Doubles and Balde Speed

Discussion in 'Lawn Mowing' started by Acme Bunny, Feb 4, 2003.

  1. Acme Bunny

    Acme Bunny LawnSite Member
    from georgia
    Posts: 15

    has any one considered changing there deck drive pully (Slowing down blade speed) when running doubles. Doubles give you twice the blade cutting surface or twice the speed. My thought is that you could use doubles at a slower tip speed ( say 25% less) and get the best of both worlds. Your would have a blade tip making a complete revelution 25% faster than a single blade and 25% slower than doubles.

    Thanks for you thoughts
     
  2. 2 man crew

    2 man crew LawnSite Senior Member
    Posts: 403

    What benefits would you be gaining by slowing the blade speed down?
     
  3. bob

    bob LawnSite Platinum Member
    from DE
    Posts: 4,254

    You could just go back to running singles then.
     
  4. Acme Bunny

    Acme Bunny LawnSite Member
    from georgia
    Posts: 15

    It has to do with a compromise. When you run doubles you can find yourself bogged down when you hit thick turf. I mow mostly burmuda grass. The stuff gets real thich if cared for properly. Usually to thick to run doubles succesfully on a regular basis.

    By changing the ratio of the gearing (blade tip speed) you sacrifice some tip speed or cut quality. But, You gain torque at the blade tip. It is like using the middle front sprocket on a 15 speed bike. It is a good compromise between the other two. One is very slow but great for climbing hills ect. The other great for speed but takes to much torque for up-hill travel.
     
  5. 2 man crew

    2 man crew LawnSite Senior Member
    Posts: 403

    Ok I see what your thinking.

    What size deck and HP are you running

    I would rather buy a mower (or engine) with more power, than sacrifice any blade tip speed.
     
  6. Acme Bunny

    Acme Bunny LawnSite Member
    from georgia
    Posts: 15

    I use a Snapper ztr, 48" deck and 20 hp engine. I love it because of the joy stick steering ( I can drink my morning coffe and mow at the same time). I am looking to make an in-expensive improvement. I currently have the deck torn down for a complete maintanance and some minor repairs. I have to replace one blade pulley. While I have the deck torn down, I thought it might be a good time to try out this idea.

    But, It woud be nice to have some imput before making the change. I think it will give me some improvement But, sometimes I can't see the forest for the trees. Just want to know if any one else has feedback on the idea.

    Drew

    p.s. mower is to old for a new engine and no $ for a new mower
     
  7. Doc Pete

    Doc Pete LawnSite Gold Member
    Posts: 3,469

    I realize I'm running a WB, but my 17 kaw (twin) and 48" deck has no problem cutting long grass with doubles. However, what "is" a bigger factor is the "sail". That's what really sucks up the power. If you want the best of both worlds, run low lift blades, or as I did for experientation, cut the sail completely off one set of doubles........Then, you will have enough power...........
    Pete
     
  8. 1grnlwn

    1grnlwn LawnSite Bronze Member
    Posts: 1,261

    Twice the surface yes, twice the speed no. Tip speed and blade sharpness are the only factors in a clean effecient cut. Degrading one of these will not result in an improvement. Your choices are to slow down, run singles or try lower lift blades. I wouldn't recomend hacking up your blades, this could be a safety issue.

    Mark
     
  9. SLS

    SLS LawnSite Bronze Member
    from Mars
    Posts: 1,540

    With doubles you do not get twice the blade tip speed...only twice the amount of blade tips which are covering the same FPS (feet per second) as the single blade setup.

    Example: More pistons moving up and down does not mean more rpms.......only more pistons (and mass) moving during the cycle.

    In fact, I believe that the added resistance of running doubles already lowers the blade tip speed by some amount due to moving twice mass (blade weight) with the same amount of force (torque/HP at spindle pulley)...and running double Hi-Lifts definately slows the blade tip speed.

    Dig back into old threads about doubles and you will find where guys have asked how to INCREASE the rpms of their engines to compensate for the loss of blade tip speed or to eliminate the engine from bogging down when the going gets tough.

    Remember that slowing the blade tip speed down will degrade the 'lift' (or vacuum) under the deck...leading to a loss of quality of finished cut. Lift, pitch, and blade sharpeness are all critical aspects of achieving a great finish. Degrade any of these and quality of finish will suffer.


    Try this: Set your throttle at half and run the mower as fast as it will go with the blades engaged over some turf that need a cut. Then look at the cut. Not only was the equipment straining but the cut suffered too.

    Faster tip speed = better lift =better cut. And happier machine.
     

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