There are many varieties of U-Joints, a few of which are very complex. The easiest category referred to as Cardan U-Joints, are either block-and-pin or bearing-and-cross types.
U Joint china U-Joints can be found with two hub models solid and bored. Solid hubs do not have a machined hole. Bored hubs currently have a hole and so are named for the hole form; round, hex, or square style. Two bored types that deviate from these prevalent 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 wrong lube can result in burned trunnions.
Unless otherwise recommended, use a superior quality E.P. (serious pressure) grease to support most vehicular, commercial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding action between two flanges that happen to be fork-formed (a yoke) and having a hole (eye) radially through the eye that is connected by a cross. They enable larger angles than flexible couplings and are being used in applications where high misalignment has to be accommodated (1 to 30 degrees).
Always make sure fresh, fresh grease is evident at all four U-joint seals.
Can be due to operating angles which are too big.
Can be caused by a bent or perhaps sprung yoke.
Overloading a travel 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 quit rolling, they remain stationary and will “beat themselves” into the surface area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Each time the drive 
shaft tries to shorten, the strain will be transmitted into the bearings and they’ll mark the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are caused by a frozen slide are generally evident on leading and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts can cause brinelling.
Most manufacturers 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.