Of course, there is lubricating oil inside the wheel bearing of the automobile, but it is only low mobility lubricating grease (yellow dry oil). Of course, the wheel bearings of cars will be damaged, but they can be replaced directly after they are damaged. Relatively speaking, the rolling bearings used by small non-load machines are life-long and maintenance-free, which is different from the regular oil change of sliding bearings.

Many friends will wonder why the engine oil and gearbox oil of the car has a fixed replacement cycle, while the wheel bearing does not need to be changed regularly. In fact, this is the choice problem in the industrial field. The bearing is divided into rolling and sliding. The contact area of the two friction surfaces of the rolling bearing is small and the speed is low. The advantage is high transmission efficiency. Therefore, rolling bearings are required at positions where the speed is not high and high efficiency is required. This is the case for the bearings of automobile wheels.

As shown in the figure above, this is a classic rolling bearing. Some friends will also call it a ball bearing. A point on the ball is in contact with the friction surface (point to surface contact), and the contact area is small, so the friction coefficient is smaller, only about 0.001-0.005. However, the problem is that there is direct contact between the balls (or other shapes) of the ball bearing and the friction surface. Even if a sufficient amount of yellow dry oil is applied, there will still be obvious wear during operation, that is, the clearance becomes larger (the gap between the shaft holes becomes larger, which generally means serious wear).

Sliding bearing is different. The sliding bearing can be subdivided into the hydrostatic sliding bearing and dynamic sliding bearing. The advantage of sliding bearing is that it can bear more load and can run at high speed. The most common example is the bearing bush of automobile and turbocharger bearing, which can even easily reach more than 100000 revolutions.

Of course, the strongest advantage of sliding bearing is that it uses lubricating oil to support the two friction surfaces and avoid direct wear between the two friction surfaces. For example, there is a layer of oil between the piston and the cylinder liner and between the crankshaft and the bearing bush to separate the two.

This is the so-called oil film. Secondly, why should there be enough oil pressure? Because the oil needs enough pressure to jack up the shaft and avoid direct contact with the bearing pad. Of course, this is a microscopic phenomenon. Does it seem that the shaft and the bearing are in direct contact with the naked eye? In fact, if the sliding bearing with a large contact surface is in direct contact with the two friction surfaces, it will be burnt in a very short time. Therefore, when the oil pressure is seriously insufficient, the surface of the parts can still be covered by the oil.

However, at this time, the engine oil loses its supporting effect and the two friction surfaces will contact directly. However, as long as the pressure provided by the oil pump is sufficient and the oil is kept clean, the wear between the sliding bearing friction surfaces is minimal.

This is actually the reason why the design life of the engine is only 200000-250000km, but the actual life is almost unlimited. As long as the engine is maintained according to the regulations, high-quality engine oil is used and regularly cleaned, and the friction of various parts of the engine can be almost ignored. Of course, the limitation on the life of the machine in terms of emissions is another matter.

The rolling bearing has no maintenance significance

Of course, it refers to the rolling bearing of passenger cars, which is a life-saving product in terms of design ideas. Because the rolling bearing belongs to point-to-surface contact and its friction coefficient is low, it only relies on the lubricating ability of yellow dry oil. However, since it is not pressure lubrication, there must be direct contact between metal and metal at the two friction base points, so wear is inevitable. Even if the yellow dry oil is replaced every day, it still cannot change its nature of serious wear.

This is different from the hydrostatic bearing, which ensures the integrity of parts through continuous oil change. The rolling bearing is more and more worn with the use. Now the wheel rolling bearing is well sealed and the service life of lubricating grease is long, so there is no need to change the oil of the rolling bearing. After running for 100000-150000 km, the bearing has a large clearance (the gap between the shaft holes is large). At this time, it is meaningless to change the oil because the hard damage to the bearing is irreparable. Therefore, the wheel bearings of passenger cars are maintenance-free, and they can be replaced directly if they are damaged.

Therefore, the life of the wheel bearing is not long, and of course, it will be damaged. However, the feature of not changing the oil regularly is easy to give friends the illusion that the wheel bearing is very durable. In fact, the service life of the rolling bearing is far longer than that of the hydrostatic sliding bearing. As long as the oil is changed and cleaned regularly, the service life of the pressure bearing is almost unlimited. Because the lubricating oil after pressurization will completely separate the two friction surfaces and play a supporting role, as long as the parts lubricated by pressure are well maintained, the service life is much longer than that of rolling bearings.

In fact, sliding bearing and rolling bearing are only two choices at the industrial level. Since the contact surface of the rolling bearing is small and the friction coefficient is low, only grease is used for lubrication, which is often used in low-speed and low-load scenarios. The sliding bearing is often the contact between surface and surface. The contact surface is large and the friction coefficient is much higher than that of a rolling bearing. So we can only use pressure lubrication to ensure the service life and constantly consume oil to protect the hardware from damage. However, the rolling bearing only relies on the lubricating capacity of the lubricating grease itself, but the lubricating oil can not play a supporting role because there is no pressure, so there is direct contact between the two friction surfaces, so the loss of the rolling bearing is much faster than that of the sliding bearing. However, it does not need regular maintenance, so it forms a durable illusion.