There are many types of U-Joints, a few of which are very complex. The simplest category called Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.

U-joints are available with two hub types solid and bored. Stable hubs do not have a machined hole. Bored hubs have got a hole and are named for the hole form; round, hex, or sq . style. Two bored models that deviate from these common shapes are splined, which have longitudinal grooves U Joint inside the bore; and keyed, which have keyways to prevent rotation of the U-joint on the matching shaft.

Using the incorrect lube can bring about burned trunnions.
Unless normally recommended, use a high quality E.P. (extreme pressure) grease to provider most vehicular, commercial 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 are fork-shaped (a yoke) and having a hole (eyesight) radially through the eye that is connected by a cross. They allow larger angles than versatile couplings and are being used in applications where huge misalignment must be accommodated (1 to 30 degrees).

Always make sure new, fresh grease is evident by all four U-joint seals.

Can be caused by operating angles which are too big.
Can be caused by 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 are not aligned. If the bearings stop rolling, they continue to be stationary and will “beat themselves” in to the area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Every time the drive shaft attempts to shorten, the strain will be transmitted in to the bearings and they will indicate the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be caused by a frozen slide are at all times evident on the front and back floors of the cross trunnion.
Improper torque upon U-bolt nuts can cause brinelling.
Most producers 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.