For me, mowing my lawn is a peaceful time. I come home from work, throw on a pair of noise-canceling headphones with my favorite music, and just mow stripes up and down the lawn.
It was a peaceful experience until my mower deck started vibrating so much, I felt like I was going to be shaken out of my seat.
Mower decks can begin vibrating when the mower blades are damaged or unbalanced; the material is wrapped around the blade shaft; the belt or pulleys are worn; the spindle or spindle housing is damaged, or debris is lodged in the deck or under your pulleys.
I am going to share the most common items that can cause deck vibration on your lawn mower and how to fix your deck.
Safety Precaution: Always remove the key from your mower and unplug the spark plug boots to prevent your mower deck from starting while working on the deck. Use jack stands or another comparable item to raise the deck.
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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.
Damaged or Unbalanced Blades
Take a look at your mower blades. You are looking for bent blades, worn blades, or blades that may have chunks missing due to hitting solid objects. Blades that are no longer balanced due to wearing or damage can cause vibration.
Sand can cause your blades to wear unevenly or it may be the result of not sharpening your mower blades correctly.
When sharpening your blades, you need your blades to be sharpened evenly on both sides to stay balanced. You can use a blade balancer or you can hang your blade on a nail on the wall.
When using the nail method, place the center hole of the blade onto the nail. Watch the blade to see if it sits level or is lower on one side or the other because of weight.
You will need to grind more of the blade on the side that hangs lower until your blade sits level. You can read more about sharpening your blades here.
Wrong or Incorrectly Installed Blades
If you have recently replaced your lawn mower blades and noticed vibration shortly after installation, you need to make sure you installed a blade that fits your lawn mower and that it was installed correctly.
Not all mower blades are the same. I’ve learned that from my many years of selling lawn mower blades for a variety of lawn mowers.
You need to make sure you have the correct length blade and center hole size when purchasing blades. Having this information will greatly increase your chances of getting the correct-sized blades for your mower.
I have found some aftermarket blades that are the correct length and the center hole size I need will not work. Sometimes the specifications listed for the blade are a little different from the blade you actually receive.
If you are not sure if you have the right size blade, match up an old blade to a new blade before installing it on your lawn mower.
Movement in the blade can happen when they are installed without the correct hardware in the right places or the center hole size is too big.
Check a parts diagram for the hardware used with your mower blade assembly and make sure you are placing the hardware in the right order.
For example, some mowers use a blade bolt and washer while others may use a blade bolt, splined bushing, and several washers to secure the blade.
Always make sure your mower blades are installed with the sail (or high side) of the blade pointed up toward the deck. Placing your blade upside down will beat up your grass instead of cutting it.
Foreign Material Wrapped Around the Blade Shaft
Even though you are vigilant and don’t mow over items that aren’t meant to be on your lawn, there is a chance that you picked up a material that wrapped itself around the blade shaft. This can be a piece of string, wire, or long weed.
Remove any items wrapped around your blade shaft and inspect the spindle and housing to make sure you didn’t have any additional damage.
Bad Bearing in the Spindle Housing
Identify whether or not a bearing is bad in the spindle housing. With a good pair of work gloves, to protect your hands from the sharp mower blades, grab a hold of both ends of the blade and rock it up and down to see if you can feel movement.
If the blade does move, the bearing will need to be replaced. Some mower manufacturers will give you the option to replace the bearing to rebuild the spindle housing and they may offer a complete spindle assembly that already has the bearing in place.
Other manufacturers will only offer the option to rebuild or the option to replace the assembly.
Repair Spindle Housing with a Bad 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.
Check the Mower Deck Belts and Pulleys
Remove any covers, if your lawn mower deck has covers, to gain access to your deck belt and pulleys. You should replace your belt if you find any cracks or a glazed look to the belt.
You need to check the tensioner pulley and tensioner arm to make sure they are in good working order to keep tension on the belt. Grease the tensioner arm so it doesn’t seize up. Your tensioner arm needs to be able to move.
Proceed with inspecting the deck pulleys. Grab a hold of each pulley and slowly turn it to check for resistance. Replace pulleys if your pulley doesn’t turn smoothly or you hear a noise.
This is an indication of the bearing failure in the pulley. You also need to check the bearing and bushings on the engagement linkage.
Debris in Your Mower Deck
Even the smallest pieces of debris can cause a bad vibration in your mower deck. Debris, like wood chips, acorns, pine cones, stones, and sticks, can get stuck under your idler pulleys or other areas of your deck.
When your belt goes over the idler pulley that isn’t sitting level, vibration is sent through the deck.
The smallest unimaginable items can cause your mower deck to vibrate. Inspect your deck carefully for loose hardware or foreign materials lodged in the deck and its components.
Damaged Crankshaft on a Push Mower
When you impact something hard with the blade spinning, there is a possibility the crankshaft will bend in a push mower.
If you find changing a bent blade doesn’t solve your vibration problem, you will want to check for a bent crankshaft. Operating your mower with a bent crankshaft can be extremely dangerous.
I highly recommend bringing your lawn mower into a small engine repair shop to have the crankshaft looked at. If your crankshaft is damaged, the engine has to be torn down to replace it.
This may often cost more than a new engine. Get a quote from your mechanic so you can make an educated decision when choosing to repair or replace your push mower.
Your Vibration May Be Coming from Other Areas of Your Lawn Mower
If none of these items solved your vibration problem, it may be because your vibration is actually coming from a different area on your lawn mower. For more places your lawn mower may be vibrating, check out my article “How to Stop Your Lawn Mower from Shaking and Vibrating”.
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.