There are many varieties of U-Joints, some 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 are available with two hub designs solid and bored. Sound hubs do not have a machined hole. Bored hubs own a hole and are called for the hole shape; round, hex, or square style. Two bored designs 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 or else recommended, use a high quality E.P. (intense pressure) grease to program most vehicular, professional and auxiliary drive 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 movement between two flanges that are fork-formed (a yoke) and having a hole (eye) radially through the attention that is connected by a cross. They allow larger angles than flexible couplings and are used in applications where high misalignment has to 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. U Joint china 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 will not allow the drive shaft to lengthen or shorten. Each and every time the travel shaft tries to shorten, the load will be transmitted into the bearings and they will indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are caused by a frozen slide are constantly evident on the front 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.