There are many varieties of U-Joints, a few of which are extremely complex. The easiest category named 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 don’t have a machined hole. Bored hubs have got a hole and so are named for the hole condition; round, hex, or sq . style. Two bored models that deviate from these common 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 incorrect lube can lead to burned trunnions.
Unless in any other case recommended, use a superior quality E.P. (intense pressure) grease to assistance most vehicular, professional and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding action between two flanges that happen to be fork-shaped (a yoke) and having a hole (eyesight) radially through the eye that is linked by a cross. They allow larger angles than versatile couplings and are being used in applications where excessive 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. U Joint china bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings prevent rolling, they remain stationary and will “beat themselves” into the area 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 tag the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that happen to be the effect of a frozen slip are usually evident on leading and back floors of the cross trunnion.
Improper torque upon 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.