A Space Tribology Handbook

E. Wyn Roberts
ESTL, AEA Technology, W arrington, U K
M. Eiden
M echanical System s D ivision, ESA D irectorate for Technical and
O perational Support, ESTEC , N oordw ijk, The N etherlands
The need for a Space Tribology Handbook
Tribology (from the G reek tribosm eaning to
rub) is the study of friction, w ear and
lubrication. Friction and w ear are generated at
the interface of contacting bodies in relative
m otion and therefore occur in com ponents
such as bearings, gears, screw s and slip-rings.
If the friction (or adhesion) forces are high, a
com ponent m ay prove dif cult to drive or, even
w orse, m ay seize; if high w ear occurs, a
com ponent w ill lose its precision and,
ultim ately, its ability to operate at all. Tribology
is, therefore, an im portant issue, especially in
m echanism s intended for use on spacecraft
for if spacecraft m echanism s fail, there is
(usually) no m eans to repair the dam age or to
apply fresh lubrication. Furtherm ore, if the
m echanism in question is critical to the
spacecrafts operation (as w ould be the case
w ith solar array drives, antenna pointing
m echanism s and deploym ent devices), then
loss of the m ission could result.
(European Space Tribology Laboratory), w hich
is now part of AEA Technology. Since then,
ESTL has generated a considerable am ount of
tribological data, advised on tribological design
issues, and tested tribo-com ponents and
m echanism s under space sim ulated condi-
tions. A s a consequence, a w ealth of
know ledge has been generated and m uch of
this has been dissem inated to the space
com m unity by w ay of publications and courses
on space tribology. These activities have
undoubtedly raised aw areness of tribology and
encouraged m echanism engineers to give due
consideration to tribological issues in their
designs. H ow ever, there w as no handbook
available detailing the tribological perform ance
of m aterials and providing guidelines on their
use in m echanism s design. It w as for this
reason that the Space Tribology H andbook
w as produced. W ith its publication, the space
com m unity now has a unique docum ent w hich
provides a source of inform ation consolidated
by test validation.
The H andbook has been w ritten w ith the aim of
assisting designers and engineers in the
im plem entation of sound tribological practices
and to help them determ ine how best to treat
and lubricate com ponents for a given
application. Specically, the H andbook:
provides a de nitive reference m anual on
space tribology
provides useful data on typical system s
w here tribology is im portant either to
perform ance or reliability
species guidelines for the selection, use
and lim itations of tribo-m aterials (encom pas-
sing lubricants, surface treatm ents and
substrate m aterials)
aids the designer in the selection of tribo-
com ponents
provides guidance on the ground testing of
tribo-com ponents and m echanism s.
Figure 1 show s the com pleted H andbooks
cover w ith exam ple sheets illustrating the
form at and style.
The production of a Space Tribology Handbook forms part of an
overall strategy, initiated by ESTECs Mechanisms Section, to improve
the quality of space mechanisms through the establishment of a set of
globally applicable space mechanisms standards and guidelines for
mechanism design. The standardisation activities are being
performed under the European Cooperation for Space
Standardisation (ECSS); the space mechanism engineering
requirements have been established in an ESA/Industry collaborative
effort in 1997. The Space Tribology Handbook provides the essential
guideline for tribology aspects of mechanism engineering, useable in
the day-to-day work of mechanism engineers. This strategy should
promote a more standardised approach to mechanism design and
testing and achieve reduced schedules, cost-effective developments
and high-quality mechanisms.
space tribology handbook
To avoid such anom alies and to ensure that
m echanism s operate to their prescribed
specification, effective designs are required
w hich incorporate good tribological practice so
that friction, adhesion and w ear are m inim ised.
In Europe, the im portance of tribology in space
w as quickly recognised by the Agency and led,
in 1972, to the establishm ent of ES TL
F ig u re 1 . T h e S p a c e
Trib o lo g y H a n d b o o k
sh o win g e x a m p le s o f
c o n te n ts
The scope and structure of the Handbook
The handbook is arranged in seven chapters,
the titles of w hich are listed in Figure 2. The rst
chapter serves as an introduction to tribology,
its purpose being to fam iliarise readers w ho are
new to the subject w ith the fundam ental
concepts of tribology. The rem aining chapters
form the H andbooks guidelines and span the
progression from the initial selection of a
tribological com ponent, through its lubrication
and, nally, to its testing at com ponent and
m echanism level.
A ow chart detailing how the various stages of
com ponent selection, lubrication and testing
are presented and cross-referenced is
illustrated in Figure 3.
Contents of Handbook
The follow ing outlines the contents of the
individual chapters of the H andbook.
Basic Tribology
Engineers involved in the design of
m echanism s need to have an understanding of
the basic concepts of tribology so that they can
recognise the issues w hich are critical to the
success, or failure, of their m echanism .
Fundam ental to this understanding is a
know ledge of how engineering surfaces
interact w hen brought into contact under load
and how this interaction changes w hen there is
relative m otion (w hich m ay be separation,
sliding or rolling) betw een the contacting
bodies.
This chapter, therefore, looks at w hat happens
w hen real surfaces com e into contact and
explains how contact areas in reality are sm all
and contact stresses high. The m anner in
w hich surfaces deform is discussed and the
conditions under w hich these deform ations
becom e perm anent (plastic deform ation) are
rbulletin 94 m ay 1998
u
1 . B a sic Trib o lo g y
2 . Trib o -c o m p o n e n t S e le c tio n , D e sig n a n d P e rfo rm a n c e
3 . S e le c tio n o f L u b ric a n t Typ e
4 . S o lid L u b ric a n ts
5 . F lu id L u b ric a n ts
6 . M a te ria ls fo r Trib o lo g ic a l C o m p o n e n ts
a n d S u rfa c e s
7 . C o m p o n e n t a n d M e c h a n ism Te stin g
F ig u re 2 . C o n te n ts o f th e
S p a c e Trib o lo g y H a n d b o o k
F ig u re 3 . F lo w-c h a rt
sh o win g th e H a n d b o o k s
stru c tu re a n d lo g ic
w hat softw are is available to assist in design
and prediction of perform ance.
Selection of lubricant type
H aving identified the com ponent to be
em ployed, the next step to address is w hether
to use uid or dry lubrication (Fig. 5). This
chapter defines the criteria by w hich this
decision should be m ade and also offers
guidelines on selection for those cases w here,
in principle, both uid and dry lubricants are
applicable. C hecklists are provided to help
select a specic lubricant once the choice has
been m ade betw een uid and dry. In addition,
suggestions are m ade on how to extend the life
of space lubricants. The chapter features the
follow ing topics:
identied. The various phenom ena, w hich give
rise to friction and w ear, are described and the
m anner by w hich these m ay be reduced
(through lubrication) is explained. Finally the
unique environm ent in w hich space
m echanism s have to operate is de ned and the
im pact such an environm ent has on tribological
system s is outlined.
Tribo-component selection, design and
performance
This chapter is intended to assist the designer
in the selection of tribological com ponents
that is, on how to choose the m ost suitable
com ponent for a given application (Fig. 4). The
areas covered are:
selection of rotary, rolling-elem ent bearings
selection of plain, spherical and rod-end
bearings
selection of ball, roller and plain screw s
selection of linear bearings
selection of gears
sliding electrical contacts
other tribo-com ponents: separation
surfaces, end stops, cam s, threaded
fasteners, brakes, seals
safety factor for tribo-com ponents.
For each generic com ponent, advice is given
on w hich type of com ponent is best suited to
an application on the basis of param eters such
as load capacity, stiffness and precision. The
m aterials, and their relevant properties, from
w hich the com ponents are m anufactured are
specied. Exam ples are given of w here specic
com ponents have been used successfully and
space tribology handbook
F ig u re 4 . E ffe c tive
m e c h a n ism d e sig n re q u ire s
a n in fo rm e d a p p ro a c h to
se le c tio n , sizin g a n d
lu b ric a tio n o f trib o lo g ic a l
c o m p o n e n ts. T h e H a n d b o o k
p ro vid e s g u id e lin e s in th e se
a re a s a n d h e lp s to e n su re
th a t th e m o st a p p ro p ria te
c o m p o n e n t a n d lu b ric a tio n
a re c h o se n fo r a g ive n
a p p lic a tio n . T h e p h o to g ra p h
sh o ws a typ e o f b e a rin g
a n g u la r-c o n ta c t) wh ic h is
c o m m o n ly u se d in sp a c e
m e c h a n ism s. T h e ra c e s a re
lu b ric a te d with a th in lm o f
m o lyb d e n u m d isu lp h id e
F ig u re 5 . T h e c h o ic e o f
lu b ric a n t is c ritic a l to th e
su c c e ssfu l o p e ra tio n o f a
sp a c e m e c h a n ism . T h e
H a n d b o o k p ro vid e s
g u id e lin e s fo r se le c tin g b o th
u id a n d d ry lu b ric a n ts.
S p e c ia lise d lu b ric a n t
p ro c e sse s m a y b e re q u ire d
su c h a s th e p la sm a -a ssiste d
d e p o sitio n syste m , sh o wn
h e re , fo r th e p ro d u c tio n o f
th in la ye rs o f le a d lm s
Guide to selection; dry versus uid
lubrication:
applications w here only dry or fluid
lubricants can be used
choosing betw een uid and dry lubrication
w hen both are applicable
checklist w hen choosing solid lubrication
checklist w hen choosing uid lubrication
labyrinth seal leak-rate calculation.
Enhancing lubricant performance:
extending life of per uorinated lubricants
extending life of solid lubricant coatings
running-in of solid-lubricated com ponents.
Solid lubricants
Solid lubrication of a bearing m ay be brought
about in one of tw o w ays. It can be provided
as bulk m aterial for a m oving surface,
transferred by rubbing from a solid m ade from ,
or containing, the dry lubricant as, for exam ple,
w ith self-lubricating cages. Alternatively it can
be applied to one (or both) counterface(s) in the
form of a lm , as w ith techniques such as
sputter deposition. Inform ation on both form s
of solid lubricant is included in this chapter. The
areas covered are:
Selection of solid lubricants:
w hen to use solid lubricants
advantages and disadvantages of solid
lubrication
types of solid lubricant.
rbulletin 94 m ay 1998
u
Solid lubricant coatings:
coating types and m ethods of application
guide to selection of solid lubricant coatings
tribological properties of thin lm s
lubricant coatings for tem perature extrem es
effect of environm ent
coatings to prevent static adhesion and
fretting dam age
storage and handling of coated com -
ponents.
Bulk solid lubricants:
polym ers and polym er com posites
non-polym eric com posites.
The section on coatings describes the various
m ethods of application and the corresponding
thickness ranges. This is follow ed by a general
guide to selection based on the coatings
tribological properties in air and vacuum , their
range of operating tem peratures, their
suitability for precision com ponents, and the
type of application and duty for w hich they are
best suited. M ore detailed inform ation on the
tribological properties of specific coatings
follow s w ith exam ples of their perform ance
w hen applied to ball bearings and gears.
The section on bulk solid lubricants deals w ith
self-lubricating polym ers and com posites. The
latter are com posites containing solid
lubricants, w hich m ay be reinforced w ith llers
in the form of particulates or bres. The types
covered are polym er com posites (for bearing
cages and gears), thin-layer polym eric
com posites (for journal and spherical bearings),
m etal/M oS
2
com posites (for m otor and slip-
ring brushes) and leaded bronzes (for bearing
cages). In each case the m echanical and
m aterial properties are specied and the friction
and w ear characteristics are listed.
Fluid lubricants
The follow ing aspects are addressed:
selection of a uid lubricant
uid lubricant database
tribological properties of uid lubricants
uid loss, retention and replenishm ent
handling, cleaning and lubrication
procedure.
The section dealing w ith selection describes
the types of uid lubricant available and gives
the relevant properties (vapour pressure and
pour point) of som e com m only used space oils
and greases. M ore extensive data is provided
by the uid lubricant database w hich lists the
know n physicochem ical and tribological
properties of over 100 uid lubricants that have
been used, or are candidates, for space
applications. The tribological properties of oils
F ig u re 6 . T h o ro u g h te stin g
o f c o m p o n e n ts a n d
m e c h a n ism s is e sse n tia l
in o rd e r to p ro ve th a t a ll
fu n c tio n a l a n d life tim e
re q u ire m e n ts a re m e t.
T h e H a n d b o o k p ro vid e s
g u id e lin e s o n a ll a sp e c ts o f
g ro u n d te stin g a n d d e ta ils
E S A s c u rre n t m e c h a n ism
ve ri c a tio n re q u ire m e n ts.
T h e p h o to g ra p h sh o ws th e
in sta lla tio n o f a n a n te n n a -
p o in tin g m e c h a n ism in a
te st c h a m b e r wh e re it will
b e su b je c te d to a th e rm a l-
va c u u m te st
space tribology handbook
and greases are described and their m easured
perform ance in com ponents such as bearings,
gears and screw s is presented.
Materials for tribological components and
surfaces
This chapter surveys the properties of m aterials
that have been used, or m ay be suitable for
use, in tribological com ponents in space
m echanism s. Its purpose is to act as a
reference source of basic m aterial properties
and as a supplem entary source of inform ation
to that provided in other chapters. Additionally,
a guide is given on the selection of base
m aterials and appropriate surface treatm ents
for com m only used tribological com ponents.
Specic topics covered are:
the com patibility of m aterials w ith vacuum
environm ents (vapour pressure and
outgassing characteristics)
physical and m echanical properties of bulk
tribo-com ponent m aterials (including
hardness, elastic m odulus, tensile strength,
m axim um operating tem peratures and
density)
listings, descriptions and suitability of
surface treatm ents
a m aterials selection guide for bearings,
gears, screw s, fasteners and separating
surfaces.
Component and mechanism testing
H ere, guidance is given on the testing of tribo-
com ponents and m echanism s for space
satellite applications. It identies and de nes
the various test environm ents and provides
guidelines on how to achieve them . Exam ples
are given of therm al-vacuum test set-ups for
com ponent and m echanism testing together
w ith notes on the testing of specic tribo-
com ponents (Fig. 6). The rationale for
m echanism testing is explained and verication
requirem ents, based on current ES A
guidelines, are presented.
Future updates
In line w ith the evolution of space tribology, it is
our intention to update the H andbook on a
regular basis. This w ill be achieved by
incorporating new ly generated data and
responding to the suggestions and reactions of
users. In this w ay, the H andbook w ill rem ain a
contem porary, up-to-date and, above all,
relevant docum ent w hich m eets the needs of
space m echanism designers and engineers.
To obtain a copy of the Space Tribology
H andbook, you can request an order form by
w riting to ESTL at the follow ing address:
Space Tribology H andbook
ESTL
R D 1/165
AEA Technology
R isley, W arrington
U K
W A3 6AT
Alternatively, telephone +44 (0)192525 3015 or
send an e-m ail to: em yr.roberts@ aeat.co.uk
Acknowledgements
M uch of the tribological data presented in the
H andbook w as generated through the
dedicated efforts of present and form er
m em bers of ESTL staff. The authors are
grateful for the constructive com m ents m ade
on draft chapters by m em bers of ESTEC s
M echanism s Section. r