Few things are more frustrating than a lawn mower blade spinning but not cutting. I’ve experienced it before, pushing a mower across a lawn with the engine roaring but not cutting. With some troubleshooting, I’ve discovered quite a few reasons why this could happen. I’m going to share what I have learned and how I got my blades spinning correctly again.
Why Is My Lawn Mower Blade Spinning But Not Cutting (The Short Answer)
I narrowed the issue down to a particular area. Any systems after the driveshaft/crankshaft, such as the pulleys or belt, can cause the lawn mower blade not to spin fast or at least not fast enough.
Check These 5 Things If Your Lawn Mower Blade Spins But Doesn’t Cut
Lawn mower blades use either a direct or indirect connection to the engine. A direct connection is where the blade connects directly to the engine by bolting onto the crankshaft. An indirect connection is where the engine attaches to a series of pulleys and a deck belt.
Direct Connection Lawn Mowers
With this design, there is only one possible reason why the lawn mower blade spins but is not cutting: the blade has come loose. If the center bolt has come loose, the blade will look like it’s spinning, but as soon as it comes into contact with the grass, it will stop.
To fix this, disengage the mower’s starting mechanism, including the spark plug, and look under the deck. If you can spin the blade without turning the shaft, you’ve found your problem.
Indirect Connection Lawn Mowers
This is a two birds, one stone kind of situation. Because the pulley system and belt all work together, you can inspect it all simultaneously. First, check that the blade is mounted correctly. If it isn’t, you’ll need to diagnose the other systems. So let’s take a look.
Is The Belt In Place?
If the belt is in place on all the working pulleys and tensioner, check to see if the belt is tight or loose. If the belt is tight and routed correctly, then the problem is with the blade. If the belt is routed correctly but is baggy, then the belt has stretched out, and here lies the issue.
Has The Belt Fallen Off?
If the belt has fallen off, you must look at the potential causes. First, check and make sure the pulleys are firmly connected to the mower and operate correctly. Pulleys on the driveshaft and the blade should be firm, and the guide pulleys should be free spinning. If you find anything working improperly in the pulley system, then you have more than likely found the problem. If all is working correctly, install the belt back on the pulleys and check if the belt is stretched out and needs replacing.
Before you reach for a new belt, look at the tensioning mechanism if your mower has one. Some mowers use the tension of the belt to do this. If you have a tensioning spring, check to make sure it’s connected and that it is under pressure. If it’s disconnected, then here’s your problem.
If it looks like the spring is stretched out but in place and still loose, you have a bad spring. On the other hand, if everything looks OK with the spring and the pulleys, then you will have to look for any problems with the belt.
Here’s an overview of what you should be checking.
- Blade Correctly Installed.
- Belt Condition.
- Deck Belt Placement.
- Pulley Operation.
- Tension Spring Function.
If you follow these items during your diagnosis, you will eliminate those that are not the cause and discover the culprit as I did. So let’s now move on and see how to fix this lawn mower blade that’s spinning but not cutting.
Fixing a Lawn Mower Blade That Spins But Doesn’t Cut
Now that you have diagnosed the cause of the problem, let’s go ahead and see how you fix each issue. Here you will find my step-by-step guide on how to tackle each problem and what tools you’re going to need.
Tightening a Loose Blade
A loose blade seems relatively simple to address, but there is a correct way to go about it.
First, you’ll need to tilt your lawn mower over the correct way to gain access to the blade. If you tilt the mower with the handle touching the ground or over on its side, just make sure it’s going to stay in place. If you don’t have a neighbor nearby that can help, putting weight on the handle will work fine as long as it’s not going to move. For those with a ride-on, you’ll just need to raise your deck and lock it into place. Using a block under the deck would be a good idea.
Then, place a blade lock–I like to use a 4×2 block–against the blade to stop it from moving. Next, grab your wrench or socket and tighten up the center bolt. Once this is done, you can remove your blade lock or 4×2 and return the lawn mower to its upright position. A simple fix.
Tools to Tighten a Lawn Mower Blade:
- Support for Tilted Mower.
- Wrench or Socket.
- Blade Lock or 4×2.
Try Adjusting the Tension
You’ll find that some manufacturers have added the option to adjust the belt’s tension. If you happen to be one of the fortunate ones that have this, you may just be able to add additional pressure to the belt and be up and running. To do this, ensure the belt is correctly routed and seated in the pulleys, then increase the tension. You’ll need either a screwdriver or a small wrench for this. Just be careful not to apply too much pressure as this could lead to new issues with the pulleys and even snap your belt.
“But how tight should a mower deck belt be?” I hear you ask. Check your mower’s technical manual as this information will be listed there. It can vary from mower to mower.
Tools to Tighten Tension Adjuster:
- Screwdriver or a Small Wrench.
Changing Out a Worn Belt
Changing out a belt on big lawn mowers can take a fair bit of strength, and you may want to find some help. For those with a walk-behind, this isn’t such a workout. Let’s take a look at the two stages of replacing a belt.
Removing the Old Belt
The first thing you will need to do is remove the old belt. I like to do this by removing it in one piece. You could cut the belt, but this is potentially dangerous for you and the lawn mower, so I would avoid this method.
Because you are changing an old belt, it should be easy to remove since it probably has enough slack. If this is the case, you can simply lift it over the pulley and remove it from the mower. You may want to wear a pair of gloves to do this.
If you have a tension adjuster, loosening it may provide enough slack to remove the belt. If you can’t loosen the tension adjuster, or if you don’t have one, then you’ll need to remove the tensioning spring.
Depending on the mower’s design, this may be a case of pushing or pulling the tension pulley to create slack. This can usually be achieved by hand. If it proves to be difficult, then look to remove the spring from its anchoring point with a prying tool such as a screwdriver. Just be careful not to deform the spring or to let it fly back and get you. Don’t forget your gloves. If the spring pinches you, you’ll feel it.
With tension out of the belt, it’s just a case of removing it from the mower. Once done, reinstall the tension spring and set the adjustable tensioner back to its original position. You are now ready to install a new belt.
Installing a New Belt
Take your new belt and thread it into position and follow the desired path. If you have multiple pulleys, take your time and check the belt diagram either on the deck or in the manual, as this can be somewhat confusing. Depending on your mower, you may just be able to roll the belt over the pulley while turning the blade quite easily. This method also works with bigger mowers such as a zero-turn.
If the belt is too tight, extend the tensioner by either backing off the adjustment or pushing or pulling the pulley. Here’s where the second pair of hands is helpful. Getting a new belt on really comes down to using your hands. I do not recommend prying the belt on, as you’ll end up replacing it immediately due to damage.
Replacing a Pulley
Replacing a pulley usually means that it’s seized, has broken bearings, or is loose. You’ll need to secure the existing pulley or install a new one.
The first task is removing the belt, which we have already covered above, then you need to get to the pulley. Walk-behinds will typically have a cover that just needs a few screws removed to gain access, whereas bigger tractors and zero-turns mainly have their pulley accessible from the top of the deck. Here, you can remove the height adjustment pin and drop the deck to the ground.
Locate the pulley in question, then back out the bolts with a set of sockets and ratchet. If the pulley proves stubborn, here’s some advice on how to remove a pulley that’s stuck and won’t move. For new and existing pulley installs, I highly recommend using a thread lock adhesive to prevent accidental loosening from vibration. When this is done, you can install your bolts. The last step is to put the belt back on. Here you can again follow the steps from above.
Tools to Replace a Pulley or to Secure an Existing Pulley:
- Socket and Ratchet.
- Screwdriver.
- Thread Lock Adhesive
Swapping Out a Tension Spring
Here’s the last item on the list that you might need to fix to get your lawn mower blade spinning and cutting properly. Replacing a tension spring will require you to remove and install the belt, which we have already covered, so we can just look at the spring. You’ll notice that when no belt is installed, the spring sits loosely in its mount, making it super simple to change.
One end of the spring is hung over a mounting pin or through a hole in the tensioner. This end you can simply lift off. The other end is similar but probably just tighter. You might need to give it a bit of a twist, and it should come free. Once it’s removed, you can go ahead and install the new one. Just be careful to orientate it correctly.
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