Babbitt Bearing Design Considerations

Journal Bearings:

The sliding bearing, also known as a journal or sleeve bearing, is a plain shell bearing that is used to support rotating shafts or journals. These bearings are anti-frictional and have a cylindrical or semi-cylindrical bushing made from a suitable, soft material. They are generally classified into full, partial, and fitted bearings depending on the extent to which the bearing envelops the journal. A Babbitt bearing is any journal or shell bearing that is coated with a layer of Babbitt material on the contact surface. Some typical applications of journal and Babbitt bearings include automotive engines like connecting rods and crankshaft that offer high temperature, speed, as well as varying load.

Bearing Materials:

An integral part of designing a bearing is selecting the material, since the bearing materials constitute an important part in the journal bearing. This maxim is particularly important for Babbitt bearings. The materials that are chosen are especially significant when considering the beginning of the hydrodynamic lubrication process, when contact occurs between the rotating shaft and the bearing. The bearing materials must be soft enough to resist the wear and endure load under gas pressure. In addition, they must also possess good compressive and fatigue strength. While the compressive strength is to withstand the temperature variations, the fatigue strength is to resist cyclic loading of the journal when it is revolving. Finally, the material should also have a low shear strength aimed at easy smoothing of surface asperities.

Some other important considerations of the bearing materials include a low coefficient of friction, reliability, and resistance to corrosion. In addition, they must also have good thermal conductivity to dissipate the frictional heat. Considering all these requirements, Babbitt alloys are one of the most commonly used bearing materials.

Composition of Babbitt Materials:

Babbitt alloys possess excellent conformability and embeddability. These materials are not used in applications where operating temperatures exceed 300°F, but have relatively low compressive and fatigue strength. The commonly used compositions of bearing materials are:

Tin-base babbitts with 89% Sn, 8% Pb and 3% Cu
Lead-base babbitts with 75% Pb, 15% Sb and 10% Sn
Copper alloys such as Cu- 10% to 15% Pb

Babbitts can also be made of other materials, such as tin or leaded bronze, copper lead alloy, aluminum bronze or alloys, and cast iron, which are widely used in many applications.

Principles of Journal Bearing Design:

The role of a bearing is to provide relative positioning and rotational freedom while transmitting the load of the shaft to the bearing housing. The factor, which controls the length of the bearing to journal diameter, is known as the L/D ratio. When this ratio is greater than unity, it results in a long bearing, while a ratio less than unity would result in short bearings.

The deflection produced by the journal within the bearing can adversely affect its load carrying ability. This deflection can be greatly reduced by increasing the diameter and decreasing the length of the journal. The resultant short bearing has a greater flow of oil at the ends, which helps in transferring the heat from the bearing and consequentially reducing the bearing's temperature.

The load carrying ability of the bearing can be increased by reducing the roughness of the sliding surface. A good quality finish and minimum clearance can ensure that the load carrying capability of the bearing is maximized. However, the clearance should not be too small, as it would result in higher bearing temperatures and reduced film thicknesses. Surface treatment also improves the bearing design.

Important Factors for Designing Hydrodynamic Bearing:

Most Babbitt bearings are operated as hydrodynamically lubricated bearings and the design considerations for hydrodynamic bearings are significant as well. To ensure minimized friction and bearing wear, the bearing material for hydrodynamic bearings should have moderate shaft deflection and adequate strength at operating temperature conditions. At the hydrodynamic bearing inlet, there must be continuous and adequate supply of clean and cool oil.

Additional grooves can be machined into the bearing for easy distribution of oil that covers the whole journal. The appropriate combination's of clearance and oil viscosity for any hydrodynamic operation condition should be chosen correctly by taking into consideration the load and time elapsed during start-up and shutdown process. Two other important considerations for designing hydrodynamic bearings include the heat dissipation and friction. The lubrication systems should be set up such that lowest temperatures for the bearings are achieved with minimum oil film thickness. Similarly, friction should be kept as low as possible to refrain from power loss.

Ron Bargman, president of Zycon.com, has been fascinated and involved with the engineering and manufacturing processes required to turn ideas into products for over 30 years. Mr. Bargman is a regular contributor of manufacturing theme articles, and his rich industry history provides insight into manufacturing and engineering events and changes that are timely, poignant, and relevant. Through Zycon, he is able to transfer his passion for the industry by assisting engineers, designers and inventors find the services, parts and components that they need to succeed.

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