There are many varieties of U-Joints, a few of which are very complex. The simplest category referred to as Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.
U Joint china U-Joints can be found with two hub models solid and bored. Sound hubs do not have a machined hole. Bored hubs have got a hole and so are named for the hole shape; round, hex, or sq . style. Two bored styles that deviate from these prevalent shapes are splined, which have longitudinal grooves in the 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 in any other case recommended, use a superior quality E.P. (intense pressure) grease to provider most vehicular, industrial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by using a telescoping shaft (sq . 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 (eyesight) radially through the attention that is connected by a cross. They allow larger angles than flexible couplings and are used in applications where excessive misalignment should be accommodated (1 to 30 degrees).
Always make sure fresh, fresh grease is evident at all four U-joint seals.
Can be caused by operating angles which are too big.
Can be caused by a bent or perhaps sprung yoke.
Overloading a drive shaft could cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears aren’t aligned. If the bearings prevent rolling, they remain stationary and can “beat themselves” into the surface of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Every time the travel shaft attempts to shorten, the load will be transmitted into the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be the effect of a frozen slip are often evident on leading and back areas of the cross trunnion.
Improper torque in U-bolt nuts could cause brinelling.
Most suppliers 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.