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  1. #1
    Gold Member SCDolphin's Avatar
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    Kubota L5240: Craftsman GT6500

    Default Correct PTO RPM to Bush Hog

    I was told by a friend that when bush hogging to run at 540 PTO rpm's (2600 engine rpms) because if I hit something the slip clutch may not actually slip at a lower rpm. I am new at this but the cutter, a Woods BB720, cuts just fine running at 300-400 rpm (1500-1800 engine rpm's).

    Any advice?? Since I just have a few hours on the tractor and I was trying to vary the rpm's and not run it near wide open.

    Thanks very much
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  2. #2
    Super Member JerryG's Avatar
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    MF 1440-4 PowerShuttle

    Default Re: Correct PTO RPM to Bush Hog

    You can set the clutch up either way. If your dealer didn't for sure adjust the clutch right before it was delivered, then you need to set it again. Even if the dealer said that they set it, I still would do it myself, because they might have set it to tight. Then it would be just like you didn't have one. I think that a slip clutch is much better than shear pins but they have to be checked and set before each season and also if the mower has seat for any amount of time in between uses. The discs and plates will stick and or rust and then the clutch won't slip and it can damage the PTO shaft, the PTO in the tractor, bust the U-joints, damage the mower gear box or damage the gears in the tractor. The owners manual should tell you how to set the slip clutch.
    That is a good looking mower. I need to buy a new larger mower and that is the model that I have in mind. I have an old Woods mower and I bet the thing has mowed several thousand acres. One thing that I really like about the Woods mowers that no one else has is the quick change blade system. It's nice to be able to change the blades with an 11/16 socket on a 3/8 ratchet instead of a huge 2.25" socket and a 6' cheater bar.

  3. #3
    Gold Member
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    Kubota L3400 HST

    Default Re: Correct PTO RPM to Bush Hog

    My bushhog manual says that it should be run at 540 RPM. I would definitely adjust the slip clutch. It's rather easy. Loosen up the bolts. Tighten them down to when you just feel that you have some engagement. Start up the tractor with the bush hog. If it slips at all when you engage the PTO, tighten the bolts a turn to turn and a half and repeat. If it doesn't, tighten the bolts a turn to turn and a half and stop. This method keeps me out of too much trouble.

  4. #4
    Super Member Farmwithjunk's Avatar
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    Default Re: Correct PTO RPM to Bush Hog

    Quote Originally Posted by SCDolphin
    I was told by a friend that when bush hogging to run at 540 PTO rpm's (2600 engine rpms) because if I hit something the slip clutch may not actually slip at a lower rpm. I am new at this but the cutter, a Woods BB720, cuts just fine running at 300-400 rpm (1500-1800 engine rpm's).

    Any advice?? Since I just have a few hours on the tractor and I was trying to vary the rpm's and not run it near wide open.

    Thanks very much
    Good question.... This MAY just get to be a topic of much discussion before it's over. I'd bet there'll be a few varying theories on this one.

    A slip clutch is INTENDED to slip (if properly maintained and adjusted) at a certain shock load/torque load. The mower itself SHOULD be capable of withstanding any shocks that are below the rated point of the slip clutch IF...IF the clutch is properly spec-ed for the mower's rating. If you've got the blades spinning at a lower speed when they hit something, the load is lessened somewhat, supposedly well below the mowers ability to cope with the impact. In that regard, the clutch might not slip, but then, potential for damage may be less also.

    A slip clutch is intended to protect the mowers gearbox, the drive shaft, and to a certain extent the tractors pto. They won't do much to protect the blades. By the time the clutch starts to slip, the blades have already made their "impact" on whatever it is that's offering up the resistance. The clutch won't provide "retro-active protection" to the blades.

    Slip clutches are "rated" (designed) for a certain input of peak horsepower as well as what resistance they'll start slipping at. There is no "constant" as far as what input hp will be applied, as the manufacturer has no clue what sort of tractor (size/hp) their mower will end up hooked to once it leaves the factory. They recommend a maximum as well as a minimum hp, but there's generally a spread between top and bottom of the power range. The specs on a slip clutch adjustment is USUALLY determined by the PEAK input hp rating.

    So... Buy a mower that has a gearbox (and a slip clutch) that's rated for 60 hp, and hang it on your 45hp tractor, and you already have the situation you described with less hp being applied. Then factor the lower RPMs of a slower running engine/pto, and you may well be WAY below the force needed to make the clutch slip.

    Now... When I first bought my KK tiller a few years back, I was working in some very rocky conditions, along with tree roots and a few stumps. I got the bright idea I could reduce the preload on the slip clutch by just a tick and make it slip that slight bit easier. WRONG! Simply turning the adjustment nuts 1/4-turn each to a lighter setting, the clutch went up in smoke in a matter of a few minutes (on a tractor with nearly 20HP less than maximum hp rating of the tiller/slip clutch) From that result, I came to the conclusion that slip clutches are designed to operate at a given setting (preload) and just plain don't work at anything different. (More OR less preload)

    How does all this effect your original question? I don't know for sure. But consider this. If I was mowing in conditions that would make a slip clutch "do its thing", I'd feel more comfortable with rpms turned down a notch, provided the tractor has enough power and the mower was cutting to your satisfaction.

    Finally, Jerry was spot on with his comments regarding adjusting and servicing a slip clutch. The longer they go without slipping, the more likely they'll "freeze" where they don't slip when they're supposed to. At regular intervals, it's a very sound practice to loosen the preload bolts/springs, force the clutch to slip a bit, then re-tighten to spec. You have a much better chance of the slip clutch working when it should that way.
    There are three kinds of men;
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  5. #5
    Super Member greg_g's Avatar
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    Default Re: Correct PTO RPM to Bush Hog

    Determine the PTO horsepower of your tractor, then adjust the slip clutch accordingly. There should be a chart in the owner's manual. The reason you match the clutch settings to the PTO horsepower is a matter of torque. During normal operation, the specified setting permits normal rotation of the shaft. When more than the specified torque is required to turn the mower blades - such as in heavy grass or when striking an immovable object - the cluch will slip, preventing damage to either the tractor or the mower. It's important to understand that those torque numbers are based upon the manufacturer-recommended PTO speed. Mowing at any other PTO speed may defeat the purpose of having a slip clutch in the first place. In your case that specific cutter has a 120hp transmission. If the clutch doesn't slip at the correct moment, excess torque transfer could tear up your 45hp PTO in a heartbeat.

    Then mow at 540 PTO rpm. Besides the slip clutch factor, any speed other than recommended by the manufacturer is at best inefficient - and can be costly in the long run.

    //greg//
    USN (Ret)
    Former Chinese tractor owner (x4)
    Current John Deere owner

  6. #6
    Veteran Member have_blue's Avatar
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    Default Re: Correct PTO RPM to Bush Hog

    Quote Originally Posted by SCDolphin
    I was told by a friend that when bush hogging to run at 540 PTO rpm's (2600 engine rpms) because if I hit something the slip clutch may not actually slip at a lower rpm. I am new at this but the cutter, a Woods BB720, cuts just fine running at 300-400 rpm (1500-1800 engine rpm's).

    Any advice?? Since I just have a few hours on the tractor and I was trying to vary the rpm's and not run it near wide open.
    RPM has nothing to do with the torque at which your clutch will slip. If your slip clutch is set correctly, it will slip as it is supposed to at any reasonable RPM. I tested mine the first time I used it. I scratched some reference marks on the clutch plates, just to see if my clutch would slip. I soon hit a fire ant nest, and stopped and checked it. Sure enough, it had slipped. I got into the habit of checking it every time I grease the U-joint, and it has slipped a little almost every time.

    BTW, 540 RPM is a maximum for implements rated at the same HP that your PTO HP is rated. Since it takes 4X more horsepower to turn a mower blade 2X faster, it's much more efficient to cut at the lowest RPM you possibly can. If your blades are sharp, you can back off on the PTO RPM and save a lot of fuel and still make very nice cut. A bonus is, your blade's edges lasts MUCH longer at lower RPM. If you are cutting heavy brush that's near the cutter's maximum rating, you should keep the PTO RPM on maximum. You need all the oomph you can get to blast the heavy stuff.
    -

    Bob

    From the heart of Cajun Country

  7. #7
    Super Star Member EddieWalker's Avatar
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    Default Re: Correct PTO RPM to Bush Hog

    One thing that I've noticed is that I get a much better cut with the blades spinning at the proper speed. 2,600 RPM is about right and where I run mine, give or take a few RPM.

    Any slower and the blades sort of tear through the grass, or get bogged down real quick. When I get into some extra tall or thick grass, I slow down the tractor speed, but always keep the RPM's up.

    Anther consideration is where does your tractor reach it's peak HP? Probably at the proper speed for bush hogging. If you are running your engine RPM's too low, you are putting undue stress on the engine. Run the engine at peak HP and it's more efficient and working as designed.

    Good luck,
    Eddie


    PS. I just read Bob's reply and I'm editing mine. Bob knows more then I do, so I'd listen to him over me. I'm still gonna run my bush hog at full RPM's, but I can see his line of reasoning too.

  8. #8
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    Default Re: Correct PTO RPM to Bush Hog

    You should run it at 540
    rotary cutters work thru speed of the big heavy blade, not sharpness of the blade.
    And if you don't have that speed, it's just not going to give a good cut, nor handle it. They are designed to run at 540 for YEARS, the only thing you are stressing is your motor by making it work harder.

    let the speed of the blade do the work.

  9. #9
    Gold Member SCDolphin's Avatar
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    Kubota L5240: Craftsman GT6500

    Default Re: Correct PTO RPM to Bush Hog

    The Woods manual does not use an amount of torque but apparently tighten to a certain gap 1/16 inch as I recall. From a novice prospective I like the way the Bush Hog brand has a liftable cover vs the Woods plastic cover. I have not had time to look at it closely yet but will have some time this week to play with it. I spent the first few days moving an internal baffle so I hope the clutches are ok.
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    6 Foot Linebaugh landscape rake
    Down2Earth 14k trailer
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  10. #10
    Super Member Farmwithjunk's Avatar
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    Default Re: Correct PTO RPM to Bush Hog

    Over the 40+ years I've been using my own tractors to bush hog with, I've worn out several mowers from hundreds, maybe even thousands of hours of use. I've yet to see any harm, and have seen plenty benifit from not running at full 540 rpms while mowing if running a few rpm slower gets the job done. It's a waste of fuel, and arguably a waste of the engines life to run harder than is needed. The results don't suffer, and the tractor lives longer. If conditions require full 540 rpms on the pto, that's what I do. If conditions allow to save a little for another day, that's the best course of action. If it was harmfull to run at slower engine speeds or slower blade speeds when that works, explain to me why I have tractors that have been with me and on the job longer than most of you guys have been on this earth and they still run like the day they were new, considering how I choose to use them.

    It is a waste to run a piece of equipment harder than it needs to be ran unless you're simply trying to wear it out as fast as you can.
    There are three kinds of men;
    1.) The ones that learn by reading
    2.) The few who learn by observation
    3.) The rest of them have to pee on the electric fence for themselves.

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