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 styles solid and bored. Solid hubs don’t have a machined hole. Bored hubs currently have a hole and are called for the hole condition; round, hex, or square style. Two bored variations that deviate from these common shapes are splined, that have longitudinal grooves inside the bore; and keyed, which have U Joint china keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can cause burned trunnions.
Unless otherwise recommended, use a high quality E.P. (excessive pressure) grease to services most vehicular, commercial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that will be fork-designed (a yoke) and having a hole (vision) radially through the eye that is linked by a cross. They enable larger angles than versatile couplings and are used in applications where huge misalignment needs to be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too large.
Can be the effect of a bent or sprung yoke.
Overloading a travel shaft can 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 will “beat themselves” in to the surface area of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each and every time the drive shaft tries to shorten, the load will be transmitted in to the bearings and they will tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be caused by a frozen slide are always evident on the front and back floors of the cross trunnion.
Improper torque upon 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 one or more bearings to be starved for grease.