Having to stop mowing to reinstall the deck belt that has fallen off the pulleys is an inconvenience. Having to continue to stop and reinstall your belt multiple times is a big pain. Finding the cause that allows your belt to come off must be repaired or it’s never going to end.

A mower deck belt will keep coming off your lawn tractor when it is worn, stretched, or shredded. Belt keeper damage; bad pulley or spindle bearings; damaged brackets; worn tensioner spring; and debris interfering with the tensioner or pulleys can also be the reason a lawn tractor mower deck belt is slipping.

Always follow safety precautions as outlined in your owner’s manual before working on your lawn tractor.

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Follow all safety instructions provided in your equipment operator’s manual prior to diagnosing, repairing, or operating.Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

9 Reasons the Lawn Tractor Belt Keeps Coming Off

Loose or Worn Deck Belt on a Lawn Tractor

A deck belt will wear from normal use. A belt showing one of these signs of wearing should be replaced as it is more susceptible to slipping off your lawn tractor:

• Shiny glazed appearance
• Stretched belt
• Loose belt
• Cracks in the belt

Belt Keeper Damage on a Lawn Tractor

Some lawn tractors use a rod that sits between 1/8” and 1/4” away from the pulley. They are known as belt keepers or belt guides. The belt doesn’t actually touch these belt keepers. It is just an extra precaution to keep the belt from falling off the pulley and coming off the deck.

When the belt has slack in it, it can hit the belt keeper causing the rod to bend. This increases the chance your belt comes off your lawn tractor.

Adjust any belt keepers that may be bent out of shape. You may need to use a hammer or other tools to straighten the keeper to get it back to its original position.

If you are unable to reshape the rod and get it to sit correctly on the deck, you should replace it with a new belt keeper.

Shredded Deck Belt on a Lawn Tractor

A shredded belt is likely to snap and break. Shredding often happens when the belt is installed incorrectly or it’s rubbing against a component on the mower deck.

To find the cause of a shredded belt, check to make sure your belt is routed correctly. There may be a diagram decal on your mower showing you how the deck needs to be routed. If you can’t find a decal, refer to your owner’s manual diagram or a part’s diagram.

Next, if your lawn tractor belt is routed correctly, look for a bracket that is out of place. This piece may have a shiny worn look to it from the belt rubbing against it. Once you identify the bracket causing the problem, adjust it or replace it so it no longer rubs.

If your bracket has a bushing in it, you may be able to replace the bushing or you may have to purchase the bracket assembly.

Take time to find what is causing your belt to shred so you don’t continue to go through multiple belts. Deck belts can be expensive and run between $80 and $250 for a good OEM belt.

Damaged or Worn Pulley Bearings on a Lawn Tractor

A bad pulley bearing can cause the belt to keep coming off your lawn tractor. A pulley with a bad bearing will not sit flat on the mower deck. The pulley will shift causing your belt to slip and come off the pulley.

To check for a bad bearing in your pulley, grab a hold of the pulley to see if there is vertical movement in the pulley that causes it not to sit level with the deck. Next, slowly rotate and spin the pulley.

If you feel resistance or hear a noise coming from the bearing or notice it isn’t sitting flat and parallel to the deck, it’s time to replace the pulley.

Damaged or Worn Spindle Bearings on a Lawn Tractor

Another bearing to check for damage or wear is the bearing in the spindle housing. To check this, put on a good pair of work gloves and grab a hold of each end of the mower blade.

Rock the blade up and down. You’re checking for movement in the blade or a knocking sound indicating a bad bearing.

When you feel this, remove the spindle housing, inspect the bearing and replace it if it is bad. You may be able to rebuild the housing and replace the bearing on some lawn tractors.

Other lawn tractors will require you to purchase the whole spindle housing assembly. Repeat this procedure to check each spindle housing in the deck.

Idler Tensioner Bracket or Spring is Worn on a Lawn Tractor

Your lawn tractor deck has a tensioner bracket and spring that holds the idler pulley in line. Usually, you’ll find a pulley on one side of the bracket and a spring on the other side. The spring or the hole in the bracket, that the spring attaches to, can become worn.

This can cause vibration in the mower deck that can cause your lawn tractor belt to come off the deck. You’ll need to replace the bracket or the spring if they break or become worn.

Debris Interfering with Proper Tension on a Lawn Tractor

The belt on your lawn tractor must have the proper tension to keep your belt from falling off your mower deck. Debris can interfere with this tension when it gets stuck under your tensioner or builds up so your tensioner is unable to move.

Keep your mower deck free of debris by cleaning it after each mowing. Not only do you want to keep it clean so your mower deck belt operates correctly, but you also need to do this for your safety.

Dry debris that collects on your mower deck can catch on fire from the friction of the belt creating heat.

Check out How to Find & Prevent Substantial Lawn Mower Deck Problems to learn more about keeping your deck clean and in good condition to extend your lawn tractor’s lifespan.

Debris in the Pulley Grooves on a Lawn Tractor

Debris can also build up in the grooves of your pulleys which causes stress on your mower belt when it needs to stretch around the additional material. Your lawn tractor belt can come off or snap.

Check for debris buildup in your pulleys and clean them so your belt sits in the groves correctly.

Oil or Rust on a Lawn Tractor Mower Belt

Rust buildup on your deck and from your pulleys can collect on your belt causing it to become dry and more prone to cracking and coming off your lawn tractor. Oil buildup from an oil leak that dripped onto your deck can cause your belt to slip around the pulleys and come off.

The belt can also build heat when it isn’t able to grab a hold of the pulleys to turn them. Repair the oil leak or remove the rust around your pulleys. If your pulleys have a lot of rust on them, replace your pulleys. Once these items are taken care of, you must replace your belt.

In Summary

Deck belts can be pricey so the longer you can make them last, the better. Here are a few items that can extend the life of your lawn tractor belt to keep it from prematurely wearing, breaking, and slipping off the tractor:

• Clean your mower deck after each mowing to remove debris
• Check for rubbing on deck components
• Keep oil and rust from forming on your belt
• Periodically check your pulleys and spindle housings for bearing failure
• Fix or replace any damaged brackets or belt keepers