# The Green Industry's Resource Center

#1
08-14-2010, 06:31 AM
 AAALOUIE LawnSite Member Join Date: Aug 2009 Location: McComb, MS 39648 Posts: 11

Is there a formula out there to calculate blade tip speed?
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#2
08-14-2010, 08:05 AM
 C & T Landscaping LawnSite Senior Member Join Date: Aug 2009 Location: Chesterfield, Virginia Posts: 432

But seriously:

It says that most mower specialists would own a "wire based" device for measuring RPM on a running mower. Apparently 18,000 and 18,500 is the normal for Commercial mowers.

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#3
08-14-2010, 08:49 AM
 MarcSmith LawnSite Fanatic Join Date: Jul 2004 Location: Washington DC Posts: 7,242
first you need to figure out the RPMs of the blade. which could be different from the engine depending on the pulley sizes.. however if the engine pulley is the same size as the deck pulley, then the blades are spinning the same speed as the engine..

now you need to calculate the circumference in feet of the circle the blade tip travels. in one revolution. multiply the circumference my the rpm you have a blade tip speed.

lets assume the blades of a 21" mower are spinning @ 3000 rpm

a 21" blade is actually 1.75 feet. that circumference is pi * D or 3.14*1.75= 5.49. so for each revolution the blade tip is moving 5.49 feet.

5.49*3000=16470 feet per minute
#4
08-14-2010, 09:53 AM
 C & T Landscaping LawnSite Senior Member Join Date: Aug 2009 Location: Chesterfield, Virginia Posts: 432
^Smarty pants lol

This is pretty cool that we can calculate things such as that with simple problems
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#5
08-14-2010, 10:27 AM
 Johnny test LawnSite Senior Member Join Date: Dec 2006 Location: Lexington,KY Posts: 568
But why would you?
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#6
08-14-2010, 10:37 AM
 MarcSmith LawnSite Fanatic Join Date: Jul 2004 Location: Washington DC Posts: 7,242
CT the hardest part is getting the accurate blade RPM...safely...

Math scares a lot of people... and I never understood why. Numbers don't lie they can tell you just about anything....the key is to make sure you are using good numbers and good formulas..
#7
08-14-2010, 10:46 AM
 Johnny test LawnSite Senior Member Join Date: Dec 2006 Location: Lexington,KY Posts: 568
But all that math changes depending on your machines RPM ratings or throtle linkage adjustments, or how old it is or if your on a slope or not or the amount resistance the blade is up against like tall grass so again. Why would you?? Honesty I'm curious...
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97' ISUZU NPR (16' bed)
John Deere F687 60" 7iron w/MCS
John Deere GS30 36"
John Deere GS45 48"
John Deere JS45 21"
Stihl FS80 Pro Trimmer
Stihl FS46 Trimmer
Stihl BG55 Blower
Stihl BR56 X2
Stihl BR600 X2
Red Roo Auger
BRD-280 Bed Redefiner
Echo PE-280 Stick Edger
Echo SHC 212 Hedge Trimmer
Bluebird 24" De-Thatcher
Olathe 88 Plug Aerator
Yard Machines walk-behind Edger
Craftsman Lawn Vacuum/chipper
2-Homelite BP250 backpack blowers
McCullock FR56 Trimmer

#8
08-14-2010, 11:54 AM
 Runner LawnSite Fanatic Join Date: Jul 2000 Location: Flint, Michigan Posts: 13,503
LOL...I remember we had this conversation a while back, but we were figuring rpm based on 18,500 ft. per minute blade tip speed. That's a little easier.
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#9
08-14-2010, 11:55 AM
 MarcSmith LawnSite Fanatic Join Date: Jul 2004 Location: Washington DC Posts: 7,242
Quote:
 Originally Posted by Johnny test But all that math changes depending on your machines RPM ratings or throtle linkage adjustments, or how old it is or if your on a slope or not or the amount resistance the blade is up against like tall grass so again. Why would you?? Honesty I'm curious...
the math doesn't change.... if your engine Rpm drops then you just have a new value to plug in. but engine rpm is the sole deciding factor if the engine slows down so does blade tip speed... So why measuer HP all the above conditions you mention affect HP...when in reality the most important thing for us is engine torque...

blade tip speed, like HP is just one factor that helps people decide what mower to buy. higher blade tip speed provided a better cut, better discharge. but if you have the fasted blade tip speed, but not enough torque form the engine to keep the speed up it is useless...

Some people are just curious about their equipment...
#10
08-14-2010, 12:19 PM
 demhustler LawnSite Bronze Member Join Date: Sep 2009 Posts: 1,322
Quote:
 Originally Posted by MarcSmith CT the hardest part is getting the accurate blade RPM...safely... Math scares a lot of people... and I never understood why. Numbers don't lie they can tell you just about anything....the key is to make sure you are using good numbers and good formulas..
: ))))
and 16470 feet per minute = 186.8182 miles perhour
norm. tip speed 18500-20000 fpm (210-230mph)

now what the easiest (cheepest) way to mesure engine rpm, shaft(pulley)rpm?

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