Blade Tip Speed Questions

Discussion in 'Lawn Mowing' started by OldLawnMowerMan81, Jun 2, 2008.

  1. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    I know the current blade tip speed limit - as of August 1, 1968 - is 19,000 fpm, and before then it was 21,000 fpm.

    19 inch cut - 4222 rpm
    20 inch cut - 4011 rpm
    21 inch cut - 3820 rpm
    22 inch cut - 3646 rpm
    23 inch cut - 3488 rpm
    24 inch cut - 3342 rpm
    25 inch cut - 3209 rpm
    26 inch cut - 3085 rpm

    18 inch cut - 4032 rpm
    19 inch cut - 3820 rpm
    20 inch cut - 3629 rpm
    21 inch cut - 3456 rpm
    22 inch cut - 3299 rpm
    23 inch cut - 3155 rpm
    24 inch cut - 3024 rpm
    25 inch cut - 2903 rpm
    26 inch cut - 2791 rpm

    These speeds can easily be achieved on mowers which have the cutting blade connected directly to the crankshaft. On other mowers, you may have to use the blade spindle speed.

    There are some exceptions to these rules.

    One - an Allis-Chalmers WB2235 22" side discharge rotary mower with a 3.5 hp Briggs & Stratton engine governed at 4000 rpm.
    Using the formula (RPM x pi x blade length / 12), I get 23,038 fpm. Are there other factors besides engine RPM that I must use here to bring the blade tip speed down to 21K fpm? There must be another factor as to this as to reduce the physical fpm by 2K. What would the blade spindle speed be (if not related to the engine's speed)?

    Two - a Toro 23267 25" commercial side discharge rotary mower with a 6 hp Tecumseh engine governed at 3200 rpm.
    Using the formula (RPM x pi x blade length / 12), I get 20,944 fpm. Are there any other factors as to this that will bring the blade tip speed down by more than 1K fpm?

    As I said, the engine and blade spindle speeds are totally different factors. In the case of the Toro 23267 mower, the cutter blade apparently is not directly connected to the engine's crankshaft (could I be wrong?). If that is the case, then what is the blade spindle speed for this model?

    Thank you,

  2. wheels910

    wheels910 LawnSite Member
    Posts: 163

    thanks for the info????

    think you meant this post for a different forum. maybe tractors.
  3. OldLawnMowerMan81

    OldLawnMowerMan81 LawnSite Member
    Posts: 66

    No, the subject matter here is all about blade tip speeds for rotary walk-behind power mowers. Although, I would like to find out about how to measure blade tip speeds on lawn tractors, too.

  4. mowtech

    mowtech LawnSite Senior Member
    Posts: 252

    The 25 inch mower most likely has a speed reduction in a belt drive. If you want to calculate the correct tip speed it's best to measure the actual blade or spindle rotational speed. The engine speed will most likely be running less than the nameplate setting due to the droop caused by the fan load of the blade. Otherwise there are no other secrets.

Share This Page