Merlin wrote:George, I'm not quite following your explanation. I don't have a bush hog, but my neighbor does, but no tractor. I have a 3 point hitch on my Super A and he uses my tractor and his bush hog 3 or 4 times a year. I've never really looked at the set up very close, but there are 2 U joints on the bush hog. When he gets it hooked up, there is no way the U joints can come loose. There is no drawbar on my tractor, just top link, lift arms, and a anti sway bar. The bush hog could be considered "ridged" as far as left or right movement is concerned. He has one swivel wheel in the rear, so when he turns, the bush hog just follows along.
Merlin,
OK, I'll explain it as well as I can. Your setup doesn't put the U-joints under stress in turns like a true trailer mower does. It's still very important for the joints to be in phase.
Most PTO shafts are made with two square shafts that telescope together. There's a 50/50 chance of timing them correctly when you slide the two halves together unless you know how to do it correctly.
The individual joint's very nature is to turn the shaft at a different speed from the input shaft. The speed is constantly changing 4 cycles per revolution. Here's the fortunate part... If the joints are in phase, one joint makes the opposite correction to the speed so they cancel each other.
To be in phase, the two yokes connected to the two ends of the main shaft
MUST lie in the same plane. Simple if you know how to do it.
If the shaft isn't timed correctly, either the engine or the implement, or both, have to run "lumpy", putting everything under unnecessary stress.
With a trailer implement the two yokes go out of phase in a turn, since they are no longer parallel. In a tight turn you will notice pronounced knocking.