Selection of anti-friction bearings to reduce losses

March 17 2013 - William E. Forsthoffer

Proper anti-friction bearing selection, careful evaluation of low loaded bearings and review of vendor bearing load experience can eliminate the possibility of bearing skidding. Complete gearbox damage has been caused by the use of high DN, low loaded anti-friction bearings in indeterminate bearing systems in multi-stage, high torque reduction gears.

Anti-friction radial bearings support the rotor, by using rolling elements to reduce friction losses. They are used in low horsepower applications (below 375 KW), high torque gear boxes and in aeroderivative gas turbines.

All bearings are designed to have sufficient bearing area to support all the forces acting on the bearing. For anti-friction bearing applications, the pressure on the bearing elements is the point contact or ‘Hertzian’ stress on the bearing elements and rings or ‘races’.

For an anti-friction bearing to be properly designed, its D-N number (measure of the rotational speed of the anti-friction bearing elements) and bearing life must be determined. Each type of anti-friction bearing has a maximum operating D-N number. If this value is exceeded, rapid bearing failure can occur. In addition, D-N numbers are typically used to determine the type of lubrication required for bearings.

A common practice in the turbomachinery industry has been to use hydrodynamic pressurized bearings when the D-N number exceeds approximately 500,000. The exception to this rule is aircraft gas turbines, which can have D-N numbers in excess of 2,000,000. In these applications, hydrodynamic bearings are not used, since the size and weight of the required lubrication system would be prohibitive.

Anti- friction bearings have a finite life which is usually specified as B-10 or L-10 life. The life of any anti-friction bearing is inversely proportional to cube of the bearing loads. As a result, a small change in the journal bearing loads can significantly reduce the bearing life.

When anti-friction bearings suddenly start failing where they did not in the past, investigate all possible sources of increased bearing loads (piping forces, foundation forces, misalignment and unbalance). Anti-friction bearings are designed for a minimum life of 25,000 hours’ continuous operation.