Methods for Calculating Axial Thrust in Turbine Pump Motors

On many occasions field engineers must make a decision on the following conditions based on thrust:

1. Bearing required to meet downthrust.
2. Bearing life in excess of 5 years average.
3. Upthrust conditions that affect life
4. Competitive bearing and thrust evaluations.

Generally thrust is the main factor in determining what bearing is used to meet pump applications. Therefore, it is necessary to be able to calculate expected life and capacity.

Factors in Life of Bearing:

1. Standard downthrust on U.S. MOTORS® products is 1 year (8800 hours) minimum and 5 years average per AFBMA B-10 method.
2. Momentary upthrust is calculated at 30% of standard downthrust. Thrust life is normally not a factor since bearings only operate in this condition for a few seconds at start.
3. Continuous up and downthrust required on some applications are rated 1 year (8800 hours) B-10.
4. Life of a bearing is inversely proportional to the speed at which it turns, and inversely proportional to the cube of the load or thrust.
5. Theoretical life is usually never attained due to contamination and inadequate maintenance procedures.

Terminology:

• C = Thrust rating from chart for 10⁶ revolutions of life.
• F = actual thrust applied plus weight of rotor.
• Y = Bearing thrust factor from chart
• N = RPM of motor.

Basic Life Formula - Ball Bearings

Life in Hours = ( C / P )³ * 10⁶ = ( C / FY)³ * 10⁶

Note: Life adjustment factors may be applied based upon temperature and environmental factors that can increase or decrease the calculated life.