There are many varieties of U-Joints, some of which are extremely complex. The simplest category named U Joint china Cardan U-Joints, are either block-and-pin or bearing-and-cross types.

U-joints are available with two hub styles solid and bored. Sound hubs don’t have a machined hole. Bored hubs have a hole and so are known as for the hole shape; round, hex, or sq . style. Two bored variations that deviate from these prevalent shapes are splined, which have longitudinal grooves within the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can lead to burned trunnions.
Unless normally recommended, use a superior quality E.P. (extreme pressure) grease to program most vehicular, industrial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by utilizing a telescoping shaft (sq . 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 (vision) radially through the eye that is connected by a cross. They allow larger angles than adaptable couplings and are being used in applications where huge misalignment has to be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too large.
Can be the effect of a bent or perhaps sprung yoke.
Overloading a travel shaft could cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears aren’t aligned. If the bearings end rolling, they continue to be stationary and will “beat themselves” in to the area of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each time the travel shaft tries to shorten, the strain will be transmitted into the bearings and they will mark the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are caused by a frozen slide are always evident on the front and back floors of the cross trunnion.
Improper torque in U-bolt nuts could cause brinelling.
Most suppliers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause one or more bearings to be starved for grease.