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There are many varieties of U-Joints, some of which are very complex. The easiest 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 types solid and bored. Solid hubs don’t have a machined hole. Bored hubs possess a hole and are known as for the hole shape; round, hex, or sq . style. Two bored models that deviate from these common shapes are splined, that have longitudinal grooves inside bore; and keyed, which have keyways to avoid rotation of the U-joint on the matching shaft.

Using the incorrect lube can lead to burned trunnions.
Unless or else recommended, use a superior quality E.P. (intense pressure) grease to services most vehicular, industrial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding movement between two flanges that are fork-designed (a yoke) and having a hole (attention) radially through the eye that is linked by a cross. They let larger angles than versatile couplings and are used in applications where large misalignment needs to be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too large.
Can be the effect of a bent or sprung yoke.
Overloading a drive 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 continue to be stationary and can “beat U Joint themselves” into the surface area of the cross.
A “frozen” slip assembly won’t allow the drive shaft to lengthen or shorten. Each time the drive shaft attempts to shorten, the load will be transmitted in to the bearings and they will tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be caused by a frozen slide are constantly evident on leading and back floors of the cross trunnion.
Improper torque about U-bolt nuts could cause brinelling.
Most producers 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.