Your Mower Deck Belt Keeps Coming Off? Here’s Why

One of the biggest pet peeves of anyone that owns a riding mower is the deck belt coming off.

It’s not usually a big problem when it slips off, and is easy enough to put back on most of the time. But if you notice that your mower deck belt keeps coming off, you need to start asking “why?”

Because if you don’t, you’re going to keep finding yourself on your hands and knees, and that’ll get old quickly.

Why Your Mower Deck Belt Keeps Coming Off

There are lots of potential reasons for a mower deck belt that keeps coming off. It might be that the belt is damaged or worn, the belt could have stretched, one or more belt keepers could be bent out of place or there could be a damaged pulley or pulley bracket.

Our Checklist to Diagnose Why a Deck Belt Repeatedly Comes Off

To get to the bottom of a deck belt that repeatedly comes off, you’ll need to set aside some time to take a closer look at all of the components that come into contact with the belt.

There will be some sort of problem with at least one of them or with the belt itself. They don’t just come off for no reason.

Run through the checklist until you find out the cause. Good luck!

Belt is Worn/Damaged

This is personally one that I’ve heard of quite a lot. I have a few buddies that run landscaping services and when the deck belt starts to really wear, you’ll find it comes off more and more.

That’s because the belt becomes thinner in these worn spots, which is how it slips off.

Solution: Your only solution in this situation is going to be to replace the deck belt, as it’s not possible to repair these worn spots.

Whatever you do, don’t get a cheap aftermarket replacement though, as they often wear super quickly – always go with an OEM belt.

I’ve also heard very good things about belts containing kevlar fibers, so that might be worth checking out.

Belt Has Stretched

If you can’t see any visible damage or worn spots on your belt, but you have been using the belt for a considerable amount of time, it’s possible it may have stretched.

“Do lawn mower belts stretch??” is most people’s response to this suggestion. But they can and do.

And if this is the case it could explain why your mower deck belt keeps coming off, as it’s now too big. Even if just fractionally too big, the belt will work its way off.

Solution: Like with the last issue, there’s no way to reverse a stretched lawn mower belt, so your only option is going to be to source a replacement.

Wrong Belt Installed

Did you replace the belt yourself the last time that it broke? If so, are you 100% sure that you bought the right belt?

There are tons of different deck belts available and if you order online it’s very easy to make a mistake.

If you ordered a belt that isn’t the exact right belt for your lawn mower model, it could be too big, which is why it keeps coming off.

It’ll usually be obvious if this is the problem, as you’ll start having this problem right after changing the belt.

Solution: If you bought the wrong belt, you need to get the right belt on order and fit it. Paying out for belts twice is annoying, but at least it’ll stop coming off.

Belt Keeper is Bent

If you take a look at the deck, you’ll notice that alongside the pulleys you have some “pins”. These are belt keepers and are designed to keep the belt in place, on the pulley.

If one or more of these should become bent, the belt will have a gap to go through.

It’s quite common that when there’s a bent belt keeper, the deck belt comes off when lowering and raising the deck.

Solution: Check each of the belt keepers to see whether they have been bent out of shape and adjust any that are not in the right place.

Be careful not to push them too close to the pulleys though as this will prevent them from spinning freely.

Pulleys that are Bent or Damaged

Each of the pulleys that the deck belt runs around should be level and spin freely.

These pulleys do occasionally get damaged through use, and should they no longer run level, it can result in the belt slipping off the pulley during operation.

To check this you’re going to need to remove the belt completely, then check each pulley individually.

Does it spin freely? Is it spinning level? If you find one that isn’t, that could be the problem.

Solution: You can try to bend them back into place with a bit of force (and maybe a hammer). Often it’s the bracket that bends. In the worst-case scenario, you may need to replace the pulley.

Split Plate/Bracket Below Pulleys

When your mower deck belt keeps coming off, most of the focus gets put on the belt itself, the pulleys and the pulley keepers. These are the primary suspects and should be the things you check first.

If you see nothing wrong with them, look closely at the pulley bracket and the plate it is attached to.

Sometimes these things can split, and when the mower is in use, that extra “give” can cause the pulley to move and the belt to slip off.

Solution: A split plate would need to be welded and a bracket either repaired or replaced to fix this issue.

Plan of Action (Quick Summary)

If your mower deck belt keeps coming off it’s a pain in the butt. It gets old very quickly.

If this is the situation you’re in, take the deck covers off and position your mower so you can get a good look at the belt and pulleys.

And then ask yourself these questions.

• Is the belt especially worn in certain places?
• Have you recently fitted a new belt? If so, are you sure it was the right one?
• Do any of the belt keepers appear to be bent out of place?
• Are the pulleys all spinning freely and level?
• Do you see any damage to the brackets/plates the pulleys are attached to?
• Is your deck belt old? If so, do you think it could have stretched?
• How tight should your mower’s deck belt be? Is it tight enough? Have a look in your manual.

If you run through this checklist, you should find out why the belt keeps coming off. Once you put it right, you can get back to what matters the most – cutting your grass!