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TRIBOLOGY OF HIGH SPEED MOVING MECHANICAL SYSTEMS FOR SPACECRAFTS - TRIBOLOGICAL ISSUES
K. Sathyan
where, s r and s b are the r.m.s surface finish of races and balls. The EHL
regime is characterized by λ ratio between 3 and 10 which corresponds
to a film thickness between 0.1 and 1 μm. It has been pointed out that a
full film can be obtained with no asperity contact only when λ > 3. If λ
< 3, it will lead to mixed lubrication with some asperity contacts [22].
The concentrated research on EHL resulted in the identification of three
subdivisions in EHL, namely starved EHL, parched EHL and
transient/non - steady state EHL [25]. In starved EHL, the pressure
build-up at the inlet contact region is low due to restricted oil supply.
As a result the lubricant film will be thinner than calculated by EHL
theory [22]. In parched EHL, the lubricant film is so thin that they are
immobile outside the contact zone [26, 27] and this regime is
particularly important for spacecraft systems bearings operating at high
speeds. In the transient/non-steady state EHL, the load, speed and
contact geometry are not constant with time. The theoretical behavior
of this regime in point contact bearings is not well understood [25] but
it is studied experimentally by Sugimura et al. [23].
Fig.3. Elastohydrodynamic lubrication
Mixed lubrication: If the bearing pressure in an elastohydrodynamically
lubricated bearing is too high or the running speed is too low, the
lubricant film will be penetrated. The asperities of the bearing surfaces
will come into contact and partial lubrication results. The behavior of
the conjunction in a mixed lubrication regime is governed by a
combination of boundary and fluid film effects [24]. The value of λ in
this case is between 1 and 5. In spacecraft bearings mixed lubrication
will occur when there is insufficient supply (starvation) of lubricant to
the working zone.
Boundary lubrication: In boundary lubrication, the interacting surfaces
are not separated by the lubricant film. The lubricant film thickness is
so narrow that direct metal to metal contact occurs. The coefficient of
friction is high (0.15) and the resultant heat generation also high. The
frictional characteristics are determined by the properties of the
interacting surfaces and the lubricant film present. The high pressure
and temperature at the contact surfaces causes the formation of a
reactive film (called boundary film) which is capable of supporting the
load without major wear or breakdown. To impart boundary lubrication
properties, most space lubricant are processed with boundary additives.
The commonly used inorganic additives are compounds of chlorine,
sulfur, phosphorus and iodine [24]. The value of film parameter (λ) at
boundary lubrication is less than 1 and the lubricant film thickness is
less than 2.5 nm. The high speed space mechanism bearings are not
preferred to operate in the boundary regime due to high friction.
PROPERTIES OF LIQUID LUBRICANTS
Since no single lubricant can meet the often conflicting requirements of
various applications for liquids, hundreds of specialty lubricants have
been developed for aerospace applications [28]. There are a number of
factors to be considered while selecting a lubricant for space
application such as operating temperature range, working environment,
load on the bearings, speed of operation, bearing frictional torque etc.
A space lubricant should have the following essential properties:
Viscosity index: Since the system has to work over a wide temperature
range (typically between 15 and 85 °C) the change in viscosity with
temperature should be the minimum to maintain the EHD film. A space
bearing is required to work with steady friction torque; otherwise the
torque noise will act as a disturbance torque on the spacecraft.
Therefore to maintain the viscous friction of the bearing constant at the
working temperature range, high viscosity index lubricant is to be
selected.
Vapor pressure: The volatilization of lubricant contaminates the system
and may have harmful effects; therefore the vapor pressure should be
low in order to minimize losses by evaporation and to limit the
pollution due to degassing. Figure 4 [25] shows the relative evaporation
rates of various aerospace lubricants.
Pressure–viscosity coefficient (α): The pressure-viscosity coefficient is
important in determining the EHD film thickness at the ball-race
contact inlet. It is observed that the fluid viscosity is an exponential
function of pressure such that between the contacting surfaces in a
loaded rolling bearing assembly, viscosity is likely to be 10,000 times
its base value at zero pressure [29]. Also, from EHL theory, the
lubricant with the largest α value should yield the thickest film at room
temperature [25]. Since the bearings will subject to severe loads during
the launch phase of the spacecraft, lubricants with high α values are to
be selected.
Creep: All liquid lubricants have a tendency to creep or migrate over
bearing surfaces. It has previously been demonstrated by Fote et al. [30,
31] that small temperature gradients cause a rapid and complete
migration of oil films toward the regions of lower temperature. The
migration was induced by capillary forces, temperature gradients and
gravity. The creep is inversely related to lubricant’s surface tension,
i.e., if the lubricant surface tension is low, there is more chance of its
migrating from the working zone of the bearing. Hence, lubricants with
high surface tension are selected for space application.
Fig.4. Evaporation rates of various aerospace liquid lubricants [25]
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