There are many varieties of U-Joints, some of which are very complex. The easiest category known as Cardan U-Joints, are either block-and-pin or bearing-and-cross types.

U-joints are available with two hub styles solid and bored. Solid hubs do not have a machined hole. Bored hubs include a hole and so are named for the hole shape; round, hex, or square style. Two bored variations that deviate from these prevalent shapes are splined, which have longitudinal grooves inside the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can bring about burned trunnions.
Unless in any other case recommended, use a high quality E.P. (extreme pressure) grease to services most vehicular, industrial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by utilizing a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that happen to be fork-designed (a yoke) and having a hole (attention) radially through the attention that is linked by a cross. They enable larger angles than adaptable couplings and are used in applications where huge misalignment needs to be accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident in all U-joint seals.

Can be caused by operating angles which are too big.
Can be the effect of a bent or U Joint sprung yoke.
Overloading a drive shaft could cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears are not aligned. If the bearings stop 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. Each and every time the drive shaft tries to shorten, the load will be transmitted into the bearings and they will tag the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that happen to be caused by a frozen slip are constantly evident on leading and back surfaces of the cross trunnion.
Improper torque in U-bolt nuts could 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.