350u pto knock

hi all well mowing last week with a pull type brush hog my 350u made a knocking sound when turning i dont hear it with any other pto drive stuff just the the brush hog. its a 1956 with t/a and live power. only makes noise when turning a corner. what is it and do i fix it or just not use a pull type hog with it.
 
The answer is in the Universal joint connections and the geometry of the shafts. For a Ujoint to work well, it must be
paired with a second joint and phased to that joint (output yoke and input yoke on the same plane so that the shaft between
them is the element that accelerates and decelerates when they are angled). This is complex geometry, YouTube it. If the
hitch point and front Ujoint are positioned correctly, the drive shaft will have equal (or almost equal) angles when
turning. I think yours has a front joint very close to the location of the hitch pin pivot, and as a result the front joint
gets 90% of the turn angle, and the rear joint gets much less. AS a result the shaft speed variation is carried to the
mower and tries to accelerate and decelerate the mass of the mower as it spins (not good). Putting a constant velocity
joint at the front ov the shaft in place of the original front joint, or using an extended hitch plate about 12 to 14 inches
behind the existing hitch pin location, or not turning so sharp will solve the problem. I own a 350U and pull type brush
cutters. I use an extended hitch plate. Jim
 
U-Joint on the PTO shaft is likely to be in a bind when you turn because it is out of timing or your turning way to sharp.
 
Pretty much normal. As the others point out the knocking is from the rapid acceleration/deceleration of the PTO whenever the input and output shafts are not parallel, as when in a turn.

Recommend disengaging the PTO before starting a turn. This will take the knocking shock load off the tractor PTO gearing, and will greatly reduce the shock load on the cutter gearbox and PTO shaft. While you may still hear some knocking coming from the PTO shaft with the clutch disengaged, it won't be harming a bunch of expensive parts!
 
If the tongue weight of the mower is more than 300 pounds, I would use 3/4" plate. (assumes it is bolted to the Fast Hitch draw bar. Making sure the the hitch sway pin is installed in the link under the left side of the hitch. If it is loose, drill it out for a larger pin, or bolt it together to reduce side to side movement. It will be a shin damaging device like a receiver hitch. Jim
 

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