There are two main reasons why your lawn mower blade is spinning but not cutting the grass. It could be either the blade is not spinning fast enough due to slow engine speed or the blade is blunt. It is that easy.
Nine times out of ten you will either need to sharpen your lawn mower blades or replace them. To get me out of a jam I will often sharpen the blades once before replacing them.
Lawnmower blades will become dull and worn over time. More so if you are running over rocks, sticks, tree roots, and dirt mounds.
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How to sharpen Lawn Mower blades.
The best way to sharpen lawn mower blades is with a double-cut file (2nd cut). It is best to remove the mower blades before sharpening them. Some lawn mowers will have a single cutting blade whereas others will have two or more cutting blades.
The quick unsafe way is with a grinder.
Warning: Lawnmower cutting blades can have sharp edges. Always wear leather gloves or take care when removing and installing lawnmower blades. Use safety glasses when sharpening the blades.
Check the direction of the mower blades by first turning off the fuel, disconnecting the spark plug lead, and lifting up the catcher. Slowly pull the pull starter and check the spinning direction of the lawn mower blades.
Most lawn mower blades rotate in a clockwise direction when viewed from above (counter-clockwise when viewed from below).
Place the lawnmower blade in a vice and sharpen the cutting edge using the second cut file. Only sharpen the leading edge, not the fan edge that generates airflow to throw grass into the catcher.
Some lawnmowers have a single cutting blade and others have 2x cutting blades. Both will wear out over time and will require resharpening or replacement.
Lawn mower leaving uncut grass
This is a good indication that your lawn mower blades are worn and blunt. Rather than cutting the grass, the blades are pushing the long grass down and out of the way. You will need to sharpen or replace the blades.
Blunt blades will cause damage to the grass and allow fungus and other contaminants to kill the damaged grass leaf. A sharp cutting blade will not stress the grass but will promote healthy growth.
Blunt or worn cutting blades can also prevent grass from going into the catcher. This is due to the wings on the cutting blades being worn. If this is the case they will need to be replaced.
Lawn mower blade not spinning
Your lawnmower blade should spin freely by hand or when pulling the starter cord. If it is not spinning there could be a restriction or build-up of mud, dirt, grass, leaves, or rust.
An obvious reason if your blades are not spinning fast is throttle control. Make sure you have the throttle in the running position. Most often this is pictured as a rabbit, not the slow turtle.
You could also have an issue with the carburetor so check out this post if your lawnmower runs for a while and then dies.
Manual hand-push lawn movers need very sharp and close-cutting blades. These can be adjusted as they wear down over time. My kids love pushing the old push mower.
How tight should lawn mower blades be?
My owner’s manual recommends torquing the lawn mower blade bolts to between 35-40 ft. lbs. I like to use never seize paste on the bolts as they are in a very wet environment. Rust, dirt, and grass make these bolts hard to remove and tighten up.
If you have access to a torque wrench then 35ft. lbs are recommended. If you don’t have access to a torque wrench then use a socket set to tighten it up fairly tight. You don’t want these bolts coming loose.
Check the bolts after mowing your lawn for the first time after replacing your blades. There may be some movement while they bed in.
Sometimes lawnmowers with two or more cutting blades spin freely by hand. They will move from side to side. This protects them if they hit a split object so they will spin out of the way.
Sharpening Electric Lawn mower blades
I have noticed that electric lawn mower blades are thinner than petrol-powered blades. As such tend to need sharpening more often when in a rocky environment.
Remember to always remove the battery when working on an electric lawn mower.
Lawn mower blade keeps coming loose
If your lawn mower blade keeps coming loose and vibrating off then you will need a locknut or some Loctite. A Nyloc locknut has a nylon collar that increases friction on the screw thread. If you don’t have the correct size you can use Loctite on the threads to glue the nut and bolt in place. Don’t worry you will still be able to unscrew them later on.
How to remove lawn mower blades?
Lawnmowers with two or more cutting blades can be removed by taking out the center bolt and removing the round cutting disc. This will give you easy access to each cutting blade.
You can remove each blade bolt without removing the cutting disc but it is a lot harder as you will need short ring spanners. A little bit of lubricating oil or spray will help the rusted-on bolts come loose.
Lawn mower blades will last a few seasons but they will wear out over time. Rocks roots, sand, dirt, toys, and sticks all will blunt the cutting blades.
Never tilt the lawn mower with the engine running to look at the blade this is very dangerous. Always disconnect the spark plug lead and turn off the fuel when working under the lawn mower.
I am a qualified Industrial Electrician for the past 20 years and I love to share my knowledge on home repair and maintenance jobs.
I love fast toys like Motorcycles, Cars, Jetskis, Boats, and Computers so writing about them is easy. Working on them is fun.
To keeps costs down I do all my own mods, repairs, and servicing. These skills I want to share with everyone. DIY is a skill everyone can learn.