MECHANICAL SEAL WITH ELASTOMERIC ROTATING ELEMENT .1. THEORY OF OPERATION

This paper presents the results of fundamental research on flexible fa ced mechanical shaft seals. The effect of stretch and preload on the c ontact stress were investigated using a nonlinear finite element model ing package. The characteristics of the rubber materials including the stress-strain curves to elongation at break were required to complete the analysis. Pressure forces resulting from the static pressure in t he pump stuffing box as well as the friction effects on all contacting surfaces were considered in the model. Stress distribution across the contact surface established the basis for the formation of multiple d istinct lips: at least one on the outside or fluid side of the seal an d the other on the inside or air side of the seal. This phenomenon is explained by the step change in magnitude of axial stress while traver sing the contact in the radial direction allowing the formation of ext erior lips and the development of an annular region trapping debris pr oducts and process fluid. Photomicrographic images of the wear track t aken with a Scanning Probe Microscope show distinct wear tracks and th e topology of each. Although the chemical composition of the debris in the annular spaces has not been determined quantitatively, increased levels of fluorine were present, suggesting fragments of the monomers in the elastomeric rotary or migration of Teflon fillers. Axial variat ions in the stretched rotary dimensions (waviness) affects seal perfor mance. Statistics for a typical batch of rotary elements are presented and a method of factoring these variations into the seal design is pr esented. The formation of axial waves on the contact vertex affects th e seal performance and is considered in the final design of the seal p air configuration. The results of this work were incorporated into a s uite of mechanical face seals covering shaft sizes from 1.125 to 5.000 inches using seven pairs of seals, each pair consisting of a stretche d rotary sliding against an unlapped silicon carbide annular ring. Eac h of these seal pairs were tested on an experimental test device to de termine the heat generation and friction coefficients for a wide range of temperature, pressure, speed, and sealing fluid. Results of the ex perimental work are reported in a companion paper.1