There are many varieties of U-Joints, a few of which are incredibly complex. The simplest category referred to 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. Sound hubs do not have a machined hole. Bored hubs include a hole and so are known as for the hole form; round, hex, or square style. Two bored variations that deviate from these common shapes are splined, that 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 lead to burned trunnions.
Unless or else recommended, use a superior quality E.P. (extreme pressure) grease to assistance most vehicular, commercial and auxiliary drive 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 movement between two flanges that are fork-formed (a yoke) and having a hole (eyes) radially through the attention that is connected by a cross. They enable larger angles than flexible couplings and are used in applications where excessive misalignment has to be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too large.
Can be caused by a bent or perhaps sprung yoke.
Overloading a travel shaft can cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears aren’t aligned. If the bearings stop rolling, they stay stationary and will “beat themselves” into the area of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each time the drive shaft attempts to shorten, the strain will be transmitted in to the bearings and they will tag the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that happen to be the effect of a frozen slip are at all times evident on the front and back areas of the cross trunnion.
Improper torque about U-bolt nuts can cause brinelling.
Most U Joint makers 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.