Sometimes the answer ‘pretty darn quickly’ just isn’t good enough!
So how fast do lawn mower blades spin?
Is there a limit on their speed? What would happen if they spun faster?
We take a closer look at the issue here and establish why blade speed is so important.
So… let’s get started!
How Fast Do Lawn Mower Blades Spin?
Lawn mower blades can not spin any faster than 219mph according to safety regulations. Most lawn mower blades spin at or around 200mph, the same as the top speed of a Lamborghini Gallardo car. There is no regulation on the RPMs of a mower, with most falling somewhere between 2,600rpm and 2,900rpm when cutting grass. This means an average mower blade rotates between 43 and 48 times per second.
Lawn Mower Blades: RPM, MPH and Regulations
The speed at which a blade can spin is regulated by the American National Standards Institute (ANSI).
According to their regulations, the tip of the blade “should spin at a speed no greater than 19,000 feet per minute, approximately 219 MPH (miles per hour).”
We can safely say the blades of nearly all mowers will spin at somewhere around 200mph.
It is fun sometimes to put this into a little context, so let’s see how a mower blade compares to some other things:
- 68mph: A cheetah.
- 74mph: A hurricane.
- 92mph: A fastball in baseball.
- 120mph: A skydiver.
- 200mph: An arrow.
- 200mph: A Lamborghini Gallardo.
- 219mph: A lawn mower blade (top speed).
- 242mph: A Peregrine Falcon.
- 767mph: The speed of sound.
- 859mph: A bullet.
So it is in pretty good company!
It is important to note that the tip speed of the blade is regulated by ANSI, but the RPM (Revolutions Per Minute) is not.
When it comes to RPM, most four-stroke lawn mowers run somewhere in the range of 2,800rpm to 3,600rpm.
Small mowers usually have higher RPMs, whilst large-deck single-blade mowers are slightly lower.
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However that figure is the fastest a blade will be driven, it will drop when the blade comes into contact with grass.
So when your mower is cutting, the RPM is usually somewhere in the 2,600rpm to 2,900rpm range.
That means the blade will revolve somewhere between 43 and 48 times per second.
Regardless of RPM, the blade speed should never exceed 219mph as per ANSI regulations.
So let’s quickly round up what we have found out:
- How fast does a lawn mower blade spin in MPH? 219mph (maximum)
- What is the RPM of a mower (when not cutting)? 2,800rpm to 3,600rpm.
- What is the RPM of a mower (when cutting grass)? 2,600rpm to 2,900rpm.
- How many times will a mower blade rotate per second? 43 to 48 times.
What Affects Blade Speed?
The Length of the Blade
There are several things that affect the speed at which a lawn mower blade spins, but the factor with the biggest influence is the length of the blade.
If you could go back and compare all the lawn mowers you have ever owned, you will find that they rarely have a blade over 21 inches in length, no matter what size the mower is.
And that is all down to the issue of centrifugal force.
Whereas a 21-inch blade would have a tip speed of around 200mph, a 24-inch blade would max out at 350mph.
This would be incredibly dangerous if you were to hit something and break the blade, as the mower deck wouldn’t be able to contain the blade due to its speed and the result, unless you got incredibly lucky, would be a serious injury.
That is why a riding mower with a 48-inch deck doesn’t use a 48-inch blade, instead it uses three 19-inch blades or something similar.
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It might sound like the nickname for an intimidating person who collects unpaid debts, but the governor is what regulates the speed of your engine.
As the name suggests it governs the RPM of your mower, so if you hit a patch of particularly thick grass the governor opens the throttle a little more to keep the RPMs consistent.
Most governors work in one of two ways:
- An internal mechanism operates on centrifugal force. A lever from the crankcase activates the throttle to keep the RPMs consistent.
- A wind vane receives air from the rotating cooling fins on top of the engine. If it isn’t getting enough wind the throttle is opened more until the RPMs are correct.
The spindle is simply the post to which a blade attaches and spins around when it is in motion.
As we have discussed blade tip speed is regulated and is usually somewhere around 200mph, give or take a few mph.
A shorter blade, with a faster spindle allows for more RPMs and more cuts per minute.
It also means more spindles can be fitted across the deck which theoretically distributes weight better and increases maneuverability.
Why You Shouldn’t Try to Increase Blade Speed
You should never try and increase the speed of your lawn mower blades, not least for our illustration in the section above which shows how dangerous it could be.
Lawn mowers are designed to operate at an optimum blade speed and the main factor for that is safety.
A faster blade speed wouldn’t mean a better cut or that you get your mowing done quicker. More horsepower would do that as it would mean thick, heavy grass wouldn’t slow the blades down as much.
However increasing your blade speed will place extra stress on the blade, deck and drive mechanism and increase the likelihood of damage to them.
It will also generate more vibration, which will loosen nuts, bolts and compromise the engine of the mower.
And of course, as touched on above, it means a hard blade strike, hitting a rock or the edge of a concrete sidewalk would be much more dangerous to you or anyone (or anything) in the vicinity.
Also if you do alter your mower in some way to allow the blades to run faster, you are almost certainly voiding the warranty of the machine.
A limited selection of manufacturers do support blade adjustment, via a dedicated blade brake clutch that allows the operator to reposition the clutch wire.
But even then measures will be in place to ensure the mower cannot exceed the ANSI regulations.
There are ways you can increase the speed of your blades, but I won’t go into them here as I don’t think it is safe to do so unless you are an expert in the field and know what you are doing and why.
However I am sure if you have your heart set on doing so, you can use Google to find the answer!
Manufacturers don’t recommend increasing your blade speed and neither do I.
It is counterproductive and dangerous.
How You Can Check Your Blade Speed
If you think your mower blades might be running too slow, see how your grass looks after cutting it.
Slow blades will tear and damage grass, rather than cutting it cleanly and efficiently. On the contrary, if your blades are too fast, as well as it being dangerous, it will waste power and fuel.
Your mower manual (if you still have it!) should specify the engine and blade speed.
You can check the RPM of your mower’s engine, with a tachometer.
In a case such as this, the issue is likely to be a mechanical one and that means it is always best to get your mower checked by a professional mechanic.
Many times they will be able to tell if there is a problem simply by listening to the engine.
Having an expert cast their eye over your mower is likely to save you time and money in the long run, as well as being a lot safer.
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Lawn mowers are governed by strict regulations from ANSI as to how fast their blades can spin.
The speed is not allowed to exceed 219mph.
There is no regulation around the RPM of the mower, although most run somewhere in the 2,600rpm to 2,900rpm bracket when they are cutting grass.
The blade speed is regulated for a reason, and that reason is safety.
Attempting to increase the speed at which your lawn mower blades spin should not be done just for the sake of it.
It should only be done if you think there is a genuine problem affecting the mower’s ability to cut grass.
And in such cases, I would always advise you to get a professional to look at your mower and diagnose the problem.