Blade Tip Speed...Whats yours?

Discussion in 'Lawn Mowing' started by TLS, Mar 22, 2001.

  1. TLS

    TLS LawnSite Fanatic
    Messages: 7,943

    I am seeing a lot of questions pertaining to blade tip speed. Obviously the faster the better, right? a point. What I am wondering is, at what speed is the best cut possible at? It would have lots of variables. Blade design is one, blade to baffle distance would be another. Deck design is also important. I'm sure that there are alot of different variables.

    Also, another variable is engine RPM and what RPM the factory "mower" rates the fpm at. As an example, my '97 Dixie with a 25 Kohler "yeah!" was delivered to me with a free load speed of 3450 RPM. That seemed a little slow, so I looked in the owners manual and found that they should be at 3600 RPM. So I adjusted it to that. Much better cut, like night and day!. Then after reading Lawnsite, and how many ran as high as 3850 RPM, I then adjusted them to 3750 just to be safe. I noticed another improvement in cut. At what speed does Dixie attain the 19000 + fpm speed? 3450, 3600, 3750, or 3850? These jumps in RPM have a measureable affect on blade tip speed as well as quality of cut.

    Next would be...If the drive pulley on the engine is the same diameter as that on the spindle (are they?) then the spindles turn at the same RPM as your engine. Are some "geared" up or down with pulleys to adjust this spindle RPM? How about the shaft driven decks, are they spinning faster or slower. The length of the blade determines tip speed, so a 72" deck needs to turn slower than a 60" deck.

    All right, all you engineering students, what is the formula for determining the blade tip speed when spindle RPM is known? How can we learn spindle RPM when it is shaft driven?

    BOY, do I need to get out there and start cutting or what!!!

    Sorry for the long post!

    Carry ON!!!

  2. Eric ELM

    Eric ELM Husband, Father, Friend, Angel
    Messages: 4,830

    There was a thread on this topic not long ago about figuring out the fpm of the blade by the rpm's. I worked the last 2 days for the first time in ages, I'm to tired to find it. :(
  3. MikeGA

    MikeGA LawnSite Member
    Messages: 101

  4. TLS

    TLS LawnSite Fanatic
    Messages: 7,943

    Thanks Mike, must have been plowin' that day! Professor Martin explained the formula's perfectley!

  5. mowingmachine

    mowingmachine LawnSite Senior Member
    Messages: 253

    That Martin guy beat me to it. I was hopen I could be the hero to explain that. His formulas are right and he did a great job of explaining it. As far as the shaft driven decks it is the same thing and it comes down to gear ratios. If the gears inside each of the gear boxes are the same size all the way through then the spindle speed will be the same as the engine. So if the input gear on the gear box is say 4" and the output gear is 2" then the spindle shaft speed would be twice as fast but you would cut the torque in half. Most gear boxes are a 1:1 ratio as well as pulley sizes from the engine to the spindle. Here is a good rule of thumb for a 60" deck. Most 60" decks have 20" blades and at about 3600 RPM the blade tip speed is 19000 feet per minute. The 19000 f/m is the industry safety standard and I would stay below it for liability reasons. If something goes wrong on one of your jobs and you are above that speed it is no longer the manufacturers fault and you will be the one getting sued. Hope this helps.
  6. mowingmachine

    mowingmachine LawnSite Senior Member
    Messages: 253

    Sorry I wasn't very clear about the rule of thumb for a 60" deck. This only holds true if the engine pulley is the same size as the spindle pulley.
  7. ncmow

    ncmow LawnSite Member
    Messages: 8

    Faster is not always better. ANSI regulates this speed because of the thrown objects test. Anything above 18,500 ft/min and the pattern dispersed fails ANSI standards.

    As for the formula, I've got one easier that the previous posts. Use a photo tachometer and a piece of reflective tape. Put a piece of tape on a spindle sheave and measure rpms. Then plug into below:

    Blade Spindle RPMs X 3.14 X Blade Length in Inches / 12

    This will give you blade tip speed in ft/min
  8. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    Let me see now, where did I leave that photo tach? What the heck is a photo tach and where do you buy it?
  9. Eric ELM

    Eric ELM Husband, Father, Friend, Angel
    Messages: 4,830

    Richard, your mower should be a 1 to 1 ratio. If the pulley on the motor and the pulley on the deck are both the same size your spindles should be turning the same speed as your motor. You have a tach which is fairly close, so if you are running 3600 rpm's and your blades are 20.5" long, you should have 19,000 fpm blade speed.
  10. mowingmachine

    mowingmachine LawnSite Senior Member
    Messages: 253

    kinda like to know where you got the ANSI standard to be 18,500 f/m. The industry goes by 19000 f/m.

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