There are many types of U-Joints, a few of which are very complex. The easiest category called Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub models solid and bored. Sturdy hubs do not have a machined hole. Bored hubs include a hole and are named for the hole form; round, hex, or sq . style. Two bored designs that deviate from these common shapes are splined, which have longitudinal grooves inside bore; and keyed, which have keyways to avoid rotation of the U-joint on the matching shaft.

Using the incorrect lube can result in burned trunnions.
Unless usually recommended, use a high quality E.P. (severe pressure) grease to services most vehicular, professional and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that are fork-formed (a yoke) and having a hole (eyes) radially through the attention that is linked by a cross. They let larger angles than versatile couplings and are used in applications where excessive misalignment needs to be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too big.
Can be caused by a bent or perhaps 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 stop rolling, they remain stationary and can “beat themselves” into the surface of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each and every time the drive shaft attempts to shorten, the load will be transmitted into the bearings and they’ll tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be caused by a frozen slip are generally evident on leading and back surfaces of the cross trunnion.
Improper torque about U-bolt nuts can cause brinelling.
Most manufacturers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause one or more bearings to be starved for grease.