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

U-joints are available with two hub types solid and bored. Sturdy hubs don’t have a machined hole. Bored hubs include a hole and are named for the hole condition; round, hex, or square style. Two bored styles that deviate from these prevalent shapes are splined, which have longitudinal grooves within the bore; and keyed, which have keyways to prevent rotation of the U-joint on the matching shaft.

Using the incorrect lube can bring about burned trunnions.
Unless usually recommended, use a high quality E.P. (severe pressure) grease to provider most vehicular, commercial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that will be fork-designed (a yoke) and having a hole (eyes) radially through the eye that is linked by a cross. They enable larger angles than U Joint china versatile couplings and are being 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 perhaps 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 quit rolling, they remain stationary and will “beat themselves” into the area of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each 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 happen to be the effect of a frozen slip are usually evident on leading and back surfaces of the cross trunnion.
Improper torque upon U-bolt nuts could cause brinelling.
Most suppliers 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.