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

U-joints can be found with two hub designs solid and bored. Stable hubs do not have a machined hole. Bored hubs own a hole and are known as for the hole form; round, hex, or square style. Two bored designs that deviate from these common shapes are splined, which have longitudinal grooves in the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can cause burned trunnions.
Unless or else recommended, use a U Joint superior quality E.P. (intense pressure) grease to services most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-shaped (a yoke) and having a hole (eyesight) radially through the eye that is connected by a cross. They enable larger angles than adaptable couplings and are used in applications where substantial misalignment needs to be accommodated (1 to 30 degrees).

Always make sure new, fresh grease is evident in all four 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 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 remain stationary and can “beat themselves” in to the surface area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Each and every time the travel shaft tries to shorten, the strain will be transmitted into the bearings and they will mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be caused by a frozen slip are generally evident on the front and back areas of the cross trunnion.
Improper torque about U-bolt nuts could cause brinelling.
Most makers 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.