JNTU B.Tech III DESIGN OF MACHINE MEMBERS - II Previous Question Paper (November/December - 2020 - R16)

Sliding contact bearings, also known as plain bearings or journal bearings, operate on the principle of sliding motion between the mating surfaces. They are categorized based on the mode of lubrication:

• Hydrodynamic Lubricated Bearings: These bearings operate with a thick film of oil that completely separates the journal from the bearing surface. The pressure is generated by the relative motion of the journal and the bearing, which drags the lubricant into a converging wedge.
• Hydrostatic Lubricated Bearings: These bearings use an external pump to supply lubricant under high pressure to the bearing clearance. This ensures a complete film of oil even at zero speed, making them ideal for heavy loads at low speeds.
• Boundary Lubricated Bearings: In these bearings, the film of lubricant is very thin, and there is significant contact between the asperities of the sliding surfaces. These are typically used where the speed is low or the lubricant supply is intermittent.
• Solid Film Lubricated Bearings: These use solid lubricants like graphite or molybdenum disulfide, suitable for extreme temperatures or vacuum conditions where liquid lubricants would fail.

Sketch description: A typical journal bearing consists of a cylindrical shaft (journal) rotating inside a stationary sleeve (bushing). In hydrodynamic lubrication, the eccentricity between the journal and bearing creates a wedge-shaped space that generates the load-carrying pressure film.

For the formation of a thick film of oil (hydrodynamic lubrication) in a bearing, the following factors are essential:

• Relative Motion: There must be a continuous relative velocity between the journal and the bearing surface. This motion is necessary to drag the lubricant into the clearance space.
• Converging Geometry: The bearing clearance must be shaped such that it creates a wedge. As the lubricant is dragged into this narrowing space, the fluid is squeezed, generating the hydrodynamic pressure required to support the load.
• Lubricant Viscosity: The lubricant must have an appropriate viscosity to maintain the film. If the viscosity is too low, the film will be too thin; if too high, it may cause excessive friction and heating.
• Clearance: A specific radial clearance between the journal and the bearing is required. This space allows for the formation of the oil wedge and accommodates the lubricant flow.
• Load and Speed Balance: The load must be within the capacity of the generated pressure film. The Petroff's Equation or the Sommerfeld Number is often used to analyze the relationship between these parameters (load, speed, viscosity, and geometry) to ensure stable film formation.

The formation of this film is governed by the Reynolds equation, which relates the pressure distribution to the film thickness and the geometry of the bearing.