Of course there is lubricant inside the wheel bearings of a car, but it is a low-fluidity grease (yellow dry oil). Of course the wheel bearings of a car can break down, but when they do, they can simply be replaced with new bearings. Relatively speaking, the rolling bearings used in small non-load machinery are maintenance-free for life, which is different from the sliding bearings that are often changed.

　　Many friends will wonder why the car oil, transmission oil have a fixed replacement cycle, while the wheel bearings do not need regular oil changes? In fact, this is the choice of the industrial field, the bearing is divided into rolling and sliding, rolling bearing two friction surface contact area is small, low speed, the advantage is high transmission efficiency. So for some speed is not high, cut the need for high efficiency of the location are required on the rolling bearing, the bearing of the car's wheel is so.

　　The difference between rolling bearings and plain bearings

　　As shown above, this is a classic rolling bearing, some friends will also be called ball bearings. A point on the ball and the friction surface contact (point surface contact), the contact area is very small so the coefficient of friction is smaller, only about 0.001-0.005. But the problem is that the ball bearing ball (or other shapes) and the friction surface have direct contact, even if coated with a sufficient amount of yellow dry oil, in operation will still produce significant wear, that is, the amount of openness becomes large (the gap between the shaft hole becomes large, generally refers to serious wear).

　　The sliding bearing is different, and the sliding bearing can be subdivided into static pressure sliding bearing and dynamic pressure sliding bearing. The advantages of sliding bearings are greater load bearing, high speed operation, etc. The most common examples are the axle shards of automobiles, turbocharger bearings and even easily reach more than 100,000 rpm. Of course the strongest advantage of plain bearings is that it uses lubricating oil to play the role of supporting two friction surfaces, avoiding the formation of direct wear between the two friction surfaces. For example, between the piston and the cylinder liner, between the crankshaft and the shaft tile, there is a layer of oil to play a role in separating the two.

　　This is the so-called oil film, followed by why there is a need for sufficient oil pressure, because the oil needs sufficient pressure to the shaft to the top to avoid direct contact with the tile. Of course, this is a microscopic phenomenon, our naked eye observation seems to be direct contact between the shaft and the tile? In fact, if it is a large contact surface of the slide bearing once the two friction surface direct contact, then it will lead to burn in a very short time. So when the oil pressure is seriously insufficient, although it can still ensure that the surface of the parts are covered by the oil.

　　But at this time the oil loses its supporting role, and the two friction surfaces will be in direct contact. But as long as the pressure provided by the oil pump is sufficient, and the oil is kept well clean, then the wear between the friction surfaces of the plain bearings is minimal. This is in fact the reason why the engine's design life is only 200,000-250,000 km, but the actual life is almost unlimited. As long as the prescribed maintenance, the use of high-quality oil, regular cleaning, then the engine components friction can be almost negligible. Of course, the emission aspect of the machine life limit is another story.

　　Rolling bearings have no meaningful maintenance

　　Of course refers to the rolling bearings of passenger cars, in the design thinking is a life goods. Because the rolling bearing belongs to the point surface contact, its own friction coefficient is low, so it only relies on the lubrication ability of yellow dry oil. But because it is not pressure lubrication, so the two friction base points are bound to have direct metal-to-metal contact, so wear and tear is inevitable. Even if you change new yellow dry oil every day still can not change the nature of its serious wear.

　　This is different from the hydrostatic bearings by constantly changing oil to ensure the integrity of the parts, rolling bearings is with the use of leading to increasingly serious wear. And today's wheel rolling bearing seal is very good, grease life is also long, then the rolling bearing is not necessary to change the oil. Run a 100,000-150,000 kilometers, the bearing openness is very large (the gap between the axle hole is large), at this time, even if the oil change is also meaningless, because the bearing hard damage has been irreparable. So the wheel bearings of passenger cars are maintenance-free, bad directly to the new.

　　So the wheel bearing life is not long, of course, will also be damaged, but do not have to regularly change the oil characteristics of easy for friends to produce the wheel bearing is very durable illusion. In fact rolling bearings are far from the long life of hydrostatic plain bearings, pressure bearings as long as the required oil changes, regular cleaning, then its life is almost unlimited. Because the lubricant after pressurization will completely separate the two friction surfaces and play a supporting role, so the parts with pressure lubrication will have a far longer life than rolling bearings as long as they are well maintained.

　　In fact sliding bearings and rolling bearings are just two options at the industrial level. Rolling bearings have a small contact surface and low coefficient of friction so they are lubricated with grease only and are often used in low speed, low load scenarios. While sliding bearings are often face-to-face contact, contact surface is large, the coefficient of friction is also much higher than rolling bearings. So it is left to use pressure lubrication to ensure longevity and constant oil consumption to protect the hardware from slow damage. The rolling bearing, on the other hand, simply relies on the lubrication ability of the grease itself, but because there is no pressure resulting in the lubricant can not play a supporting role, so there is direct contact between the two friction surfaces, so the loss of rolling bearings is far faster than sliding bearings, only it does not require regular maintenance, so it creates the illusion of durability.