Blade tip speed calculation?

Discussion in 'Lawn Mowing' started by AAALOUIE, Aug 14, 2010.

  1. AAALOUIE

    AAALOUIE LawnSite Member
    Posts: 11

    Is there a formula out there to calculate blade tip speed?:confused:
     
  2. C & T Landscaping

    C & T Landscaping LawnSite Senior Member
    Posts: 480

  3. MarcSmith

    MarcSmith LawnSite Fanatic
    Posts: 7,157

    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..

    you need to measure your blade. tip to tip in feet.
    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. C & T Landscaping

    C & T Landscaping LawnSite Senior Member
    Posts: 480

    ^Smarty pants lol

    This is pretty cool that we can calculate things such as that with simple problems
     
  5. Andover Landscape Co.

    Andover Landscape Co. LawnSite Senior Member
    Male, from Lexington,KY
    Posts: 572

    But why would you?
     
  6. MarcSmith

    MarcSmith LawnSite Fanatic
    Posts: 7,157

    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. Andover Landscape Co.

    Andover Landscape Co. LawnSite Senior Member
    Male, from Lexington,KY
    Posts: 572

    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...
     
  8. Runner

    Runner LawnSite Fanatic
    Posts: 13,496

    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.
     
  9. MarcSmith

    MarcSmith LawnSite Fanatic
    Posts: 7,157

    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. demhustler

    demhustler LawnSite Bronze Member
    Posts: 1,322

    : ))))
    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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