There are many types of U-Joints, a few of which are very complex. The simplest category referred to as Cardan U-Joints, U Joint china happen to be either block-and-pin or bearing-and-cross types.
U-joints are available with two hub variations solid and bored. Sturdy hubs do not have a machined hole. Bored hubs currently have a hole and are called for the hole condition; round, hex, or sq . style. Two bored designs that deviate from these prevalent shapes are splined, which have longitudinal grooves inside the bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.
Using the incorrect lube can result in burned trunnions.
Unless otherwise recommended, use a high quality E.P. (serious pressure) grease to services most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply using a telescoping shaft (square shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-designed (a yoke) and having a hole (eyes) radially through the attention that is linked by a cross. They let 
larger angles than adaptable couplings and are being used in applications where great misalignment must 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 large.
Can be the effect of a bent or perhaps sprung yoke.
Overloading a travel shaft could cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings quit rolling, they stay stationary and can “beat themselves” into the surface area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Each and every time the drive shaft attempts to shorten, the load will be transmitted into the bearings and they will indicate the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be the effect of a frozen slide are generally evident on leading and back areas of the cross trunnion.
Improper torque about U-bolt nuts can cause brinelling.
Most producers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause one or more bearings to be starved for grease.