The right friction bearing for gear pumps

What role do friction bearings play in gear pumps?

Gear pumps are rather simple in design because they consist of only a few, but high-quality components:

  • Friction bearing
  • Gear shafts
  • Housing parts
  • Axial shaft seal

Gear pumps are usually equipped with medium-lubricated friction bearings, but in rare cases also with roller bearings. The bearings play an essential role in gear pumps. They ensure that the gear shafts can run evenly and roundly. The special feature of the bearings is that they are lubricated by the pumped medium.

 

 

 

Landscape

What is the functional principle of a friction bearing?

The functional principle appears simple at first glance.

A bearing bush accommodates a journal. The bearing is used to support the load during shaft rotation. With the gear pump, the load is created by the differential pressure.

The differential pressure is the difference in pressure ratios between the suction side (pump inlet) and the pressure side (pump outlet).

The friction bearing has one, if not the most important task in the gear pump. Optimally dimensioned, it ensures wear-free shaft rotation in the bearing shell (even under high load).  By building up a hydrodynamic lubricating film, the two surfaces (journal and bearing shell) are separated from each other.

The hydrodynamic lubricating film is created by the rotation of the gear shaft. The rotating movement drags the lubricant (in this case the pumped medium) into the bearing gap. The medium is compressed and accumulated at the narrowest point in the gap.

At this point the hydrodynamic pressure build-up takes place. The pressure build-up is coupled to the viscosity of the medium and the rotational speed of the shaft. If the pressure build-up is sufficiently large, the shaft is lifted in the bearing shell and the surfaces are separated. If the lubricating film is not sufficiently thick, there is a risk of tarnishing, which in the worst case leads to seizure of the shaft in the bearing. To avoid this risk, it is important that the bearing is matched to the shaft, viscosity and speed in the dimensioning. This also applies with regard to the materials used.

How are friction bearings designed?

The bearing system is designed in such a way that the pumped medium builds up a supporting lubricating film through grooves and channels provided by the shaft rotation. The medium flows into the bearing at one point and out again at another point. By reducing the gap between the journal and the bearing wall, a hydrodynamic lubricating film is built up between the bearing and the gear shaft. This type of bearing is called a hydrodynamic friction bearing. The gear shaft is supported by the hydrodynamic lubricating film in the bearing. In order to ensure that this is reached, a certain shaft speed is necessary, which draws the medium into the groove or the bearing gap and compresses it. The speed required is strongly dependent on the viscosity of the medium.

In contrast to hydrostatic friction bearings, where the lubricant is supplied by an external pump, hydrodynamic friction bearings do not require any additional units.

What should be considered when selecting materials?

To ensure reliable operation of the pump, the material of the friction bearing must also be matched to the material of the gear shaft.

As a rule, different materials are used, whereby the bearing material is chosen to be somewhat softer than the gear shaft material.

In the case of low viscosities (media with low hydrodynamic load-bearing capacity), care must also be taken to ensure that the gaps (clearances) between the bearing and the journal are correctly selected so that the pump can also pump the desired quantity. If the clearance is too large, the pumps may have poor efficiency and not deliver the desired performance. If the clearance is too small, the shaft will start to run in the bearing and, in the worst case, seize.

When selecting materials, it is imperative to consider the thermal expansion coefficient of the material in relation to the other materials used. Only in the sum of the right material combinations, taking into account the thermal expansion coefficient of the material, the right speed as well as the sufficient load capacity of the medium, can the pump be operated reliably.

What are the bearing types?