There are many varieties of U-Joints, some of which are extremely complex. The easiest category referred to as Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub models solid and bored. Solid hubs don’t have a machined hole. Bored hubs own a hole and so are known as for the hole shape; round, hex, or square style. Two bored styles that deviate from these prevalent shapes are splined, that 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 in any other case recommended, use a superior quality E.P. (serious pressure) grease to services most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that will be fork-formed (a yoke) and having a hole (eye) radially through the attention that is linked by a cross. They let larger angles than flexible couplings and are used in applications where substantial misalignment should be accommodated (1 to 30 degrees).
Always make sure new, fresh grease is evident at all U-joint seals.
Can be due to operating angles which are too large.
Can be caused by a bent or perhaps sprung yoke.
Overloading a travel shaft could cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears aren’t aligned. If the bearings prevent rolling, they remain stationary and can “beat themselves” into the surface of the cross.
A “frozen” slip assembly will not allow the drive 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 tag the cross trunnion. U Joint Unlike brinnell marks due to torque, brinnell marks that are the effect of a frozen slip are constantly evident on the front and back surfaces of the cross trunnion.
Improper torque about U-bolt nuts could cause brinelling.
Most makers 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.