Bearing and bearing types is an important point of production. A bearing facilitates the low-friction relative motion between two machine elements while simultaneously transmitting the forces between them as well as guiding and positioning them relative to each other.
Bearings allow relative rotational and translational motion between two machine parts. Apart from transmitting radial and axial forces (in rotary bearings) and forces and moments transverse to the direction of motion (in linear bearings), they define the relative positions of the machine elements supported by the bearings.
Bearings are core components of all production machines and are often of great importance for the performance of these machines. In a machine tool, for example, different types of bearings are used for different applications: Linear guidings are used for feed axes. Rotary bearings are used for rotary axes, threaded spindle drives for linear axes, and bearings for main spindle shafts, among other things.
In machine tools, the load capacity, the stiffness, and the speed capabilities of the bearings determine the cutting performance. The stiffness also affects the achievable machining accuracy. Bearing friction, in turn, contributes to the machine’s energy consumption, and bearing life can directly affect the downtimes of the entire system.
General bearing types
Bearing Types
As shown above Figure, the same basic bearing principles and bearing types can be used in rotational and translational applications.
In hydrostatic and hydrodynamic bearings, a lubricant film separates the contact surfaces of the two mated elements. Whereas the necessary oil pressure is provided by a pump in hydrostatic bearings, in hydrodynamic bearings, the lubricant film is only formed during operation through the relative motion of the two surfaces. The main advantage of a hydrostatic bearing is that a stable lubricant film is present regardless of the operating state and hence the bearing is also suitable for low speeds or regular acceleration and braking operations. The disadvantage is the additional need for a pump and pipes. Hydrodynamic bearings, in contrast, are only suitable for applications with adequately high speeds and infrequent acceleration and braking because they would otherwise often run in mixed friction mode and wear would accordingly occur. Both bearings share a limited suitability for extremely high speeds due to the relatively high friction between the driven components and the lubricant.
These types of bearings can be found, for example, in high-precision machining systems in which their high stiffness, good damping characteristics, and very good concentricity are utilized.
In aerostatic bearings, which are basically designed in the same way as hydrostatic bearings are, air or any other gas is used for separating the supported elements. This produces very low bearing friction due to the significantly lower viscosity of the gas and the high volumetric flow rate, making these bearings suitable for extremely high speeds.
On the other hand, given comparable dimensions, these bearings exhibit much lower load capacity, stiffness, and damping than hydrostatic bearings do. Application areas for these bearings include ultrahigh-precision machines in which low machining forces are generated at very high speeds.
Magnetic bearings use electromagnets to transmit forces between two machine elements and position them relative to each other. Because the gap between the two components is filled with air here, too, these bearings exhibit a similarly low amount of friction as aerostatic bearings do and are hence likewise suitable for extremely high
speeds. At the same time, the bearing control system allows bearing properties such as stiffness or damping to be adapted during operation. In this way, the vibration behavior of a main spindle can be actively manipulated, for example, to suppress chatter.
In sliding (or plain) bearings, the two mating elements are separated by an intermediate layer made of a material with a minimum coefficient of friction with respect to the supported machine elements. This is extremely important because, in these bearings, solid-state friction always occurs between the two machine parts and the
intermediate layer, which in many cases is made of a polymer or a nonferrous metal. This friction severely limits the maximum relative surface speeds. However, these bearings are also suitable for applications with especially high demands on cleanliness due to the complete lack of lubrication.
The most commonly used bearings are roller bearings. Rolling elements are arranged between the parts to be supported, yielding a rolling motion between the supported machine elements and the rolling elements. This ideally results in pure rolling friction; in reality, however, there can be slidingrolling friction overlaps due to factors such as the bearing design or the operating conditions. The widespread application of this bearing principle can be explained by the comparatively low costs, the ease of use, the high degree of standardization, and the diversity of properties that can be obtained with different roller bearing designs, among other things. Accordingly, further discussion will focus on roller bearings.
Below Table provides an overview of the various bearing types and their main characteristics.
Comparison of bearing type properties