"These arrangements have a problem with the telescopic shaft either bottoming out or coming apart during certain conditions. Specifically, the telescopic shaft can bottom out when the tractor pitches up and/or rolls toward the implement when the tractor is turning and can separate when the tractor pitches down and/or rolls away from the implement when the tractor is travelling more or less straight ahead."
Patent US5427192 - Sliding pedestal shaft for PTO driven ...
http://www.google.com/patents/US5427192

"In a draft implement having an input power driveline and a tongue adapted to be articulately connected to a tractor, an arm is pivotally mounted on the tongue and supports the driveline rearwardly of a forward telescoping assembly of the driveline which includes forward and rear universal joints. Centering tension springs extending along the tongue and interconnecting the pivotal arm and the tongue maintain the arm in a centered position in which the rear universal joint is positioned the same distance rearwardly of the tractor/implement articulate connection point as that at which the forward universal joint is positioned forwardly of the connection point. The springs will yield to allow the arm to pivot laterally to either side of the tongue to accommodate bottoming out of the telescoping assembly during sharp turns of the tractor relative to the implement. Once the tractor is re-articulated out of the sharp turn relative to the implement, the springs return the arm to its centered position."
http://www.google.com/patents/US4020913

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Note added at 4 hrs (2015-01-04 15:34:50 GMT)
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"PTO afjustement
Make sure the drive shaft length is compatible with the different working positions of the implement. If the PTO shaft is too long it might bind and damage may occur to unit /
PTO / tractor. In the collapsed position, the driveline should be approximately 2” from bottoming out."
e it is dynamically balanced at, 3000 RPM for Light ... length of the driveshaft.
[PDF]PTO Adjustment - Newton Crouch, Inc.
http://www.newtoncrouch.com/files/faq/pto_adjustment.pdf