Page 187 - Tribology in Machine Design

172 Tribology in machine design

4.15.11. Lubric ati on of seals
The initial assumptions used in analyses of narrow seal face lubrication are
based on the one-dimensional incompressible Reynolds equation

where r is the radial coordinate, h is the film thickness, p is the pressure, /i is
the viscosity, co is the angular velocity and 0 is the angular coordinate.
Fluid film models for seals do not allow for the dynamic misalignment
and other motions that are characteristic of all seal faces; in real seal
applications there are important deviations from the concepts of constant
face loads and uniform circumferential and radial film thicknesses. Also, the
interface geometry is markedly influenced by the manufacturing processes,
deformations and the interface wear processes, as well as by the original
design considerations for film formation. The properties and states of the
fluids in the seals vary, so that solid particles, corrosive reactions, cavitation
phenomena and theology changes may be critical to the formation of a
lubricating film. Also, it has been observed that the size of the wear particles
and the surface roughness can determine the leakage gap and thereby
establish the film thickness. Circumferential waviness in seal faces may
result from planned or unplanned features of the manufacturing processes,
from the geometry of the structure supporting the nose-piece or the primary
ring, from the mechanical linkage, i.e. drive pins, restraining radial motion
in the seal assembly and perhaps from several other factors. These fluid
film-forming features seem to occur because of random processes that cause
inclined slider geometry on both macro and micro bases. Micro-geometry
of the surface may be determined by random wear processes in service. It is
reasonable, however, to anticipate that desired macro-geometry waviness
can be designed into a sealing interface by either modifying one or both of
the sealing interface surfaces or their supporting structures.
Hydrodynamic effects of misalignment in seal faces have been analyti-
cally investigated and shown to provide axial forces and pressures in excess
of those predicted for perfectly aligned faces. Misalignment of machines,
however, cannot usually be anticipated in the design of seals for general
industrial use. Misalignment can be designed into either the mating ring,
the primary ring or the assembly supporting the primary seal ring. Using a
floating primary seal ring nose-piece, misalignment can be conveniently
achieved. However, with a rotating seal body (including the seal ring) the
misalignment would be incorporated into the mounting of the mating ring.
Hydrostatic film formation features have been achieved in several commer-
cial face seals (in several instances with a converging gap) by a radial step
configuration, and by assorted types of pads and grooves. These are
essentially so-called tuned seals that work well under a limited range of
operating conditions, but under most conditions will have greater leakage
than hydrodynamically-generated lubricating films at the sealing
interfaces.