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TRIBOLOGY OF HIGH SPEED MOVING MECHANICAL SYSTEMS FOR SPACECRAFTS - TRIBOLOGICAL ISSUES
K. Sathyan
in high friction. However, this method is still used in some components
where friction is not so critical such as in clamps, release mechanisms
etc. Vacuum deposition technique is used to give a thin uniform coating
of lubricant to bearings used in precision mechanisms such as solar
array drive, slip rings and brushes, scanner bearings etc. In this process,
the film thickness can be accurately controlled. The film thickness is
dependent on the surface roughness and cleanliness of the substrate.
Therefore, before the coating process, the bearing surfaces are cleaned
by the sputtering technique. Usually, the thickness of the lubricant film
will be less than a micron. Sputtering and ion-beam techniques are used
to give uniform coating. This method is widely used to plate MoS 2 and
lead (Pb) ion on precision bearings that are exposed to hard vacuum. Of
these two commonly used solid lubricants (MoS 2 and Pb [11, 13-15]),
the lead ion has limited life in the presence of air due to the formation
of oxides. Therefore the spacecraft systems with lead ion plated
bearings are to be protected with inert gas during testing phase. In the
space environment, these films show extremely low friction. Gold and
silver are plated to tribological surfaces function as electrical
conductors such as the slip rings and brushes in a solar array drive
mechanism.
Liquid Lubrication
As mentioned above, most of the high speed systems used in spacecraft
are lubricated by liquid lubricants. The primary advantage obtained
with liquid lubricants is that the bearing surfaces separated by the
hydrodynamic film of the lubricant, have virtually no wear, and thereby
have potentially infinite lives. Depending upon the thickness of
lubricant film present between the interacting surfaces, four well
defined lubrication regimes are identified, such as hydrodynamic,
elastohydrodynamic (EHD), mixed and boundary lubrication regimes
[9,16-19]. These four regimes are clearly understood from the
Stribeck/Hersey curve (Stribeck performed a series of journal bearing
experiments in the early 1900's [20]. He measured the coefficient of
friction as a function of load, speed, and temperature. Later, Hersey
performed similar experiments and devised a plotting format based on a
dimensionless parameter, ZN/P [21].), which shows the coefficient of
friction as a function of dimensionless bearing parameter (ZN/P), where
Z is the lubricant viscosity, N is the velocity and P is the bearing load.
These regimes are depicted in Figure 2 [18]. A space bearing with
liquid lubrication undergoes the last three regimes namely EHD, mixed
and boundary before it fails due to lubricant starvation. The
characteristics of these regimes are briefly presented here.
Hydrodynamic lubrication: In hydrodynamic lubrication, the thickness
of the lubricant film is sufficiently thick to separate the interacting
surfaces. This will occur when the lubricant viscosity and or speed are
sufficiently high and the load on the bearing is low. The film thickness
will be greater than 0.25 µm and no metal to metal contact occurs. This
kind of lubrication is not suitable for space bearing because it is not
possible to store and supply such a high quantity of lubricant required
for longer periods.
Fig.2. Stribeck / Hersey curve [18]
Moreover, the liquid lubricants are prone to contamination by
evaporation, and this will have harmful effect on other components. For
this reason, the space bearings are lubricated by minimum quantity and
the bearing systems are hermetically sealed.
Elastohydrodynamic lubrication (EHL): In EHL [19,22-24] the bearing
pressure increases to a level where the lubricant viscosity provides
higher shear strength than the interacting metal surfaces. Here, the
lubricant is carried into the convergent zone approaching the contact
area. As a result, the metal surfaces deform elastically in preference to
the highly pressurized lubricant, which increases the contact area
(Figure 3). In other words, the load is carried by the elastic deformation
of the bearing material together with the hydrodynamic action of the
lubricant. A bearing operating in EHD region shows an indefinite life
with least friction torque (Figure 2). The most interesting practical
aspect of the EHL theory is the determination of lubricant film
thickness which separates the ball and the races. The generally used
equation for calculating the film thickness is the one developed by
Hamrock and Dowson [19]:
0.68
0.49 -0.073 -0.68k
H
min
= 3.63 U
s
G W 1 - e
[1]
and
H = min
h
min
R x
, [2]
where, H min is dimensionless minimum film thickness, U s is the
dimensionless speed parameter, G is the dimensionless material
parameter, W is the dimensionless load parameter, k is the ellipticity
parameter, h min is the minimum film thickness and R x is the effective
radius.
The effectiveness of EHL is described by the film parameter or λ ratio,
which is the ratio of central film thickness at the hertz contact zone to
the r.m.s surface finish of the rolling element surface;
h min
=
1
[3]
2 2 2
s r + s
b
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