On May 12, 11:00�am, "Karl Townsend"
wrote:
I'm repairing the same issue on my airblast sprayer for about the fifth
time. Own something long enough and the same problems keep repeating. the
sprayer has a vibration in it at 2500 tractor RPM, 540 PTO RPM. The
vibration is not like a tire out of balance but its a bucking forward to
reverse.

Universal joints are widely used to transfer rotational motion
through shafts which are not perfectly aligned. A feature of these
devices is that the output shaft does not rotate in precise synch with
the input shaft. There is a cyclic "lead" and "lag" introduced by the
mechanism. A nice explanation of this phenomenon is presented in the
wikipedia posting on universal joints. Use google with search words
"universal joint" and pick up on the Wikipedia link.
In practice, universal joints are used in pairs, aligned so that
the cyclic lead and lag introduced by the first joint is cancelled out
by by the second joint. This correcting action is dependent on having
the two joints mounted in phase with each other, and this is why many
setups use a rectangular telescoping shaft to connect the two joints.
That way, it is impossible to get them connected out of phase. If
mounted incorrectly, a pair of U-joints can aggravate the lead-lag
problem rather than correct it.
The symptom described in the case of Karl's sprayer is the sort
of pulsing torque which would be introduced by either a single U-joint
or a pair of U-joints which got mounted out of phase. My belated
input to this discussion is a suggestion that Karl to grab a cool
refreshment and take a reflective look at his setup. Are the U-joints
in phase? (Refer to the wiki photo to see the right way.) Are the
PTO shaft and sprayer shaft set up to be parallel, and not too far out
of alignment? With power off, rotate the shaft by hand to feel for
increased resistance at some point in a complete rotation.

Pat