There are many types of U-Joints, some of which are very 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 variations solid and bored. Solid hubs do not have a machined hole. Bored hubs possess a hole and are known as for the hole condition; round, hex, or square style. Two bored designs that deviate from these prevalent shapes are splined, which have longitudinal grooves within the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can result in burned trunnions.
Unless normally recommended, use a high quality E.P. (extreme pressure) grease to services most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by using a telescoping shaft (sq . 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 (vision) radially through the attention that is connected by a cross. They let larger angles than flexible couplings and are being used in applications where large misalignment must be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too large.
Can be the effect of a bent or sprung yoke.
Overloading a travel shaft can 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 continue to be stationary and can “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Every time the drive shaft attempts to shorten, the load will be transmitted in to the bearings and they will mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be the effect of a frozen slip are generally evident on the front and back areas of the cross trunnion.
Improper torque upon U-bolt nuts can cause brinelling.
Most suppliers U Joint china 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.