Stiffness Ratio

This is another name for L3/D4 . You will also hear it referred to as the:

  • Slenderness ratio or
  • Flexibility factor

It is a guideline for determining mechanical seal reliability in single stage centrifugal pumps with an overhung impeller. It is an indicator of how seal and bearing reliability relates to pump shaft stiffness.

In this ratio the length of the shaft cubed is compared to its diameter to the fourth power. The number should be below 60 in imperial units (inches) and 2.0 in metric units (millimeters).

The length of the shaft is measured from the center of the radial bearing to the center of the impeller vane. The diameter is measured on the solid shaft and beneath the sleeve if one has been installed

Please see “shaft bending, the L3/D4 formula” for information on how to calculate the L3/D4 of a single stage centrifugal pump with an overhung impeller.



  • On February 17, 2018