Friction bearings for gear pumps

What is a friction bearing?

The friction bearing is located in the pump housing and accommodates the gear shaft to give it stability during rotation and to ensure low-wear rotation. This is reached by the build-up of the hydrodynamic lubricating film, which absorbs the shaft load caused by the differential pressure.

In gear pumps, two bearings are used per gear shaft, one in front of and one behind the gear rim. The bearings are recessed in the housing. The gear pump has a total of 4 bearing bushes. The bearings are protected against rotation by the use of a feather key or torsion protection.

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What is meant by medium-lubricated friction bearings?

Medium-lubricated friction bearings are bearings that are lubricated by the pumped fluid and thus do not require separate lubrication by additional lubricants such as greases or oils. In order to avoid contact between the bearing bore and the rotating journal, the pumped medium must be able to form an appropriate supporting film.

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How does a medium-lubricated friction bearing work?

The friction bearing is lubricated by part of the product flow. Although this bearing-lubrication flow represents a leakage flow, it is relatively small, accounting for about 1 % of the main product flow, and is absolutely necessary for the reliable functioning of the pump. The pressure built up by the pump moves the pumped medium into and through the friction bearings, from where the pumped medium circulates through grooves and bores back to the suction side into the main product flow. The bearing-lubrication flow is proportional to the differential pressure, therefore it is essential to ensure a minimum differential pressure adapted to the viscosity when operating the gear pump. Otherwise, there is no longer a sufficient exchange of media in the friction bearings. Due to overheating and/or excessive shearing of the lubricating fluid, the journals in the friction bearings may start to tarnish and the pump may even fail.

In addition, special channels and grooves in the bearing support the bearing lubrication with highly viscous fluids. Normally, with low-viscosity fluids, the cylindrical bearing bore is sufficient to build up a sufficiently load-bearing lubricating film. The selection of the correct bearing clearance is important for low-viscosity media. With highly viscous media, the bearing clearance is negligible. Due to the rotation of the gear shafts in the bearing, the fluid is distributed along the direction of rotation. At the narrowing of the gap, a pressure is created which lifts the shaft and through its rotation the medium is dragged along, thus providing lubrication. The required hydrodynamic lubricating film is formed.

The lubricant film thickness depends on:

  • The speed
  • The viscosity
  • The bearing load (pressure load that the bearing must absorb)
  • The bearing geometries (lubrication bore reliefs, bearing clearances)

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