There are many varieties of U-Joints, some of which are extremely complex. The simplest category known as Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.
U-joints are available with two hub variations solid and bored. Sound hubs do not have a machined hole. Bored hubs currently have a hole and so are named for the hole condition; round, hex, or square style. Two bored models that deviate from these prevalent shapes are splined, that have longitudinal grooves inside 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 otherwise recommended, use a superior quality E.P. 
(intense pressure) grease to program most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply by using a telescoping shaft (square shafting or U Joint splines). U-Joints function by a sliding action between two flanges that happen to be fork-shaped (a yoke) and having a hole (attention) radially through the attention that is connected by a cross. They let larger angles than versatile 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 in all four 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 won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings end rolling, they remain stationary and will “beat themselves” in to the surface area of the cross.
A “frozen” slip assembly won’t allow the drive shaft to lengthen or shorten. Each and every time the drive shaft attempts to shorten, the strain will be transmitted in to the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be caused by a frozen slip are usually evident on leading and back areas of the cross trunnion.
Improper torque in U-bolt nuts can cause brinelling.
Most producers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause a number of bearings to be starved for grease.