Being the most common employed is roller bearings and their properties are explained this passage. There are three main types of roller bearings used in machine tool main spindles: Angular contact ball bearings (so-called spindle bearings) with a contact angle between 15 and 25° can carry radial and axial loads and are mainly characterized by the best speed capabilities of all roller bearings. Tapered roller bearings can also carry combined loads and have a higher load capacity and stiffness than spindle bearings of comparable size do. Due to the line contact between the rolling elements and the raceways and the greater amount of friction arising from it (in comparison with the point contact occurring in spindle bearings), the suitability of these bearings for high-speed applications is limited. Cylindrical roller bearings are extensively used in combination with spindle bearings in applications in which high radial stiffness and load capacity are required. However, their internal geometry makes bearings of this type very sensitive to differences in the thermal expansion of the inner and outer ring and restricts their range of applications.
Properties and Operational Behavior of Roller Bearings
Because the operating behavior of the bearings has a significant effect on the operating characteristics of the overall machine, the suitability of the selected bearings for the planned application case must be investigated carefully. The main criteria in the first phase of bearing selection are the load capacity and the speed capabilities. The load capacity of a bearing mainly depends on the materials used, the inner and outer geometries as well as the size of the bearing, the number of rolling elements, and the bearing type. The relationship between the load rating and the bearing loads expected in operation can be used to estimate the bearing life. This is a statistical value, which gives the probability of survival of the bearing for a given number of load cycles (revolutions).
The limiting speed dn, also called high-speed coefficient or realizable speed coefficient, is often expressed by the multiplication of bearing rotational speed (n) and bearing pitch diameter (dm). It represents the limit up to which a bearing can be operated. It depends on various factors and is mainly influenced by the bearing friction, which increases with increasing speed and load, and the associated heating up of the bearing. This increase in temperature can cause damage to the lubricant and the bearing’s polymer components and, in extreme cases, even the steel used in the bearing. In grease-lubricated bearings, the maximum allowable speed can also be limited by the maximum allowable shear stress for the grease or the ability of the grease to reliably supply the bearing gap with lubricant, even at high speeds (not always the case due to the limited flow of the grease). At very high speeds, the additional bearing load arising from the centrifugal forces acting on the rotating parts must also be taken into account. Depending on the bearing type, the bearing properties may also change with changing speed. For example, in spindle bearings at high speeds, the rolling elements undergo radial and axial displacement, resulting in a decrease in the contact angle at the outer ring and hence a decrease in the axial bearing stiffness.
