There are many varieties of U-Joints, a few of which are extremely complex. The simplest category named Cardan U-Joints, are either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub types solid and bored. Sturdy hubs do not have a machined hole. Bored hubs include a hole and are called for the hole form; round, hex, or sq . style. Two bored designs that deviate from these prevalent shapes are splined, that have longitudinal grooves in the bore; and keyed, which have keyways to avoid rotation of the U-joint on the matching shaft.

Using the incorrect lube can result in burned trunnions.
Unless usually recommended, use a high quality E.P. (excessive pressure) grease to support most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-formed (a yoke) and having a hole (eyesight) radially through the attention that is connected by a cross. They enable larger angles than adaptable couplings and are used in applications where excessive misalignment has to be accommodated (1 to 30 degrees).

Always make sure new, fresh grease is evident for all 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 drive shaft could cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears aren’t aligned. If the bearings end rolling, they remain stationary and will “beat themselves” into the area 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 load 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 the effect of a frozen slide are U Joint generally evident on the front and back floors of the cross trunnion.
Improper torque upon U-bolt nuts could cause brinelling.
Most 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.