is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (eg graphite, MoS2) a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (greases) or exceptionally a gas.
In the most common case the applied load is carried by pressure generated within the fluid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces.
Grease Fittings are almost a thing of the past. You will find that most all front wheel drive cars do not have any grease fittings anywhere on the vehicle .However, you will still find plenty of then on a rear wheel drive car or truck. They are usually located on the steering components and the drive shafts and U-Joints. Make sure these are greased at every oil change go cut down on pre-mature wear.
A single tapered roller bearing
Older cars and trucks had tapered bearings that could be removed and repacked with grease and was required for maintenance.
Tapered roller bearings are bearings that can take large axial forces (i.e. they are good thrust bearings) as well as being able to sustain large radial forces.
Now cars and trucks alike have Hub Bearing assemblies that are maintenance free or non serviceable.
To the right is an example of a hub bearing. They are completely sealed and non-serviceable. The axle shaft of the car fits through the center. It has the wheel studs that the wheel bolts to attached to it as well.
The average life of a hub assembly is about 85,000 to 100,000 miles. Consequently, you may have to change one or more of these units more than once during the life of the vehicle.
In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles. More specifically, a ball joint is a steel bearing stud and socket enclosed in a steel casing. Ball joints are the pivot between the wheels and the suspension of an automobile. Ball joints play a critical role in the safe operation of an automobile's steering and suspension. Ball joints can also be found in most linkage systems for motion control applications, and should not be confused with spherical rod end bearings, which are a different design.
Sealed ball joints do not require lubrication as they are "lubed for life" but standard ball joints must be lubed from time to time. It's best to inspect standard ball joints once a year. Generally speaking, standard ball joints will outlive sealed ones because eventually the seal will break, causing the joint to dry out and rust. While there is no exact life span that can be put on a sealed ball joint, they can fail as early as 80,000 miles. Signs of a failing ball joint start with a clicking or snapping sound when the wheel is turned and eventually turns into a squeaking sound at the end of a stop, when the gas pedal is used and/or also when hitting bumps.
If a ball joint fails, the results can be dangerous as the wheel's angle will be unconstrained, causing loss of control. Because the tire will be at an unintended angle, the vehicle will come to a screeching halt damaging the tires. Also, during failure, debris can damage other parts of the vehicle.