You’re mowing the lawn and you feel excessive shaking of your lawn mower. From my experience, it may not be as bad as you think. There are many items that can cause vibration in your lawn mower.
Why is your lawn mower vibrating? Your lawn mower may be experiencing vibration from loose engine mounting bolts, the electric clutch, unbalanced blades, foreign mater wrapped around the blade shaft, bearings in the spindle housing, bad pulleys, or debris wedged in a mower.
I had a friend call me up in a massive panic. He said, “Hey, you need to come to check out my mower. When I engage the blades it feels like I am riding a jackhammer!
The entire mower is going to vibrate apart!” Have you ever been in this situation? Maybe you’re at that point now and that’s what brought you to my blog.
As a good friend, I grabbed my tools, headed to his home, and checked out his “jackhammer”. I went over the regular maintenance steps he should check while I inspected his mower. I then went through the steps, one at a time, to show him how to isolate his vibration problem.
This post may include affiliate links. Purchases made through these links may provide a commission for us, at no extra cost to you. As an Amazon Associate, we earn from qualifying purchases.
Follow all safety instructions provided in your equipment operator’s manual prior to 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 Steps to Check Your Lawn Mower for Vibration
1: Remove the Spark Plug Boot for Safety
Do not skip this very important step. Removing the spark plug boot will prevent the engine from starting while you work under the mower deck. It’s important to ensure your safety while working on your lawn mower.
2: Look for Loose or Missing Engine Mounting Bolts
Inspect the engine mounting bolts, also known as motor mounts, to make sure they are tight. Replace if the bolts are missing, worn or broken.
These bolts are designed to reduce vibration when the engine is running. With a push mower, you can check the engine bolts if you have a vibration when you engage your blades.
3: Check the Clutch for Bearing Failure
The electric clutch has a ball bearing in it that can fail. This can bring on a vibration. A manual engagement clutch with belts and pulleys will have bushings in the linkages that will wear and cause a vibration.
You will not have an electric clutch on a push mower, but you will have a belt engagement lever you should check.
4: Inspect Your Mower Blades for Unbalanced Blades
Look at your mower blades. An unbalanced blade can cause vibration. A blade can become unbalanced when it is worn more on one side than the other. This can be caused by sand wearing it out unevenly or from being sharpened incorrectly.
You can read more about inspecting and sharpening your blades here.
If you have recently replaced your blades, make sure you installed them correctly using the correct length blade and center hole size. It’s important to install balanced blades.
A good way to check your blades before installing is using a blade balancer or hanging on a nail to ensure the blade isn’t heavier on one side.
5: Check for Foreign Material Around Mower Blade Shaft
Even though you spend time picking up items in your yard before mowing, there is a chance you will mow over items that aren’t supposed to be there and that you didn’t see while mowing.
It’s important to inspect your lawn mower deck if the prior items you checked are not the cause of the shaking.
Look at the top of the blade to check to see if there is string, wire, or any other foreign material wrapped around the blade shaft or spindle shaft as the professionals call it. If any items are found, remove them and make sure the object didn’t cause any additional damage.
6: Check for Bad Bearings in the Spindle Housing
Look at the bearings in the spindle housing which are sometimes referred to as quill assemblies on smaller units. With a good pair of work gloves, to protect your hands from the sharp blades, grab a hold of both ends and rock it up and down to see if the bearing is bad.
If you feel movement, the bearing needs to be replaced. You can replace only the bearing or sometimes it’s just easier to replace the whole cutter housing assembly if that is an option available for your lawn mower deck.
Fix a Bad Lawn Mower Bearing
- Remove the belt from the pulley
Find something narrow to put between the belt and the groove of the bad pulley and carefully turn the belt by hand. Let the narrow object follow the belt to the pulley and pry up on the belt so it comes off of the pulley.
- Remove the pulley
Get a socket that will fit the nut on top of the pulley and remove it. In most cases, it will be a normal thread so turn it to the left or counterclockwise. Depending on what type of mower you have, there may or may not be set screws holding the pulley onto the shaft.
These may be allen screws or hex screws. They may not come off very easily and you may need to use a puller to remove them.
- Remove the spindle
There should be 4 to 8 bolts holding the spindle in place. Remove the bolts and the assembly will fall out of the bottom of the deck.
- Replace bearing
Please the housing assembly on a flat hard work surface. Check to see what you will need to disassemble the housing. Sometimes a snap ring will hold it together and other times you will just replace the entire housing which includes the bearings.
Replace the bearing if your assembly allows you to. Grease the bearing and attach it to the deck in the reverse steps you used to take it apart.
- Reinstall housing, pulley, and belt
Once you bolted the housing to the deck and install the pulley, make sure everything is tight and there isn’t any movement. Reinstall the belt. Turn the belt by hand to make sure it is on all of the pulleys.
7: Check Belts & Pulleys
Remove the covers on the deck if your lawn mower has covers. This is an important area to keep clean and free from debris. It’s a good idea to clean out the belt and pulley area when conducting your lawn mower cleaning for performance and also to prevent a fire.
With dry grass packed into the top of the mower deck the belt housing area heat can build up and, when you least expect it, you can have a fire on your lawn mower. Avoid this costly repair by keeping the area clean.
Look at the belts and pulleys and check the bearing and bushing on the engagement linkage.
8: Check for Debris Lodged in Mower
In my buddy’s case, I found a wood chip wedge in the pulley. Every time the belt went over it, the belts would get vibration and would rattle the complete mower deck. Little rocks, wood chips, and tree debris can get stuck in your mower.
So when you are looking at the cause of your lawn mower vibrating, you need to look at all options and details.
No matter how small or weird a recommendation may sound you should check it out. You may be surprised at what you find. The strangest things can cause vibration in your lawn mower.
What causes my mower deck to vibrate or shake when the blades are engaged?
Your mower deck vibration can be caused by unbalanced blades, bad bearings in cutter housing, and foreign material wrapped around the blade shaft or lodged into the deck and pulleys.