Bearing

1-Plain bearing

The plain bearing is an element frequently used in centrifugal pump construction that allows a moving component to slide within a stationary component. A distinction is made between radial plain bearings volcano for radial forces (transverse forces) and axial (or thrust) plain bearings for axial forces (longitudinal forces).

Radial plain bearing Volcano

On radial plain bearings, the moving part is the pin or journal of the axle or shaft; the stationary part is the bearing shell.

Bearing  Volcano shells and other variants

1. a Cylindrical bearing shell
2. b Two-face, lemon-bore bearing shell
3. c Two-face, radially offset bearing shell
4. d Three-face bearing shell
5. e Three-face and multiple-face bearing shell with lubrication grooves or pockets
6. f Rubber bearing
7. g Multiple-face bearing with tilting radial pads

This wide range of bearing designs is required to cater for the dynamic operating behaviour characteristic of centrifugal pump rotors. The vibration characteristics of rotors fitted with plain bearings depend largely on the rotor mass, the mass distribution, the shaft stiffness and the dampening characteristics of the bearings at a given load.

Appropriate rotor bearing design allows both types of lateral rotor vibrations(forced and self-induced) to be eliminated or reduced to an acceptable level for the machine. The dynamic bearing coefficients can be optimised. The choice of bearings is a key element of this optimisation, as each bearing offers different performance characteristics.
A bearing clearance is achieved through the appropriate sizing and mutual adjustment of both the moving and stationary bearing components.

This is filled with a liquid or solid (grease type) lubricant in order to avoid sliding friction. When the bearing journal reaches a sufficient circumferential speed, the bearing clearance allows the lubricant to form a load-bearing wedge. The lubrication wedge separates the sliding faces from one another, meaning that the bearing is operating on full lubrication. This process is typical of hydrodynamic plain bearings.

Advantages and disadvantages of hydrodynamic plain bearings Advantages:

• Simple manufacture; the lubricant is fed either unpressurised or at very low feed pressure into the bearing during operation
• Very little or no energy is required for the oil supply system

Disadvantages :

• During start-up and run-down, full fluid film lubrication is impossible, resulting in wear on the running surfaces (mixed friction) see Fig. 4 Plain bearing
• Another bearing type is the hydrostatic plain bearing. Here, the liquid lubricant is fed into the individual bearing pockets under high pressure.

Forces are absorbed as a result of pressure differences:

• High static pressure in the pockets on the loaded side of the running surface (small clearance during operation, therefore very small decrease in pressure in the lubricant layer)
• Low static pressure in the pockets of the unloaded side of the running surface (large clearance during operation, therefore considerable pressure drop in the lubricant layer)

Advantages and disadvantages of hydrostatic plain bearings:

• Advantage: full fluid film lubrication at all times, including start-up and run-down, therefore no excessive wear risk
• Advantage: smaller dimensions and lower friction losses in comparison with hydrodynamic bearings of equal load-bearing capacity
• Disadvantage: more expensive to manufacture than hydrodynamic plain bearings (several manufacturing operations)
• Disadvantage: more expensive to operate because pressure boosting is required for the lubricant, leading to increased investment and energy costs

Hydrodynamic and Hydrostatic Types

Hydrodynamic and hydrostatic types can be combined. In the case of plain bearings which operate hydrodynamically in the steady state, the increased friction during start-up and run-down and the wear associated with it can be reduced by providing auxiliary hydrostatic lubrication at a high pressure via longitudinal grooves which do not extend to the edge of the bearing shell.

The auxiliary lubricant feed is shut off during normal operation to ensure that the hydrodynamic pressure is maintained in the lubricating clearance gap.

Friction conditions in a plain bearing

• a. Dry friction:
  without a separating lubricant layer between the stationary and moving components
• b. Mixed friction:
  a combination of dry and fluid friction
• c. Fluid friction:
  with a separating lubricant layer (ideal situation)

All three friction types can occur in hydrodynamic plain bearings during the three phases of operation: start-up, operation, run-down. Start-up is the operating phase from standstill to full operating speed. As the sliding velocity increases, hydrodynamic plain bearings experience mixed friction, with the amount of dry friction gradually giving way to fluid friction as speed increases further. Finally the transition point is reached when the surfaces separate from one another and full fluid film lubrication with a minimum of friction losses is established.

As the sliding velocity increases further, the thickness of the lubricant film also increases, but friction losses rise again slightly.