Transmission V-belts failures and breaks analysis

How identify when your V-belt drive has a complication?

Obvious warnings of a problem drive include important noise, vibration and heat; so a first checking involves looking, listening and touching.
A belt that chirps or squeals, makes a rubbing, slapping or grinding sound, or even an exceptional loud drive is a sign of problems. So is a belt flopping in the sheave or excessive vibration. A hot belt is another warning sign.

Additional analysis admits checking the belt. Any sign of exceptional wear points to a possible problem with the drive. Check for irregular cracking in the back ,wear patterns, notches or undercord, burned spots ,frayed covers, hardening  and swelling.
Detailed fixing needs shutting down the drive and leads a thorough checking of all components: belt(s), sheaves, belt guard, bearings, and shafts.

In the next parts, we’ll define the clues that point to specific kinds of abnormal belt wear, and how to make fixing.

Premature V-Belt Failure

Brocken v-beltWhen a V-belt fails untimely it could break (see Figure 6), delaminate, or slip to the area where it can’t carry the load. These symptoms are almost the same as normal failure at the end of a belt’s effective lifetime, so how do you call the difference? Time. If the drive design provide a two-year service life and the belt rupture after two months, you can be sure the failure is exceptional. Begin to looking for an anomaly with the drive and fix it before changing the belt.

An under-designed drive or important shock load also might cause the untimely break of belt. Fixing this problem imply redesigning the drive. A different cause of a clean break might be an item falling into the drive, which may be resolved by giving a higher guard protection. Or the installer could have pried or rolled the belt onto the sheave, damaging the traction cords. This problem is resolved by giving the right take-up when mounting a new belt.

V-belt edge cord failure

Another type of untimely failure is edge cord damaged (see Figure 7) cause to sheave misalignment or a damaged tensile member. Fixing this problem involves inspecting and fixing the alignment, and the right  installation procedures when mounting a new belt.

When a belt fails to carry the load for no discernible reason, the drive could have been under-designed. Go back and control the manufacturer’s drive design advices. A tensile member could have been damaged, which need following the right installation method to fix. Or the sheave grooves could be worn and the sheaves require a replacement. A different possible cause is centre distance movement, which must be re-inspecting while operation.

Severe or Abnormal V-Belt Wear

A V-belt doesn’t have to break untimely to be weak.
Your inspecting task could reveal signs that something is not common with the drive.

When inspecting a V-belt, look at these areas for signs of uncommon wear:

  • 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 might be the result of the belt rubbing against the guard, or by a fail of the idler. Control these areas, and fix or change the guard and/or idler to correct the anomaly.

V-belt wear on top cornersWear on the top corners (see Figure 8) of the belt could be the sing that the belt is smaller for the groove in the sheaves. Changing the belt for the right sheave will fix the problem.

V-belt wear on belt sidewalls

Severe wear along the belt sidewalls (see Figure 9) may be due to several factors. The belt may be slipping due to a bad tension. In that case, increase tension until the slipping stops. Another possible problem is sheave misalignment, which needs to realigning the drive. In this case, change the sheaves. Or the belt could simply be the inappropriate size and needs to be change with the right size.

V-belt wear on belt bottom cornersWear on the bottom corners of the belt (see Figure 10) may be causes by worn sheaves or an improper fit between belt and sheave. Control the sheaves for wear and change them if needed, or find the right belt/sheave match.

V-belt bottom surface wearBottom surface belt wear (see Figure 11) may be due to fragments in the sheaves, sheave wear, or the belt bottoming out against the sheave grooves. Bottoming out is due to an improper match between belt and sheave, and can be fixed by choosing the proper match. If the sheaves are worn, change them, and if fragments has gotten into the sheaves, clean them.

V-belt undercord crackingUndercord cracking (see Figure 12) could be cause by a plenty of factors. Environmental conditions (severe heat or cold) or a bad storage could be responsible. Solutions involve checking the belt drive environment and following the right storage and handling protocol.

Another cause could be belt slip, fixed by re-tensioning the belt to the manufacturer’s suggestions. A sheave that is smaller for the belt section, causing the belt to wrap too tightly around the sheave, could crack the undercord. Changing the small sheave with a larger one may fix the problem. Likewise, a backside idler with a small diameter may be the problem, corrected by increasing the size of the backside idler.

V-belt sidewall burning

Sidewall burning or hardening (See Figure 13) may be due to worn sheaves or shaft movement or under-designed drive. A slipping belt must be re-tensioned to the manufacturer’s protocol.

A worn sheave must be changed. If the drive is under-designed, ask the manufacturer’s recommendations to redesign it. Shaft movement may be due to a changes in the center distance between the sheaves, and must be controlled and adapted.

V-belt surface hard or stiffIf the belt surface is hard or stiff (see Figure 14), it may be cause by a severe hot environment or to belt slip. Fix the problem by given more ventilation to the drive or adapting belt tension.V-belt surface flaking

A belt surface that is swollen ,flaking or sticky (See Figure 15) could have been contaminated by oil or chemicals. Eliminate the contamination and its source, and never apply belt dressing.

Banded (joined) V-belt problems

Banded V-belts (multiple belts with a common cover that serves as a tie-band) could show signs that point to a drive complication. 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

V-belt Tie-band separation

Tie-band separation (See Figure 16) may be the cause of a bad groove spacing. Control the sheaves to be sure that they have been manufactured to industry requirement. Another cause may be worn or wrong sheaves, which needs changing the sheaves. Also control to see if the sheaves are misaligned, which may force a separation of the tie-bands. Realign the drive to fix the problem.

V-belt top of Tie-band damagedIf the top of the tie-band is damaged, worn or frayed, (See Figure 17), decide if the belt is interfering with the guard and adapt the guard as desired. Another possible cause is worn or wrong sheaves. Change the sheaves to resolve the problem. Debris in the sheaves could also damage the tie-band, so clean the sheaves if required.

When a banded belt jumps off the sheaves, two cases are possible. Either items has penetrated into the sheaves, or they are misaligned. Align the drive to fix any misalignment problems. If item is a problem cause by the type of use, clean out the sheaves and use single belts rather than a banded belt.

V-belt rib outside sheaveA belt that has ribs running outside the sheaves (see Figure 18) could be under-tensioned. Control the manufacturer’s protocol and re-tension the belt. Another possible cause is sheave misalignment. Realign the drive to fix it.

Problems Common to Single and Multiple V-Belts

V-belt turn over or comes off sheaveTwo problems current to both single V-belts and Poly V-belts include belts coming off the sheave or turning over (See Figure 19), and belts stretching beyond the available take-up.

There are a plenty of probable causes and remedial actions for single or Poly V-belts turning over or coming off the sheave:

Table A

Probable Cause Corrective Action
Shock loading or vibration Check drive design; use banded (joined) belts
Foreign items in grooves Shield drive and grooves
Sheave misalignment Realign drive
Worn sheave grooves Replace sheaves
Subminimum diameter sheave Replace sheaves with correct diameter







When multiple V-belts stretch unequally beyond the available take-up, the probable cause and corrective action could be: 

Table B

Probable Cause Corrective Action
Misaligned Drive Realign drive and retension belts
Debris in sheaves Clean sheaves
Broken tensile member or chord Replace all belts, install properly
Mismatched belt set Install matched belt set
Belts from different manufacturers Replace all belts with belts made by the same manufacturer







When a single V-belt (or multiple belts) stretches evenly beyond the available take-up, check for the cause and corrective action: 

Table C

Probable Cause Corrective Action
Insufficient take-up allowance Check take up; use allowance specified by manufacturer
Grossly overloaded or underdesigned drive Redesign to manufacturer’s specifications
Broken tensile members Change belt or the whole belt set and install properly

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