There are many varieties of U-Joints, some of which are incredibly complex. The easiest category known as Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub variations solid and bored. Sturdy hubs do not have a machined hole. Bored hubs have got a hole and are named for the hole shape; round, hex, or sq . style. Two bored variations that deviate from these prevalent shapes are splined, that have longitudinal grooves inside bore; and keyed, which have keyways to prevent 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. (excessive pressure) grease to support most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that will be fork-designed (a yoke) and having a hole (eye) radially through the eye that is connected by a cross. They enable larger angles than adaptable couplings and are being used in applications where substantial misalignment must be accommodated (1 to 30 degrees).

Always make sure new, fresh grease is evident in all four 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 drive 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 quit U Joint china rolling, they remain stationary and will “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. 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 caused by a frozen slide are often evident on leading and back floors of the cross trunnion.
Improper torque in U-bolt nuts could cause brinelling.
Most producers 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.