Mechanical bearings can be divided into several subcategories: rolling-element bearings, hydrodynamic journal bearings, gas bearings, squeeze-film bearings, and hydrostatic bearings.
Rolling-element bearings are probably the most common type of bearing used in machinery. They are relatively compact, can transmit heavy loads of various forms, and can be installed and serviced easily. Several specific design types with a range of stiffness are available. Damping in rolling-element
bearings is very small but may be enhanced by squeeze-films.
Journal bearings consist of a circular section of shaft (the journal) rotating inside a bearing ‘‘bush,’’ which is nominally circular. The clearance space between the two is partially filled by the lubricating fluid, which is pressurized by the motion. In addition to reducing the friction in rotational motion, this hydrodynamic film behaves like a complicated arrangement of springs and dampers and so influences the critical speeds and imbalance response. If designed improperly, the film can contribute to rotor instability, which is the result of self-excited vibration. Hydrostatic bearings differ from hydrodynamic bearings in that the lubricant pressure required to separate the bearing surfaces is supplied from some external pressure source and is not a result of journal rotation.
Gas bearings are similar to oil-lubricated bearings, but because the gas is compressible, the behavior differs. Compared with hydrodynamic bearings of similar surface area, gas bearings carry much smaller loads and generally operate with smaller film clearances. The advantage of gas bearings is that they tend to have much smaller rotational losses because the viscosity of a gas is much smaller than that of a liquid. Several types of gas bearings are available: self-acting, externally pressurized, porous wall, and flexible vane.
Squeeze-film bearings are a special case of hydrodynamic bearings. This type of bearing consists of a conventional rolling-element bearing and an external structure for damping radial motion. The outer race of the rolling-element bearing is surrounded by a damper casing. A small radial clearance between the outer race and the inner bore of the casing accommodates a thin lubricating film. The fit is so close that the outer race of the conventional bearing does not rotate. However, the squeezed film is pushed from one region to another by the radial force of the
outer race. The lubrication pressure is built up purely by the action of squeezing the film in this clearance. Friction between the lubricant and the races, and inertia in the lubricant itself, produces a radial stiffness.
If the rotor of the flywheel is rotating in air, any of the preceding bearings can be used. If the rotor operates in vacuum, the number of bearing choices is limited and the vapor pressure of the lubricant must be low. Solid-lubricated ceramic bearings (e. g., those made of silicon nitride [SiN] and lubricated with molybdenum disulfide or other proprietary solid films) are often used in this situation.