There are many varieties of U-Joints, some of which are extremely complex. The easiest category U Joint called Cardan U-Joints, are either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub styles solid and bored. Sound hubs don’t have a machined hole. Bored hubs have a hole and are called for the hole shape; round, hex, or square style. Two bored styles that deviate from these prevalent shapes are splined, that have longitudinal grooves within the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.
Using the incorrect lube can cause burned trunnions.
Unless otherwise recommended, use a superior quality E.P. (severe pressure) grease to services most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that happen to be fork-shaped (a yoke) and having a hole (eye) radially through the eye that is connected by a cross. They enable larger angles than flexible couplings and are being used in applications where substantial misalignment should be accommodated (1 to 30 
degrees).
Always make sure new, fresh grease is evident for all U-joint seals.
Can be caused by operating angles which are too large.
Can be the effect of a bent or perhaps sprung yoke.
Overloading a drive shaft can cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears aren’t aligned. If the bearings quit rolling, they remain stationary and can “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Every time the drive shaft tries to shorten, the strain will be transmitted into the bearings and they will indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be the effect of a frozen slide are constantly evident on leading and back areas of the cross trunnion.
Improper torque in U-bolt nuts can cause brinelling.
Most companies publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause one or more bearings to be starved for grease.