Bearings and their uses
If while driving you hear a noise or a metallic knock in the area of the wheel hub, the bearings may be damaged.
The older the car, the more likely they are to wear out.
The most common signs of wear in rolling bearings include: loud operation of the support bearing with a characteristic whine, sounds of metallic friction, rattling and noise from the hub area. With severe wear of the bearings, the vibration of the road wheels and the vibration of the steering wheel are felt. In addition to natural wear, in practice, the destruction of bearings is common. 
caused by the ingress of water, which causes corrosion which, if present for a long period of time, will block the bearing.
Designers set bearing units for 15 years of operation. However, road wheel bearings wear out earlier, which is affected by driving technique, road surface conditions and general vehicle operating conditions.
During operation, the bearings make multimillion-dollar revolutions. Abrasive wear is minimal, fatigue in the form of flaking of the raceways and chipping of metal pieces predominates. A bearing damaged in this way must not be used.
The bearings are very precisely manufactured and rarely fail. The failure causes incorrect assembly, poor preload adjustment, or the use of the cheapest substitutes. To obtain high durability of bearings during their installation, it is necessary to observe exceptional cleanliness, and all work must be carried out in accordance with the manufacturer's technology. When dismantling bearings, use appropriate pullers and assemble using presses, not hammers.
As a rule, two tapered roller bearings of different diameters were used to fasten the wheel hub, in which the axial play is fixed by a central nut. Newer designs use double row angular contact ball bearings. Most often these are bearings with sealing rings and a constant supply of lubricant. In practice, there are two modifications to this solution, in one of which the inner race of the bearing is a properly hardened journal, and in the other the outer ring is part of the hub.
Rolling bearings are produced by many companies around the world. Their products differ in quality and price. For example, a set of wheel bearings for an Opel Astra I costs PLN 60, Ford Focus front wheels PLN 200, and Ford Focus rear wheels PLN 392 (repair kit). Replacement, depending on the complexity of the design, costs from 100 to 180 zł.
