Mower drive belt keeps coming off, why - Garden

Everett, Do you mean drive belt keeps coming off front pulley?
Possible causes, accumulated dirt/debris above deck or on trans axle, damage,stretched,wrong size belt, belt tensioner seized/missing spring if equipped, belt retainers/guides/keepers damaged /missing/ adjusted to far from pulley, damaged/worn pulleys, loose engine, belt way out of alignment,etc.
Check the attached links,instruction and guides, Good luck
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What Are the Causes of Belt Coming Off Riding Mower
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Need help with mower deck belt keeps coming off Craftsman LT2000...
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Make sure the belt is in line and the idler pulleys are free and not seized and check the adjustment is correct might be the back axle is slotted for the adjustment.

If your drive belt comes off, it's usually a sign of either the belt isn't right, too long... OR the belt guides are either missing or installed incorrectly. There should be guides next to each pulley which keeps the belt close so that when the blades are disengaged and loose, they won't flip off once activated

There are no belt adjusters on most mowers, they gave spring loaded constant adjusters..if the belt is coming off it because the belt is worn..your mower should have one or two adjustable belt protectors around the the motor and deck pulleys they keep the belt from coming off during on and off operations the design is to be set close enough to the pulleys that will not allow the thickness of the belt to pass through...so if the belt is Worn too much or the proctectors are out of adjustment the belt will come off..usually during non mowing or just when engaging the deck or drive..kind of tricky to get at..but can be done yourself...do check them out if they are more then about 1/4-3/8 inch they are out if adjustment..if not the belt is worn out...good luck..

Once the drive belt comes off once - it's probably stretched and twisted and won't stay on. You'll need a new belt - be sure to get one that's Kevlar so you won't be replacing as often.

They may make a Kevlar belt that fits your mower, which may stop the breaking. But may cost twice the amount of the original belt. Seems like it may be worth it to me?

You answered your own question. The source of the wobble may also be a worn bearing or bushing supporting the shaft the pulley is mounted on. Be certain that you are installing the belt over the proper path of pulley's I know that My personal Mower WEEDEATER 42" can have the belt installed wrong it works but will fall off in heavy grass.

Check that the pulley isn't seizing up and causing the belt to snap, also be sure you loosen up the self propelled mechanismn by spraying down with WD 40, too much belt tension can cause this, but it rather seems like the belt pulley seizes and the engines drive pulley keeps turning causing the belt to overheat and break or stretch and come flying off.

Check to see if the idler arm at rear of machine where belt comes off PTO is bent or bushings wore out.Ask your dealer if there is any guides you can install around spindle pulleys.

check the alignment of the ideler pully to make sure it is even withe the drive pulleys. if not in line it will cause the belt to break. There is a spacer that is on the idler that should be on top and not on the bottom of the pully.