There are many varieties of U-Joints, a few of which are very complex. The simplest category known 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. Sound hubs do not have a machined hole. Bored hubs possess a hole and so are called for the hole shape; round, hex, or sq . style. Two bored variations that deviate from these prevalent shapes are splined, which have longitudinal grooves inside bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.

Using the incorrect lube can cause burned trunnions.
Unless usually recommended, use a high quality E.P. (serious pressure) grease to assistance most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding motion between two flanges that will be fork-designed (a yoke) and having a hole (eyesight) radially through the attention that is connected by a cross. They enable larger angles than versatile couplings and are used in applications where substantial misalignment needs to be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too large.
Can be the effect of a bent or perhaps sprung yoke.
Overloading a U Joint 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 stop rolling, they remain stationary and can “beat themselves” in to the area 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 load will be transmitted in to the bearings and they will indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are caused by a frozen slide are constantly evident on the front and back floors of the cross trunnion.
Improper torque on U-bolt nuts can cause brinelling.
Most manufacturers 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.