Lawn Mower Vibration: How to Diagnose and Repair

The Importance of Preventing Lawn Mower Vibration

It's imperative that you learn how to diagnose and repair lawn mower vibration issues, in the event your lawn mower suddenly starts vibrating more than usual. Lawn mowers that excessively vibrate, can eventually lead to internal engine damage/failure, if not addressed in a timely manner. The engine's internals are not designed to be subject to such violent and rapid vibrations, and it is always best to resolve this problem sooner rather than later.

Looking for Clues

It is important to note at what point did the vibration begin? Was it after an event such as striking a hard object with the blade, like a rock, stump or other hard obstruction? If you can confidently remember such an event, it will prove helpful in trying to decipher the root cause of your vibration problem, ultimately increasing the likelihood of reaching an accurate diagnosis and eventual repair. Also, take a look at your lawn mower's blade. Is there evidence of it striking a hard object?

The Importance of Blade Balance

Lawn mower blades normally spin at a rate of 1200-2000 RPM. Lawn mower engines are designed to be run with the weight of a blade spinning, to counteract the rotation of the engine's internals; namely the spinning flywheel and rotating crankshaft. If the blade becomes unbalanced, it essentially means that more weight is on one side of the blade than the other. Even a subtle imbalance can create a condition that can cause a lawn mower to noticeably vibrate.

Remedy: Remove the blade and check balance. To do so, first remove the park plug wire from the lawn mower to prevent accidental starting. Next, tilt your lawn mower with the carburetor and air filter side of the machine up (detailed instructions on how to do this safely can be found here: https://www.maverickmowers.com/post/how-to-tilt-lawnmower) to gain access to the machine's undercarriage. Now, rotate the blade and pay attention to the distance between the outermost section of the blade and the undercarriage of the deck. Does the space remain consistent throughout the entire circular rotation, or does the space shrink or expand in certain spots? Next, select an appropriate sized socket to remove the blade mounting hardware fastening it to the crankshaft protruding from the bottom of the engine. To prevent the blade from spinning while trying to loosen this hardware, place a piece of wood to wedge the blade against the underside of the deck. Then break free the bolt by loosening counter-clockwise. Next, examine the mounting hardware of the blade for worn, elongated holes as this will cause an out of balance blade condition.

The space between where the green arrows are pointing should remain consistent when rotating the blade. If gaps increase or decrease a bent blade or crankshaft is likely.

Examine the Blade's Cutting Edge

With the blade removed, examine the cutting edge for nicks, gouges or missing material. Pay special attention to cracks in the blade. If any cracks are seen, the blade must be replaced. If possible, hang blade with a string and gently strike with a hammer, if a sharp ringing sound is produced the blade is good, if a dull thud sound is produced the blade is bad and contains cracks; it must be replaced.

Examine the Blade for Straightness

Visually examine the blade by holding it away from you, lengthwise, and pay attention to the blade's end down its entire length. If available, hold the blade down against a known flat surface to check.

Testing a Blade for Balance

To test a blade for balance simply clamp a bolt in a vice parallel to the ground. Next, place the blade's mounting hole through the bolt and release your hold on the blade examining which side of the blade drops down. The heavier side of the blade will require more material to be removed following your method of blade sharpening. (Safe practice and proper blade sharpening procedures are beyond the scope of this article, however they must always be followed. Always wear proper protective equipment when sharpening blades.) The blade is balanced when you are able to place the blade on the bolt and it remains horizontal and does not drop either way.

A balanced blade will rest in this position.

Sheared Flywheel Key

A flywheel key is a small key made of a softer metal. It resides within the crankshaft and mates with the key ways found on the crankshaft end and the flywheel. The flywheel then is bolted to the top end of the crankshaft. It keeps the engine in "time" by ensuring that spark is fired at the precise moment necessary for combustion to take place in the engine. The spark must be timed with the magnets on the flywheel. If the blade of a lawn mower is to strike a hard object, the flywheel key may or may not deform enough to stop the engine and prevent catastrophic engine damage such as a bent crankshaft from occurring, with the impact of the blade striking something. A bent crankshaft usually means a lawn mower is not worth repairing. It is always best to examine the area you are about to mow prior to mowing for any hard objects to prevent crankshaft damage.

This key has become deformed due to an obstruction hitting the blade of a lawn mower. Its should have nice squared or slightly rounded corners.

Remedy: If you suspect you have hit a hard object with your lawn mower's blade, or if it suddenly has become impossible to start, or is vibrating violently and running poorly, you may need to remove the flywheel and inspect the flywheel key.

The circled portion is where the flywheel key mates with the flywheel. The crankshaft's top end goes through this center hole. Examine the key way as shown here on the flywheel and on the crankshaft's key way itself for damage.

To access the flywheel you need to remove the engine cover on your lawn mower. It will usually be fastened by a few screws or bolts.

The flywheel underneath will be fastened to the top end of the crankshaft with a flywheel nut. It is often necessary to wedge the blade with a block of wood to keep the flywheel from spinning as you loosen this nut.

The flywheel is usually press fit to the crankshaft with a taper. This means you will not be able to simply lift off the flywheel. Often it must be strategically pried off or a special flywheel removal tool must be used, depending on make and model. Note: The flywheel is often made of aluminum and can easily be cracked or damaged if struck hard, always be cautious when removing.

The flywheel key must be removed and examined, as damage to it may not be easily observed until it is completely removed. Examine all its sides for any deformities. Replace if necessary.

Loose Engine Cover Fasteners

Sometimes, simply having loose engine cover fasteners will cause the lawn mower to appear as if it is vibrating when all that is necessary is a quick tightening of these bolts or screws to solve this problem.

Remedy: Simply ensure these fasteners are hand tight using appropriate ratchet and socket.

An excessively vibrating lawn mower condition should never be ignored. A few simple checks, like the ones outlined in this article, are all that is necessary to diagnose and repair lawn mower vibration, and prolong the service life of your machine!

Never the Norm!