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dumb blade tip speed question

Discussion in 'Lawn Mowing' started by milo, Jul 4, 2008.

  1. milo

    milo LawnSite Bronze Member
    Messages: 1,360

    do blades really go around like 18,000 times a minute? or do they go around 9,000 times but because of 2 sides they say 18,000 rpm????
  2. ed2150

    ed2150 LawnSite Member
    Messages: 174

    Blade tip speed is usually expressed a feet per minute, referring to how far one tip of the blade travels in one minute's time.

    The 18,000 figure you mentioned is not referring to rpm but fpm instead. It refers to the speed of each and every blade tip and is not affected by the number of blades.
  3. Nosmo

    Nosmo LawnSite Bronze Member
    Messages: 1,216

    Milo answered the question correctly but I'll answer your question as if the speed were measured in RPM's. If measured in RPM's it would be a complete rotation of the blade. Just think of the spindle shaft making a complete rotation.

  4. JimQ

    JimQ LawnSite Bronze Member
    Messages: 1,139

    Typical specs for a 61" deck....

    21" blade

    3300 blade RPM

    18,133 FPM (Feet per minute - blade tip speed)

    In one minute, each of the blade tips has made 3300 revolutions.

    The distance each blade tip has traveled in that minute is 18,133 feet.

  5. OP

    milo LawnSite Bronze Member
    Messages: 1,360

    ok, that makes sense. i was going to say to go even 9,000 rpm's the pulleys would have to be way smaller like 1/3 the size to go 9,000 rpm's
  6. whoopassonthebluegrass

    whoopassonthebluegrass LawnSite Platinum Member
    Messages: 4,304

    At 9,000 rpm's we'd probably all be in prison for involuntary manslaughter. :dizzy:
  7. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Messages: 66

    For those of you who have either a gear or belt driven mower, another way would be to determine what pulley size is right for your machine and figuring out what RPM the engine pulley and belts are to be operated at.

    I would hint that the engine pulley on my 1993 MTD 123-478C mower is operating at the speed of about 1620 rpm. The pulley size is 5-1/16" (according to the owner's manual) and, if you multiply this number by the engine speed which is 3200 rpm, then you have 1620 rpm coming out of the pulley.

  8. mtcjb

    mtcjb LawnSite Member
    Messages: 37

    Ok I am no mathematician by any means but these figures just do not seem right, any and all corrections will be greatly appreciated. If I remember correctly circumference which would be the distance traveled by the blade tip is 2*pie*r (pie=3.14 and r is radius of the circle which would be 1/2 of blade length) My 7 Iron II deck is rated at 18000 fpm. one revolution of a 20" blade would be 2*3.14*10=62.8"=5.23'......ok nvm this does come out to roughly 3441 rpm.... remember kids don't drink and MATH!!!! :confused::drinkup:
  9. Heatman

    Heatman LawnSite Member
    Messages: 47

    That would be 55 revolutions per second.... that can't be possible can it?
  10. HighlandLC

    HighlandLC LawnSite Member
    from CT
    Messages: 202


    Hey, they have rase rocket engines turning over 10,000 rom.

    IIRC, tip speed for mowers is regulated by law.

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