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Journal bearing works satisfactorily with L/D ratio in the range of: [where, L = Length of bearing, D = Diameter of journal]

Journal bearing works satisfactorily with L/D ratio in the range of: [where, L = Length of bearing, D = Diameter of journal] Correct Answer 1 - 2

Following Parameters are important in the selection of Bearing

Length to diameter ratio (L/D) ratio: In the design of bearings the diameter of the shaft is designed by the strength and rigidity criterion not on the bearing capacity.

The length to diameter ratio (L/D) affects the performance of the bearing. As the ratio increases, the resulting film pressure increases. A long bearing has, therefore, more bearing capacity compared with a short bearing. A short bearing has greater side flow which improves the heat dissipation.

The long bearings are more susceptible to metal to metal contact at the two edges, when the shaft is deflected under load. The longer the bearing, the more difficulty is there to get sufficient oil flow through the passage between the journal and the bearing. Therefore the design trend is to use (L/D) ratio as 1 or less than one.

Bearing Pressure: It depends upon number of factors such as bearing material, operating temperature, the nature and frequency of load and service conditions.

Start-up Load: The unit bearing pressure for the starting condition should nit exceed 2N/mm2. It mainly consists of the weight of the shaft and its attachments.

Radial Clearance: The radial clearance should be small to provide necessary velocity gradient.

c = (0.001) r

Minimum Oil Film Thickness: This is the lower limit beyond which metal to metal contact occurs.

ho = (0.002) r

Maximum oil film temperature: Lubricants tend to oxidise when the operating temperature exceeds 120°C. Therefore the operating temperature should be within the limits.

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