PEEK  bearing cages for refrigeration screw compressor

SKF PEEK cages boost refrigeration screw compressor speeds by 17%

PEEK cages
The challenge
A leading North American producer of transport temperature control systems needed a bearing performance upgrade solution for a screw compressor redesign.  Applications would include transport refrigeration for trailers as well as bus air conditioning.

The maximum speed of the bearing (or the rotor) used in the previous compressor design was 12,000 rpm, for which a pressed sheet brass bearing cage worked quite well. Needing to widen that operating range, the company looked to SKF.

The solution  
SKF engineers worked closely with the compressor manufacturer, considering several possible new bearing cage materials. These included pressed sheet steel and  machined steel cage and brass. None of them were able to handle the increased operating speeds. 

Extended operation range has been accomplished by changing the cage material and the cage design. The extended operation range corresponds to decreased viscosity of the base lubricant, increased operating temperature and increased operating
speed of the bearing. Ultimately, a moulded, glass fiber reinforced polyether ether ketone (PEEK) material was selected.

The moulded material help minimize costs, but it needed more room than the original pressed sheet brass cage. SKF compensated by reducing the number of balls from ten to nine. Typically this would decease the bearings basic load rating by 10 %, but SKF further compensated by using optimized, heat-treated steel and improving the surface finish on the balls and the raceways. 

The result
The maximum discharge temperature (bearing temperature) increased
to 148.9 °C (300 °F). The maximum speed of the bearing (or the rotor) was increased from 12,000 to 14,000 rpm – a 17 % speed increase. The value of the increased operating range more than balanced the individual bearing cost increase.
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