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

U-joints can be found with two hub models solid and bored. Sound hubs do not have a machined hole. Bored hubs possess a hole and so are called for the hole condition; round, hex, or square style. Two bored styles that deviate from these prevalent shapes are splined, that have longitudinal grooves within the bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.

Using the wrong lube can result in burned trunnions.
Unless in any other case recommended, use a high quality E.P. (excessive pressure) grease to assistance most vehicular, commercial and auxiliary travel 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 movement between two flanges that will be fork-designed (a yoke) and having a hole (eye) radially through the eye that is linked by a cross. They allow larger angles than adaptable couplings and are used in applications where excessive misalignment should be accommodated (1 to 30 degrees).

Always make sure fresh, fresh grease is evident by all 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 end rolling, they remain stationary and can “beat themselves” into the surface 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 attempts to shorten, the strain will be transmitted in to the bearings and they'll mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be the effect of a U Joint frozen slide are at all times evident on the front and back areas of the cross trunnion.
Improper torque in U-bolt nuts could cause brinelling.
Most companies 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.