There are many varieties of U-Joints, some of which are very complex. The simplest 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 variations solid and bored. Sound hubs don’t have a machined hole. Bored hubs include a hole and are called for the hole shape; round, hex, or square style. Two bored designs that deviate from these prevalent shapes are splined, that have longitudinal grooves inside bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can bring about burned trunnions.
Unless usually recommended, use a high quality E.P. (excessive pressure) grease to service most vehicular, professional 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 motion between two flanges that will be fork-formed (a yoke) and having a hole (attention) radially through the attention that is connected by a cross. They enable larger angles than versatile couplings and are being used in applications where huge misalignment should be U Joint china accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident by all four 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 could cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears are not aligned. If the bearings stop rolling, they continue to be stationary and can “beat themselves” into the surface of the cross.
A “frozen” slip assembly won’t allow the drive shaft to lengthen or shorten. Every time the drive shaft tries to shorten, the strain will be transmitted into the bearings and they will mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be the effect of a frozen slide are constantly evident on leading and back areas of the cross trunnion.
Improper torque in 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.