Page 176 - Tribology in Machine Design

162 Tribology in machine design

where y is the surface tension and R t and R 2 are the radii of the meniscus in
mutually perpendicular planes. In the case of parallel plane surfaces R can
be taken as infinity and RI as approximately h/2, where h is the separation
of the surfaces. Assuming a surface tension of 0.02 N m~ *, the thickness of
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the fluid film is about 5 x 10~ m and the pressure difference resisted by the
4
seal can amount to 8 x 10 Pa. Thus, in the situation depicted in Fig. 4.63,
4
where the fluid wets the surface, a pressure of 8 x 10 Pa acting from right to
left can be resisted. However, in the absence of this pressure the fluid would
Figure 4.63
continue to be drawn into the cavity with the interface advancing to the
right. It has been shown experimentally, that when the meniscus reaches the
end of the constricted passage it begins to turn itself inside out as indicated
in Fig. 4.63.
Owing to the contamination of engineering surfaces, the contact angles of
oil against synthetic rubber and steel under industrial conditions are found
to be high, so that the sealed oil does not spread along the steel shaft. In
addition to the equilibrium meniscus effect, any local variation of the
surface tension of a liquid induces a driving force to a fluid. This is known as
the Marangoni effect and its implications for the action of seals have been
investigated. When a temperature gradient exists on a solid surface, a
droplet of liquid laid on that surface will spread out more rapidly towards
the lower-temperature side. Even when conditions are generally isothermal,
differential evaporation of the constituents of a multicomponent liquid may
produce local variations in surface tension which markedly affect spreading
behaviour. The constituents of mineral oils having higher molecular
weights will tend to spread more rapidly by reason of their greater surface
tensions. This process promotes segregation of the constituents of blended
oils, thus depriving the high-temperature side (where lubrication is more
difficult) of the more effective components. Thus, the Marangoni effect can
account for differences in the sealing behaviour of apparently similar oils.
Generally, those with a narrow-ranged molecular weight distribution are
easier to seal than are blended oils characterized by wide-ranged
distributions.

4.15.2. Utilization of surface tension
The bearings of watches and fine instruments are lubricated by droplets of
fine oil which are kept in place by a surface tension mechanism known as
epilaming. The surface of the metal surrounding the joint is treated with a
surface active substance such as a fatty acid which prevents the lubricant
applied to the bearing from spreading.

4.15.3. Utilization of viscosity
If pressure is applied to a seal over and above that required to overcome the
surface tension, an estimate of the volume of leakage may be made using the