Journal Bearing Design Parameters and Selection - Ques10

In most of the cases, preferably in the preliminary stages of journal bearing design, it is required to select suitable values for the following parameters:

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(i) length-to-diameter ratio - The length to diameter ratio (l/d) affects the performance of the bearing. As the ratio increases, the resulting film pressure increases as shown in Fig.

A long bearing, therefore, has more load carrying capacity compared with a short bearing. A short bearing, on the other hand, has greater side flow, which improves heat dissipation. The longer the bearing, the more difficult it is to get sufficient oil flow through the passage between the journal and the bearing.

(ii) unit bearing pressure - The unit bearing pressure is the load per unit of the projected area of the bearing in running condition. It depends on a number of factors, such as bearing material, operating temperature, nature, and frequency of load and service conditions.

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$(P)=\frac{W}{L \times D}$

(iii) start-up load - The unit bearing pressure for starting conditions should not exceed 2 MPa. The start-up load is the static load when the shaft is stationary. It mainly consists of the deadweight of the shaft and its attachments.

(iv) radial clearance - The radial clearance should be small to provide the necessary velocity gradient. However, this requires costly finishing operations, rigid mountings of the bearing assembly and clean lubricating oil without any foreign particles. This increases the initial and maintenance costs. The practical value of radial clearance is 0.001 mm per mm of the journal radius.

(v) minimum oil film thickness - The surface finish of the journal and the bearing is governed by the value of the minimum oil film thickness selected by the designer. There is a lower limit for the minimum oil film thickness, below which metal to metal contact occurs and the hydrodynamic film breaks. This lower limit is given by $h_o = 0.r$