There are many types of U-Joints, a few of which are extremely 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 models solid and bored. Sturdy hubs do not have a machined hole. Bored hubs currently have a hole and so are named for the hole form; round, hex, or sq . style. Two bored styles that U Joint deviate from these common shapes are splined, which have longitudinal grooves within 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 in any other case recommended, use a superior quality E.P. (extreme pressure) grease to service most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply utilizing a telescoping shaft (square shafting or splines). U-Joints function by a sliding action between two flanges that happen to be fork-formed (a yoke) and having a hole (eyes) radially through the attention that is connected by a cross. They allow larger angles than adaptable couplings and are used in applications where excessive misalignment has to be accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident at all U-joint seals.

Can be due to operating angles which are too big.
Can be caused by 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 aren’t aligned. If the bearings end rolling, they stay stationary and will “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each and every time the travel shaft tries to shorten, the strain will be transmitted into the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are the effect of a frozen slip are generally evident on the front and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts can 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.