There are many varieties of U-Joints, some of which are very complex. The easiest category referred to as Cardan U-Joints, happen to be 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 are known as for the hole condition; round, hex, or square style. Two bored models that deviate from these common 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 wrong lube can result in burned U Joint china trunnions.
Unless in any other case recommended, use a high quality E.P. (intense pressure) grease to assistance most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding movement between two flanges that happen to be fork-shaped (a yoke) and having a hole (vision) radially through the eye that is connected by a cross. They allow larger angles than flexible couplings and are being used in applications where substantial misalignment must be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too big.
Can be the effect of a bent or sprung yoke.
Overloading a travel shaft can cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings quit rolling, they remain stationary and will “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each and every time the travel shaft attempts to shorten, the strain will be transmitted in to the bearings and they will mark the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be the effect of a frozen slip are at all times evident on the front and back floors of the cross trunnion.
Improper torque upon U-bolt nuts can 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.