Metal Trumps Plastic in Lawnmower

One of the two front wheels that propel my Sears Craftsman self-powered rotary lawnmower stopped working, while the other wheel started working intermittently. When I removed the plastic cover over the drive mechanism, I saw what appears to be a worm-drive transmission that was coupling power from the engine via a belt to a shaft driving the two front wheels.

While squeezing the control bar on the handle, I could see the drive shaft turning, but one of the wheels was not turning. Once I removed the front wheels, I noticed that at least half of the teeth had been almost ground away inside of one wheel, while a smaller section of the teeth had been ground away in the other wheel.

When the drive control bar on the handle is squeezed, the worm-drive transmission is pivoted forward, presumably on a bracket attached to the deck. When that happens, the pulley tightens up against the belt, drawing power from the engine. As the transmission pivots, the drive shaft is moved forward, engaging the gears on the ends of the drive shaft into the teeth inside the front wheels. The front wheels are made of plastic with gear teeth molded inside the rim.

The gears on the ends of the drive shaft are made of metal and are engaged into these plastic teeth when the mower's self-propulsion system is engaged. If the operator doesn't fully squeeze the control bar up against the handle bar, the metal drive gears are not fully engaged and literally chew away the plastic teeth inside the front wheels. Replacing the wheels is only a temporary fix, as the new wheels will suffer a similar fate.

This entry was submitted by Andy Morris and edited by Rob Spiegel.

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