Blade tip speed??

Discussion in 'Lawn Mowing' started by gogetter, Sep 9, 2001.

  1. gogetter

    gogetter Banned
    Messages: 3,256

    I use a 21" Craftsman push mower for some real small areas that the exmark walk behind won't fit into.
    The cut isn't great. It's a new mower. Blades have been sharpened. It's got a 6.75hp motor. However, it doesn't sound like it's running high enough. So I'm thinking that the blade tip speed isn't high enough causing the crappy cut.

    Just kinda wondering if anyone knows what the average blade tip speed is for a 21" craftsman as compared to a commercial type 21" like a Toro, Exmark, etc.??

  2. awm

    awm LawnSite Gold Member
    Messages: 3,354

    they come muffled dn a lot more these days plus goveners are set at a low max rpm. combine that with 101/2 inch length on each side of spindle and i think you could be right . low blade speed. i think its against the law to change the settings .
    but it can be done.
  3. 65hoss

    65hoss LawnSite Fanatic
    Messages: 6,360

    Speed is part of the problem. These mowers are not known for the best cut.
  4. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    I'm not for sure but I think the maximum tip speed is 19,200 feet per minute. So a small blade can spin faster than a large blade. I don't know why it's 19200 because a 21 inch blade turning 3600 rpm has a tip speed of 19845 fpm. It would be safe to set you high idle speed to 3600 rpm.
  5. EJK2352

    EJK2352 LawnSite Bronze Member
    Messages: 1,150

    Check out the blade on your Craftsman & compare it to the ones found on a Toro, Lawn-Boy, Honda, etc. The blade on those name brand mowers is alot stiffer so it doesn't flex and will give a cleaner cut. Your mower is probably running a little on the slow side also. I can't remember the max engine speed for a 21" mower engine. You might want to check with a reputable dealer on this, as they publish it in the Briggs & Stratton service manual.:) ;) :) ED
  6. gogetter

    gogetter Banned
    Messages: 3,256

    Thanks for the replies so far guys.

    Ya, I know the Craftsman isn't meant for the best cut, but it doesn't get used much and I couldn't justify spending $700 to $1,000 for a commercial unit.

    I did a search about raising the rpm by adjusting the governer, but seems that this greatly reduces engine life. But then again, seeing as I use it very little, for very short periods, maybe it would be worth it anyway.

    I'll look into a better blade too. Thanks again.
  7. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    Most small gas engines are rated at 3600 rpm. Yours is no different. Have a shop check and raise the rpm's to 3600. There won't be any problems with tip speed. Yes, raising the rpm's will shorten the life of an engine. But a slower running, lugging engine will overheat and it too will have a shorter life. 3600 rpm is where it should be running.
  8. SLS

    SLS LawnSite Bronze Member
    from Mars
    Messages: 1,540

    Hey gogetter,

    I just looked in my Briggs & Stratton single cylinder 4-cycle repair manual.

    The table provided in the 'governors' section shows as follows:

    18" @ 3800 rpm (No Load)
    19" @ 3600 rpm //
    20" @ 3400 rpm //
    21" @ 3250 rpm //
    22" @ 3100 rpm //
    23" @ 2950 rpm //
    24" @ 2800 rpm //
    25" @ 2700 rpm //

    Idle speed 1750 for aluminum engines.
    // // 1200 for cast-iron engines.

    The manual also states:

    'Run engine at half throttle to allow the engine to reach normal
    operating temperature before measuring speed with tachometer. To account for tolerances, which may be required by tachometer manufacturers, we suggest taht the top governed speed of the engine be adjusted at least 200 RPM lower than the maximum speeds shown."

    I hope this helps.
  9. awm

    awm LawnSite Gold Member
    Messages: 3,354

    according to mr exmark on that forum .longer blade means higher bladetip speed ,unless i misunderstood what he was saying.
    remember we talkin blade tip speed.
    i assumed that meant that say a 60 cut would have higher
    bladetip than my 52. assuming rpm the same. now ill
    mabe learn something i didnt know. ill tune in tomorrow
    later now
  10. 65hoss

    65hoss LawnSite Fanatic
    Messages: 6,360

    That was the way I understood it also AWM.

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