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

U Joint china U-Joints are available with two hub models solid and bored. Stable hubs do not have a machined hole. Bored hubs have a hole and so are called for the hole condition; round, hex, or sq . style. Two bored variations 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 otherwise recommended, use a high quality E.P. (severe pressure) grease to service most vehicular, commercial 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 action between two flanges that are fork-designed (a yoke) and having a hole (vision) radially through the attention that is connected by a cross. They allow larger angles than flexible couplings and are used in applications where large misalignment has to be accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident for all 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 travel shaft can cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings stop rolling, they remain stationary and will “beat themselves” in to the area 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 load will be transmitted into the bearings and they’ll mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be caused by a frozen slide are always evident on the front and back floors of the cross trunnion.
Improper torque on U-bolt nuts could cause brinelling.
Most suppliers 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.