Machine Design

Bearings that bear up

By M. M. Khonsari
Prof., Mechanical Engineering
Louisiana State Univ.
Baton Rouge, La.

E. R. Booser
Consulting Engineer
Niskayuna, N.Y.

Bearing selection looks at several key design metrics including anticipated loads, lifetime and reliability, ambient conditions, vibration, and cost. This information lets you choose between one of four basic bearing types: dry, semilubricated, fluid-film, or rolling-element bearings.

Dry bearings are probably the simplest and least expensive of all. They blend polymers, such as PTFE and nylon, with molybdenum disulfide, graphite, and other inorganic powders to lower friction and add strength. Dry bearings closely conform to the shape of their mating shaft or thrust surface and are mostly for applications with small loads and low surface speeds.

Some polymer bearings incorporate silicone or other oils (semilubricated) to boost operating envelopes. Also in the semilubricated category are bearings made of sintered powders of bronze, iron, and aluminum. Such designs may also operate with a limited supply of oil from wicks, oil mist, air-oil feed, or individual oil or grease applicators. This greatly improves load and speed limits over semilubricated operation, but still to only moderate levels.

Wear is a common failure mechanism of both dry and semilubricated bearings. Suppliers typically relate bearing wear rate and limiting temperature rise in what is called a PV factor, where P = bearing load on a projected area, lb/in.2, and V = surface velocity, ft/min. Surface heating limits surface operating speeds to about 100 to 500 m/min.

Fluid-film bearings, as the name implies, use a thin film of liquid (oil, water, even fuel in some cases) or gas to fully separate moving surfaces. Fluid-film bearings are further classified by the method of fluid feeding. Self-acting types produce their own pressure to support loads through relative motion between a shaft or thrust runner. Externally pressurized fluid-film bearings, by comparison, generate load support by feeding lubricant under pressure to the bearing/shaft gap. Lowspeed and gas-fed bearings use this technique because self-pumping action is inadequate. Feedrate is proportional to bearing length, width, clearance, and surface velocity. For example, oil-film bearings in a typical steam turbine-generator at an electric power station require about 4 m3/min (1,000 gallons/min).

There are a wide variety of rollingelement bearings though most use one of a few basic designs. Needle bearings work for radial loads only. Ball and straight roller bearings handle radial and limited thrust loads. Ball-thrust bearings manage thrust loads only, while tapered roller bearings handle both radial and thrust loads. Common practice limits rolling-element bearings with oil lubrication to a DN value (mm bore rpm) of 500,000 to 1,000,000, corresponding to surface speeds of 1,600 to 3,100 m/min (5,000 to 10,000 ft/min). Applications with DNs less than 300,000 can be serviced by factory sealed, greased rolling-element bearings.

Lubricants need only cover surface roughness of working surfaces. Less than one drop is adequate for many small and medium-sized ball and roller bearings. Heavy loads and high speeds require additional lubricant, not for lubrication, but to remove heat and maintain reasonable operating temperatures. Ball and roller bearings made of high-carbon, low-alloy steels such as AISI 52100 and case-hardening alloys are generally limited to 300°F and can be specially stabilized for operation to about 400°F. Tool steel and ceramic bearing materials boost operating temperatures to 1,200°F when combined with suitable solid-film lubricants.

Fluid-film bearings are generally the most temperature limited of all types. They are commonly made from tin and lead babbitt soft-metal bearing materials that limit operating temperatures to about 300°F. At relatively low temperatures (–5 to 50°F) the mineral lubricating oils become highly viscous and don't flow freely through oil feed and drain passages. Load capacity is a function of rotation speed and oil viscosity because these two factors influence oil-film formation. Fluid-film bearing DNs can easily exceed those of rolling-element bearings. Surface speeds in fluid-film turbine bearings may reach 8,700 m/min (30,000 ft/min), for example.

Bearing damping is another issue. It's desirable to have a certain amount to absorb vibration energy of rotating parts and reduce vibration amplitude. Ball and roller bearings provide virtually no damping themselves, but oil or friction damping can be introduced through specially designed housing mounts. These external dampers quell severe vibration at critical (resonant) speeds, especially important for jet engines and other aerospace applications. Fluid-film bearings, on the other hand, provide damping response via the film itself.

For further details on this subject, see author's "Applied Tribology-Bearing Design and Lubrication," Wiley Book Co., N.Y. 2001

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