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. When it gets a little lose, in a few more hours of run it will damn near shake the tractor off the ground. Its also a natural harmonic at this RPM, slow down and the vibration goes away. Unfortunately, this REALLY reduces sprayer performance.
I'm replacing the entire PTO shaft and tightening up the clearance in the gear box again. if everything is tight, the problem is less severe. The is working on the symptom, not the cause.
This is a long shot, anybody know about balancing this sort of vibration or changing the natural harmonic frequency?