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Blade tip speed. Gas VS diesel

Discussion in 'Lawn Mowing' started by davidcalhoun, Oct 24, 2005.

  1. davidcalhoun

    davidcalhoun LawnSite Bronze Member
    Posts: 1,022

    I have been considering switching to a diesel ZTR but I have a couple of questions. It appears that the diesel engines run at a slower operating RPM than their gas counterparts (3000 to 3200 RPM's for diesel and around 3600 RPM's for gas).

    1) Do the OEM's use the same decks (as in pulley sizes) on both gas and diesel ZTR's? I guess I am concerned that the blade tip speed would decrease some with a diesel ZTR compared to the gas ZTR unless the OEM has made some adjustment.

    2) Anyone know the mathematical formula for calculating blade tip speed? I would like to see just how much difference there is if I am only running at 3000 RPM VS 3600 RPM.

  2. rookiemower

    rookiemower LawnSite Senior Member
    Posts: 766

    dont worry the blade tip speed is the same if not higher. a good thing about a diesel is they wont bog down in heavy wet growth because of the high amount of torque they have.:waving:
  3. davidcalhoun

    davidcalhoun LawnSite Bronze Member
    Posts: 1,022

    I am well aware of the "lugging" capacity of a diesel engine (family farm switched to diesel many many years ago). I though "mowtech" did an excellent job explaining this concept to rest of the forums members in another thread.

    Yes, I agree that the diesel engine will handle tougher conditions more consistantly when the deck starts cutting grass at a heavier load. But there is no way that the blade tip speed can be greater on a diesel (running at 3000 RPM) VS a gas (running at 3600 RPM) if the decks are equal.
  4. TClawn

    TClawn LawnSite Silver Member
    Posts: 2,036

    actually, this diesel engine is governed to 3850rpm. http://exmark.com/lazerzxpspecs.htm

    what you have to look at is the RPM of the blades. the larger deck you get, the larger blades, meansa larger blade circumferance, and a faster blade tip speed per rpm. but if you have a large blade, you are not turning as many rpms. infact, my quick 36 with 12" blades turns a lot faster than my lazer hp's blades, even thought the exmark is at 18,500fpm and my quick 36 is at 17,250 fpm.
  5. mowtech

    mowtech LawnSite Senior Member
    Posts: 252


    Good question. Tip speed is important to the performance of the cutting unit, so most manufacturers would make sure that they maintain tip speed between a diesel powered unit versus a gaser. To keep the deck the same this would most likely be done through a different clutch pulley (if the engine were running slower this pulley would be larger). But it could also be accomplished by using smaller spindle pulleys. If in doubt, check the manufacturers specifications--tip speed should be listed as a spec item on both gas and diesels units.

    As for tip speed formula:

    speed (fpm) = Pi X D X S / 12


    Pi =3.14159
    D = blade diameter (inches)
    S = spindle speed (rpm)

    Remember engine speed and spindle speed are not necessarily the same depending on the drive ratio. However, if you were to slow an engine down without changing the drive ratio the the tip speed reduction would be proportional to the change in engine speed. i.e. 3600rpm reduced to 3000 rpm reduction would be 3000/3600=.833, so tip speed reduction would be 18500fpm X .833 = 15420fpm
  6. wowlawncare

    wowlawncare LawnSite Member
    from TN
    Posts: 62

    Before I switched to dixie chopper I had a grasshopper diesel. It didnet have near the tip speed of the gas. When I got into the thick grasses here the dielel wouldnt keep up. I ended up selling. BUT one of the big benefits of it was much better fuel comsuption. Had a 4galon tank on the diesel and 2 4galons on the gas. Id fill them up at the same time. Plus the longjevity of having a diesel. But as for cutting wise, I perfer gas. Just my oppinion!
  7. TClawn

    TClawn LawnSite Silver Member
    Posts: 2,036

    wowlawncare, were both mowers a grasshopper?
  8. Texas Mower Parts

    Texas Mower Parts LawnSite Senior Member
    from Texas
    Posts: 472

    Most of your blade tip speed can be controlled with pulleys. We had a 225 Grasshopper demo that we installed an inch smaller pulley on the center spindle running from the clutch to deck. Man, that was a mean machine, sounded like a damn helicopter. We estimated 26,000 fpm.
  9. TClawn

    TClawn LawnSite Silver Member
    Posts: 2,036

    and are these pulleys available?
  10. TLS

    TLS LawnSite Fanatic
    Posts: 7,940

    On a mule drive belt driven ZTR, ideally you want the same diameter pulley from the clutch to the deck with the idlers spaced out to that exact width of these 2 pulleys.

    Exmark adjusts the BTS via the OTHER deck pully which is above the deck-to-PTO pulley.

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