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ENGINEERING QUESTION - Why 19,000fps blade speed?

Discussion in 'Lawn Mowing' started by MOW ED, Nov 9, 2001.

  1. MOW ED

    MOW ED LawnSite Fanatic
    Posts: 5,028

    Why is the blade tip speed a MAXIMUM 19,000fps?

    Is it some type of LAW?

    When was it established?

    What kind of testing was done and why this number?

    I appreciate and welcome opinions too but can someone show me HARD FACTS and literature to back it up?

    I guess I am not happy knowing that there is a standard without knowing the reason.

    So I know there are those engineering types and others who are interested and welcome the input. I would like to also know what a minimum blade tip speed would be and does horsepower enter the equation? I know that a 26hp motor can spin something very fast and that the speed is geared down by pullies. Would a faster spin give better results, why not 20,000 or 30,000? With the new materials and technology today, I would believe that anything is possible.

    HOWARD JONES LawnSite Member
    Posts: 233

    OK, here is a semi-engineering answer: I have been unable to get the spec on the web, but several sources name ANSI B71.1-1990 and ANSI B71.4 (commercial) as the governing safety spec "USA standard safety specification for powered lawn mowers" and list the reason for the limit as hazard for thrown objects. By the way, the limit is 19,000 fpm, not fps. 19,000 fpm equates to about 215 mph, 19,000 fps would be almost 13,000 mph - much past supersonic, which would be the practical limit of blade tip speed (just like propellers) if there were no safety concerns.
  3. Richard Martin

    Richard Martin LawnSite Fanatic
    Posts: 14,700

    Jeeze, can you imagine the noise a mower would make if the blades did spin faster than the speed of sound? On each and every rotation there would be a sonic boom. The US Air Force did build a prop that exceeded the speed of sound at the blade tips but the noise was deafening and would cause the pilot and ground crew to get sick.
  4. MOW ED

    MOW ED LawnSite Fanatic
    Posts: 5,028

    Ok I was wrong on the s or m but thrown objects are the reason.

    I have seen some objects thrown by a mower and I dont care if its 16,000 fpm or 25,000 fpm its gonna kill someone if it hits em.

    I am not doubting your response but I still am wondering HOW they came up with this number.

    Did they stick a dummy on a chair and shoot a rock at it at 19000fpm and see if it just dented it or blew its melon off.

    What was the research and when was it done?

    Thanks so far guys.
  5. Eng Mwr Guy

    Eng Mwr Guy Gravely Manufacturer
    Posts: 249

    Yes. Something like that.

    All the more reason to keep the chutes down guys.....sorry for that reality check.

    Mfg's following the ANSI standards severely limits a plaintiff's ability to build a case.

    Testing was done to create the standard.

    Steel balls, nails and unbalanced blades tests are all done to keep a dangerous machine safe.

    I will talk to the resident safety expert and post what I learn.

    If you like I can send you a copy of the standard.

  6. Ric

    Ric LawnSite Fanatic
    Posts: 11,956

    Mow Ed

    "Clip Rate" is the blade speed to ground speed. The higher the blade speed and slower the ground speed the better the cut. Walker has a slower blade speed but also has a much slower ground speed therefore the clip rate is higher that a Dix Chop traveling at 15 MPH. My .02 LOL
  7. Kent Lawns

    Kent Lawns LawnSite Senior Member
    from Midwest
    Posts: 870

    That really doesn't always hold true.

    I don't think even a sharp blade would cut grass spinning the speed of a record player even if the machine were standing still.
  8. Ric

    Ric LawnSite Fanatic
    Posts: 11,956


    Is that at 45 or 78 rpm?? Come on Kent! I have read your posts you are an intelligent man don't bust my chops.
  9. MOW ED

    MOW ED LawnSite Fanatic
    Posts: 5,028

    They don't make record players anymore do they, mayby a CD player?;)

    Thanks for the replys and I really am looking forward to seeing copies of the standard. I still am curious as to WHY? I can understand how they throw ball bearings, nails and unbalanced blades but what makes this the majic number? Why not 16 17, 18k? Does it have to do with distance of a thrown item? I know that my Walker can shoot a rock 50' faster than you can blink and I know it would penetrate skin of anyone near and my Walker is no where near the 19000 fpM mark.

    Ric, I get what you are saying as I have a Walker. I can see Clip Rate as being variable as a Dixie can cut as -slow- as a Walker if it wants to but a Walker can't cut as fast as a Dixie. We know that persuing a discussion on the quality of cut here will get many pictures posted and many "Z"ealots screaming so I won't give my opinion on that.:D
  10. Richard Martin

    Richard Martin LawnSite Fanatic
    Posts: 14,700

    My Gravely dealer explained to me a few years back that there is a metal fatigue factor invloved when you rotate these blades too fast. It probably involves sub-harmonics that can neither be felt or heard.

    The reason that Walker decides to rotate their blades slower than just about anybody else is probably due to two reasons.

    The first is that a spindle will last more hours if you rotate it slower. Even with the best grease and maintenace routines the bearings will only rotate so many times before they need replacing. If you spin them slower they last longer.

    The second reason probably has to do with the GHS and the rate at which grass can be fed through it and/or the airflow under the deck only works properly at a given airspeed.

    And in an effort to keep things simple Walker doesn't use different sized pulleys on their different decks This is evidenced by the fact that all of the decks rotate the blades at 2900 rpm regardless of blade length.

    Other companies use different sized pulleys to keep the BTS up at or near to the maximum.

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