Tip speed, How fast???

Discussion in 'Lawn Mowing' started by Doc Pete, Jun 3, 2002.

  1. Doc Pete

    Doc Pete LawnSite Gold Member
    Posts: 3,469

    So, if we can spin 2 blades at 3,700 rpm, how fast is it reasonable to spin one blade??? IOW, I'm wondering much we can increase single blade speed and not run into problems? Blade speed or tip speed is what will give a better cut, that is, IMO.
    I'm running doubles and every now and then it still leaves some uncut whimpy grass
    I realize we need to worry about belt and pulley speed, but is there much difference in throwing a stone or whatever between 3,700 rpm and 4,000 rpm. It's kinda like will a faster bullet kill you quicker??
    Exmark talks about 19,000 fpm blade speed and my machines are down around 17,000 fpm.
    Thanks,
    Pete
     
  2. awm

    awm LawnSite Gold Member
    Posts: 3,354

    jmo. but 4000 might be pushing the envelope for longterm use. u do it and let us know.
     
  3. Richard Martin

    Richard Martin LawnSite Fanatic
    Posts: 14,700

    It's not RPMs, it FPM.

    Example:

    A 12" blade tip travels 37.68 inches each time the blade makes a complete rotation. Multiply that by 3600 RPM's and you get 135,648 inches traveled in 1 minute. Divide that by 12 to convert it to feet and you get 11,304 FPM.

    A 22" blade tip travels 69.08 inches each time the blade makes a complete rotation. Multiply that by 3600 RPM's and you get 248,688 inches traveled in 1 minute. Divide that by 12 to convert it to feet and you get 20,724 FPM.

    I'm sure they did a lot of testing to come to the conclusion that a hard object thrown at about 210 mph (roughly 19,000 FPM) will do less damage than one thrown at 250 mph.
     
  4. Jman

    Jman LawnSite Member
    Posts: 217

    It's like which would you rather hit a cement wall at, 210 mph lor 250 mph.:D
     
  5. Doc Pete

    Doc Pete LawnSite Gold Member
    Posts: 3,469

    Here's my exact thought. OHSA or whomever seems to be OK with allowing a "tip speed" (Feet per minute) of almost 20,000 fpm on a 60 inch machine. This would be the speed or force of which an object would be propelled out of the shoot (as an example)
    Now, if it's OK to allow that amount of "feet per minute", what's wrong with speeding up a 48/ 52/54 inch machine to the same speed??? I'm sure the belts and pulleys aren't a problem.


    Pete
     
  6. TLS

    TLS LawnSite Fanatic
    Posts: 7,937

    Running that fast! Exmark claims 18,500 fpm on all their mowers. So they must compensate with different pulleys on the shorter bladed machines, right? I don't know, as my small mower is a SCAG, and I can spin my blades by hand faster than this machine!!! :eek:

    I'd have to go and measure the pulley sizes to make sure.
     
  7. rodfather

    rodfather LawnSite Fanatic
    Posts: 9,501

    I dunno Pete...

    ... on my big Toro Groundsmasters, I am sure the rpm or fpm is not near 17 or 18k. Probably more like 13K. But when I mow with them (72" machines), nothing is left standing. And I mean nothing. They will take down grass that is literally waste high, and on the first pass.

    They are 4 wheel diesels with a heck of a lot of torque. They don't even so much hiccup in the heavy s***, under any conditions...kind of remind me of a brush hog, LOL.

    Just wondering if rpm or fpm is everything to think about.

    Just my 2 cents.
     
  8. CHAN

    CHAN LawnSite Member
    from AL.
    Posts: 132

    Hey Richard what is the formula you used to get those figures. I thought it was Pie x Radius to get the full circle or how far the tip traveled or about half of what you stated. Is this not right. Seems we not turning as fast as thought. Me and another guy were just talking about this today.
    Later
    Chan
     
  9. Doc Pete

    Doc Pete LawnSite Gold Member
    Posts: 3,469

    It's TT (pie) times the diameter for distance and TT pie times radius squared for total area.
     
  10. rodfather

    rodfather LawnSite Fanatic
    Posts: 9,501

    D*** Pete,

    You an engineer, work for NASA, or into quantum mathematics???
     

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