There are many types of U-Joints, some of which are very complex. The simplest category called Cardan U-Joints, are either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub variations solid and bored. Stable hubs don’t have a machined hole. Bored hubs currently have a hole and so are named for the hole form; round, hex, or U Joint china square style. Two bored styles that deviate from these prevalent shapes are splined, that have longitudinal grooves inside the bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.

Using the wrong lube can bring about burned trunnions.
Unless otherwise recommended, use a superior quality E.P. (serious pressure) grease to assistance most vehicular, professional and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that are fork-designed (a yoke) and having a hole (attention) radially through the attention that is connected by a cross. They enable larger angles than adaptable couplings and are being used in applications where excessive misalignment needs to be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too big.
Can be the effect of a bent or sprung yoke.
Overloading a drive shaft can cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears aren’t aligned. If the bearings prevent rolling, they stay stationary and will “beat themselves” into the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each time the travel shaft attempts to shorten, the load will be transmitted in to the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are the effect of a frozen slip are at all times evident on leading and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts could cause brinelling.
Most producers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause a number of bearings to be starved for grease.