There are many varieties of U-Joints, some of which are very 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 variations solid and bored. Solid hubs do not have a machined hole. Bored hubs have a hole and so are called for the hole form; round, hex, or sq . style. Two bored variations that deviate from these common shapes are splined, which have longitudinal grooves within the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can cause burned trunnions.
Unless or else recommended, use a high quality E.P. (excessive pressure) grease to assistance most vehicular, professional and auxiliary travel shaft applications.
Mechanically U Joint china flexible U-Joints accommodate end movement by simply using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that happen to be fork-shaped (a yoke) and having a hole (eye) radially through the attention that is linked by a cross. They enable larger angles than flexible couplings and are used in applications where substantial misalignment has to be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too large.
Can be the effect of a bent or perhaps 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 stop rolling, they stay stationary and will “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each and every 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 are the effect of a frozen slide are at all times evident on leading and back floors of the cross trunnion.
Improper torque about U-bolt nuts could cause brinelling.
Most suppliers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause one or more bearings to be starved for grease.