Tightening the bolts holding a lawn mower’s blades in place is a critical operation. Over or under-tightened bolts can expose your lawn mower to problems, such as engine failure. How can you apply the right pressure to ensure that these bolts are tightened correctly?
You need a torque wrench to change lawn mower blades as it ensures that the bolts are not over or under-tightened. The torque wrench will help you apply the correct pressure to avoid engine failure or any other problems that might arise from incorrect bolt tightness.
In this article, I’ll discuss the amount of torque your lawn mower blades need and how to find it. I’ll also cover the best way to tighten your mower’s blades and factors that can cause them to come loose. Keep reading!
How Much Torque Do Lawn Mower Blades Usually Need?
Lawn mower blades usually need 30-foot pounds (360-inch pounds) of torque for a secure fit, and are normally tightened with 9/16-inch (1.43 cm) sockets. This is the level of torque that you need to apply so that the blades don’t come loose while using them.
According to Khan Academy, torque is the force an object needs to rotate about an axis. In this case, it’s the force your mower blade needs to rotate freely and cut the grass.
You might be wondering how you can measure 30-foot pounds (360-inch pounds) of torque. This is where the torque wrench comes in. A torque wrench allows you to measure precisely 30-foot pounds (360-inch pounds) of torque to avoid over or under-tightening your bolts.
Follow the process below to help you set the torque on your torque wrench:
- Loosen off the tightener by about one-half turn.
- Turn the torque wrench to the 30-foot pounds (360-inch pounds) setting. You should keep adjusting until you attain the exact mark.
- Tighten the locknut clockwise to prevent your torque specs from changing.
- Tighten the bolt until you hear a click from the torque wrench.
It’s worth noting that one torque wrench click completes one cycle of tightening, indicating a secure fit. Ensure you don’t over-tighten the bolts, as this will damage the threads.
Where To Find the Exact Torque for Your Lawn Mower Model
Torque specifications for mower blades vary depending on the model you have. The best way to find the correct torque for your lawn mower is by checking the owner’s manual.
If you don’t have the owner’s manual, you can find the torque specifications on the manufacturer’s website. Just enter your lawn mower model in the search bar and look for the blade installation instructions.
However, most manufacturers recommend the following:
- Riding lawn mowers: Most riding lawn mowers need 70 to 90-foot (840 to 1,080-inch) of torque pressure.
- Walk-behind mowers: Most walk-behind mowers need 35 to 50-foot (420 to 600-inch) of torque pressure.
Expert Tip: Never assume that the torque for your model is the same as another model. Always consult the owner’s manual or the manufacturer’s website to be sure.
The Best Way To Tighten Your Mower’s Blades
Now that you know how much torque is needed to secure lawn mower blades, it’s time to learn how to tighten them.
The process of tightening lawn mower blades is straightforward and can be done in a few simple steps, as discussed below:
- Disconnect the spark plug wire from the spark plug to prevent the engine from accidentally starting.
- Remove the bolts holding the lawn mower blade using a socket wrench.
- Take off the old blade and put on the new one. Make sure that the cutting edge is facing the right way. Check out my article here where I explain which way the blades need to be mounted.
- Reattach the bolts and hand-tighten them.
- Apply the correct amount of torque pressure using the torque wrench. A click indicates that you’ve attained the maximum torque.
- Reconnect the spark plug wire to the spark plug.
Your lawn mower is now ready for use!
Consequences of Not Applying the Right Torque
If you ignore the installation specs in your mower’s manual, you might end up causing extensive damage.
Some of the consequences of this include:
- Under-tightening the blades: Under-tightening your mower’s blades will give you a poor cut. A wobbly blade produces uneven cuts, which can negatively affect your lawn’s appearance. Such a blade will also vibrate, causing damage to other lawn mower parts. An under-tightened blade can also come loose while you’re mowing. This can be dangerous as the blade can fly off and cause serious injuries.
- Over-tightening the blades: You risk over-tightening the blade by applying excessive torque. An overly tight blade will destroy your mower by overworking the engine. In such a case, the engine has to work harder to rotate a blade that’s too tight. This can, in turn, damage the engine beyond repair. An over-tightened blade also causes poor cuts. The blade will be difficult to rotate, which affects its cutting ability.
Reasons a Lawn Mower Blade Keeps Coming Loose
Everything happens for a reason. So, if you notice that your mower’s blade keeps coming loose, it’s probably due to one of the following reasons:
The Lawn Mower Is Old
An old lawn mower is more likely to have worn-out parts. This includes the lock nuts and bolts that hold the blade in place.
The only way to fix this problem is by replacing these parts with new ones. Thankfully, you can do this easily by ordering new parts online or buying them from a local lawn mower repair store.
The Lawn Mower Has Been Misused
Another reason a lawn mower blade might keep coming loose is if it’s misused. For instance, hitting the blade on stones and other hard objects can damage the lock nuts and bolts, making them unable to hold the blade in place.
Using your lawn mower as intended is advisable to prevent such damage. You should remove hard objects like stones from your lawn before mowing.
The Blade Is Poor Quality
A lawn mower blade that’s improperly tempered will also keep coming loose. Tempering is a process of hardening and stress-relieving metal to make it more durable. A poorly-tempered blade will be too soft, making it unable to withstand the rigors of mowing.
Buying lawn mower blades from reputable brands is advisable to ensure they’re adequately tempered.
The Lawn Mower Is Used on Uneven Surfaces
Mowing uneven surfaces can also make a lawn mower’s blade loose. Such surfaces include hills and slopes. When you mow on such surfaces, the blade tends to hit the ground more often, increasing the chances of the lock nuts and bolts getting damaged. This makes them unable to hold the blade in place.
If you must mow on uneven surfaces, be extra careful to avoid damaging the blade. You can do this by:
- Taking short passes: This will help reduce the number of times the blade hits the ground.
- Reducing the blade’s cutting height: This can also help reduce the number of times the blade hits the ground.
- Avoiding mowing when it’s wet: Wet grass can be slippery, which makes it challenging to control the lawn mower.
How Often Should You Replace Lawn Mower Blades?
It’s advisable to replace your lawn mower blades after 100 to 200 mowing hours. After mowing for these hours, the blades will be too dull to produce clean cuts. They will also wear out, making them unable to stay in place.
It’s a good idea to keep track of the number of hours you’ve used your lawn mower. This will help you know when it’s time to replace the blades.
You should also inspect the blades regularly for damages. If you notice any damage, replace the blades immediately to prevent further damage to your lawn mower.
Blades with specialized steel or tungsten edges are an exception to this. You can use such blades for up to 400 mowing hours without replacement because they’re designed for long-term use.
You should make sure never to skip regular maintenance intervals to keep your lawn mower in good shape for a long time. You can check out my article here where I talk about how often you need to service a lawn mower.
Final Thoughts
A torque wrench is an essential tool when changing lawn mower blades. You want the blades to fit securely with the right pressure to avoid straining your engine. The best way to achieve this is by using a torque wrench when tightening the bolts.
A torque wrench ensures that the bolts are tightened to the correct specifications. This helps to prevent the blade from coming loose and damaging your lawn mower.
It’s also essential to buy lawn mower blades from reputable brands. Such blades are adequately tempered and designed for long-term use.