Are the bearings on car wheels lubricated? Why are they never replaced?

Of course, there is lubricating oil inside the wheel bearing of a car, but it is a grease with low fluidity (yellow dry oil). Of course, the wheel bearing of the car will be broken, but it can be replaced with a new bearing directly after it is broken. Relatively speaking, the rolling bearings used in small non-heavy machinery are maintenance-free for life, which is different from the frequent oil change of sliding bearings.

Many friends will wonder why the car's oil and transmission oil have a fixed replacement cycle, while the wheel bearings do not require regular oil changes? In fact, this is the choice of the industrial field, the bearing is divided into rolling and sliding, rolling bearing two The friction surface has a small contact area and low speed, which has the advantage of high transmission efficiency. Therefore, rolling bearings are required for some positions where the speed is not high and higher efficiency is required. This is the case with the bearings of automobile wheels.

The difference between rolling bearing and sliding bearing

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

Sliding bearings are different, sliding bearings can be subdivided into hydrostatic sliding bearings, dynamic pressure sliding bearings. The advantages of sliding bearings are that they can withstand greater loads and can run at high speeds. The most common examples are automobile bearings and turbocharger bearings that can easily reach more than 100000 revolutions. Of course, the strongest advantage of the sliding bearing is that it uses lubricating oil to support the two friction surfaces, avoiding direct wear between the two friction surfaces. For example, between the piston and the cylinder liner, between the crankshaft and the bearing bush, there is a layer of oil to separate the two.

This is the so-called oil film. Secondly, why is it necessary to have enough oil pressure, because the oil needs enough pressure to lift the shaft to avoid direct contact with the tile. Of course, this is a microscopic phenomenon. It seems that the shaft and the tile are in direct contact with each other. In fact, if the sliding bearing with large contact surface is in direct contact with the two friction surfaces, it will burn out in a very short time. Therefore, when the oil pressure is seriously insufficient, it can still ensure that the surface of the parts is covered by the oil.

But at this time, the oil loses its support, 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 maintains good cleanliness, the wear between the friction surfaces of the sliding bearing is minimal. This is actually the reason why the design life of the engine is only 20-250000 kilometers, but the actual life is almost unlimited. As long as the required maintenance, the use of high quality oil, regular cleaning, then the engine parts friction can be almost negligible. Of course, the limit on the life of the machine in terms of emissions is another matter.

Rolling bearings have no sense of maintenance

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

This is different from the hydrostatic bearings through continuous oil change to ensure the integrity of the parts, rolling bearings are more and more serious with the use of wear. Today's wheel rolling bearings are well sealed and the grease has a long life, so there is no need to change the oil for the rolling bearings. When running for 10-150000 kilometers, the bearing has a large gap (the gap between the shaft holes is very large). At this time, even oil change is meaningless, because the hard damage of the bearing is irreparable. Therefore, the wheel bearings of passenger cars are maintenance-free, and they are replaced directly if they are broken.

Therefore, the life of the wheel bearing is not long, and of course it will be damaged, but the characteristic of not having to change the oil regularly will easily give friends the illusion that the wheel bearing is very durable. In fact, the rolling bearing is far from the long life of the hydrostatic sliding bearing. As long as the oil is changed and cleaned regularly according to the requirements, the life of the pressure bearing is almost unlimited. Because the pressurized lubricating oil will completely separate the two friction surfaces and play a supporting role, as long as the parts that use pressure lubrication are well maintained, the life span is much longer than that of rolling bearings.

In fact, sliding bearings and rolling bearings are only two options at the industrial level. Rolling bearings have small contact surfaces and low friction coefficients, so only grease is used for lubrication, which is often used in low-speed, low-load scenarios. The sliding bearing is often the contact between the surface and the surface, the contact surface is large, the friction coefficient is far higher than the rolling bearing. Therefore, we can only use pressure lubrication to ensure life, and constantly consume oil to protect the hardware from slow damage. The rolling bearing only depends on the lubricating ability of the grease itself, but because there is no pressure, the lubricating oil cannot play a supporting role, 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, but it does not need regular maintenance, so it forms a durable illusion.