Consider that modern machines mostly use short bearings due the following reasons: 1) l/d of the most modern bearings is in the range of 1.4 to 2 2) No end leakage of oil from the bearing 3) Shaft deflection and misalignment do not affect the operation 4) Can be applied to both hydro-dynamic and hydrostatic cases Which of the above are correct?

Consider that modern machines mostly use short bearings due the following reasons: 1) l/d of the most modern bearings is in the range of 1.4 to 2 2) No end leakage of oil from the bearing 3) Shaft deflection and misalignment do not affect the operation 4) Can be applied to both hydro-dynamic and hydrostatic cases Which of the above are correct? Correct Answer 1 and 3

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. In practice it varies from 0.5-2.

For short bearing shaft misalignments and deflections are handled by the fluid film forces. Hence, operation is not affected.

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.

Hydrodynamic bearing is used for long bearing so that oil reaches the end. End oil can only go in small bearings, hence end leakage is possible. Small bearings are most suitable for hydrostatic bearing.