Bearings in compressor and pump development: a guide

A wide range of industries require pumps and compressors for a variety of processes. Often they perform in the background, but any failure can result in a sudden loss of productivity and maintenance costs can spiral. Pump and compressor manufacturers, however, continue to reduce the size of their product to provide a more compact offering while also looking to extend the service life and the mean time between failures (MTBF).

This mix of client and designer demands places increased pressure on the ball and roller bearings installed in each piece of equipment to satisfy those needs, which is where continuous investment in development is crucial in order to keep pace.

Depending on the design, the bearings in a pump can run under axial loads, radial loads or a combination of both. Running speeds can vary enormously, as can the lubrication conditions, but the requirement for long life, or a high MTBF, remains a constant challenge in the tough conditions of the pump environment.

In compressor design, the bearings are also crucial in maintaining the position of the rotors in both the axial and radial direction - this is very important as the efficiency and performance of the compressor is directly linked to absolute control over the rotor position. As a result, the bearing design must ensure that the axial and radial deflection is kept to a minimum, while still providing a high MTBF.

Pumps come in many shapes and sizes, including centrifugal, piston, vortex, impeller and screw types as well as specialist designs such as those dedicated to the cryogenic and LNG (Liquefied Natural Gas) process for example. Each of these has their own requirements in bearing design, which has resulted in NSK providing specific options for each application.

Increasing dynamic loading

The centrifugal pump requires a bearing that can withstand high axial loads while ensuring minimal axial free play. One approach is the High Capacity Deep Groove ball bearings (HR series), which have larger rolling elements than standard deep groove ball bearings. This design can increase the dynamic loading by up to 19 per cent as well as improving the life of the equipment.

Piston pumps operate under heavy axial and radial loads, with a requirement for precise bearing mounting and compatibility with the lubricating oil. In this case, NSK has developed a cylindrical roller bearing with the special L-PPS cage, which has superior heat and wear resistance plus chemical stability.

Submersible pumps are widely used in a variety of applications such as construction, waste water, mining, agriculture and general industry. Most submersible pumps comprise a vertical electric drive motor coupled directly to the impeller. They are required to operate for long periods with minimal maintenance. The top bearing in the pumps should have a very long life, but this can be drastically reduced by creep. The combination of a light radial load and a loose fit in the housing can result in premature bearing failure. NSK has developed a creep-free product which integrates double o-rings into the external surface of the bearing. By increasing the MTBF of this component, maintenance costs are reduced as well as improving productivity.

LNG pumps operate at very low temperatures, down to -196degC and operate under very poor lubrication conditions. The bearings are immersed in LNG, which means the bearing needs to be self-lubricating and have good corrosion resistance while operating at speeds up to 3600rpm.

To cope with these tough conditions, NSK has developed deep groove ball bearings using special materials. Rings and balls are made from martensitic stainless steel to provide corrosion resistance. The bearing cage is made from Fluororesin, a material that offers the very useful property of being self-lubricating, even at very low temperatures. This is significant because during operation, the cage - a two-piece unit held together by special rivets - actually transfers a thin film of Fluororesin from the retainer pocket to the ball, and then to the raceway surface, maintaining good lubrication conditions. NSK LNG pump bearings extend service intervals in this very demanding environment.

The variety of compressors covers screw type, roots blower and piston compressor, which again have their own individual requirements. NSK has again developed a specific approach to match the product.

Screw compressors require bearings that offer specific axial and radial clearances to provide precise screw guidance. In the case of an oil-free screw compressor the bearings must have good high-speed performance as well as excellent heat resistance.

On the other hand, the oil-injected screw compressor bearings must withstand higher axial and radial loads with a cage that will be compatible with special oils. NSK has developed the cylindrical roller bearing with L-PPS cage and the HPS Angular Contact Ball bearing, with L-PPS cage.

Extending bearing life

This bearing has an increased load capacity, up by 5 per cent compared with the previous series, which can extend bearing life by up to 18 per cent. Improved manufacturing technology has also improved the axial clearance and preload tolerance, which is now 8-12um, to ensure precise shaft positioning.

Roots blowers, or positive displacement blowers, are a very basic form of pump, commonly used as an air compressor or supercharger. As such, they operate in an environment with medium to high temperatures and some vibration. The bearings required for such a function are a combination of cylindrical roller bearings, angular contact bearings and deep groove ball bearings, which need to provide a high MTBF so as to avoid expensive down-time and loss of productivity.

The piston compressor operates on the same principles as the internal combustion engine and as such places a high radial load on the bearings due to the reciprocating motion. Again there is a combination of cylindrical roller bearings and High Capacity deep groove ball bearings required, whose construction must ensure long bearing life under tough conditions. By installing the HR series of bearings, the dynamic load rating is improved by up to 19 per cent, improving the overall service life of the bearing.

More information can be found on this vast subject in NSK's free "guide to bearings for use in compressor and pump applications'. It contains detailed information on these specific applications and the bearings most suitable to ensure efficient and reliable service. It also demonstrates the most recent developments as well as industry case studies. The guide is available in both paper and electronic format; the pdf can be downloaded directly from the NSK website.