There are many types of U-Joints, a few of which are extremely 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. Sound hubs do not have a machined hole. Bored hubs have a hole and are named for the hole shape; round, hex, or sq . style. Two bored variations that deviate from these prevalent shapes are splined, which have longitudinal grooves inside bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.

Using the incorrect lube can cause burned trunnions.
Unless normally recommended, use a high quality E.P. (intense pressure) grease to service most vehicular, industrial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by utilizing a telescoping shaft (square shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-shaped (a yoke) and having a hole (attention) radially through the attention that is linked by a cross. They let larger U Joint angles than adaptable couplings and are used in applications where excessive misalignment should be accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident at all 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 travel 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 end rolling, they remain stationary and can “beat themselves” in to the area of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each and every time the travel shaft tries to shorten, the strain will be transmitted in to the bearings and they’ll tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be caused by a frozen slip are constantly evident on the front and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts could cause brinelling.
Most manufacturers 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.