How to figure blade tip speed?

[LAWNGODFATHER]

Or feet per a minute? Anyone know?

[Sooners]

LGF, first you'll need to know the RPM of your engine, plus the pulley size for the engine shaft and the pulley size on one of your blade mandrels. Also the blade length. I'll try to work up the formula in the mean time.

[dave80]

Actually, I think you really only need to know the speed of your engine and the size of your blade to determine blade tip speed (at least the theoretical value). The pulley connected to the engine is going to turn at the same speed as the engine regardless if it is 1 inch or 1 foot. Think of the pulleys and belts as a complete system, so all of the pulleys should be spinning at the same speed (the speed of the engine). Theoretically this is true, but in the real world you have to account for friction, which will make the calculations off a little.

Engine speed is determined in rpm, but blade tip speed is determined in feet/min. All that you need to do is to be able to convert rpm to fpm. Exmark's specs say that their blades turn about 18,500 feet/min. Let's see how they figure this out:

Assume your engine turns 3500 rpm (revolutions per minute). While the blade is spinning, the tip is moving in the shape of a circle, so to determine the distance that the tip travels in one revolution, you have to find the circumference of the circle. A 60" Exmark uses 20.5" blades, but to convert to feet you must divide by 12. The formula for the circumference of a circle is Pi (3.14)*diameter.

So, assuming that your engine turns 3500 rpm, here is the formula:

(3500 rev/min)(20.5*3.14/12 ft/rev) = 18775 fpm. Of course this is just a rough estimate, but it will give a good idea to get you somewhere in the ballpark.

[BSDeality]

the size of the pulley is the KEY to the math. its like a drive train, each gear is a different size.

[yourlawnguy]

Originally posted by BSDeality
the size of the pulley is the KEY to the math. its like a drive train, each gear is a different size.
correct: if the pulley on the spindle is smaller than the engine pulley it will be turning at a higher RPM.

[Richard Martin]

It's simple.

Engine rpm times pulley ratio times blade length (in inches) times pi divided by twelve equals fpm

Example:

My Dixie uses 20.5" blades, turns 3750 rpm and has a 1 to 1 pulley ratio so....

3750 X 1 = 3750
3750 X 20.5 = 76875
76875 X 3.14 = 241387
241387/12 = 20115 fpm

[hubb]

All you need to know is the length of the blade and the RPM's of the motor.
Blade length multiplied by Pi. That gives you circumference. Now multiply that times your engines RPM's and divide by twelve.
If I'm not mistaken, the pulley that drives the blades should be the same size as the one on the crankshaft of most mowers. This gives your blade the same rpms as your motor. I could be wrong about that. Maybe some engineer on here could tell you more about that I don't know.

Example:
Blade length = 18"x Pi (3.14)=56.52
That gives you circumference.
Now all you have to do is multiply that by your RPM's (3650)

56.52 x 3650 = 206,298

Now divide that by 12 ie; 206,298/ 12= 17,191.5
I think that's it.

LGF you just did this to see if we would all get brain lock didn't you. :dizzy:

[greenman]

Originally posted by hubb


LGF you just did this to see if we would all get brain lock didn't you. :dizzy:

Kinda what I was thinking. Guess he had a brain fart. Then again, Mikey JUST THINKS he knows everything. :rolleyes: ;) :D Anywho.....thanks for the info, I did not know how to figure blade tip speed.

[Richard Martin]

hubb wrote:

If I'm not mistaken, the pulley that drives the blades should be the same size as the one on the crankshaft of most mowers.

Ahh. I believe you're mistaken. Exmark and Dixie Chopper both use different ratios on different sized blades so they can achieve tip speeds close to or (in Dixie's case) above that magical 19,000 fpm no matter which blade length you buy. Walker mowers use 1 ratio for all of their blade lengths and their blade tip speeds are all over the place. That and the GHS is why Walker mowers are designed to go so ssslllooowww.

[Sooners]

Dave80, Richard Martin, & Hubb all have the right formula, but the pulley dia. is the key. Evidently my eXmark is a 1:1 ratio to put my blade tip speed at around 19,800 fpm. I haven't measured the blade exactly, but I used 21" for my calculations.

[hubb]

I have a Scag an MTD pro and an Exmark and all of them the diameter of the pulley that drives the blades and and one on the crank are the same diameter. (within 1/16") That gives them a
1-1 ratio so its easy to figure the blade tip speed of those. Other mowers however may be different. I wish an engineer from one of our sponsers would get in on this one. That might help to answer this one.

[parkwest]

Why do you want to know?

Is there something wrong with your mowers?

Thinking of a class action against someone?

Is this important for a business owner to know?

[walker-talker]

Call the manufacturer

[Richard Martin]

Parkwest wrote:

Is this important for a business owner to know?

Nah. But sometimes we just like to shoot the breeze. Maybe expand our knowledge a little.

[BRL]

Yea now that Parky is having all that fun with the women of LS these days, he has forgotten we like to just chat about this technical kinda stuff to excercise our brains a little. Because we can. ;)

[OldLawnMowerMan81]

Is it true that regardless of pulley size, it will be spinning the same speed as the engine if the pulley is directly mounted on the engine crankshaft?

~Ben (OldLawnMowerMan81)

[Richard Martin]

Is it true that regardless of pulley size, it will be spinning the same speed as the engine if the pulley is directly mounted on the engine crankshaft?

~Ben (OldLawnMowerMan81)

That depends on what "it" is? If by saying "it" you mean the blade then...

No. If the pulley on the spindles or any other pullies involved is a different size than the crank pulley and it's not compensated for then the blade RPM will be different from the engine.

[OldLawnMowerMan81]

That depends on what "it" is? If by saying "it" you mean the blade then...

No. If the pulley on the spindles or any other pullies involved is a different size than the crank pulley and it's not compensated for then the blade RPM will be different from the engine.


Of course "it" is the blade. Thus, if I were to put a big 1.15" dia. pulley on the shaft (no other pulley sizes involved elsewhere on the mower; esp. if not self-propelled), and then the blade, then if the engine speed is 3200 RPM, the size of the pulley will reduce that to about 2792 RPM. Right?

~Ben

[ALC-GregH]

Of course "it" is the blade. Thus, if I were to put a big 1.15" dia. pulley on the shaft (no other pulley sizes involved elsewhere on the mower; esp. if not self-propelled), and then the blade, then if the engine speed is 3200 RPM, the size of the pulley will reduce that to about 2792 RPM. Right?

~Ben

You can't change the engine RPM by changing a pulley. Blade tip speed is determined by the pulley size and engine rpm. Well, blade length too.

[OldLawnMowerMan81]

You can't change the engine RPM by changing a pulley. Blade tip speed is determined by the pulley size and engine rpm. Well, blade length too.

If the Toro 23267 commercial mower had a 25" blade, then its engine should be governed to no higher than 2900 RPM. This of course contradicts its specifications that say the engine speed is 3200 RPM and gets its 3 MPH ground speed from that.

TORO COMMERCIAL 25-6 (MODEL 23267) SPECIFICATIONS (ABRIDGED):
Engine: 6 HP @ 3600 RPM. Governor set at 3200 RPM. 4-cycle. 27-oz. oil capacity. 13.53-cubic inch displacement, dry element air cleaner, recoil starter (Tecumseh).
Ground speed: 3 MPH at 3200 RPM.

If the governor were set to 27/2900 RPM, then wouldn't that hinder the 3 MPH ground speed?

~Ben

[Richard Martin]

If the Toro 23267 commercial mower had a 25" blade, then its engine should be governed to no higher than 2900 RPM. This of course contradicts its specifications that say the engine speed is 3200 RPM and gets its 3 MPH ground speed from that.

TORO COMMERCIAL 25-6 (MODEL 23267) SPECIFICATIONS (ABRIDGED):
Engine: 6 HP @ 3600 RPM. Governor set at 3200 RPM. 4-cycle. 27-oz. oil capacity. 13.53-cubic inch displacement, dry element air cleaner, recoil starter (Tecumseh).
Ground speed: 3 MPH at 3200 RPM.

If the governor were set to 27/2900 RPM, then wouldn't that hinder the 3 MPH ground speed?

~Ben

Yes, the mower will go slower. But by so small a margin you probably wouldn't notice it.