There are many types of U-Joints, some of which are extremely complex. The easiest category referred to 
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. Solid hubs do not have a machined hole. Bored hubs have got a hole and are named for the hole shape; round, hex, or sq . style. Two bored designs that deviate from these prevalent shapes are splined, that have longitudinal grooves within the bore; and keyed, which have keyways to avoid rotation of the U-joint on the matching shaft.
Using the incorrect lube can cause burned trunnions.
Unless usually recommended, use a high quality E.P. (serious pressure) grease to service most vehicular, industrial 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 motion between two flanges that are fork-formed (a yoke) and having a hole (attention) radially through the eye that is connected by a cross. They allow larger angles than adaptable couplings and are used in applications where substantial misalignment must be accommodated (1 to 30 degrees).
Always make sure new, fresh grease is evident in all 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 drive shaft could cause yoke ears to bend. U Joint china 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 and every time the travel shaft tries to shorten, the load will be transmitted into the bearings and they’ll tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be the effect of a frozen slide are constantly evident on leading and back areas of the cross trunnion.
Improper torque on U-bolt nuts could cause brinelling.
Most suppliers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause a number of bearings to be starved for grease.