Page 175 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 161

lip seals. Lubrication of the sealing interface varies from nil to full
hydrodynamic and wear can vary accordingly. Wear of abradable shroud
materials is utilized to achieve minimum operating clearance for labyrinth
seals and other gas-path components like turbine or compressor blade tips
to achieve minimum leakage. The functions of seals are also of great
importance to the operation of all other lubricated mechanical com-
ponents. Wear in seals can occur by a variety of mechanisms. A cause of
wear in many types of mechanical systems is contamination by abrasive
particles that enter the system through the seals. Design features in seals
that exclude external contamination from mechanical systems may be of
vital importance. Seals are also important to energy conservation design in
all types of machines. The most effective leakage control for contact seals is
achieved with a minimum leakage gap and when both sliding faces, moving
and stationary, are flat and parallel. This condition is perhaps never
achieved. That is probably fortunate, since a modest degree of waviness or
nodel distortion can give rise to fluid film lubrication that would not be
anticipated with the idealized geometry. With distortion, wear of either
internal or external edges can cause the nose piece to form a leakage gap
that can be convergent or divergent. Changes in the leakage gap geometry
have significant effects on the mechanics of leakage, on the pressure
balance, and on the susceptibility of lubrication failure and destructive
wear.
One of the wear mechanisms which occur in seals is adhesive wear. With
adhesive wear, the size of the wear particles increases with face loading. An
anomaly of sealing is that as the closing forces on the sealing faces are
increased to reduce the leakage gap, the real effect can be larger wear
particles that establish and increase the gap height and thereby increase
leakage. Also, greater closing force can introduce surface protuberances or
nodes from local frictional heating, termed thermoelastic instability, that
may determine the leakage gap height. The leakage flow through a sealing
gap obeys the usual fluid mechanics concepts for flow. In addition, there are
likely boundary layer interactions with the surfaces in an immediate
proximity. In this chapter, design considerations to control the wear and to
optimize wear reducing fluid lubrication will be discussed. For guidance on
the selection of a proper seal for a particular application, the reader is
referred to ESDU-80012 and ESDU-83031.

4.15.1. Operation fundamentals
The most important mechanism for sealing fluids between solid bodies is
that of surface tension. By using various concentrations of a surface-active
agent in the water phase, it is easy to demonstrate that the rate of leakage in
an oil-water system depends on the oil-water interfacial tension. The usual
formula to calculate the pressure due to capillarity is