Each year in our service department, we see several instances of customers who have experienced a bent blade on their lawn mowers. In every case, the spinning mower blade had come into contact with an immovable object, causing the blade to suddenly stop, resulting in a bent blade. In some cases, the force of impact was significant enough to cause the engine’s crankshaft to bend, causing damage to the engine internally. Symptoms of a bent blade include excessive vibration, an uneven cut, potential damage to the mower’s deck, the scrolling and pose a serious safety risk. In addition, a bent crankshaft usually requires the engine be rebuilt and/or replaced, resulting in an expensive repair bill and it may even mean buying a replacement mower. Bent blades, crankshaft and/or engine damage caused by blade impact are issues that are not covered under your mower’s manufacturer warranty.
As a lawn mower owner, it is important to understand the reason why a crankshaft can bend. When the mower is running and the blades are engaged(spinning), modern lawn mowers can rev as high as 50+ revolutions per minute (Each revolution is one full 360 degree turn of the blade). Essentially, this means the blade is spinning at approximately 200 miles per hour. Considering this fact, the kinetic energy created by the blade spinning at this speed is approximately the same as the ground impact from a rock weighing 50 pounds dropped from a third story window. When the blades make contact with an immovable object such as a tree stump or water meter cover and are forced to suddenly stop, the laws of physics dictate that all this energy must be dissipated.
In this case, the energy is dispersed among the blades and when the sudden stop happens, the crankshaft twists, but can also bend due to the exposed length of the crankshaft between the bottom of the lawn mower’s engine and the blades. This distance creates a lever effect, resulting in torque that causes the crankshaft to bend. Modern lawn mower components such as the crankshaft are designed to be strong and resist breaking, however it is impractical to assume no damage will occur when a spinning lawn mower blade comes into contact with an immovable object such as a tree stump, rock, or even a curb.
1Know The Location Of Immovable Objects in your Lawn
Before you mow, walk your lawn and clearly mark the location of any and all hidden immovable objects in the mowing path. The most common hazards in your lawn include water meter covers. stumps/exposed tree roots, large rocks, and sprinkler heads Bright colored small utility flags work well, are inexpensive and can be purchased at your local hardware/home improvement store.
2Be Aware of Areas With Hard Drop-offs And Edges
3Know When/Where To Use Your Mower’s Self Propel Function
If your mower is equipped with a self propel system, it is a good idea to refrain from using the self propel function when mowing around these areas of your lawn, instead, manually push your mower when navigating near and around these areas.
4Comprehensive List of Items To Watch Out For
1. Water Meter Covers/Utility box covers
2. Protruding/loose rocks (pea gravel and smaller rocks also pose a safety issue as these items can become projectiles)
3. Exposed Tree Stumps/Roots
4. Sprinkler Heads/Control box covers
5. Sticks/fallen branches
6. Children’ toys, Pet Toys and Athletic Gear (such as Balls, Pads, Rackets, shoes, etc)
7. Hand tools/small metal objects
8. Curbing, Terrace and retaining walls (risk of wheel dropping off the edge)
9. Cables, power lines, and exposed pipes
10. Be cautious on slopes/hills to avoid possible injuries from falls while mowing
Finally, it is important to realize that all lawn mowers were designed and purpose built to cut grass. Any solid objects in your lawn pose the threat of blade and/or engine damage. Here are a few tips you can use to help avoid a bent blade, and potential damage to the engine:
As always, if you have a possible bent blade/bent crankshaft, It is recommended that you take your mower to a reputable and knowledgeable repair shop so any potential damage can be accurately diagnosed and then properly repaired to avoid possibly injury and additional damage to your mower.