Blade Tip Speed....Does It Really Matter That Much?

Discussion in 'Lawn Mowing' started by Tonyr, Sep 25, 2005.

  1. Tonyr

    Tonyr LawnSite Bronze Member
    Posts: 1,973

    Just wandering....and yes, I have seen this topic discussed before, but curious for fresh opinions.

    How important is it that blade tip speed be 18500 fpm plus etc?

    I ask because, I have a new J.D 997 Rear Discharge Fabricated deck, it's tip speed is only 17480 fpm and it can cut full speed 11.5 mph and not leave one stringer, not one uncut grass leaf, cuts cleaner and much faster than my Toro with TFD ever could and it's blade speed was 18500 fpm.

    And the Toro would have much higher vacume than the rear discharge deere, hmmmmmmmm

    So is blade speed a selling point....or does it really matter?

    obviously different decks require more/less vacume to perform best, just curious.......
     
  2. SproulsLawnCare

    SproulsLawnCare LawnSite Senior Member
    from IN
    Posts: 365

    I run John Deere products and I see no problem with the cut either. Maybe faster blade speed would perform even better, but I think there is a point to where it wouldn't be worth extra speed. Then it would only create more wear on the moving parts. I am quite sure that all of the manufacturers who design these machines have tested them with different blade speeds and decided which speed works best with the particular design of the deck. Either that, or built the deck around the blade speed that the machine will produce. Either way, the blade speed surely matches the deck design to produce the best cut possible for each machine regardless of the manufacturer.
     
  3. PMLAWN

    PMLAWN LawnSite Gold Member
    Posts: 3,535

    I always felt that the blade speed gave you more passes with the edge to the blade of grass so if the first pass missed or hit a nicked spot the next one would get it.
    I felt that the deck design and depth with the blade lift affected the grass lift.
    But I may be wrong.
    I am still very pleased with the cut of the turbo force deck and only have problems if the blades get nicked bad.

    By the way are you happy with the Deere.
    What made you switch as I know that you really liked the Toro.
     
  4. Precision

    Precision LawnSite Silver Member
    Posts: 2,995

    blade tip speed is a function of the length of the blade.

    I don't know/want to do the math, but a 10" blade spinning at the same RPM of an 18" blade will have a much lower blade tip speed. So if blade tip was the only issue, then the single blade 30 and 32" mowers would give the best cut around.

    Now we all know that isn't true.

    There is a relationship between blade tip speed and ability of the blade length to keep that up. Obviously you can bog down a 32" blade much faster than a 10" blade.

    I know it isn't apples to apples, but I have a toro 36" WB with 2 18 inch blades and 15 hp. I also have the new scag rider 36" with 3 12" blades and 19hp. Guess which one bogs quicker. Big hint it isn't the one I sit on. And yes I know the rider has 4 more hp but it also is dragging around 350 or more extra pounds. and it can cut at 7 mph.

    My very unscientific view on blade tip speed. Marketing idea with no merit.
     
  5. mowtech

    mowtech LawnSite Senior Member
    Posts: 252

    This can depend on deck size. The difference between 17480 fpm and 18500 fpm is not that significant, about 6%. Per the fan law air flow is proportional to the rotational speed. So in this case, if you have the same blade size, the Toro would move about 6% more air. On the other hand, if you are comparing a 72-inch deck to a 60-inch deck, then you have to look at the blade size. Given the same rotational speed, the air flow is a cubic function to the blade diameter. But, we are talking tip speed not rotational speed, so to get the same tip speed the larger blade of the 72-inch deck would be spinning slower than the 60-inch blade. But that would be offset by the cubic function which means is that a 72-inch deck could move a lot more air at the same tip speed as the 60-inch.

    As for the number of passes the blade makes, at 11.5 mph a 60-inch deck at 18500 would travel about 0.27 inches during one revolution of the blade while at 17480 it would travel about 0.29 inches. Again, not that significant. On the other hand with a 72-inch deck at 18500 you would travel about 0.36 inches per blade rotation. At 17480 you would travel about 0.38 inches. 3/8th inch is starting to get up there!

    Higher tip speeds are generally better because more airflow promotes more vacuum to lift the turf, but there are other factors that can significantly effect the quality of cut: sail height and style, general design of the deck to promote vacuum, and the location of the casters. Higher tip speeds for side discharge decks also promotes better dispersion of the grass, but with rear discharge you don't care how far it is thrown.
     
  6. Tonyr

    Tonyr LawnSite Bronze Member
    Posts: 1,973

    Thanks everyone, we all see it the same....

    good replies guys!


    PMLAWN
    Mate, the deere has some issues with the rear screen that j.d are looking at a cure for due to our dusty conditions, and rear discharged dust gets sucked up and clogs the radiator.

    I bought the j.d as dealer is 20 mins away, toro 1.5 hrs, I needed/wanted rear discharge for liability, safety, versatility etc, and j.d had just released a 60'' ztr.....and hustler, but their 60'' deck model wouldn't be here till next month. The toro was great, but was getting a bit tired needing some work, and my dealers mechanic is just not good enough, he botched the last job, not again.

    plus, i was keen to go bigger hp, and diesel....I'm very happy with the power and torque and feul consumption of the j.d.

    why rear discharge? These are faster, cut stringers, weeds, stalky stuff clean as, where my toro struggled, they eat long rough ferral jobs so easy, my jobs are about 20% - 30% faster with the deere.

    the j.d is a lot better built than the toro, the finishing touches are great, unlike my toro, better paint etc, ya pay more for a deere, but ya get more machine....

    sorry for rambling....answer is, rear discharge deck on a diesel ztr.
     
  7. dvmcmrhp52

    dvmcmrhp52 LawnSite Platinum Member
    from Pa.
    Posts: 4,205

    Tony,
    I believe the blade tip speed is much more important in a side discharge deck than it is in a rear discharge due to the movement of cuttings from one side of the deck to the other. With rear discharge there isn't near the work to be done by the blades as with side discharge or a mulching set up.
     
  8. Tonyr

    Tonyr LawnSite Bronze Member
    Posts: 1,973

    G'Day Dave,

    I think you would be right mate.

    Love to try meg mos on it, but total cost shipped to hear is around $600, ouch! lol.

    doubt I would get a set sent to demo 1st.....bummer lol.
     
  9. dvmcmrhp52

    dvmcmrhp52 LawnSite Platinum Member
    from Pa.
    Posts: 4,205




    Why not just try some doubles and see what happens? It sure beats buying meg mos and finding out there's no real improvement after (ouch!) $600...............If the doubles help you out then maybe consider the meg mos.
     
  10. Tonyr

    Tonyr LawnSite Bronze Member
    Posts: 1,973

    good point mate....
     

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