Redbug said:
So, in real life what's the bottom line here? Leave the clutch set for the max rpm and load of 540...even if you are running the bush hog at a lower rpm most of the time? Or let up on the clutch bolts a little and run it?
I'm going to make one final post in this maelstrom of a thread. I'm one of those doubting Thomas's that has to try things for myself. I refuse to take someones word for something if I doubt their findings. I've tried adjusting slip clutches to tailor them to my needs. I trashed one and needed a rebuild for another to convince myself of their lack of adjustability.
THEOREM. a preposition that can be proved from accepted premise, law or principal.
FACT. A thing that has actually happened or is true.
We can draw conclusions based on an engineering theorem, or we can actually trash a few slip clutches and determine the facts.
Slip clutches would seem to be adjustable by tightening or loosening the preload bolts/springs. And to a certain extent we can. Problem is, they don't adjust on an accurate sliding scale like we think they should. At least they aren't accurately adjusted beyond a very narrow scope.
Mowers are built to withstand a certain amount of abuse. Same applies to the tractor powering that mower. A slip clutch is designed to limit shock loads when that amount of abuse is approached or surpassed. Shock loads less than that SHOULD BE tolerated by tractor and mower. That's where the hp rating of the gearbox and drive shaft come in to play. Their rating SHOULD BE ample to withstand those shock loads below the slip clutch's rating. (IF....if properly spec-ed) Now we all know that SHOULD BE isn't a definate. But engineers SHOULD BE sharp enough to adaquately design the drive shaft, the gear box, the tractors pto system, and even those poor blades that take the lions share of abuse.
When you operate at blade speeds generated below the rated pto rpms, the shock loads SHOULD BE less than when the blades strike something at full rated speed. Very simple premise. (Hit a tree with a car @ 25 mph. Hit the same tree with the same car @ 40 mph. Which speed does greatest damage? Which impact has to deal with the greatest energy? at which of the 2 speeds does the bumper recieve the greatest shock load?)
The slip clutch is there to handle shock loads greater than the rest of the package can handle. It's the bumper and air bag all rolled into one.
I deliberately destoyed a slip clutch. I found a new one on EBAY for a paultry sum of money and decided to experiment. The older clutch was still functional, but getting old. The u-joint attached to the slip clutch was failing. The new one came with a new u-joint. I had a perfect opportunity to see just what would happen with an adjustment to the preloading. The plan was to loosen the preload at 1/4-turn increments. I didn't count on the clutch going up in smoke on the very first adjustment, but it did. After that, even by TIGHTENING the preload several turns, it wouldn't hold up to normal use. It did BEFORE my experiment. (6' King Kutter tiller in reasonably friable soil) It was trashed by a very slight adjustment. (some consideration must be given to the fact that it was a used slip clutch that had seen a normal amount of "abuse" in its life prior to my little experiment)
A very true statement was made in an above post. You DO NOT want the clutch to slip even a small amount while in what would be considered normal use. It should only slip when shock loads are exceeding the capacity of any of the major components in the chain. This would tell me that you shouldn't exceed the tractors pto hp by a vast margin on the gearbox/driveshaft/slip clutch rating, in spite of the "common sense" idea that doing so would provide a saftey net of sorts. In other words, match all components as closely as possible with mower/tiller/ect rating slightly above tractors pto hp rating.
In the owners manual for all 3 of my tractors, it says "RECOMMENDED ENGINE RPM's for 540 rpm pto operation" , NOT "REQUIRED ENGINE SPEED". That tells me there is some leeway in how fast you power a given implement from the tractors point of view. In the manual for my Bush Hog mowers and the Woods mower it says "SUGGESTED PTO RPM's". I know the difference between a good cut and a poor cut with a mower. If it does the job right, and doesn't do any damage, why not take it easy on the equipment if and when you can?
With tractors of 36 years of age and 29 years of age that have been used in the above manner and have ZERO ill effects to show for it, along with very good results from the mowers used on those tractors, WHY NOT TAKE IT EASY ON THE EQUIPMENT WHEN YOU CAN?
I rest my case......
