There are many varieties of U-Joints, some of which are extremely complex. The simplest category called Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.
U-joints are available with two hub models solid and bored. Sound hubs do not have a machined hole. Bored hubs include a hole and are known as for the hole shape; round, hex, or sq . style. Two bored styles that deviate from these common 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 bring about burned trunnions.
Unless in any other case recommended, use a superior quality E.P. (serious pressure) grease to services most vehicular, industrial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply utilizing a telescoping shaft (square shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-designed (a yoke) and having a hole (attention) radially through the attention that is linked by a cross. They allow larger angles than adaptable couplings and are being used in applications where excessive misalignment should be accommodated (1 to 30 degrees).
Always make sure new, fresh grease is evident in all U-joint seals.
Can be caused by operating angles which are too large.
Can be caused by 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 aren’t aligned. If the bearings quit 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. Each and every time the drive shaft tries to U Joint shorten, the load will be transmitted in to the bearings and they’ll mark the 
cross trunnion. Unlike brinnell marks due to torque, brinnell marks that happen to be the effect of a frozen slip are constantly evident on leading and back areas of the cross trunnion.
Improper torque upon U-bolt nuts can cause brinelling.
Most companies 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.
