How to Inspect a lawn mower blade or Where’d my blade go?


It’s steel, it’s sharp and it’s spinning at an incredible velocity. It’s designed to strike things and come out winning and when you’re cutting grass it does just that. So you’re halfway through with the back yard when suddenly there’s a teeth jarring bang! Your engine powers down briefly. The tension in the air is palpable. But within moments, the engine recovers, powers back up and you shake it off.

At least twelve more passes to go before you can break out the edger. Better get a move on!

Kids. Little toy dropping monkeys. Dogs too. Debris blowing in. Rocks, roots, branches….there’s just no telling what’s going to end up in your lawn. But one things for sure… you’ll find it with your mower! It never fails.

Now, in that little scenario the real problem could have been that we just didn’t inspect the yard adequately before cutting. If you’re guilty of that, welcome to the club. Pick up your name tag at the table by the door.

But where we really dropped the ball (pun intended) was that once the whatever hit the fan (another pun intended), we probably should have shut the old girl down and inspected the blade.

What’s the big deal?

A couple of potential disasters.

The average lawn mower spins that blade at around 3,000 revolutions per minute. That’s what the professionals call “fast”. At that speed, it doesn’t take much to wreck your mower.

Awww snap!

Sudden impact can cause fractures in the blade. Over time, a fractured blade will deteriorate through use. Small cracks in the blade will worsen as the metal thins out. The significant risk here is that most of the time, these loose pieces of steel are discarded as the blade is turning!   Now your lawn mower has been converted into the “Ninja Star-o-matic” high velocity projectile disbursement device. Run Spot! Run!

Damage to the blade adapter can result in excessive vibration or looseness in the blade mounting. The blade can strike the mower housing or the ground and break. In this case, I hope you have insurance. Damage to the shaft can result in….well…buying a new mower or personal injury. The internet is full of pretty pictures of lawn mower blade injuries. Feel free to test your gag reflex. I’ll wait here.

I’m feelin kind of dizzy

Ever watch a top spinning? When it’s balanced properly it remains upright and turns smoothly. Like a little ballet dancer. If its lopsided, the top is very quickly thrown out of balance. In order for your blade to work properly it needs to be balanced. When the blade strikes an object, it can cause the blade to bend which may throw it off balance.  It wobbles, topples and crashes to the ground.

Now, attach a 19 inch steel blade to the top of it. See what I’m getting at?

Getting the shaft

There is a steel shaft that come out of your engine called the “crankshaft” and it is an essential part of the engine. Many times, the blade is connected to the shaft through a part called a”blade adapter”. Blade adapters are designed to be the “breaking point” between the blade and the crankshaft. They are smaller, less expensive and easier to replace than an engine and when they work, it’s a good thing to have.

But sometimes the blade attaches directly to the shaft with a bolt. When that happens we could see more immediate issues.

When everything remains in balance, the engine performs as it was designed. Parts remain intact and in contact with each other. Seals remain sealed, stuff keeps doing its thing and everything is right with the world.

But when it all starts to shake, things come apart.

Seals get worn and parts no longer connect with each other the way they should. It’s a vicious circle. Wobble loosens parts, which increases the wobble, which makes parts looser and so on and so forth. If we were talking about an easy chair, that might not be a game changer. But we’re talking about an internal combustion engine and that’s bad.

What can I do?

The trick is to catch these things before they become real problems, because when it comes to engine vibration…once you realize you have a problem; it’s too late to fix it.

So, the most important piece of advice I can offer is to inspect your blade on a regular basis.

  • Look for signs of excessive wear. Thin spots, bends, or curls in the steel. This is supposed to be a solid pieces of metal, not a Christmas ornament!
  • Remember that balance is important. When the blade is sharpened, it should be balanced too. If you’re not sure how to do that, let your local technician do it for you. They can probably take care of it quickly and you’ll know the job is done right.
  • Rounded cutting corners are a clear sign that its time to change the blade. Small cracks in the steel, especially near the center hole can result in a blade being dislodged. If the blade is chipped, broken, bending or curling at the corners…you got your moneys worth out of it. Get a new blade.
  • Nicks in the edge are common and do not necessarily warrant replacing the blade. Many can be ground out when the blade is sharpened. When in doubt, take your blade to a technician and let them make the call. Warning: they will probably err on the side of safety because they don’t want to see you lose a foot! So be prepared to replace the blade.

Sometimes blades just wear down and chip apart on their own. If you have sandy soil, the sand has probably been doing a real number on your blade and will reduce its life expectancy. Take the time to look them over. When in doubt, let the tech figure it out. Be safe. Don’t take chances with a large, spinning blade.

It’s pretty cut and dried. Hah!

– Bill

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Bill Brown

Bill is the head of content creation for the LawnMowerPros Blog and DIY section. He’s been in the Outdoor Power Equipment Industry for years and he’s still learning new things everyday. You can often find him creating featured articles, DIY guides, videos, graphics and much more.