Blade Tip Speed...Whats yours?

Discussion in 'Lawn Mowing' started by TLS, Mar 22, 2001.

  1. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    I am new to this forum and I would like some help here in regards to blade tip speeds of my lawn equipment listed below.

    I have four lawn mowers (and one lawn tractor):

    1 - a 1987 Sears Craftsman 38321 20" rear bagger
    engine - Craftsman, model 143.374012 (Tecumseh TVS90-43352E), engine speed 3200 rpm (high) and 2100 rpm (low)
    Blade tip speed (from ncmow's post) - 16,755 fpm (high) and 10,996 fpm (low)

    2 - a 2002 Sears Craftsman 38761 22" side discharge mower/mulcher
    engine - Craftsman Eager-1, model 143.024200 (Tecumseh LEV105-347001A), engine speed 3050 rpm
    Blade tip speed (from ncmow's post) - 17,567 fpm

    3 - a 1973 Sears Craftsman 9026 20" side discharge
    engine - Craftsman Eager-1, model 143.234062 (Tecumseh ECV100-145035A), engine speed 3000 rpm
    Blade tip speed (from ncmow's post) - 15,708 fpm

    4 - a 1993 MTD 123-478C084 21" rear discharge, self-propelled
    engine - Briggs & Stratton Quantum XTE, model 124802-3110-01, engine speed 3200 rpm
    Blade tip speed (from ncmow's post) - 17,593 fpm

    5 - a 2006 Sears Craftsman 27681 LT3000 42" deck lawn tractor
    engine - Briggs & Stratton Intek OHV (with AVS), model 31P977-0635-E1, engine speed 3300 rpm
    What is the blade tip speed for this tractor? What formula do I use to determine blade tip speeds for tractors?

    Thank you,



    Ben (OldLawnMowerMan81)
     
  2. mowtech

    mowtech LawnSite Senior Member
    Posts: 252


    The easiest way is to measure the rotational speed of the spindles with a tach and use that speed for calculating the tip speed. If you actually want to calculate it from the engine speed you can do this by calculating the ratio of the drive pulleys--engine to spindles, and using that ratio to calculate the spindle speed. This is easier said than done because the actual pitch diameter of the pulleys is hard to measure without the proper tools. A close approximation would be to use the pulley diameters. The drive can either speed up the spindles or slow them down. The device with the smallest pulley will be spinning the fastest.
     
  3. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    So the formula for blade tip speed is as follows:

    For walk-behind push-type mowers:
    Measure blade circumference (tip to tip) (ex. 20" blade = 5.236' circumference)
    Find physical engine speed (ex. 3200 rpm)
    BTS: 3200 x 5.236' = 16,755.2 fpm

    For walk-behind self-propelled mowers:
    Measure circumference of drive pulley (ex. 5.06" pulley = 1.325' circumference)
    Measure circumference of engine pulley (ex. 5.5" pulley = 1.44' circumference)
    Find physical engine speed (ex. 3200 rpm)
    Measure blade length (tip to tip) (ex. 21" blade = 5.498' circumference)
    BTS: 1.325' / 1.44' x 3200 x 5.498' = 16,189 fpm
    (This formula could be different for those mowers whose engines have PTOs. Could anyone please give me an alternate formula for self-propelled mowers whose engines have their own auxiliary driveshafts?)
     
  4. mowtech

    mowtech LawnSite Senior Member
    Posts: 252

    Not quite. You don't actually need the circumference of the pulley because the circumference is proportional to the diameter. (Pi x Dia)

    For a belt driven mower:

    Tip Speed (feet/minute) = Engine Speed (RPM) x (Pitch Dia Engine Pulley / Pitch Dia Spindle Pulley) x 3.1415 x Blade Length (inches) / 12

    For a shaft driven mower:

    Tip Speed (feet/minute) = Engine Speed (RPM) x Gearbox Ratio x (pitch Dia Gearbox Pulley / Pitch Dia Spindle Pulley) x 3.1415 x Blade Length (inches) / 12

    This assumes gearbox is driven directly off the engine.
     
  5. Ooomwizard

    Ooomwizard LawnSite Senior Member
    from atlanta
    Posts: 296

    My blade speed is about 2 mph.. then I have to shave again the next morning.
     
  6. Lawn-Sharks

    Lawn-Sharks LawnSite Senior Member
    Posts: 912

    My blade speed on my snapper is 1,000,000.00 Fpm..not really but is sure would feel like that if i stuck my hand under the deck!
     

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