Bearing cage technology improves screw compressor reliability

NSK's patented L-PPS bearing cage technology is providing highly effective- both in terms of long-term performance and competitive cost - for a manufacturer of screw compressors in the refrigerant sector. The manufacturer was receiving frequent complaints from customers about compressor failures, which were resulting from damage to bearings operating under continuous high temperatures above 70degC.

The problem experienced by the manufacturer was not uncommon in an industry that is demanding ever increasing trouble-free life and longer intervals between overhauls for compressors. In addition to the greater load carrying capacity and improved lubrication capability demanded from bearings in such compressors, other key requirements are excellent thermal and chemical stability.

NSK was called in by the screw compressor manufacturer following the failure of bearing cages in a competitor's range of bearings. The cages were manufactured either from brass or polyamide but, unfortunately, both of these materials were failing prematurely. The brass cages exhibited stress corrosion cracking, while the operation of the polyamide cages was impaired by lubricant at high operating temperatures.

After examining the failed bearings, NSK recommended the use of bearings with cages made from L-PPS (linear polyphenylene sulphide) material. Covered by global patents, L-PPS was developed by NSK using a special manufacturing process. This has enabled bearings equipped with L-PPS cages not only to offer greater performance and life, but provide these benefits at a competitive price compared with most competitors' products, which are either brass or Peek.

L-PPS is a glass fibre-reinforced plastic designed for high temperature use, at up to 190degC. The material also has a high level of chemical resistance and thermal stability. Bearings equipped with L-PPS cages also offer greater load-carrying capacity and have much improved lubrication capability.

Following the introduction of bearings equipped with L-PPS cages, complaints of failed bearings to the screw compressor manufacturer ceased. This meant that the manufacturer was able to minimise his costs regarding repair and replacement of bearings and provide enhanced guarantees as to the long-term operation of his compressor products.

Although initially developed for compressor bearing applications, the advantages of bearings employing L-PPS plastic cages are applicable in other applications where high temperature and/or special conditions exist. Additional benefits of the material are that its strength and chemical stability change little, even when used with compressor oil, refrigerator oil or ammonia gas. Cages manufactured from L-PPS are available on both the new types of cylindrical roller and angular contact bearings that NSK has developed for use in screw compressors. In both types of bearings, substantial improvements in load capacity have been achieved by redesigning the bearing interior to take full advantage of the L-PPS plastic cages.

For further information about NSK's L-PPS bearing cage technology, go to the website at www.nskeurope.com.