L3/D4 L002

Sometimes called:

  • The stiffness ratio
  • The slenderness ratio.
  • The flexibility factor

A guide line for determining mechanical seal reliability in single stage centrifugal pumps with an overhung impeller. Its an indicator of how seal and bearing reliability relates to pump shaft stiffness. In this ratio the length of the shaft cubed (L3) is compared to its diameter to the fourth power (D4). The number should be below 60 in imperial units (inches) and 2.0 in metric units (millimeters) to resist shaft bending and deflection.

  • 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:

  • How to calculate the L3/D4 of a single stage centrifugal pump with an overhung impeller. S039
  • To see a practice calculation go to S040
  • L3/D4 and premature mechanical seal failure L003


  • On February 16, 2018