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

U-joints are available with two hub designs solid and bored. Stable hubs do not have a machined hole. Bored hubs own a hole and are known as for the hole shape; round, hex, or square style. Two bored models that deviate from these common shapes are splined, that have longitudinal grooves inside bore; and keyed, that have keyways to avoid 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. (intense pressure) grease to service most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding action between two flanges that are fork-designed (a yoke) and having a hole (eyes) radially through the eye that is linked by a cross. They allow larger angles than flexible couplings and are used in applications where substantial misalignment should be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too big.
Can be the effect of a bent or perhaps 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 can “beat themselves” in to the surface area of the cross.
A “frozen” slip assembly won’t allow the drive shaft to U Joint china lengthen or shorten. Each time the travel shaft tries to shorten, the load will be transmitted into the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are the effect of a frozen slide are constantly evident on leading and back areas of the cross trunnion.
Improper torque upon U-bolt nuts can cause brinelling.
Most manufacturers 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.