There are many U Joint varieties of U-Joints, a few of which are very complex. The simplest category named Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub styles solid and bored. Solid hubs don’t have a machined hole. Bored hubs have got a hole and so are called for the hole shape; round, hex, or square style. Two bored types that deviate from these common shapes are splined, which have longitudinal grooves inside bore; and keyed, which have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can cause burned trunnions.
Unless in any other case recommended, use a superior quality E.P. (excessive pressure) grease to support most vehicular, commercial 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 happen to be fork-designed (a yoke) and having a hole (attention) radially through the attention that is linked by a cross. They allow larger angles than adaptable couplings and are used in applications where excessive misalignment has to be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too large.
Can be the effect of a bent or sprung yoke.
Overloading a drive 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 stop rolling, they remain stationary and will “beat themselves” into the surface area of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Every time the travel shaft attempts to shorten, the strain will be transmitted into the bearings and they will tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be the effect of a frozen slide are often evident on the front and back surfaces of the cross trunnion.
Improper torque about U-bolt nuts can cause brinelling.
Most producers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause a number of bearings to be starved for grease.