There are many types of U-Joints, some of which are very complex. The easiest category called 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. Solid hubs do not have a machined hole. Bored hubs possess a hole and so are known as for the hole condition; round, hex, or sq . style. Two bored variations that deviate from these prevalent shapes are splined, which 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 normally recommended, use a superior quality E.P. (extreme pressure) grease to program most vehicular, industrial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding motion between two flanges that happen to be fork-shaped (a yoke) and having a hole (vision) radially through the attention that is connected by a cross. They enable larger angles than versatile couplings and are used in applications where excessive misalignment should be accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident for all four U-joint seals.

Can be due to operating angles which are too big.
Can be the effect of 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 are not aligned. If the bearings quit rolling, they stay stationary and can “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each time the travel shaft tries to shorten, the load will be transmitted in to the bearings and they will tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are the effect of a frozen slip are always evident on the front and back surfaces of the cross trunnion.
Improper U Joint torque in U-bolt nuts could cause brinelling.
Most makers 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.