# How to figure blade tip speed?

Discussion in 'Lawn Mowing' started by LAWNGODFATHER, Jul 9, 2003.

1. ### LAWNGODFATHERLawnSite Fanaticfrom St. Louis, Missouri Gateway to the westPosts: 6,750

Or feet per a minute? Anyone know?

2. ### SoonersLawnSite Memberfrom ArkansasPosts: 190

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.

3. ### dave80LawnSite Memberfrom Central PAPosts: 42

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.

4. ### BSDealityLawnSite Silver Memberfrom Fairfield County, CTPosts: 2,849

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

5. ### yourlawnguyLawnSite Memberfrom Cocoa, FloridaPosts: 228

correct: if the pulley on the spindle is smaller than the engine pulley it will be turning at a higher RPM.

6. ### Richard MartinLawnSite Fanaticfrom Greenville, NCPosts: 14,700

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

7. ### hubbLawnSite Memberfrom Saint Charles MOPosts: 192

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.

8. ### greenmanLawnSite Addictfrom Little Rock, ARPosts: 1,405

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

9. ### Richard MartinLawnSite Fanaticfrom Greenville, NCPosts: 14,700

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.

10. ### SoonersLawnSite Memberfrom ArkansasPosts: 190

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.