May 15, 2008
Pump Tip
Slenderness Ration
Slenderness may be good for your waistline but it is not very desirable for your overhung pump shafts. A quick way to ensure that your overhung pumps have the robustness to withstand operating far from the best efficiency point is to determine their slenderness ratio. The ratio is calculated by taking the cube of the length (L) from the center of the inboard bearing to the overhung end of the shaft and dividing it by the diameter (D) of the pump shaft at the stuffing box (under the sleeve) to the fourth power. As you can see from the formula, large diameter shafts that have limited overhangs from the inboard bearings are desirable. Ratios of twenty-five or less should handle the unevenly distributed pressure in the pump volute caused by throttling without allowing shaft flexing that causes premature seal or bearing failures. It is a good exercise to check the slenderness ratio of your problem pump to see if there are opportunities for improvement.
Tip provided by Jim Hudson
Trico Corp.
800-558-7008 (USA only)
262-691-9336
