Lawn Mower: Vibrates Excessively

A bad vibration in your lawnmower might be the result of a damaged or out-of-balance blade. The tip of your mower blade can be moving at nearly 220 MPH! At these kinds of speeds, it doesn’t take much damage to throw a blade out of balance, causing a severe vibration. There are a couple of different ways a blade can become out of balance. If the blade strikes something solid, it might be damaged. Striking a tree root, rock, metal post, or pipe can cause a piece of the blade to break or chip away. The blade should be inspected any time a strike like this occurs. The blade can be cracked, which can lead to part of the blade breaking free and flying away from the mower later. A broken or cracked blade should be replaced immediately. The blade can also become out of balance due to improper sharpening. When a mower blade is sharpened, the exact same amount of metal must be removed from each side of the blade. Sharpening one side more than the other will cause the blade to become unbalanced. When sharpening a mower blade you will need to test it for balance. There are devices you can buy to do this or you can simply hang the blade through its center hole on a nail in a wall. The heavy side of the blade will drop lower than the other. Continue sharpening the heavy side a little at a time until the blade balances evenly on the nail. Removing and replacing a blade is easy to do. Start by tipping the mower on its side so you can access the blade. Tip the mower so the carburetor is up, otherwise fuel will spill from it. Depending on the mower, you might need to drain most of the fuel from its tank before tipping the mower, or fuel might leak from the gas cap. Once on its side use a wrench to remove the blade. If the fasteners are very tight you might need to wedge a scrap of wood between the blade and the mower deck to stop the blade from rotating with the wrench. Once the blade is removed, clean any debris from the mounting bracket and washers before installing the new blade. Install the new blade by tightening the fasteners by hand and then use a torque wrench to finish tightening. Refer to your owner’s manual for the correct torque spec for your mower.

The crankshaft is a central component of your lawnmower’s engine. The piston in the engine is attached to, and drives, the crankshaft. The end of the crankshaft passes through the engine’s crankcase and in the case of most walk-behind lawnmowers, the blade attaches directly to the crankshaft. Most lawnmower vibration issues concerning the crankshaft are the result of the blade striking a solid object. Tree roots, rocks, buried stakes, or pipes can all cause serious damage to the crankshaft. The tip of the lawnmower blade can travel at upwards of 220 MPH. When the blade strikes a solid object and comes to an abrupt stop, all that energy must go somewhere. Often it is transferred to the crankshaft, which causes it to bend. It doesn’t take much of a bend to cause vibration in the mower – usually just a few thousands of an inch will do it. The easiest way to diagnose a bent crankshaft is to first eliminate any other possible source of vibration. If the mower blade struck something hard enough to bend the crankshaft, the blade will also be badly damaged. So start by replacing the blade with an undamaged one and then test the mower again. If the vibration is still there you will need to do some more complex testing. The next step is to check the crankshaft runout using a dial indicator. First, remove the blade, the blade mounting hub, and the spark plug from the mower. The dial indicator can then be attached to the mower deck and positioned on the crankshaft. Rotate the engine slowly over and read the amount of runout on the dial indicator. The engine manufacturer will have a spec for acceptable crankshaft runout, but as a rule anything over .003” would indicate that damage was done. Repairing a mower with a bent crankshaft is a major repair and it is not suggested for a novice repair person. However, if you have the skills and the tools to do it yourself, you can save a substantial amount of money. The entire engine will need to be torn down to remove and replace the bent crankshaft. If that seems like more of a repair job than you are comfortable with, you might want to consider taking the mower to a small engine shop for a repair quote. Often having a repair like this performed by a shop will be substantially more expensive than simply replacing the entire engine.