The belt on your mowing deck is a crucial component that must work correctly to enable your lawnmower to operate.
Unfortunately, the belt can fail, leaving the mower unable to drive the blades and cut grass. One such bothersome belt problem is when it keeps coming off.
Whenever this happens, it disconnects the blades from the crankshaft, making them unable to turn.
Thankfully, you can fix most of the reasons why the belt keeps coming off riding mower without the help of a technician.
This article discusses nine problems that can cause your mower’s belt to come off and how you can address each problem.
Why Belt Keeps Coming Off Riding Mower
A lawnmower belt may keep coming off if the belt keeper is crooked or damaged or due to bent pulleys or bad bearings. Using the wrong mowing belt or worn-out belt can also cause the problem. The other common culprits include accumulated debris and incorrect belt routing.
When the belt on your mower keeps coming off, any one or more of these problems can be responsible.
Knowing what to look for and how to diagnose the problem can help you identify and fix the issue.
Before troubleshooting the problem with your mower, always ensure your safety first. Wear eye protection and protective gloves.
Next, pull out the spark plug wires and ground them against your engine to keep the motor from accidentally starting while you are working.
Once that is done, proceed based on the problem you are addressing out of the following nice:
1. Damaged or Bent Belt Keeper
The belt on your mower goes around a system of pulleys, each with belt keepers at specific locations around it to help maintain the belt in position.
If these belt keepers or belt guides become crooked, they may not perform their role properly.
As a result, the belt may come off the pulley and drop onto the deck while raising and lowering the deck.
The belt keeper is supposed to lock the belt in place, but a crooked one allows the belt some space to pass through and fall off the pulley.
You must fix or straighten the crooked keeper or belt guide to stop the mower belt from falling off.
What to do
Closely examine the belt keepers around each pulley on your deck for any signs of crookedness. If any keeper is bent away from the pulley, it may be the point where the belt falls off.
You will likely need to unscrew the bolt holding the pulley and remove the pulley to straighten the belt keeper before reinstalling the pulley.
While straightening a crooked keeper, ensure you do not bring it too close to the pulley so that it prevents the pulley from rotating freely.
Ideally, the rod should sit about 1/8 to ¼ inches from the pulley.
2. Accumulated Debris Under the Pulleys and Tensioners
Over time, grass, wood chips, twigs, and stones may form a mass of debris jammed in the space beneath the tensioners or in the pulley grooves.
When this happens, the accumulated debris competes for space with the belt. As the belt moves, the debris may push it out of place, forcing it to fall off the mower’s pulleys system.
In serious cases, the dirt and debris can nip the belt, causing it to break and fall off.
Solution
If accumulated dirt and debris under your mower dislodges the belt from its track, you must remove the buildup to stop the problem from occurring.
After that, you can prevent similar future buildups by cleaning the mower deck and the area around the pulleys system after every mowing session.
Removing solid particles and pieces from this area once you finish mowing will go a long way in preventing them from accumulating and creating a buildup that displaces the mower belt in the future.
3. Worn or Slacked Mower Belt
After using your mower for a while, the belt may become worn out or loose.
Worn mower belts may lose entire fragments, remaining with thin sections that can easily pass through the small space between the belt keepers and the pulleys and fall off.
A worn-out belt may also be slacked and stretched, losing its ability to maintain its position within the pulley grooves.
When slack, the riding mower belt can easily slip from time to time. Additionally, it can also create friction that could endanger your machine.
What to do
Inspect the belt for any signs of fraying, slackness, separation from the rest of the pulley assembly, or cracks.
If the belt is old and frayed, you should see the damaged parts or looseness if it has lost the required tension.
The only solution for a damaged mower belt is to replace OEM belt with a new one.
4. Incorrect Belt or Belt Size
The belt assembly and track are designed to fit a specific belt. Using the wrong belt would mean a mismatch that could create issues.
For instance, the wrong belt may be too long, resulting in slackness. As already mentioned, looseness endangers your mower and may lead to slipping off of the belt as the mower runs.
The wrong belt may also be too small, allowing it to pass through the small space between the pulley and the belt keepers.
These problems can cause the belt to fall off occasionally while mowing or lowering and raising the deck.
Possible fix
When replacing the belt on your mower, check your lawn mower model number and order the correct belt with matching specifications or dimensions.
Without checking these details, you could end up with the wrong belt that causes problems for your mower.
If the wrong belt is the problem, check your mower’s details and order the correct belt size to resolve the issue.
Once you have the right belt, fit it out and try running the mower to see if the problem is solved.
5. Damaged Bearings in the Spindle Housings
The spindles on your lawn mower transfer the engine’s rotational force to the blade through the system of pulleys on the mower.
See: Do lawn mower blades have a reverse thread?
The engine rotates the pulley on one end of the spindle, which spins the blade on the other end. Inside the spindle housing assembly are bearings that enable this high-torque, high-speed process.
If these bearings get damaged, they can wobble and cause the belt to slip off while operating.
You can tell if the bearings are damaged from the excessive vibration the spindle will create while rotating.
What to do
Examine the idler pulleys on your mower, feeling them with your hands to see if they are firm and parallel to the deck. The spindle pulley should run flat and parallel to the deck surface.
If the pulley is angled differently, it may be the reason your deck belt starts to come off. Ensure you wear sturdy leather gloves when approaching this part.
Rock the mower blade up and down while listening in for any knocking sounds to tell whether the spindle housing has damaged bearings.
If movement is inside the spindle housing, disassemble it and inspect for damaged components.
Replace only the bearing if the housing is unsealed. However, if it is sealed, replace the entire spindle housing.
Read: Is 300 hours a lot for a riding mower?
6. Misalignment or Incorrect Belt Routing
Looping the belt incorrectly around the pulleys can cause problems leading to its coming off the assembly.
What to do
When taking off your belt to service the deck, consider taking pictures before unlooping the belt to ensure you get it correctly when returning the belt.
Also, ensure the belt goes through all the belt guides. Looping the belt incorrectly or failing to pass it through all the belt guides may keep it out of place, letting it off the pulleys.
7. Improperly Mounted Deck
The deck carries the pulley assembly. It needs to be mounted correctly to stay balanced. Otherwise, it may create problems when engaging the blades.
An off-balance mower deck may affect the belt orientation, potentially causing it to fall off when raising or lowering it.
What to do
You can fix this problem by checking how the mowing deck hangs. Refer to your owner’s manual for the correct way of mounting the deck.
If anything is off, correct it to ensure the deck is mounted properly. This may include replacing any missing parts likely to keep the deck from being balanced.
Related Read: John Deere 42 inch mower deck parts diagram
8. Bent or Damaged Belt Pulleys
Belt pulleys should spin freely if they are sound. If any is bent, it can affect the blade’s movement, allowing it to slip off.
Solution
Unloop the belt and check the deck for damaged or bent pulleys by rotating and inspecting each.
Try using a hammer to straighten any bent pulleys or replace them if they are in bad shape.
9. Torn Bracket/pulley Plate
Sometimes the pulleys may be okay, but the bracket on which they are mounted is not.
Over time, the plate or bracket holding the pulleys can get torn, affecting the belt orientation and potentially causing it to slip off.
Solution
Check the idler tensioner bracket for any noticeable damage and weld any split parts likely to cause the belt to slip off its track. You could also replace it if it is badly damaged.
Why Specific Mower Models Belts Keep Coming Off
Why Craftsman riding mower deck belt keeps coming off
Craftsman riding mowers may keep coming off due to a damaged, worn, or stretched belt. The problem can also occur due to bent belt keepers or damaged pulleys.
Why Troy Bilt deck belt keeps coming off
One of the common problems with the Troy Bilt riding lawn mower is its deck belt coming off if the belt is damaged, stretched, or worn out. They can also slip off the idler pulley system due to faulty spindle bearings in the spindle housing or bent pulleys.
Husqvarna mower deck belt keeps coming off
If your Husqvarna mower deck belt keeps coming off, the mower may have damaged pulleys, belt keepers, and bearings in the spindle housing.
John Deere mower deck belt keeps coming off
John Deere mower deck belt may keep coming off if the lawnmower has an incorrect or shredded belt on its deck. Damaged pulleys or bearings in the spindle housing can also cause the problem.
Zero-turn drive belt keeps coming off
Your zero-turn drive belt can keep coming off if the bracket holding the pulleys on the deck is split or torn. The problem can also occur if the belt keepers are crooked or the pulleys are damaged.
MTD deck belt keeps coming off
MTD deck belts can slip off the pulleys due to bent belt keepers, damaged pulleys, or faulty bearings in the spindle assembly housing.
Why does my Serpentine belt keep coming off?
The reasons your Serpentine belt keeps coming off may include stretched lawn mower belt, incorrect mower belt, damaged pulleys, or bent belt keepers.
What causes the belt to come off of Cub Cadet Mower
The deck belt may come off Cub Cadet Mower due to damaged pulleys, crooked belt keepers, an imbalanced deck, bad bearings in the spindle assembly housing, and incorrect belt looping.
How do I know if my lawn mower pulley is bad?
You can check if your lawnmower pulley is bad by spinning all the pulleys on deck one at a time. Good pulleys will spin freely, while the bad ones will have some play or resistance. You can also see signs of wear and damage inside any bad pulleys.
Final Thoughts
A mower belt that keeps falling off can waste a considerable amount of your time. Thankfully, you can fix most of these problems if you know where and what to look for.
This article provides a detailed account of the common problems likely to cause your mower belt to come off. It also suggests the solutions for each problem.
We hope this post helps you get your mower back up and running quickly. Let us know your thoughts in the comments.