There are many types of U-Joints, a few of which are incredibly complex. The easiest category named Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub designs solid and bored. Solid hubs don’t have a machined hole. Bored hubs have a hole and so are called for the hole form; round, hex, or sq . style. Two bored types that deviate from these common U Joint china shapes are splined, that have longitudinal grooves within the bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.
Using the incorrect lube can lead to burned trunnions.
Unless normally recommended, use a high quality E.P. (serious pressure) grease to support most vehicular, professional and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-formed (a yoke) and having a hole (eyes) radially through the eye that is connected by a cross. They allow larger angles than flexible couplings and are being used in applications where huge misalignment needs to be accommodated (1 to 30 degrees).
Always make sure fresh, fresh grease is evident at all four U-joint seals.
Can be caused by operating angles which are too large.
Can be the effect of a bent or sprung yoke.
Overloading a drive shaft can cause yoke ears to bend. Bearings won’t roll in the 
bearing cap if the yoke ears aren’t aligned. If the bearings quit rolling, they stay stationary and can “beat themselves” into the area of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Every time the travel shaft tries to shorten, the load will be transmitted into the bearings and they’ll mark the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be the effect of a frozen slide are generally evident on leading and back surfaces of the cross trunnion.
Improper torque about U-bolt nuts could cause brinelling.
Most suppliers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause one or more bearings to be starved for grease.