Engaging Mower Blades

Discussion in 'Mechanic and Repair' started by Roachy, Jul 26, 2012.

  1. Roachy

    Roachy LawnSite Member
    from NJ
    Posts: 125

    Just wondering what your opinions are here. I just recently took my ztr to the toro dealer and had him change my pto belt. He charged me $190 in labor and im sure the belt wasnt that hard to put in. Well less than 30 hrs later the belt just breaks in half when I go to start up the blades. I end up telling him that I usually engage the blades at half throttle. He tells me that this is the reason that the belt broke and if i had the engine at full throttle it would have never snapped. I kept saying how it was a defective belt and hes just throwing me a load of crap. Is it time for me to find a dif dealer? I have always heard you should engage at half throttle to have less wear on the pto, was my dealer right or not?
     
  2. blakescape

    blakescape LawnSite Member
    Posts: 108

    You always engage the pto at half throttle and disengage at full throttle. Its the same for mowers and leaf vacuums. There is twice as much torque and rpm's being applied to the belt when it is engaged at full throttle. I have had mowers for 5 years without any problems with belts breaking.
    The belt was either defective or to pto is out of alignment, ie too much tension is being applied to the pto belt. I would ask the dealer to replace the belt (assuming it broke within a day or two of purchasing).
     
  3. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    Your dealer is right on the money. When a belt just snaps, it's because of shock load.

    When you engage the PTO at say 1/2 throttle, there is far less slip in the clutch and a MUCH higher shock load is placed on the belt. This load can exceed the tensile strength of the belt

    You've got the guys that will argue that it's hard on the clutch… well, yes, the clutch is forced to slip more but how many clutches have you seen that were replaced because of worn out friction material? Not many.

    With the high cost of deck belts, snap a few and that's the same as the cost of a new clutch.

    I actually had a guy stop in the shop earlier this season complaining that he was going through deck belts every 20 to 30 hours. Fortunately he had saved a couple of the broken ones. Just like you explained, they were a clean snap. I asked about how he engaged his PTO. "About 1/2 throttle" he says. I got him a new belt and told him to engage at full throttle. I saw him the other day. He thinks he has an additional 75 -100 hours on the machine/belt and believes the issue is behind him.

    Some deck drive systems just don't like being engaged at less than full throttle while other systems will tolerate it.

    There are a lot of contributing factors to this issue.
    Deck/blade size, clutch size and health, static belt tension, idler spring rate, idler pulley travel, belt construction/cord material....

    I don't think there is any single "right" answer to the question "At what RPM should I engage the blades?" Find what works best for your machine and stick with it.

    Clearly your machine is not happy with the blades being engaged at 1/2 throttle. Engage them at full throttle. You may be pleasantly surprised at how long your belt lasts.

    Listen to your dealer. His living depends on knowing the product he sells and treating you right. Heed his advise.

    Q
     
  4. Jimslawncareservice

    Jimslawncareservice LawnSite Platinum Member
    from mn
    Posts: 4,134

    How come so many manufactures recommend and write by the pto to engage at half throttle? Just a question, not a smart azz remark
    Posted via Mobile Device
     
  5. watsmi57

    watsmi57 LawnSite Senior Member
    Posts: 329

    I think it depends on your make and model mower. According to the manual for my 252 you are to engage blades at the 3/4 throttle positon. Disengage at full throttle.
     
  6. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    Fair question.

    Honestly, if I were to speculate, I'd say because they don't know any better and don't have the data to back up the claim. I feel like it's a left-over from days gone by.

    I have to admit, without much thought, 1/2 throttle makes loads of common sense to me. 1/2 throttle with a manually engaged PTO also makes a lot of sense.

    We live in the age of "spec wars". Engines are getting bigger, clutches are getting stronger. Now we've got these monster 999cc engines equippted with 250-350 ft/lb clutches powering the same ol' deck sizes. Imagine the forces they can produce when there is no slip in the clutch....

    Q
     
  7. Pietro

    Pietro LawnSite Senior Member
    Posts: 857

    I engage/disengage at less than half throttle. Out of 3 ZTR's we've changed 1 deck belt....it lasted 1500 hrs. A clutch is a clutch. If you have manual trans in a car and dump the clutch at 6500 rpm it's going to cause more stuff to break....ease it in at 2500 and it will last longer. My men and I keep the throttle below half and have had lots of luck. One z has 2500 or so hrs and it's on belt #2...the others are approaching 1000 hrs....both original belts.
    Posted via Mobile Device
     
  8. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    Don't miss the point.

    What I am suggesting is that, in effect, by engaging the clutch at full RPM you ARE "easing it in" by creating some clutch slip.

    In some cases, at a lower RPM there is not as much or no clutch slip and a higher load is applied to the belt.

    Your car analogy is not a good one for this situation. With an electric PTO clutch, there is no way to "ease it in". There is no way to control the clamping force over time. You apply power to the coil, the clutch energizes, snaps together and holds with all its got.

    I'm not saying what you're doing is wrong. Obviously it’s working for you. Don't change it. It just can't be assumed that the same will work for every machine.

    Q
     
  9. american dream

    american dream LawnSite Senior Member
    Posts: 294

    I have always gone by the owners manual and never any belt problems.
    Posted via Mobile Device
     
  10. Mickhippy

    Mickhippy LawnSite Platinum Member
    Posts: 4,250

    I think engaging at half or so (at least not full) revs is the go. That shock on the belt should be taken up by the deck idler. Reduced revs is to minimise wear on the clutch, not to help the belt.
    Ive been told to engage at lowest revs, just enough to NOT stall the engine. I engage at higher than that though.

    At 30hrs I'd say the belt was faulty unless there was a stick or rock mixed up in it. It happens!
    Try a different belt like (brand) Hi Power or something.
     

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