Lawn Mower Gear Box Testing

Ponderosa Associates was hired to study and determine the mode of failure in the gearboxes of a commercial grade lawn mower. To start the investigation, a special test fixture was designed and built.

The test fixture, which is specified by design standards for testing lawn mowers, included a large spring-loaded metal rod that was ejected upward from a concrete-encased cylinder into the spinning blade of a mower. As catastrophic as that may sound, the tests proved to be much less dramatic than anticipated; however, the source of the problem was discovered. Sheer pins attaching the blade to the drive train are designed to break in the event the blade strikes a solid object. Measuring strain to the drive train during the blade stop tests, it was determined that the rotational inertia of the rotating blade was instantaneously converted into a shock load when the metal rod stopped the blade. Interestingly, the size of the shear pin played no role in the failures observed to the gearboxes. The fundamental design requirement of having a gearbox drive the blades turned out to be the critical flaw in the system’s failure to meet the design standard.