There are many types of U-Joints, some of which are incredibly complex. The easiest category named Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub variations solid and bored. Solid hubs don’t have a machined hole. Bored hubs currently have a hole and are named for the hole condition; round, hex, or sq . style. Two bored variations that deviate from these common 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 bring about burned trunnions.
Unless otherwise recommended, use a high quality E.P. (serious pressure) grease to assistance most vehicular, industrial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding motion between two flanges that will be fork-formed (a yoke) and having a hole (attention) radially through the attention that is connected by a cross. They allow larger angles than flexible couplings and are being used in applications where high misalignment needs to be accommodated (1 to 30 degrees).
Always make sure fresh, fresh grease is evident by all four U-joint seals.
Can be caused by operating angles which are too large.
Can be caused by a bent or sprung yoke.
Overloading a drive shaft could cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears aren’t aligned. If the bearings prevent rolling, they continue 
to be stationary and can “beat themselves” into the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each and every time the travel shaft attempts to shorten, the load will be transmitted into the bearings and they will tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are caused by a frozen slide are at all times evident on the front and back floors of the cross trunnion.
Improper torque about U-bolt nuts can cause brinelling.
Most suppliers publish the recommended torque for a U-bolt nut.
Improper lube U Joint china procedures, where recommended purging isn’t accomplished, can cause one or more bearings to be starved for grease.