Principles of journal bearing design

Monday, October 12, 2009

Tags :  bearings

bearingsThe purpose of a bearing is to support a load. Deflection of the journal within the bearing can adversely affect the load carrying ability of the bearing. This deflection can be greatly reduced by increasing the diameter of the journal and decreasing its length. This results in a short bearing with a consequential greater flow of oil out of the ends of the bearing. This outflow of oil transfers heat from the bearing and helps to reduce the bearing temperature.
The factor which controls the length of the bearing to the journal diameter, is known as the L/D ratio. For ratios greater than unity a long bearing would result and for ratios less than unity a short bearing results. If the roughness of the sliding surfaces is reduced, the load carrying capability of the bearing is increased. The requirement for a very good quality finish of the bearing surfaces is an important means of increasing load carrying capacity. Applying suitable surface treatments is a useful addition to bearing design.

Clearances should be small enough to obtain the maximum load carrying capability of the bearing. If the clearance is too small the bearing temperature will be too high and the minimum film thickness will be too low. As the bearing wears, the effect on the bearing performance must be considered, as this leads to a decrease in the bearing temperature and an increase in the flow of oil through the bearing with a knock on effect on hydrodynamic and boundary lubrication.

Lubrication

The success of modern journal bearings is due to the understanding that lubrication, the function of which is to separate the surfaces, is an integral part of journal bearing design. A journal bearing may be subject to four types of lubrication, namely: Hydrodynamic, hydrostatic, boundary and solid film. However, only hydrodynamic and boundary lubrication will be considered here.

Hydrodynamic lubrication does not depend upon the introduction of lubricant under pressure, but it does require an adequate supply of oil at all times. The oil film pressure is created by the moving surface dragging the lubricant into a wedge shaped zone at a sufficiently high velocity to create the pressure to separate the surfaces. Heat is generated within the bearing due to the work performed by shearing of the oil film as the journal rotates in the bearing. A pressure fed system is used to force greater oil flow through the bearing to improve cooling. Hydrodynamic lubrication is sometimes referred to as full film, fluid, or thick film lubrication.

Boundary lubrication occurs when the build up of full film lubrication is not possible. This may occur with a change in operating conditions leading to very high bearing temperatures, or during periods of start up and shut down. Bearing wear which results from thin-film lubrication may also be improved by the inclusion of additives to the lubricant or surface treating the bearing. When a bearing operates under hydrodynamic and thin film conditions together, mixed film lubrication is said to exist.

Instability of the oil film known as Oil-whirl or Oil-slip has not been considered here, while solid film lubrication has also been ignored.

Bearing Materials

A bearing material must possess compressive strength to withstand the gas pressure loading and fatigue strength to withstand the cyclic loading of the rotating journal and temperature variations. It must be soft enough to allow for wear and the embedding of foreign particles, but it must also have a low modulus of elasticity to allow for deformation, within its elastic limit. The rate of wear of the bearing material and a low coefficient of friction are also important considerations while other considerations of bearing materials include reliability and corrosion resistance.

Typical bearing materials for high performance engines would be copper-lead and lead-bronze. Some copper-lead bearings may be given a thin coating of pure lead followed by an even thinner film of indium. The lead and indium diffuse into one another, providing a coating to the copper-lead bearing. This coating is intended to last the life of the bearing, assists in the running-in process and provides surface protection.


Written by John Smith.

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Comments

The soft, low strength very thin surface layers (tin-lead, nickel, copper, etc.) of a typical automotive journal bearing don't have sufficient compressive strength for the loads applied.  The perform critical functions such as embedability, improving heat transfer, and providing improved adhesion between layers.  The steel backing is what provides the compressive strength.  Peak hydrodynamic oil film pressures can easily exceed 20,000 psi, and the bearing will be subjected to a very high number of load cycles.  So having a bearing shell material with adequate compressive fatigue strength is very important.

The drawback with steel shells is that the steel has relatively low thermal conductivity.
Well written. For further reading I would suggest a descriptive piece by  Fred C. Wiesinger, Jr. http://www.pioneer1.com/the-double-edged-sword-of-high-pressure-jacking-oil-system-of-hydrodynamic-tilting-pad-thrust-bearings/