What causes a crank shaft to bend? Main Image

We’ve all been there once before, its mid-summer and you’re out in the lawn mowing when…BAM! That sneaky tree stump, rock, or some other hidden object stops your mower dead in its tracks. Before you can start to understand the damage that may have just occurred to your mower it helps to know the amount of energy being exerted by your mower and its moving parts.

While you’re leisurely walking along mowing your lawn, your mower blade (or blades) are zipping around at racecar speed. On average, the blade tip on a 21” walk-behind lawn mower can reach speeds close to 200mph. With the blade moving so fast (about 51 revolutions per second), it’s important to consider the amount of kinetic energy being created by this movement. The kinetic energy generated by the mass of the rotating blades is comparative to the energy of dropping a 50lb weight from 17ft.

When your lawn mower blade encounters a sudden stop as a result of hitting an immovable object in your lawn, this kinetic energy has to go somewhere. The majority of this kinetic energy is dissipated through the blades and crankshaft. When the impact initially occurs, the crank shaft will twist. With the combination of the crankshaft twisting and the distance between the bottom of the engine and the lower end of the crankshaft acting like a lever, enough torque is created to bend the crankshaft.

Manufacturers are aware that these parts are prone to impacts so lawn mower blades and crankshafts are built strong to withstand these impacts without breaking. Even though the parts are built to withstand impacts, when there’s that much energy, it’s impossible to believe that these parts will incur no damage when making contact with a solid or immovable object.

At the end of the day, lawn mowers are designed to cut grass lawns. Not rocks, stumps, pipes, sprinklers, curbs or any number of other immovable objects. Since these objects can easily be hidden by grass growth, we recommend spending a little extra time before mowing your lawn to carefully mark any immovable objects in your lawn to avoid running into them. We also recommend using extra caution while mowing around curbs and planter edges where the wheel of your mower can suddenly drop. Pushing your mower around these types of obstacles instead of using the mowers self propel system (for mowers with this feature) is a more cautious and safe approach. By spending a little extra time checking your lawn and mowing cautiously around stationary obstacles, you can avoid the collateral damage of getting “stumped".