If mowing your lawn is causing your teeth to chatter because the lawnmower is shaking violently, you obviously have some kind of problem with your machine. A few reasons account for why a lawnmower will shimmy and shutter, and diagnosing the problem is the first step to getting your mower back to the smooth ride (or push) it once was.
If you have removed the blades of your mower recently for sharpening or have made any adjustments, they may have been reinstalled improperly. If the blades of a mower are not level or get out of balance, the fast spinning of the blades will cause a severe vibration in the entire mower, according to the Lawnmower Man website.You must fix this problem before continuing to use the mower. If a blade is not installed properly, it could potentially fly out during use and seriously injure or kill someone, or it could destroy the mower. An unbalanced blade can also be the result of an accident. Running over a piece of concrete or a metal stake or any heavy, solid object could potentially result in a bent blade, which will throw off the balance.
Debris on Blades
Sometimes the blades of a mower go through something they can't sling off. Clay or mud might be stuck on the blades, or perhaps twine or clumps of grass have managed to become attached to the blade. As the blade revolves, the extra weight form the debris will cause additional force on one side of the blade but not the other. This off balance rotation can result in a slight shaking. This problem is easily remedied by turning off the mower and cleaning the debris off the blades.
Mount or Crankshaft
Since a lawnmower shakes a little all of the time, the nuts and bolts that hold everything together may eventually get a little loose, which can cause a significant increase in vibration and shaking if the bolts holding the motor mount or crankshaft are loose. The vibration of the motor intensifies if it is not secured tightly to the frame, and if the mount is broken the problem will get much worse. Keep the motor mount secure as well as all of the other parts of the machine by routinely inspecting all nuts and bolts for tightness to prevent this problem.