There are many types of U-Joints, a few of which are very complex. The simplest category referred to as Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.

U-joints are available with two hub designs solid and bored. Stable hubs don’t have a machined hole. Bored hubs U Joint currently have a hole and are known as for the hole form; round, hex, or sq . style. Two bored types that deviate from these prevalent shapes are splined, that have longitudinal grooves in the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the incorrect lube can lead to burned trunnions.
Unless in any other case recommended, use a superior quality E.P. (excessive pressure) grease to program most vehicular, commercial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding motion between two flanges that are fork-formed (a yoke) and having a hole (eyes) radially through the eye that is connected by a cross. They enable larger angles than versatile couplings and are being used in applications where substantial misalignment should be accommodated (1 to 30 degrees).

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

Can be caused by 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 are not aligned. If the bearings quit rolling, they remain stationary and can “beat themselves” in to the area of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Every time the travel shaft tries to shorten, the load will be transmitted into the bearings and they’ll mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are the effect of a frozen slip are generally evident on leading and back surfaces of the cross trunnion.
Improper torque in U-bolt nuts can cause brinelling.
Most makers 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.