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timing&tip speed

Discussion in 'Lawn Mowing' started by bababooie, Feb 21, 2001.

  1. bababooie

    bababooie LawnSite Member
    Messages: 134

    how do measure the timing of the machines,and the tip speed?
  2. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    Timing can be done with a regular automotive timing light. Just put marks on the pulleys and connect the timing light like you normally would. Then start your engine up and engage the blades. Then just observe the marks on the pulleys to see if one is spinning faster or slower than the others.

    To determine blade tip speed you need to know how fast the spindles are spinning and the circumference of the blade arc.

    To determine the spindle speed you need to measure the circumference of the various drive and driven pulleys at the point where the belt actually touches the pulleys. Some belts sit down inside of the pulleys and this is where the measurement must be made.

    So if the drive pulley on the end of the crank is 8" around and the pulley on the spindle is 7" around you would have a ratio of roughly 1.14. That means that the spindle pulley rotates 1.14 times for every time the crank pulley rotates once. So if the engine is running at 3,600 rpm times 1.14 then the spindle is rotating the blade at 4,104 rpm.

    To determine the circumference of the blade arc you need to multiply the length of the blade by 3.14. So if you have a 17" blade times 3.14 you get a circumference of 53.38 inches.

    Now multiply your spindle rotation speed by you blade circumference arc. It looks like this: 4,104 times 53.38 equals 219,071 inches per minute divided by 12 (there are 12 inches in a foot) and you get 18,256 feet per minute blade tip speed.

    Simple isn't it?
  3. Eric ELM

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

    I have one for you to figure out since you are good at this. I have a diesel that is supposed to be turning 3750 at the crank. The crank has 2-6" pulleys that turn 2-6" pulleys on the mule drive shaft. On the end of this shaft is a 7" pulley that turns a 7" pulley on the mower deck with 20.5" blades. How fast are these blades spinning?

    I had someone tell me this is still a 1 to 1 ratio, but the outside of that 7" pulley has to be turning faster than the out side of the 6" pulley. I realize the shaft is still turning 3750, but the 1" increase in size has to be speeding things up a bit.

    I have put the 22 HP Kohler next to the diesel and run the gas engine at 3750 rpm's with the blades on, which has a 1 to 1 ratio. I then start the blades on the diesel and push the throttle up to about 3/4 throttle and by listening to the harmonics of this, they sound the same. This is where I run the diesel at when mowing unless I get into some thick tall grass, I shove it all the way forward for more power and tip speed.
  4. John DiMartino

    John DiMartino LawnSite Silver Member
    Messages: 2,555

    Eric,i think the sound of the newer deck is different,that what is throwing you off.I mowed with a friends 60"dixe,its a 98,it had the old deck,mine sounds like its spinning much faster too,I looked at his tach,and it was at 4000 rpm's.I m only guessing,but mine cuts as good at 3000 rpm's as his does at 4000.And mine will throw clipping as far at 3500 as his at 4000.I think its in the depth and baffling of the deck.If you really want to know tip speed,have a helper with you,mark the crank of the motor,mark a spindle,hand turn the engine with a wrench until the crank has made a full rotation,and measure the amount the spindle went around,then divide that by the rpm's to get the spindle RPM.So if the soindle went 300 degrees,that is 5/6 of the engine RPM.So at 3600 Rpm,s the spindle is going 3000.That, with the length of the blade you can get your tip speed.It is much more accurate than maesuring pulleys,because belts dont always ride theoretically the same in a 6" and a 4" pulley,this will throw off true tip speed.
  5. Eric ELM

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

    Thanks John. :)

    I'll try your method, but it would be interesting to see what Richard comes up with to compare.

    Now if I can find a wrench long enough to turn that 23 to 1 compression ratio over, I will have it made. :)
  6. John DiMartino

    John DiMartino LawnSite Silver Member
    Messages: 2,555

    Eric,pull the spark plugs, LOL.
  7. Eric ELM

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

    Good idea. :)
    I'm still looking for them. I just can't find them, but if I do find them, I will pull them out. :)
  8. thefarmer4

    thefarmer4 LawnSite Member
    Messages: 71

    Shouldn't the timing be set from the factory. What would throw the timing off? I run my exmark metro at full trottle I always assumed thats how they were ment to run, am I wrong?
  9. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    Your friend is correct. The outside of the 6" pulley does not rotate faster than the outside of the 7" pulley in relationship to the center of the axis since they are on the same axis. The outside of the 7" pulley will rotate faster than the 6" pulley in relationship to any stationary object. It is a 1 to 1 ratio. So it would be 20.5 X 3.14 = 64.37 X 3,750 = 241,387/12 = 20,115 feet per minute. But... if you take this same engine and run it at 3600 rpm it will only be turning at 19,311 fpm or only 311 above OSHA rules.

    That's why I said to measure the pulley where the belt rides and not the outside of the pulley. Re-read my 3rd paragraph. Your system of guesstimating whether the pulley has rotated 5/8's of the way around or 11/16's of the way around could cause the numbers to be off by thousands. But, if you were to put a degree wheel on both axis that would provide the most accurate ratio measurement under the circumstances that the mower may be in at the time.

  10. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    The timing <b><i>should</b></i> be set from the factory. And yes, you should run the mower at full throttle. Usually the only thing that will upset timing on a single belt mower (like most 36s) is one of the pulleys is worn more than the other. On a dual belt system like my Gravely 50" it could be the pulleys or one belt has worn more than the other.

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