Increasing blade tip speed?

Discussion in 'Mechanic and Repair' started by MikeKle, Jul 15, 2008.

  1. MikeKle

    MikeKle LawnSite Platinum Member
    Posts: 4,253

    I have been thinking about trying to increase the blade tip speed on my turf tracer lately too see if I can cut better and faster. Has anyone "re-pulleyed" their mowers? Pros and cons of doing this? any info would be appreciated. Thanks!
     
  2. Jason Rose

    Jason Rose LawnSite Fanatic
    Posts: 5,858

    How old is it? I'm no expert, but I believe that Exmark pretty much pushed the MAX blade tip speed allowed for many years now. I really don't know the physics of it, or how fast blades can really spin and still be structually OK, but I feel like the speeds these commercial machines are running are fast enough!

    More than likely, just a guess, if you feel like the blades aren't turning fast enough, is the engine running at the proper RPM's? A slow engine, even by a few hundred rpm's can kill the preformance of the blades. The engine should be running 3650 rpm or so. Depending on the make of the engine and mfg. year I know I've seen some that spec up to 3800 rpm on the engine.
     
  3. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    Hi,

    Is it true that on lawn mowers that are self propelled (whether the power drive is by gears, chains or belts), that if you engage the drive it will reduce the blade tip speed and not increase it?

    On a 1968 Allis-Chalmers WB2235S mower, the engine (a 3.5-hp Briggs & Stratton) is governed by the mower manufacturer at 4,000 rpm (I kid you not!).

    Without the power drive engaged, the blade tip speed is 23,038 fpm (unfortunately, this figure exceeds 2,000 fpm above the now-outdated 21,000 fpm limit established by the USASI, called ANSI as of mid-1969). For mowers with a 22" blade, until 1969, the maximum engine speed allowed was up to 3646 rpm (today that's 3299 rpm max). Thereafter, it's up to 19,000 feet per minute (fpm).

    The engine has a 8.5:1 gear ratio with a 1/2" diameter PTO shaft extension. Plugging all these figures together:

    Without power drive engaged:
    4000 rpm x pi x 22 / 12 = 23,038 fpm (oh no!)

    With power drive engaged:
    4000 rpm x 8.5 gear ratio to 1 x (0.5" dia. PTO shaft ext. / 4.875" drive pulley dia.) x pi x 22 / 12 = 20,085 fpm

    ~Ben
     
  4. perfect_cut

    perfect_cut LawnSite Member
    from Akron
    Posts: 14

    Well said...I will be as smart as you one day :weightlifter:



     
  5. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    On the other hand, if the drive pulley should be 4.25" in diameter to compensate for the 8.5:1 gear ratio and the 1/2" PTO shaft diameter so as not to sacrifice any blade tip speed under load, then the governor spring should be changed to part no. 260875 for the correct speed setting of 3500 rpm.

    ~Ben
     
  6. bigmower1

    bigmower1 LawnSite Member
    Posts: 158

    What kind of mower is it?
     
  7. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    In that case I was referring to the 1968 Allis-Chalmers WB2235S. The engine on it is a Briggs & Stratton 3.5 hp model 92982 type 0385-01.
     
  8. bigmower1

    bigmower1 LawnSite Member
    Posts: 158

    A 3.5 hp motor really inst much power so i dont think its wise to try to increase the blade speed, you will loose power and probably over heat the engine.
     
  9. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    I have a 1984 Toro 20672 21" mower with a Toro (Tecumseh) TNT100 4-cycle engine on it. Does high torque really kill performance of cutting grass?

    The TNT100 and TNT120, as I'd heard from other people, are high-torque engines. Also, Toro governs certain engines, both by Tecumseh and Briggs & Stratton, to figures below 2600 RPM as a result of the aluminum deck not strong enough to handle ultimate projectile flings.

    ~Ben
     
  10. topsites

    topsites LawnSite Fanatic
    Posts: 21,653

    Not to rain on the parade but the higher the speed of the blade, the better the chances that it could
    split into pieces when it hits an obstacle, you can forget flying rocks here.
     

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