In the case of a rotating shaft, the most widely used method of support is by using one or a number bearing. A considerable number of different types of bearings are commonly available. The system selected is a function of the loads and speeds experienced by the system; for very high-speed application air or magnetic bearings are used instead of the conventional metal-on-metal, rolling contacts.
The bearing arrangement of a rotating component, e.g. a shaft, generally requires two bearings to support and locate the component radially and axially relative to the stationary part of the machine. Depending on the application, load, running accuracy and cost the following approaches can be considered:
- Locating and non-locating bearing arrangements.
- Adjusted bearing arrangements.
- Floating bearing arrangements.
Locating and non-locating bearing arrangements:
The positioning bearing on one end of the shaft provides a radial support and at the same time locates the shaft axially in both directions. Therefore, you should look at the position on both the shaft and the housing. Suitable bearings are radial bearings that can accommodate combined loads, eg. Eg deep groove ball bearings.
The second bearing provides an axial location in both directions but must be mounted with radial freedom (ie, have a clearance adjustment) in its housing.
Adjusted bearing arrangements:
In a tight bearing arrangement, the shaft is located axially in one direction by the bearing and in the opposite direction to the other bearing. This type of arrangement is called crossed location and is generally used on short axes. Suitable bearings include all types of radial bearings that can accommodate axial loads in at least one direction.
Floating bearing arrangements:
The floating bearing arrangements are also located transverse and are suitable when the demands with respect to the axial location are moderate or where other components on the shaft serve to locate it axially. Deep groove ball bearings will satisfy this arrangement.
Air bearings can either be of an aerostatic or an aerodynamic design. In practice, aerodynamic bearings are used in turbomachinery, where speeds of up to 36 000 rev min−1 in high-temperature environments.
In an aerostatic air bearing, the two bearing surfaces are separated by a thin film of pressurised air. The compressed air is supplied by a number of nozzles in the bearing housing. The distance between the bearing surfaces is about 5 to 30 μm.
Advantages of using air bearing:
- A high rotation accuracy typically greater than 5 × 10-8 m can be achieved and will remain constant over time as there is no wear due to the absence of contact between the rotating shaft and the housing.
- Low frictional drag, allow high rotational speeds; shaft speed of up to 200 000 rev min−1 with suitable bearings can be achieved.
- High stiffness which is enhanced at speed due to a lift effect.
In a magnetic bearing the rotating shaft is supporting in a powerful magnetic field, and as with the air bearing gives a number of significant advantages:
- No contact, hence no wear, between the rotating and stationary parts
- Operating through a wide temperature range, typically−250◦C to 220◦C: for this reason magnetic bearings are widely used in superconducting machines.
- A non-magnetic sheath between the stationary and rotating parts allows operation in corrosive environments.
- The frictional drag on the shaft is minimal, allowing exceptionally high speeds.