There are many types of U-Joints, some of which are extremely complex. The easiest category called Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.

U-joints are available with two hub models solid and bored. Solid hubs do not have a machined hole. Bored hubs possess a hole and so are called for the hole condition; round, hex, or sq . style. Two bored styles that deviate from these common 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 bring about burned trunnions.
U Joint Unless usually recommended, use a superior quality E.P. (intense pressure) grease to service most vehicular, industrial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that are fork-shaped (a yoke) and having a hole (attention) radially through the attention that is linked by a cross. They allow larger angles than versatile couplings and are used in applications where large misalignment has to be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too big.
Can be the effect of a bent or sprung yoke.
Overloading a travel 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 end rolling, they continue to be stationary and will “beat themselves” into the surface of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Each time the travel shaft attempts to shorten, the strain will be transmitted in to the bearings and they’ll mark the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be the effect of a frozen slide are generally evident on leading and back surfaces of the cross trunnion.
Improper torque about U-bolt nuts can cause brinelling.
Most manufacturers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause a number of bearings to be starved for grease.