The problem with a single U-joint is the vibration mentioned earlier...as power at a constant speed (lets say 600 rpm) is transmitted through a Hooke's joint (U-joint) an angle, the speed on the other side will vary, with two angular accelerations and two angular decelerations per revolution (590 to 610 rpm, in this example). Now if there is a typical pto shaft (Cardan shaft), with a U-joint on each end, the deflection angle the same at each end and in proper "phase", the speed variations will be damped out and the output will be a constant 600 rpm in the above example. Being "in phase" means that the yokes at each end of the intermediate shaft are in the same plane...if the yokes are even one spline tooth (at the slip joint) out of phase, heavy vibration, short cap bearing life, etc., will be there to plague you.
Each Cardan shaft also has a "critical" speed, which is determined by intermediate tube wall thickness, and length of the tube.
Another little often-ignored detail on these devices is that if run straight-through, with no deflection, the cap bearings will Brinnell and fail.
There are at least four types of CV joints...Citroen, Rzeppa, Bendix and a "double-U-Joint". The Bendix type is seldom seen, unless you have an old Vietnam era duece-and-a-half or 5-ton Army truck (anyone ever replaced a steering axle seal on an Army 2 1/2 ton?). A Rzeppa joint is usually on the outboard (steering) joint on a modern front-drive auto, with a Citroen joint (3 cam followers and a tulip) inboard, next to the transmission. A "double-U-joint" is a back-to-back set of U-joints.
I have seen heavy industrial Cardan shaft units run at 1200 rpm, although the cap bearings had short life (about 18 months). I have also seen a failed Cardan shaft on the back side of a paper machine...talk about a nasty "radius-of-destruction"...
Suggest you seriously look at a Lovejoy (spider-type) coupling if you are using direct drive.
Swedgemon
GM-90 6/1
Somewhere in Kentucky