There are many varieties of U-Joints, some of which are incredibly complex. The easiest category referred to as Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub styles solid and bored. Stable hubs do not have a machined hole. Bored hubs possess a hole and so are named for the hole form; round, hex, or square style. Two bored designs that deviate from these prevalent shapes are splined, that have longitudinal grooves inside 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. (severe pressure) grease to support most vehicular, professional and auxiliary drive shaft applications.
U Joint Mechanically flexible U-Joints accommodate end movement by simply by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that will be fork-formed (a yoke) 
and having a hole (vision) radially through the attention that is linked by a cross. They enable larger angles than versatile couplings and are used in applications where large misalignment has to be accommodated (1 to 30 degrees).
Always make sure new, fresh grease is evident at all U-joint seals.
Can be caused by operating angles which are too big.
Can be caused by a bent or sprung yoke.
Overloading a travel shaft can cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears are not aligned. If the bearings end rolling, they continue to be stationary and will “beat themselves” in to the area of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Every time the travel shaft tries to shorten, the strain will be transmitted into the bearings and they will mark the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are the effect of a frozen slip are often evident on leading and back areas of the cross trunnion.
Improper torque in U-bolt nuts can cause brinelling.
Most companies 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.