There are many varieties of U-Joints, some of which are extremely complex. The easiest category known as Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.

U-joints are available with two hub styles solid and bored. Sturdy hubs don’t have a machined hole. Bored hubs have got a hole and are called for the hole form; round, hex, or sq . style. Two bored types 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 lead to burned trunnions.
Unless or else recommended, use a superior quality E.P. (extreme pressure) grease to provider most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by by using a telescoping shaft (U Joint square shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-formed (a yoke) and having a hole (vision) radially through the attention that is linked by a cross. They let larger angles than versatile couplings and are being used in applications where huge misalignment must be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too large.
Can be the effect of 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 are not aligned. If the bearings prevent rolling, they stay stationary and will “beat themselves” in to the surface 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 will indicate the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be the effect of a frozen slip are often evident on leading and back surfaces of the cross trunnion.
Improper torque about U-bolt nuts can 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.