You’re out mowing your lawn when you feel like your arms are just going to shake off. The push mower has developed a significant vibration that is making your mowing task very uncomfortable. It’s time to find and fix the problem so you can gain control of your arms!
A push mower begins vibrating when the engine mounting bolts are worn or missing; the engagement lever is worn or loose; the blade adapter is damaged; the mower blade is bent or unbalanced; the crankshaft is bent, or the flywheel key is sheared.
Before performing any work on your push mower, follow the safety guidelines in your operator’s manual to prevent injury. Never work under your mower deck without ensuring your mower will not start. This includes removing the spark plug wire.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
8 Reasons a Push Mower is Vibrating
Engine Mounting Bolts
Most of the “normal” vibration coming from a push mower is the result of the engine vibrating. A lot of this vibration is absorbed by the engine mounting bolts, also known as motor mounts.
Over time, these bolts can become worn, loose, broken, or go missing so they are no longer able to reduce the amount of engine vibration you feel when operating your mower.
Tighten and replace any bolts that are missing or not in good condition.
The engagement lever can cause vibration. This is the lever that is used to engage the blade. Check for loose or worn parts. Tighten the parts and replace any parts that are showing signs of wearing.
When the mower blade has a significant impact with a stationary object or a series of smaller impacts, the blade adapter can become damaged and fail.
The blade adapter can also wear when the crankshaft is damaged, the wrong blade is installed or the blade is unbalanced.
A blade adapter that is worn or broken must be replaced. Check for additional damage to the crankshaft. New push mower blades must also be installed when replacing the blade adapter.
Bent Mower Blade
Impact on a push mower blade can bend the blade. A bent blade will send a vibration through the mower. To check for a bent blade, park your push mower on a flat-level surface.
Measure from the ground to the blade tip. Rotate the blade 180 degrees and measure from the same spot on the ground to the blade tip. If there is more than a 1/8” variance between the two measurements, replace the mower blade.
Another way to determine whether you have a bent blade is by removing it and matching it to a new mower blade. If they don’t fit together and there are gaps remaining between the old and new blades, the blade is bent and must be replaced.
Don’t attempt to straighten and reuse a bent mower blade. The metal is weakened when you straighten a blade making it more prone to cracking.
A piece of metal from a cracked mower blade can fly out from the mower deck at high speeds. This can put you and others in the area at risk of injury.
Unbalanced Mower Blade
An unbalanced mower blade is one that is heavier on one end of the blade than the other. This blade can wobble and cause a significant vibration. There are a couple of ways a blade can become unbalanced.
The first way is uneven wearing. Dirt is drawn into the mower deck and wears the blade. When it doesn’t wear evenly, the weight of the blade will not be equal on each end of the blade,
The second way is by incorrectly sharpening the mower blade. More metal may have been removed from one end of the blade causing the unequal weight.
To correct this, use a blade balancer to check for equal weight. Remove a little metal from the heavier end of the blade.
If you don’t have an inexpensive blade balancer, another method is to place a nail in the wall with the nail head about 1 inch away from the wall. Place the center hole of the blade onto the nail.
A balanced blade should sit level. If it doesn’t sit level, remove a little metal from the side of the blade that hangs lower until it sits level.
Bent Mower Crankshaft
A bent crankshaft will definitely cause your mower to shake. You can perform the following tests to determine whether the crankshaft is bent. Be careful and protect your hands from the sharp blade.
With your spark plug removed and a clamp holding the safety lever to the handle to disable the flywheel brake, carefully spin the blade and watch the center bolt. If the bolt is moving all over the place, your crankshaft is most likely bent.
If you are unsure after rotating the blade, remove the blade and blade adapter. Turn the crankshaft and measure to make sure the shaft is remaining in the center.
You can use a dial indicator or some form of a stationary object to gauge whether the crankshaft is moving off-center indicating it is bent.
If it does not remain centered and shifts to one side, the crankshaft is bent and it’s time to replace your lawn mower. Replacing the crankshaft is usually more expensive than purchasing a new mower.
Some choose to try to bend the crankshaft back in place, but this poses a significant safety risk. I highly recommend purchasing a new lawn mower.
Sheared Flywheel Key
The key located between the flywheel and the crankshaft can become sheared and cause a vibration. Remove the nut off the top of the crankshaft to inspect the flywheel.
You will find a small key in the keyway. If it is damaged, it must be replaced.
Check for loose hardware and worn parts on your mower that can cause items on your mower to vibrate.
Still Having Problems with Your Push Mower?
Push mower owners encounter many common problems over a mower’s lifespan. Every mower develops issues over its lifespan even when purchasing a top-of-the-line lawn mower.
To help you identify the causes of these problems and how to fix them, I put together this guide to help: Common Push Mower Problems & Solutions.