Also inspect other drive components, such as shafts and bearings, for signs of unusual wear. Bent or broken shafts and over-heated bearings can point to a problem with the V-belt drive.
Analysis and Maintenance Pay Off
It pays to inspect V-belt drive systems on a regular basis. If V-belts require frequent replacement (every few weeks or months), take time to analyze the belts and sheaves. They can indicate whether the drive system is functioning properly. The reward is a better-performing, longer-lasting drive.
Upstream Pumping Solutions, Winter 2012