Transmission Belt Keeps Coming off Pulleys

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You're mowing with your riding lawn mower and it suddenly stops moving. The engine's running fine, and the mower blades are spinning, but the mower just won’t move. You wonder what's gone wrong. A cursory inspection reveals the transmission drive belt has come off. You do the necessary maneuvers to get it back in place and once again your mower will move. A few minutes later, the same problem happens. Now what should you do? This is one of the many frustrations of life, but can be remedied with a careful and conscientious following of suggestions in this article.

Belt Condition

Examine the condition of belt. Is it a twisted “floppy rag”, or is it stiff and straight. Floppy belts need replaced as they will continue to run off pulleys. If the belt has any twists or kinks, it will need replaced as when the twist or kink comes to a pulley it most likely will jump off the pulley. A belt that is stiff and straight may be used again.

gloved hand testing engine transmission belt

Pulley Condition

Check pulleys for rotation and straightness. If idler pulleys will not rotate they most likely have seized bearings and will need replaced. Spring loaded idler pulleys need to be able to maintain belt tension, so be sure they can move freely on their pivots. Tension springs need to be connected at both ends to maintain belt tension.

Pulley Alignment

Carefully check the alignment of the pulleys including motor drive pulley, any and all idlers, and transmission(s) input pulley(s). Yes the Zero Turn mowers have two transmissions usually powered by the same belt. Any up/down movement of pulleys will contribute to belt coming off. If you find the pulleys misaligned, determine the cause. Once you figure out the cause of pulley misalignment you can correct the problem.

Transmission Input Pulley

Check the tightness of input pulley fastener as a flopping pulley will “throw” the belt. Be sure the cooling fan that is integral with the input pulley is not contacting the belt and causing it to come off the pulley.

Transmission Mounting Points and Bolts

transmission belt on gears and pulleys

Broken mounting brackets and/or missing/broken bolts will cause misalignment and that in turn will result in belt coming off pulleys. Carefully check for bolt tightness and tighten them as needed. Replace any missing or broken bolts. A good maintenance practice is to check all transmission mounting bolts at each oil change time. The vibrations inherent in riding mowers will cause bolts to loosen and the strains imposed on transmission mounting points leads to their failure. Loose bolts will add to the failure of the transmission mounts. Loose or missing bolts will allow the transmission to tilt and that will misalign the pulleys and cause belt to come off pulleys.

Engine Mounting Bolts

Check for the presence of and tightness of the bolts that fasten engine to mower frame. If these bolts are loose or missing it will allow engine to get out of alignment and in turn the belt can come off. The vibration mentioned earlier can loosen any bolt on the mower causing misalignment. It's good practice to check all bolts for presence and tightness on a regularly scheduled interval of time or use.

Engine Pulley and Belt Stabilizers

The engine pulley is held on with a bolt up through center of pulley and threaded into engine crankshaft.

engine crankshaft

If this bolt gets loose the pulley will lower out of alignment and belt will come off. This bolt can fall out of pulley causing the pulley to come off the mower engine. The engine pulley has some type of guide pins/belt stabilizers to aid in keeping belt on pulley. If these become loose or fall off they will not be able to maintain belt and pulley alignment.

Getting Things Back to Correct Position

Follow all the steps listed here and your mower should perform well and belt should stay aligned in pulleys.