Top 8 causes of belt failure

We've put together a list of problems and possible causes to help you identify the cause of your v-belt failure, with tips on how to prevent it.

Key Takeaways

Worn/damaged pulleys are typically the root cause of failure, with sidewall wear, spin burn, cog loss, and cracking all tracing back to it. If a pulley wall looks bright and shiny, it needs replacing.
Incorrect tension can greatly damage v-belts. Use a tension gauge to ensure your tension is correct, or you risk stretching, glazing, and even snapping belts.
Always consider re-tensioning belts 24–48 hours after installation, even "fit and forget" belts.
Contamination (oil, grease, moisture, dust) softens or degrades belts, causing swelling, fraying, paste build-up in pulleys, and loss of grip.
Extreme heat can be the source of multiple types of failure, including hardening and cracking in standard rubber belts, softening/melting in urethane belts, and permanent stretching.
Misalignment causes uneven wear and stress, contributing to fraying, cog separation, and sidewall damage. Laser alignment tools are recommended for accurate checks.
Never mix old and new belts, or belts from different manufacturers, on the same drive. Mixing belts can often lead to multiple failures at once.
Glazed belts can't be repaired. Once the sidewall turns smooth and hard, replacement is the only option.
Belt guards are a simple, low-cost preventative measure, protecting against dust, oil, and grease contamination.
Consider EPDM rubber belts for extreme temperature environments, and composite belts (e.g. Fenner PowerTwist) for oil/grease-heavy environments.
Coolant exposure can break down rubber and cause cracking/drying. Resolve leaks first, then clean the belt or replace it if possible.
Excessive start-stop cycling accelerates wear, so tension and pulleys should be checked regularly in these applications.

Belts are a low-maintenance solution suitable for mid- to high-power belt drives, delivering a high level of efficiency. This makes belts amongst the most common components in modern machinery, found in almost every industry, from agriculture to pharmaceuticals.

We have been speaking to Power Transmission Product Manager, Paul Speight, about what exactly it is that causes premature belt failure in a range of industries, and how we can help you to get the most out of your belts.

Visually examine your belts to check for any of the following symptoms to combat premature failure and unplanned downtime:

Download our v-belt troubleshooting guide here for easy fault detection

Belt Failure Causes

1. Rapid sidewall wear

Although sidewall wear is inevitable over time, rapid sidewall wear can be a sign of a bigger problem, as most commonly it can be attributed to worn or damaged pulleys. However, there are many other possible influencing factors, such as using belts in extreme heat, excessive oil, grease, moisture and other generally abrasive environments, which can also cause this type of wear.

Solution: Ensuring pulleys are replaced when changing belts is important to be certain that you are getting the most from your belt systems. If the pulley wall is bright and shiny then it’s time to replace it.

2. Worn cover on back

Most commonly, back wear is a direct cause of the belt being forced or pried onto the pulley, or there is some misplaced slack within the belt, which is causing damage by vibrating against the idler during operation. On some occasions, defective or worn backside idlers can contribute to worn backside covers, although this is less common.

Solution: Making sure that your belts have the correct tension on installation is essential. Even belts that are measured correctly and considered ‘fit and forget’ such as Optibelt Red Power 3, need to be re-tensioned within 24-48 hours of installation.

3. Belt soft, swollen

Excessive exposure to oil or grease can be a recipe for disaster for some belts, as these products can cause belts to soften and become swollen, and thus no longer fit properly into the pulley groove. This sort of problem could also be similarly attributed to the use of belt dressing.

Solution: If belts are working in extreme conditions, likelihood is that it’s not reaching its full potential. A simple belt guard can prevent foreign bodies including oil & grease from contaminating the drive, while it’s always worth noting that belt dressings are to be avoided. If oil and grease is still a problem, consider using a composite v-belt such as Fenner PowerTwist belting as they are generally unaffected by oil, grease, water and common industrial solvents.

4. Belt slips, squeals (spin burn)

Spin burn can be caused by worn or damaged pulleys and incorrect tensioning, which can wreak havoc on your belts, causing damage and ultimately, premature failure. This can also be less commonly caused by using the wrong belt cross-section or type, excessive oil, excessive grease or moisture and insufficient wrap on a small pulley.

Solution: This type of wear means it’s time to check your pulleys and belt tension as under-tensioned belts and smooth pulleys mean unnecessary wear. You may well find that you need new pulleys or to re-tension your belt.

5. Split belt cover

There’s a lot to be said about the damage that can be caused by forcing a belt onto a pulley or running your belts with the incorrect tension, as this is the most common cause for many types of belt damage, including cover splits.

Solution: If you’re sure that there are no foreign objects in the grooves that could be causing the issue, then ensuring regular belt tensioning and that the correct size belts are ordered every time, you can help to avoid this issue.

6. Underside cracked

Cracking in belts can cause serious issues, particularly if they are left untreated as they can affect drive efficiency and eventually cause belt failure. Cracks commonly begin to appear on belts that are being exposed to excessive heat. Although it cannot always be exclusively attributed to this, as pulleys and backside idlers that are too small or misaligned can also cause this cracking.

Solution: If your belts are working in extreme temperatures, consider an EPDM rubber belt that is designed to withstand these temperatures without premature or unnecessary failure.

7. Missing cogs

Missing cogs can cause belts to slip and fail. If teeth are found to be missing, the belt should be changed immediately. The most common reasons your belt could be missing a cog are excessive heat, the pulleys are too small or misaligned. It can also be caused by improper or prolonged belt storage.

Solution: Once again, if your cogged v-belts are working in extreme temperatures, it may be ideal to consider belts designed for these environments. Additionally, establishing that your pulleys are correctly aligned using a laser alignment tool can eliminate the possible cause and improve machine running.

8. Cracked Bushings

Having cracked bushings means your drive is in dangerous territory, if the damage becomes too severe, your pulleys may not be attached securely to the shaft. Bushings can become cracked due to excessive tension from the drive or using worn or damaged pulleys.

Solution: Over-tensioning belts can be just as damming to your v-belts as under-tensioning, using a belt tension gauge to check belt tension can help you avoid unnecessary bushing damage.See our full range of v-belts and pulleys here.

Top tips to prevent premature failure

I find that belt drives really don’t need a lot of attention to be efficient, but getting the basics right can avoid these types of failure and really improve the life span of your belts and pulleys to ensure you get the optimum efficiency from your components.

Don’t replace brand new belts onto worn pulleys, as it can reduce the life of a belt by up to 50%. If the pulley wall is bright and shiny, then it’s time to replace it.

If the pulley is only worn on one side, alignment may be the fault.

Check the alignment of your pulleys; laser alignment tools are by far the best way to check alignment and can increase the belt life.

Although some belts are now manufactured and pre-tensioned, almost a fit-and-forget, most V-belt drives need to be tensioned on installation and then re-tensioned within 24-48 hours. Over-tension and under-tension can contribute to belt failure.

Never mix old and new belts on the same drive.

A simple guard can prevent foreign bodies from contaminating the drive. if your drive is in a dusty environment and there is oil and grease present, this can create a paste that eats away at the most robust components.

Other environmental factors, such as extreme temperatures, when there is either a high or very low ambient temperature, try choosing an EPDM rubber belt.

Speak to our experts today to discuss your v-belts

At Acorn Industrial Services, we specialise in the supply of industrial parts, including name-brand V-belts like SKF, Gates, ContiTech, ProTorque, and Optibelt.

We also have experts like Paul on hand to provide V-belt advice so you can be confident your industrial machinery is working at maximum capacity.

Speak to an expert today, or shop our range of V-belts at your leisure.

Frequently Asked Questions

Can you identify failure from a photo?

Yes, our team may be able to deduce the cause of your v-belt failure if you provide the photo alongside some key information, like the product application, running temperatures, etc.

Why is my V-belt cracking across the ribs?

The most common reasons for v-belt cracking across the ribs are:

Consistent bending stress: Bending stress is often caused by running a belt around pulleys that are too small or not aligned correctly. This can lead to additional stress, which may cause the rubber to crack.
Belt slippage: Belt slippage can happen if the belt is loose or worn. When this happens, it tends to generate excess heat, which cracks the rubber.
Extreme heat: Extreme heat is generally caused by friction or high operating temperatures.

What causes sidewall wear?

Sidewall wear is most typically caused by:

Damaged pulleys
Extreme temperatures (outside of tolerance levels)
Contamination from moisture
Contamination from abrasive particles
Over-lubrication

Monitor for the above issues if you spot wear on your sidewalls, and speak to our experts if you need any further support.

Why is the belt glazing?

There are a number of causes for v-belt glazing (where the sidewall becomes smooth and hard instead of its normal rubbery texture). These causes include:

Chronic and consistent under-tension: Chronic under-tension is where the belt has been slipping for an extended period of time. This makes the contact surfaces smooth and hard.
Worn sheave grooves: If your grooves are worn, this provides less grip surface. From there, the belt may microslip and begin to glaze.
Heat exposure: If your v-belt is close to a heat source, it may begin to glaze. You can also see this issue if the ambient temperature is too high.

If any of the above issues affect your belt, you should consider replacing it, as a glazed belt cannot be restored. You should also consider inspecting and replacing worn sheaves and reviewing whether the belt’s tension is to specification.

If you find that the glazing was caused by heat, consider upgrading to a cogged belt. Alternatively, if you can resolve the heat issue and prevent it, you may still be able to use a standard v-belt.

Why is the belt fraying?

The most common causes of v-belt fraying include:

Pulley misalignment: When the pulleys aren’t aligned as they should be, it can cause the belt to enter and exit the grooves at an angle. Over time, this lateral rubbing causes the sides of the belt to wear down and fray.
Contamination: Contamination from external factors can cause V-belts to fray over time, particularly alongside any heat generated. Monitor if chemicals like oil, power steering fluid, and fuel, etc., get onto the belt, and plug any potential leaks. Replace the belt immediately if you notice it has become gummy, spongy, or completely dried out.
Incorrect or improper tension: Proper tension is key to ensuring the longevity of your v-belt. Always follow the recommendations and avoid:
- Tension that is too loose. Too loose tension will cause the belt to slip, which in turn creates heat and friction that will degrade the structural cords of the belt and could burn the rubber.
- Tension that is too tight. Too-tight tension puts excessive stress on the belt and bearings, which can cause the reinforcement cords to snap
Wear and tear: Wear and tear will always happen regardless of how well optimised the belt is for consistent use. You may find that after extended use, the rubber will begin to harden and turn brittle, which in turn will cause splitting and fraying. Before you reach this point, you should consider replacing your v-belt.

What causes belt stretching?

Most commonly, stretching is caused by worn v-belt grooves or by combining v-belts from different manufacturers. Other common causes for v-belt stretching include:

Over-tensioning: Always follow the manufacturer's specifications for how tight your v-belt should be. Over-tightening will cause excessive tension, causing it to stretch.

Contamination: Spilling chemicals onto the v-belt, like coolant or oil, may affect the friction coefficient, which can soften the rubber. This may cause it to stretch.

Heat: Consistent friction caused by high temperatures or belt slippage may damage the rubber and reinforcement cords, which can result in stretching.

Shock overloading: If you consistently operate machinery beyond its designed torque capacity, the belts are forced to stretch to meet the additional weight and resistance, resulting in longer-term wear.

If you find that your v-belt is stretching, you should consider replacing it and examining which elements of your process are causing the stretching. This can help to save money on replacement parts and reduce downtime.

Why are the cogs separating?

If you are finding that the cogs on your V-belt are separating, consider the following causes:

Heat: When temperatures exceed the belt’s limits, the rubber can harden and lose elasticity, causing cogs to separate.

Belt tension: Ensure your belt is set up to the tension standards recommended by the manufacturer. Over-tension can increase the bending stress of the belt. Under-tension generates additional heat through friction and causes the belt to slip.

Chemical contamination: Contamination from harsh chemicals, oil, and grease can soften and swell the rubber. This causes the rubber to lose structural integrity.

Dust and debris contamination: Abrasive debris and dust can get trapped within pulleys, which can accelerate cog wear, resulting in the likelihood of them separating being higher.

Misaligned pulleys: When pulleys are misaligned, the belt begins to track unevenly, which causes excessive sidewall wear. This wear on the sidewall creates stress on the cogs, which may result in them separating.

Cog separation on your V-belt often requires a rapid replacement.

Why has the belt snapped?

Belt snapping can be a dangerous, critical issue and may result in excessive downtime, depending on the application. These are some of the most common reasons that a belt may snap:

Overloading the belt: Belts may snap if there is a jammed component or exceptionally heavy startup, resulting in extreme initial torque.

Incorrect belt sectioning: If your belt is too small for the load, it will likely snap even under normal operating conditions.

External damage: If an external tool or piece of debris is caught between the belt and the sheave, this can cause excessive wear and snapping.

Wear and tear: If your belt has been running with slipping, edge wear, or cracking for a long period of time, the likelihood of it snapping is much greater.

If your belt has snapped, you should identify the cause of the snapping as soon as possible, to prevent the issue from happening again and causing additional downtime and expense.

Why are multiple belts failing at the same time?

The most common cause for multiple belts failing at the same time is a length mismatch. This is where v-belts aren’t all replaced at once, resulting in the newer v-belts handling more of the load, resulting in additional wear, and snapping.

You may also find that if pulley grooves wear down unevenly, the belts may ride at different depths, stretching them and causing failures.

Additional causes may include:

Pulley misalignment forces the belts to twist or ride against the groove edges, resulting in increased friction and damage across all belts.
Belts with poor tension management may slip if they are too loose, which can lead to overheating, or be too tight, resulting in stretching and breaking.
Shock loading or heavy vibration may cause belts to slap, which can lead to belt turnover and cord breakage.

Can coolant damage V-belts?

Yes, coolant can damage v-belts, as it can cause the belts to slip and the chemicals can break down the v-belt rubber, resulting in it drying out and cracking.

If your belt has been exposed to coolant, you should:

Locate the source of the leak and resolve it.
Clean the belt and pulleys with hot soapy water, being sure to remove any slippery residue.
Check the belt for signs of damage, including: Glazing, fraying, cracking, or any other structural damage.
Replace the belt if damaged.

Does dust shorten belt life?

Dust can shorten the lifespan of a v-belt.

Abrasion: Fine particles that could be considered ‘dust’, like dirt or sawdust, can grind between the metal pulley and belt, which can cause additional wear.

Paste contamination: Dust particles can mix with lubricants, which can create a paste-like substance, which will damage both belts and pulleys.

Traction loss: If dust gets caught up in the pulley grooves, it can create an uneven surface. This uneven surface makes it difficult for the v-belt to grip as it should do, which can result in slipping, and higher temperatures.

Protect your v-belt by installing belt guards, cleaning them regularly, checking belt tension, or by switching to a heavy-duty v-belt.

How does high ambient temperature affect belts?

Excessive heat can cause compound hardening on standard rubber belts, where the upper continuous limits (approx 60°C - 85°C) are consistently broken. This results in a belt that turns stiff and brittle, and eventually cracks.
Excessive heat can also cause a permanent stretch in v-belts, which in turn causes stretching and slippage, resulting in further heat through friction.
If you use a urethane belt, be aware that excessive heat can cause the belt to soften or melt.

You should always review the manufacturer recommendations for standard belt limits, and work to reduce the ambient temperature of your v-belt as much as possible, or risk needing to replace belts prematurely.

Is excessive start-stop cycling causing premature wear?

Yes, you should avoid excessive start-stop cycles, or risk momentary slipping or stretching due to the sudden change in speed and inertial forces brought on by the engine stopping and starting.

Be sure to inspect the tension, pulleys, and listen out for noises like squeals when start-stopping.