If you own a lawn mower, chances are you have had a belt come off your mower and wondered why. If you have a belt that won’t stay on your lawn mower or mower deck you need to check over your lawn mower to make sure your belts are in good condition and there isn’t damage to other parts of your lawn mower.

Drive belts and mower deck belts are necessary functions of your mower. If one of the belts falls off your lawn mower, your mower will fail to move or fail to cut. It’s important to keep your belts, pulleys, and brackets in good shape. Read more about the different types of belts.

A lawn mower belt keeps coming off the mower because the belt guides and brackets are out of adjustment; the debris is collecting under your pulleys and tensioners; the brackets, bearings, or bushings are damaged, or the mower belt is worn.

Before you begin working on your mower, make sure to remove the spark plug for safety reasons. Let us explain why your belt may come off your mower.

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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.

8 Reasons Why Your Mower Belt Keeps Coming Off or Breaking

1. Belt Guide Damage Causes Lawn Mower Belt to Come Off

The belt guides, sometimes called belt keepers, are small rod-shaped posts that go around the pulleys. On some lawn mowers belt guides travel alongside the belts depending on how long the belt must travel.

Belt guides can move out of place or become bent when a belt snaps or the belt has a lot of slack in it. The extra slack can cause the belt to hit the guide and bend the belt guide back toward the pulleys sometimes causing the belt to jump off the pulley and slip off your mower.

Keep in mind, belt guides should never touch the belt. Instead, they should be very close to the belt remaining 1/8″ to 1/4″ away from the pulley. Brackets can be adjusted by hitting them with a small hammer or using a vice grip to move the brackets back into place.

2. Loose Belt Causes a Lawn Mower Belt to Slip

Inspect your belt to check if it seems loose. This can be a sign it is time to replace it. We like to replace our mower belts at the same time because one is pretty worn or stretched from normal use; the other is probably due to being changed.

3. Shredded Belt Causes Lawn Mower Belt to Snap or Fall Off

If you find your deck belt or drive belt is shredded, most likely the belt is rubbing on a bracket or a belt guide. It also may not be tracking correctly. Look at the components that are near the belt to see what the belt has been rubbing against.

A shiny spot on a bracket or belt guide is an indication that the belt is rubbing against this area.

Once you isolate the items causing the shredded belt, adjust the bracket or belt guide back in place so it no longer rubs against the belt. Check bushings in the brackets to make sure they are not bad.

Some brackets have replaceable bushings while others do not. The whole bracket must be replaced.

4. Damaged Bearings in Pulleys or Cutter Housings Cause Deck Belt to Come off

Next, look at the pulleys that the belt rides on. If any of them are not running flat and straight, a bearing in the pulley can be bad. Most of the time, you will find the bearing that fails is located in an idler pulley.

The idler pulley has a sealed bearing in it so you will have to replace the whole pulley if you find a bad bearing. Not every pulley will have a sealed bearing. You may find the drive pulleys or spindle housings will have replaceable bearings so you are able to complete repairs with fewer parts.

You can find bearing failures in spindle housings by grabbing a hold of the blades and rocking back and forth.

If you feel movement, the bearings in the cutter housings or a complete cutter housing need to be replaced. Spindles that are making noise and excessively vibrating can cause your belt to come off the mower deck.

5. Debris in Grooves of Pulleys Causes the Mower Belt to Slip

Look at the grooves in the pulley and check for rocks, wood chips, and other debris that may get caught in the pulleys. There have been times when I have found debris and even dried mud caked in the grooves of the pulley.

A mower belt can break when the debris is caught in the deck and under the pulleys. The belt is unable to stretch around the additional debris. The stress of being stretched can cause the belt to break.

Belts don’t always break because they are old. Many times, new belts snap because a foreign material is caught in the pulleys.

6. Idler Tensioner Bracket or Spring is Worn Cause the Mower Belt to Come Off

The brackets that hold the idler pulleys in line usually have a spring on one side and a pulley on the other. If the hole where the spring attaches is worn or the spring is worn, a constant vibration is created.

The bushing on the idler bracket can also become worn out if it isn’t greased. This can result in the pulley running on an angle.

The pulley can cut into the deck or the bottom side of the mower if it is a transmission belt. This vibration along with the pulley running on an angle can throw the belt off the mower.

7. Debris Interfering with Proper Tension on Belt Causing the Mower Belt to Come Off

The deck needs to be kept clean of debris. Dry grass and debris can collect under the lawn mower or on the deck which can cause the tension pulley not to move properly.

The tensioner pulley is no longer able to keep proper tension on the belt. If the tensioner can’t move, then the belt can’t stay tight.

This is one of the reasons it is very important to keep your lawn mower clean under the belt covers and under the mower body. Also, the cleaner you keep your lawn mower, the easier it is to find parts that may be failing before they fail.

Check out our articles, “How to Clean Your Lawn Mower for Best Performance” and ” How to Find & Prevent Substantial Lawn Mower Deck Problems ” to learn more on how to keep your mower clean and in good condition to extend your mower’s lifespan.

8. Oil or Rust on the Belt Causes Mower Belt to Slip

An oil leak on your lawn mower may collect on the belt. This is another problem that can cause the belt to come off. The pulleys need to be clean of oil and rust to keep your belts in good shape. Oil is a petroleum product that will make the belt swell if the belt remains in oil for a long time.

An oil-covered belt will also begin to slip on the pulleys and cause the belt to not be able to grab and just build heat. Rust will cause the belt to dry out and crack. If your belts have any types of cracks in them the belts need to be replaced as soon as possible.

Related Items

How Long Do Mower Belts Last

Lawn mower belts can be pretty pricey depending on the manufacturer and type of mower you are running. Drive belts and deck belts are two of the most common lawn mower belts. Belts can cost anywhere from $20 to $200.

I recommend purchasing an OEM belt from the manufacturer of your lawn mower. You can probably find a cheaper belt online or have a local auto parts store try to match the belt for you, but they rarely last as long or perform as well as an OEM belt.

I go into more depth with my article, “This is How Long a Mower Drive Belt Should Last“.

You can also find out more information on belts in “V-Belts: What Are They?”