This is a common question from people who are new to lawn mowers. Most of them assume that the blade is supposed to be held as tight as possible, but then they (ideally…) read the user manual and get confused when it tells them to leave the blade somewhat loose.
So, what does that mean? Are lawn mower blades really supposed to be loose and, just how loose is too loose?
Generally speaking, the blade should be tight enough so that it doesn’t move around on its own, but you should be able to move with your hand by applying some pressure. The manufacturer should provide a proper tightening level expressed in foot-pounds, commonly found in the mower’s manual.
Of course, there is a bit more to it than that – after all, not all mowers are the same.
Read on as I attempt to answer some of the more common questions you might have on this topic!
How Tight Should a Lawn Mower Blade Be?
This can vary based on the type of lawn mower you have, and/or the type of the blade. Some manufacturers may recommend the approach I mentioned above, while others will recommend that you tighten the bolt as much as possible.
This is because some blades are more resistant to the force of a potential impact than others. Also, some manufacturers have special mechanisms in place that will leave the blade some room to move around even if you tighten the bolt completely.
The user manual should always have information on this topic. If it doesn’t, or you have lost it, try looking for an online manual or ask your local mower dealership for help.
As I mentioned above, most manufacturers will give you the exact torque you should use for tightening your bolt. This value is measured in ft/lb or foot-pounds.
Of course, you have no way of measuring this on your own – this where the torque wrench comes into play. It’s a special tool used to apply a specific pressure to screws and bolts.
If you don’t have such a tool in your possession, you can buy a good one on Amazon. The TEKTON 1/2 Inch Drive Click Torque Wrench is one of my favorites. Just make sure to get a socket that is the right size for your bolt if you decide to buy one!
Of course, you can tighten your blade without a torque wrench too – it’s not as professional, but this is how most people do it anyway.
Can You Overtighten a Lawn Mower Blade?
Unfortunately, you can overtighten your mower’s blade if you aren’t careful. This can damage the blade itself in some cases – the blade can even break after hitting a hard object if it’s overly tight.
Apart from that, the bolt that’s holding the blade in place can also get damaged (this is often the case).
If you apply too much pressure, you can break the bolt’s head right off. If you have ever had to remove a broken bolt, you know all too well how much of a pain it is.
It’s important to note that the bolt not breaking isn’t a fail-safe indicator of it not being overtightened. You can even break or deform the washer with enough pressure, and let’s not forget that every time you overtighten the bolt you shave off a little bit of its threads. If you keep doing that, eventually you won’t be able to tighten it at all.
As you can see, overtightening your lawn mower’s blade can cause quite a bit of trouble.
This brings us to my next point:
What to Do if The Mower Blade Won’t Tighten
This is usually caused by overtightening the bolt that holds the blade in place. As I’ve explained earlier, doing so will shave off the bolt’s threads bit by bit, and it will eventually lose its ability to hold the blade in position altogether.
If this is the case, I’d suggest buying a new mower blade bolt. Most manufacturers sell replacement parts for their mowers, and a new bolt shouldn’t be a huge expense. Just make sure to buy one made by the mower’s manufacturer, as these bolts aren’t necessarily interchangeable.
Unfortunately, you can’t really fix a damaged bolt. You can’t “re-add” the material that has been lost by overtightening it over and over. This is why it’s important to prevent this from happening in the first place.
Is it Normal for a Lawn Mower Blade to be Spinning Freely?
This is not normal at all, and can be caused by a number of things. It most likely means that the blade has become loose somehow.
To fix it, try tightening the bolt. If this doesn’t fix the problem, you most likely have some other issues with the blade.
Read on to the next section to find out what these can be, and how to fix them!
What Causes a Lawn Mower Blade to Keep Coming Loose, and How to Fix it?
Every now and then you’ll run into a lawn mower blade that just keeps coming loose no matter how many times you tighten it.
Here are some of the most common causes:
Overtightened Bolt
This can happen if you overtighten the bolt holding the blade in place, like I explained before – a damaged bolt won’t be able to keep your blade in position for very long. If this is the case, buying a new bolt should fix the problem.
Bolt Not Tight Enough
But it can just as easily be caused by not tightening the bolt quite enough. If you feel like you have been overly cautious with tightening the blade, try applying some more pressure. Just make sure you don’t go overboard. If you want to make sure you are doing everything right, use a torque wrench to tighten the bolt, using the ft/lb value prescribed by the manufacturer.
Deformed Blade
Another common cause is when you hit something really hard, such as the edge of a sidewalk and the blade becomes deformed. In some cases, the unnatural shape of the blade can be enough to prevent you from tightening it properly.
A bent shape can also mess with the blade’s overall balance. An unbalanced mower blade can produce vibrations that will loosen the bolt over time. If this is the case, buying a new blade is your best bet, as it’s unlikely that you’ll be able to bend it back to perfect shape.
Unbalanced Blade
Speaking about balance – the blade can also become imbalanced due to uneven sharpening. Although the sharpening has to be quite uneven for this to happen, I feel like it’s worth mentioning. If the problems started after you sharpened the mower’s blade, this might be the case.
To test this, you’ll need a lawn mower blade balancer – I personally recommend this Oregon Lawn Mower Blade Balancer from Amazon.
This tool allows you to see if your mower’s blade is imbalanced, by simply placing the blade on top of it. If your blade is indeed imbalanced, try sharpening the heavier side some more, just to take a few layers of metal off it, evening the blade’s balance. If your blade is perfectly balanced, it should stop coming loose.
The Blade Not Being Centered
Another things that can cause your blade to come loose is the blade itself not being centered. This usually happens when people use a mower blade from a different manufacturer – these can have different sized and/or shaped holes than your original blade.
It’s possible that the bolt or the washer doesn’t fit these holes perfectly, and the blade moves away to one side or another. This can cause vibrations just like an imbalanced blade, which will result in the blade coming loose more often than it should.
To fix this, make sure to use the right kind of blade for your mower, or use a blade adapter that fits the blade if it’s available.