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Blade tip speed

Discussion in 'Lawn Mowing' started by JimmyStew, Mar 23, 2008.

  1. olde_blue

    olde_blue LawnSite Senior Member
    Posts: 349

    High blade tip speed increases air flow, but more importantly it increases impact energy of the mower blade hitting the grass blade. Impact energy increases approximately proportional to the square of velocity, so a blade tip with a speed of 19,000 fpm would have 41% greater cutting impact than a 16,000 fpm blade tip (kenetic energy KE=1/2mv^2, so ratio of KE=v1^2/v2^2).

    All else (blade length, mower speed) being equal, higher blade tip speed will mean that your mower cuts less grass with each revolution, but this relationship is linear. So the 19,000 fpm blade will cut off 16% less grass per revolution than a 16,000 fpm. The ammount of grass cut with each revolution is directly proportional to blade length and mower speed.

    For example (at 19,000 fpm blade speed, 8 mph ground speed, running single blades--so you get two "cuts" per revolution):

    12" blade: 6048 rpm (!), 0.70 inchs of ground travel between cuts (in/cut)
    16.5" blade: 4398 rpm, 0.96 in/cut
    18" blade: 4032 rpm, 1.05 in/cut
    21" blade: 3455, 1.22 in/cut (rpm near 3600, why 21" used on push mowers)
    24" blade: 3023, 1.40 in/cut

    So, increasing to a 21" blade from a 16.5" blade will decrease cuts per inch more than going from 19,000 fpm to 16,000.

    Also, the super high rpms required of 12" blades make for high spindle wear, or force lower blade tip speeds.

    At high mowing speeds (15 mph), a 72" mower will only cut once for every 2.6 inches of ground it covers!

    formulas:
    rpm= [fpm x 12 ] / [3.1415 x (blade length)]
    in/cut= [mph x 5280 x 12] / [rpm x 60 x 2]

    :dizzy:
     
  2. J&J Customs

    J&J Customs LawnSite Member
    Posts: 165

    ^ mans a genius haha but yes higher tip speed better the cut, better the cut healthier the grass, healthier the grass more money you make!
     
  3. nosparkplugs

    nosparkplugs LawnSite Gold Member
    Posts: 3,445

    Like I said, when you get up around 10MPH on a Midmount ZTR the cut quality decreases, I sold my Scag tiger cubs, 48" velocity decks; because the blades just cannot keep up with the ground speed of the mower.

    Great job olde blue :usflag::clapping:
     
  4. tb8100

    tb8100 LawnSite Bronze Member
    from TX
    Posts: 1,627

    bad boys cut at 18950fpm on nearly all models. :)

    This is about 213mph, just 2mph under the ANSI limit. Very few mowers have this high of a tip speed. Wright and Encore are like the only other ones that come to
    mind.
     
  5. tacoma200

    tacoma200 LawnSite Fanatic
    Posts: 5,427

    Blade tip speed is only one of many factors in cut quality. Faster isn't always better but as a general rule it is. Exmark first slowed the blades down on the new Triton before raising them back up. When cutting wet damp grass slower is often better. There are just too many factors to make a statement like faster is always better.
     
  6. nosparkplugs

    nosparkplugs LawnSite Gold Member
    Posts: 3,445

    Please, "speed" does not matter? the Midmount "mulchers" rely on speed, thats the whole point, This is why I sold my Midmouts to much BS,BS,and more BS. I save so much time with my Walker, you mulcher guys just twist, and bend physics, the fact remains a Walker SLOWER cutting speed is better than these speed demon Midmounts. Furthermore, the Walker "slow" pearless gear driven deck, is far superior in wet thick summer growth turf than faster belt driven deck.
     
  7. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    What Husqvarna walk are you looking at? I don't know of a Husqvarna mower with such a low blade tip speed. (not in the US anyway) They are generally somewhere between 18K-19K FPM.




     
  8. JimmyStew

    JimmyStew LawnSite Senior Member
    Posts: 367

    WH4817E -- http://www.usa.husqvarna.com/products_product_details.aspx?pid=822 -- although the online page does not include all of the specs. According to the 2008 product catalog, Blade-tip Speed, ft/min = 15,900 for this model and all of their other 48" models. All other cutting widths, 36" to 52" have tip speeds of 17,600 ft./min or greater. I gather from the early math laden post :dizzy: this may be an atribute of blade length to engine speed ratio?
     
  9. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    Well I'll Be...! :eek: You learn something new every day. I just grabbed a machine and checked for myself. The liturature doesn't lie. Thanks for the the schoolin'! That's what I love about this site, Good information!

    Q
     
  10. tb8100

    tb8100 LawnSite Bronze Member
    from TX
    Posts: 1,627

    must be nice to have Husqvarnas and Scags and what not right at your fingertips! Have you had a chance to check out Bad Boys? They're a really cool "outside the box" design. That would rock to work in R&D!!
     

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