Irregular belt wear: How to spot problems before they break down

By FEC Consulting Last updated: June 12, 2026

Irregular wear on a belt is rarely the problem itself. It is usually a symptom of something else: misalignment, incorrect belt tension, worn pulleys, overloading, or an environment with heat, oil, dust, or chemicals.
If you spot the signs early, you can often avoid costly downtime.

What is irregular wear?

All belts wear over time. This applies to V-belts, timing belts, round belts and other industrial belts.
Normal wear occurs relatively evenly across the belt's working surface. Irregular wear, on the other hand, means that the belt wears unevenly, quickly, or in a way that indicates a fault in the transmission.

Normal wear and tear

The belt is uniformly matte, has no deep ctiming bars, no loose fibers and no significant differences from side to side. There may be slight rounding of edges and normal signs of wear.

Irregular wear

The belt has a clear difference between the right and left sides, shiny or hard areas, ctiming bars, discoloration, uneven thickness, protruding fibers, or signs of heat and slipping.
It should always be checked before simply installing a new belt.

The most common signs of belt problems

1. Wear on only one side

If only one side of the belt is shiny, frayed, or noticeably more worn than the other, misalignment is one of the first things you should check.

This may be due to:

• Parallel displacement
• Angular error
• Crookedly mounted pulleys
• Axles that are not aligned correctly
• Motor or btiming baret that has moved over time

Use a straightedge, string, or laser alignment to check the pulleys. A string can be used as a quick test, but for critical installations, laser alignment is more accurate.

2. Ctiming bars in the sides or underside of the belt

Ctiming bars can be caused by age, heat, incorrect tension, pulleys that are too small, or the belt running in an environment it is not suitable for.
A belt with a few superficial ctiming bars can sometimes be monitored briefly, but deep ctiming bars, multiple ctiming bars, or ctiming bars along with heat/discoloration mean the belt should be replaced.

Typical reasons are:

• The belt is old and dried out
• The belt gets too hot during operation
• The belt is tightened too tightly
• The pulleys are too small for the type of belt
• The belt is exposed to oil, chemicals or UV light

3. Glossy or glazed surface

A glazed belt surface is shiny, hard and smooth. It is often seen when there is slipping between the belt and the pulley.

Slippage can be caused by:

• Belt tension too low
• Worn pulleys
• Oil or grease on the belt
• Incorrect belt profile
• Undersized belt drive
• Too high load

A glazed belt should not be simply tightened indiscriminately. First, you should check the pulley grooves, the belt profile, the load and whether there is oil or dirt on the system.

4. Uneven thickness or uneven shape

If the belt is not the same shape or thickness all the way around, the cause may be worn or damaged pulleys, vibrations, the wrong belt type, or a belt that has been exposed to heat, chemicals, or improper storage.

Check in particular:

• Whether the pulley grooves are symmetrical
• Whether there are nicks, scratches or sharp edges in the pulleys
• Whether the belt runs smoothly
• If the system vibrates
• Whether the belt fits the pulleys correctly

5. Fibers or cords sticking out

If cords, threads or fibers start to protrude from the belt, this is a serious sign. The supporting structure of the belt is affected and the belt should be replaced as a starting pu .

At the same time, examine:

• The grooves of the pulleys
• Sharp edges
• Incorrect alignment
• Worn or damaged pulleys
• Whether the belt has run against a guard or flange

If you simply install a new belt without finding the cause, the new belt may suffer the same damage.

Why does irregular wear occur?

Reason 1: The pulleys are not aligned correctly

Incorrect alignment can cause uneven load on the belt, increased heat, noise, vibration and shorter life.
This can happen after:

• Replacing pulleys
• Engine or gear repair
• Loose engine foundations
• Worn bearings
• Axles or fittings that have seized
• Lack of inspection after installation

Use a straightedge or string as a quick check. For longer center distances, frequent problems or expensive downtime, you should use laser alignment.
Important: ISO 4184 should not be used as a source for a fixed alignment tolerance. That standard is about V-belt lengths in the datum system. Instead, use the manufacturer's recommendations for the specific belt type and transmission.

Reason 2: The belt tension is incorrect

A belt must be tensioned enough to transmit power without slipping, but not so tight that the belt, bearings, shafts and motor are overloaded.

Too low voltage can cause:

• Sliding
• Heat
• Squeaking sound
• Glossy/glazed surface
• Rapid wear

Too high a voltage can cause:

• Overloading of bearings
• Excessive load on axles
• Ctiming bars in the belt
• Shorter lifespan
• Unnecessary energy loss

The correct belt tension depends on the belt type, profile, pulley, center distance, speed and load. As a rough field check, deflection at the middle of the free span is often used, but the correct method is to use the manufacturer's recommended values, a belt tension meter or a sonic tension meter, where applicable.

Reason 3: The pulleys are worn out

A worn pulley can destroy several new belts in a row.
If the groove is worn, rounded, uneven or filled with dirt, the belt is not properly positioned. This can cause slipping, heat and rapid wear.

Signs of worn pulleys:

• The belt sinks too deep into the groove
• The sides of the groove are rounded
• There is rust or sharp edges
• A new belt wears out quickly again
• The belt is dusty black
• The belt squeaks or slips under load

Use a profile gauge or the manufacturer's measurements for the specific belt profile. Avoid guessing based on eye measurements alone.

Reason 4: The drive is overloaded

If the machine is running heavier than before, or if the process has changed, the belt may be too small for the task.

Overload can cause:

• Heat
• Sliding
• Ctiming bars
• Rapid wear
• Frequent belt breakage

This often happens when a machine has been upgraded, runs faster than before, or when the load in production has changed without the belt drive being recalculated.
Check the motor nameplate, measure the current consumption with a clamp ammeter, and assess whether the drive is still suitable for the current load. If in doubt, the drive should be recalculated.

Reason 5: The environment damages the belt

Oil, grease, dust, sand, heat, UV light and chemicals can significantly shorten the life of the belt.
A belt that works fine in one environment may fail quickly in another.

Typical environmental problems:

• Oil and grease can reduce friction and affect the rubber.
• Dust and sand can act as an abrasive in pulleys and on belt surfaces.
• Heat can make rubber hard, brittle or discolored.
• Chemicals can attack the material and cause swelling, stickiness or ctiming baring.
• UV light can degrade certain types of rubber over time.

The solution is not always just a new belt. Often the environment needs to be improved, or a belt type needs to be chosen that is better suited to the conditions.

Diagnosis: How to find the fault step by step

Step 1: Stop safely and visually inspect

Turn off the system and follow company safety procedures before working on the belt drive.

Look for:

• Ctiming bars
• Blank areas
• Fibers or cords protruding
• Oil or grease
• Uneven wear
• Loose parts
• Damage to shielding
• Dirt in the pulleys
• Black dust around the drive

Please take pictures of the belt and pulleys. This will make it easier to compare if the problem occurs again.

Step 2: Examine the old belt

Don't throw away the old belt right away. It often tells you what's wrong.

Typical signs:

• Wear on one side often indicates misalignment.
• A shiny surface often indicates slipping.
• Ctiming bars and a hard surface can indicate heat, age or incorrect pulley diameter.
• Fibers or cords indicate severe wear or mechanical damage.
• Oil on the belt indicates a leak or contamination.
  
  

Step 3: Check the pulleys

Clean the grooves and check for:

• Sharp edges
• Worn grooves
• Rust
• Dirt
• Wrong profile
• Chops or scratches
• Uneven bottom of the groove

Use a profile gauge if you have access to one. A new belt will not last long if the pulleys are already worn or damaged.

Step 4: Check alignment

Use a straightedge, string or laser. Remember to check both parallel displacement and angular error.
Always correct the alignment before assessing the final belt tension.

Incorrect alignment can cause:

• Wear on one side
• Heat
• Noise
• Vibrations
• Belts that jump off
• Rapid wear on both belt and pulley

Step 5: Check the belt tension

Use the manufacturer's recommended method and values.
If you only use finger pressure, you will get a rough indication at most. In case of repeated problems, the voltage should be measured with a suitable tool.

Please note:

• Too low a voltage often causes slippage and heat.
• Too high a tension puts stress on bearings, shafts and belts.
• New belts may require inspection after a short break-in period.
• The tension must be checked after alignment.

Step 6: Check load and temperature

Measure motor current and belt temperature if you have the equipment.
If the belt becomes unusually hot, smells burnt, or black dust is visible around the drive, the cause should be found before replacing the belt again.

Check:

• If the machine runs heavier than before
• Whether the production speed has increased
• Whether the belt is correctly sized
• Whether the pulleys match the belt type
• Whether there is enough ventilation around the drive

Troubleshooting table

Symptom

• Wear on one side
• Glossy/glazed belt
• Ctiming bars Fibers sticking out
• Squeaking sound
• Belt jumps off Black
• Dust around the drive
  

Probable cause

• Misalignment or skewed pulley
• Slippage, too low tension, oil or worn pulleys
• Heat, age, incorrect tension or pulleys that are too small
• Severe wear or mechanical damage
• Slippage or contamination
• Misalignment, too low tension or incorrect belt profile
• Slippage, uneven wear or incorrect belt/pulley match
  
  

Quick test

• Straightedge, string or laser
• Check voltage, traces and contamination
• Visual inspection and temperature measurement
• Visual inspection
• Listen at startup and under load
• Check profile, tension and alignment
• Inspect belt surfaces and grooves
  

Solution

• Straighten pulleys and shafts
  
• Clean, adjust tension and replace worn pulleys
  
• Replace the belt and find the heat source
  
• Replace the belt and inspect the pulleys/rims
  
• Check tension, pulleys and oil
  
• Install correct belt and straighten drive
  
• Find the cause before installing a new belt
  

How to fix the problem

If the alignment is incorrect

1. Stop the system safely.
2. Check the pulleys with a straightedge, string or laser.
3. Loosen the motor or adjustable device.
4. Align the pulleys so that they are correctly aligned with each other.
5. Tighten bolts and check again.
6. Finally, check the belt tension.

If the belt tension is incorrect

1. Find belt type, profile and center distance.
2. Use the manufacturer's recommended voltage data.
3. Adjust the tension gradually.
4. Run the system briefly.
5. Stop safely and check again.
6. Avoid over-tensioning to “release” slippage before pulley and load have been checked.

If the pulleys are worn

1. Clean the pulleys thoroughly.
2. Check the grooves with a profile gauge or the manufacturer's measurements.
3. Replace pulleys if the grooves are worn, warped, or damaged.
4. Install new belt.
5. Adjust alignment and tension.
6. Check after a short break-in period.

If the environment is the problem

For oil:
Stop the leak first. Clean pulleys and surfaces. If the belt is affected by oil, it should be replaced frequently.

For dust:
Install or improve screening. Clean the area regularly.

In case of heat:
Improve ventilation, reduce slippage and possibly choose a belt type with higher temperature resistance.

For chemicals:
Choose a belt made of material that is suitable for the environment.

Belt drive maintenance schedule

During daily operation

• Listen for squeaks, banging sounds, or abnormal noises.
• Watch for vibrations.
• Check for dust, oil or visible wear.
• Check if the belt runs smoothly.

Weekly or as needed

• Check the surface and edges of the belt.
• Clean the area around the belt drive.
• Check whether the belt runs stably in the pulleys.
• Look for new ctiming bars or bare areas.

Monthly for critical facilities

• Check belt tension using a suitable method.
• Visually check the pulley grooves.
• Check alignment if problems occur repeatedly.
• Measure the temperature if the belt seems hot.
• Record observations so that changes can be grooveed over time.

At every belt change

• Inspect the old belt before discarding it.
• Clean and check the pulleys.
• Check alignment.
• Never install the belt by forcing it over the pulleys with tools.
• Check the voltage after a short break-in period.
• Note the date, belt type and reason for replacement.

Frequently asked questions about irregular belt wear

How often should a V-belt be replaced?

It depends on operating time, load, environment, belt type and maintenance.
A belt should be replaced if there are obvious ctiming bars, heavy wear, slipping, protruding fibers, or if it can no longer maintain proper tension.

How can I tell if the pulleys are worn?

Look for rounded or uneven grooves, sharp edges, rust, dirt, or a new belt quickly wearing out.
The best way to check is with a profile gauge or the manufacturer's measurements for the specific profile.

Can I clean a belt that has gotten oil on it?

Minor contamination can sometimes be removed with mild cleaning, but the leak must be stopped first.
If the belt has become soft, swollen, sticky or slipping, it should be replaced.

How tight should a V-belt be?

The correct tension depends on the belt profile, center distance, pulley, load and manufacturer's data.
Preferably use a belt tension gauge or the manufacturer's recommended deflection method.

Why is my belt howling or squeaking?

Squeaking noises are often caused by slipping, low tension, oil/dirt or worn pulleys.
Check tension, pulley grooves and whether there is any contamination on the belt.

Why does the belt only wear on one side?

This is often due to incorrect alignment, skewed pulleys, angular deviation or the shafts not being positioned correctly.
Check the drive with a straightedge, string or laser.

Can I reuse a used belt?

It is not normally recommended on critical installations.

A used belt may have hidden damage, be stretched or have adapted to old pulleys. Use a new belt when operational reliability is important.

What does B68 mean on a V-belt?

B usually indicates the belt profile.

On many classic B-belts, the number indicates a length designation in inches, but whether it refers to inner, outer or datum/pitch length depends on the standard and manufacturer.

Therefore, always check the manufacturer's data sheets or measure the belt correctly.

Why does the belt get hot?

Heat can be caused by slipping, excessive load, incorrect tension, worn pulleys, pulleys that are too small or poor ventilation.

Heat is a sign that should be investigated before the belt fails.

Do I need to change the pulley every time I change the belt?

Not necessarily.

But the pulleys should be checked every time the belt is changed. If the pulleys are worn, warped or have damaged grooves, they can quickly ruin a new belt.

Conclusion

Irregular wear is a warning sign.

If you only change the belt without finding the cause, you risk the new belt failing in the same way.

Start with the five most important checks:

1. Belt wear pattern
2. Condition of the pulleys
3. Refugee
4. Belt tension
5. The environment around the drive

If you have repeated belt breakage, uneven wear or problems choosing the correct belt, FEC Consulting will be happy to help with advice, sizing and choosing the correct belt type.

Related products

• V-belts in multiple profiles
• Rubber timing belts
• PU timing belts
• Round Belts
• V-belt pulleys and pulleys

Contact FEC Consulting

Do you need help with troubleshooting, belt selection or sizing?

Telephone: +45 48 24 85 45
Email: info@fecconsult.dk
Address: FEC Consulting, Industrivænget 10, DK-3320 Skævinge
Website: www.fecconsulting.dk

Sources and technical basis

• Gates Preventive Maintenance Manual
• ISO – Belt drives and their components
• Continental – V-Belts product range
• FEC Consulting – Contact