SCDolphin said:
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.