Roller traction performance basics, a test program to measure performance, and the need for and typical use of the information are outlined. A test rig was designed and fabricated to develop this information. Parametric conditions and specimen materials were chosen so that the resulting data will be valuable to the design and development of advanced, roller-driven space mechanisms, from precision positioning devices to telerobot joints

Rohn, Douglas A.

English

NASA Technical Reports Server

N88-23241
ROLLER DRIVE MATERIALS PERFORMANCE
Douglas A. Rohn
Structural Dynamics Branch
NASA Lewis Research Center
ABSTRACT
Roller drives offer several beneficial characteristics in servomechanism appli-
cations. The best use of these inherent qualities in a given design is often
dependent on the performance of the materials chosen for traction rollers.
This presentation outlines roller traction performance basics, a test program
at Lewis to measure performance, and the need for and typical use of the
information.
Smooth rollers can transmit torque when pressed together in a rolling con-
tact. The amount of torque depends directly on the available traction coeffi-
cient and indirectly on the normal load capacity of the roller materials.
Durability and life are related to both rolling element fatigue and wear.
Application of roller drives to space mechanisms requires this performance
data under typical traction conditions in suitable environments, for example,
vacuu.ql.
A test rig has been designed and fabricated to develop this information.
Parametric conditions and specimen materials have been chosen so that the
resulting data and understanding will be valuable to the design and develop-
ment of advanced, roller-driven space mechanisms, from precision positioning
devices to telerobot joints.
1-203
https://ntrs.nasa.gov/search.jsp?R=19880013857 2020-03-20T06:22:45+00:00Z
OVERVI EW
BENEFICIAL CHARACTERISTICS OF ROLLER TRACTION DRIVES
Roller drives offer several beneficial characteristics in servomechanism
applications. Potential aerospace applications include antenna or solar array
positioners, control moment gyro actuators, and robotic joints. In these and
similar applications the zero backlash, low torque ripple, high stiffness,
high efficiency, and ability to run without liquid lubrication of roller trac-
tion drives are important. Application of these inherent qualities to a given
aerospace mechanism often depends on the roller material performance, in terms
of available traction coefficient, load capacity, and wear rate under the
design conditions.
INCREASING
MATERIAL
PERFORMANCE
DEPENDENCE
1
DRY-RUNNINGABILITY (SOLID FILMS/SELF-LUBEPOLYMERS)
I HIGH TORSIONALSTIFFNESS
LOW SLIDING; HIGH EFFICIENCY
HIGH SPEED CAPABILITY
[ NONJAMMING
SMOOTH& QUIET; NEGLIGIBLETORQUERIPPLE
I NEARLYINFINITE POSITIONRESOLUTION
I ZERO BACKLASH
_B POWERTRANSMISSION SERVOMECHANISMAPPLICATIONS W APPLICATIONS
CD-88-31961
1-204
FRICTION AND WEAR OF SEVERAL POLYMERS
Particularly in the case of space mechanisms, performance in the thermal-
vacuum environment is critical. A large body of friction and wear data exists
for sliding contact in air, as well as a fair amount for sliding in vacuum.
Comparatively less information is available for rolling traction contacts
(i.e., rolling and sliding) in air and very little for traction in a vacuum.
Extrapolation from sliding to traction contacts is generally possible; how-
ever, exact condition data are preferred. In addition, the differences in
performance in air and vacuum can be quite large.
FRICTION
COEFFICIENT
WEAR RATE,
M3/M OF
SLIDING
.80
D 50 PERCENTRELATIVEHUMIDITY TESTS
D VACUUMTESTS (0,13 Pa)
-- _ VACUUMTESTS (0.00013 Pa)
:::;:;::: :::::::: ....
:+:+: :.:.:.:.'",,
• ,,,'.'., '.'.'.', r///
::::::::: _ :::::::: /
E_,,j, MAXIMUM
•_- AVERAGE
_ _"_"MINIMUM
10-12_
FILLED
TORLON
.:.:.:.:
:!:i:i:i :::::2
.......
:::;:::: ....... r / / J
:.:,:.:. .:.:.:. • _,
_':':':': ///A "'""" ////
I:.:.:.:- :':':':
t-:.:.:.: ///A .:.:.:. ////
_:::::::: ///A ::::::: f///
l:i:i:i:i Ilia IIII
:::::::: ///,.1 I//J
i:!:i:i:////I V/Z
COMMERCIALLY100PERCENT
AVAILABLE PMDA
POLYIMIDE POLYIMIDE
80PERCENT GRAPHITE
PMDA/20 FIBER
PERCENTBTDA REINFORCED
POLYIMIDE POLYIMIDE
i ,,m.,,i.
iiill III
::::I///
..... III
_7"R
::::::::: ///
i!i!iiiii
.:<o:.:
::::::::: ///
::::::::: II/
:i:i:i:i:II/
::::::::: III
::::::::: I//
i:i:i:i:i///
:.:.:.:. ///
,',',',' ,,,
100 PERCENT
PMDA
POLYIMIDE
(FILM)
CD-88-31962
1-205
ENVIRONMENTSIMULATIONROLLERCONTACTPERFORMANCERIG
An experimental program is under way at NASALewis to evaluate materials and
coatings for application to roller drives in space mechanisms. Central to
this effort is a test rig which properly duplicates conditions to provide val-
uable data for understanding rolling traction performance phenomena and to
support current and future space mechanism projects.
TORQUE/THRUST TRANSDUCER"
CHAMBER
rNORMAL LOAD GIMBAL
BRAKE
DRIVE MOTOR
TEST ROLLERS(EXTERNAL OR INTERNAL CONTACT)
t I I
MISALIGNMENT POSITIONER
CD-88-31963
1-206
POSTER PRESENTATION
BENEFICIAL CHARACTERISTICS OF ROLLER TRACTION DRIVES
An interesting class of mechanisms utilize traction as the means to transfer
torque. Applications range from dry contacts, such as the locomotive wheel
against the rail and elastomer-coated rollers in paper handling equipment, to
lubricated contacts in industrial adjustable-speed traction drives (Loewenthal
et al., 1983).
As power transmissions, few mechanical drives match the low noise, smooth
torque transfer characteristics, high speed capability, and speed regulating
accuracy of traction drives. For servo drive system applications, their abil-
ity to provide a smooth transfer of motion with relatively low hysteresis
losses and high torsional stiffness while producing no detectable "backlash"
upon direction reversal are obviously beneficial qualities. (Loewenthal
et al., 1985).
These characteristics are inherent in the nature of roller traction drives
(i.e., smooth rollers running against other smooth rollers). However, the
degree to which the characteristics can be put to use depends on the perform-
ance of the roller materials.
INCREASING
MATERIAL
PERFORMANCE
DEPENDENCE
1
DRY-RUNNINGABILITY (SOLID FILMS/SELF-LUBEPOLYMERS)
J HIGH TORSIONALSTIFFNESS
LOW SLIDING; HIGH EFFICIENCY
HIGH SPEEDCAPABILITY
__1
[ NONJAMMING
SMOOTH & QUIET;NEGLIGIBLETORQUERIPPLE
I NEARLY INFINITE POSITION RESOLUTION
[ ZEROBACKLASH
_ POWERTRANSMISSION SERVOMECHANISMAPPLICATIONS _ APPLICATIO S
CD-88-31961
1-207
TRACTION CURVE
A simple roller traction drive consists of a pair of rollers pressed together
with a normal load N. The traction force T capable of being transmitted
through the contact is a function of many parameters: normal load, rolling
speed U, temperature, materials, and lubricants, when present. The relation-
ship between these factors is typically shown by a set of traction curves.
The speed variation between traction rollers due to torque transfer is gen-
erally referred to as creep (Loewenthal and Zaretsky, 1985). The traction
coefficient is typically plotted against this value for a variety of speeds,
temperatures, loads, etc. The shape of any one of the family of curves follows
the range boundary lines of this figure.
Roller drives are designed to operate in the linear ascending portion of the
traction curve at some point below the peak. Operating creep rates range from
0.i to 0.2 percent for dry contacts or those lubricated with traction fluids
at low speeds, to 3 or 4 percent for lightly loaded, high-speed contacts lubri-
cated with mineral oils. This speed difference is not due to slip between
driver and driven roller but is, in fact, the accumulated lost motion due to
the tangential stretching and compressing motion or compliance occurring at
the roller contact interface. A "locked" or "zero slip" region exists at the
leading edge of the contact, and only at the peak traction point (i.e., point
of impending slip) will this region completely disappear. In the case of a
lubricated contact, lost motion due to viscous shear of the lubricant film in
the contact is also added.
TRACTION CURVE
_N "5 F DRY CONTAC S :i:iiiiiiiiiii!i!ii,i:i:i_i:i;i!iiiiiiiiij!iiii!;i:i:::i!iiii!iiiiii!ijiiii:i:i:i_i;i!iiiiii!iiiiiiiiiii!i'ii'i'i'i'i'ii!iiiiiiii,
l D i!iiii!!i , ii!iiii!i:MATERIALi
_ COEFFJ;IENT'.2TRACTION.3 _- :!iiii iii_iiiiiiiijiiiiii_SPEED:iiiiiiiiiiiiii!iiiiiiiiiiiIiii!iiiii_iiiiiiii__iiiii_iiiiiii
:::::::::::::::::::::::::::::::::::::::
0 .005 .010 .015
U i= riooi
CREEP, U1-U2
U
CD-88-31964
1-208
TRACTIONCAPACITYOFPOLYMEROLLERS
The torque and power capacity of roller traction drive contacts are dependent
on the material combination's peak available traction coefficient and the
amount of normal load sustained before failure. The failure modesof lubri-
cated roller drive contacts are very similar to those of ball and roller bear-
ings. Rolling-element fatigue, or pitting, is the likely modewhen the quality
of lubrication is good. Whenthe lubrication quality is not good, and also in
the case of dry-contact roller drives, the failure modeshifts from fatigue to
one that is predominantly wear.
Dry-running contacts, whether of polymer rollers or solid lubricant films, are
obviously advantageous where liquid lubrication is impractical. The traction
coefficient and particularly the load capacity of somecommercial polymers are
surprisingly high. Data shownhere were generated in air, with a polymer
roller running against a steel roller (Tevaarwerk, 1985).
1200!--
800
FAILURE
LOAD,
Ib/in.
400 B
.,...°,o
::::::::
::::::::
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TORLON
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,..° ° °.
 i!ililil
FILLED
TORLON
LOAD
TRACTION
COEFFICIENT
m
ff
fj f J
fj f J
f j _ f J
m .... J
EXTRUDED MONOCAST
NYLON6/6 NYLON
_ MAXIMUM
m,
t / MINI-
_ MUM ,-,.
ACETAL
COPOLYMER
.4
.3
PEAK
TRACTION
COEFFICIENT
.2
.1
CD-88-31965
1-209
FRICTIONANDWEAROF SEVERALPOLYMERS
In order to makeuse of polymer rollers in space mechanismapplications, their
friction and wear properties must be known in representative environments. At
present, data exist only for contacts undergoing full sliding. Data for sev-
eral representative commercial and experimental polymers in air and vacuumare
presented here (Fusaro, 1987).
FRICTION
COEFFICIENT
WEAR RATE,
M3/M OF
SLIDING
100 PERCENT SLIDING
t D 50 PERCENTRELATIVEHUMIDITY TESTS
D VACUUMTESTS (0.13 Pal
.80 _ VACUUMTESTS (0.00013 Pa)
_,, MAXIMUM
I--t<- AVERAGE
.m-- ,m._
.4O
:::::::::
:.:.:.:.:
,,,.,...
....,...
:;:::;1:1
:i:i:i:i:
10-12
10-14
10-1e
mmm ,..,..°..
,.o,. •.
.......... ¢///
i:i:i:i:i.... :::::::::I//J
• ",'.'." """ //I/
.... ///,_ ;::::;:.:
i :::::::::///,1 ;!:i:!:!:////
::::: ///,,1 ...... ////
::::: ///,! 1:::: ////
I:::: ////1 I:::: '///
FILLED COMMERCIALLY100PERCENT 80PERCENT GRAPHITE
TORLON AVAILABLE PMDA PMDA/20 FIBER
POLYIMIDE POLYIMIDE PERCENTBTDA REINFORCED
POLYIMIDE POLYIMIDE
::: ::: t//_
:.:.:.: i//J
::::::: ///J
• :,',.:. 1///
• .....
:.:.:.: f///
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:.:.:.: , •,,
:.:.:.: _/ /
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lOOPERCENT
PMDA
POLYIMIDE
(FILM)
CD-88-31962
1-210
FRICTIONOFGOLD-PLATED440C
Another promising roller drive material combination is steel rollers with soft
metal films (lead, silver, and gold) or hardfaced coatings (TiN and TIC).
These materials serve as solid lubricants or extremely wear-resistant barriers
(respectively) between the rollers which would otherwise cold weld in the
nonoxidative vacuumenvironment. The data shownhere (Spalvins, 1985) display
friction as well as wear life information for gold in vacuum. Friction
increases dramatically when the initial coating layer wears off and the 440C
substrate is rapidly damaged.
This and the previous data charts illustrate typical available data, which are
primarily for full sliding. Performance data in terms of traction coefficient,
load capacity, and wear rate for all materials in a roller traction contact
(combined sliding and rolling) under air, vacuum, and other environmental
conditions are needed for roller drive space mechanismdesign.
.6
COEFFICIENT .4
OF
FRICTION,
p. .2
m ,,- 440C ON BASE
/
/
-- // /
I I I I
2000 4000 6000 8000
CYCLES
CD-88-31966
1-211
ENVIRONMENTSIMULATIONROLLERCONTACTPERFORMANCERIG CAPABILITIES
An in-house program is under way to determine roller contact performance in
nonatmospheric environments. Understanding the data and phenomenawill sup-
port efforts in Lewis' Microgravity Mechanismsand Robotics Program, as well
as in the space station, telerobotics research, and future space missions.
A rig has been designed and is being fabricated to satisfy the experimental
criteria outlined here. Data on rolling traction contacts in a vacuumare
essentially nonexistent today. Proposed specimenmaterials include the full
range, from liquid lubricated Steel, to solid film coatings, to polymers, and
to ion-plated or vapor-deposited hardface materials.
PARAMETERS:
• LONGITUDINAL AND SIDE-SLIP TRACTION (ROLLING/SLIDING)
• AIR, VACUUM, GAS
• 200 Ib NORMAL LOAD, 500 rpm, 600 in.-Ib
OUTPUT DATA:
• TRACTION COEFFICIENT
• LOAD CAPACITY
• WEAR RATE
• EFFECTS OF MISALIGNMENT
SPECIMENS:
• SOLID ROLLERS
-- METALLIC
-- POLYMER
• COATINGS/FILMS
-- ION-PLATED METALS
-- HARDFACED LAYERS
-- POLYMERS
-- ELASTOMERS
• LIQUID LUBRICANTS
CD-88-31967
1-212
ENVIRONMENTSIMULATIONROLLERCONTACTPERFORMANCERIG
An illustration of the rig concept is shownhere. Test rollers are supported
in vacuum-rated rotary feedthroughs and surrounded by a chamberwhich can be
evacuated to 10-6 torr or filled with suitable gases at or below ambient pres-
sure. A dc variable-speed motor provides rotation, and a magnetic particle
brake absorbs power for longitudinal traction torque transfer testing. A more
sensitive method of testing, side-slip traction (where the rollers are deliber-
ately misaligned and the axial thrust is a measure of traction force), can
also be accommodated. Misalignment is also a potential source of power loss
in an actual roller drive mechanism; thus its effects on torque transfer will
also be studied. Short-term, accelerated wear measurementswill be possible.
Presently planned future additions to the rig include the meansto heat and
cool the rollers to represent thermal conditions.
TORQUE/THRUST TRANSDUCER-
CHAMBER
L.LOAD GIMBAL
DRIVE MOTOR
TEST ROLLERS (EXTERNAL OR INTERNAL CONTACT)
, I I
MISALIGNMENT POSITIONER
CD-88-31963
1-213
SPACESTATIONROTARYJOINTPINCHROLLERDRIVECONCEPT
The data from this effort have been used to develop a concept for the space
station powermodule alpha joint. Loewenthal and Schuller (1986) demonstrated
the feasibility of a pinch roller drive. Several performance advantages rela-
tive to gears were noted, including overtorque or jamming protection, inherent
acceptability to dry or self-lubricating materials, ease of manufacture, and in
situ assembly and maintenance. Data to predict wear rates were extracted from
plasma-nitrided steel gear data in vacuumand from pin-on-disk, I00 percent
sliding data for polyimides in vacuum. While use of such data was madecon-
servatively, actual rolling traction capacity and wear data are certainly
required before committing to space flight hardware fabrication.
TRACTION PINCH 1-_ r DRIVE MOTOR
ROLLERS _
.oTAT,.G<, STAT,O.AnY
STRUCTURE 8- TO 15-ft _ \ J STRUCTURE
DIAM RING --'/ '_.j _ _
ROLLER/RING ALPHA JOINT DRIVE
I /
SPACE
STATION ! //
/// ---,(MAIN KEEL
//
ALPHACORREC-
_TRANSITION TION ANGLE
TRUSS
WEAR LIFE ESTIMATE:
• 76 mm DIAMETER ROLLERS
• 50_m EXP POLYIMIDE COATING
• > 16yr SERVICE LIFE
LST '88 CD-88-31968
1-21_
ORIGI:.J.q.! ?ASZ 13 
OF POOR QUALITX 
ORNL BENCH-TEST TELEROBOT JOINT WITH ROLLER DRIVE DIFFERENTIAL 
Another application of advanced roller drive actuators is in robot manipulator 
joints. The Oak Ridge National Laboratory (ORNL) developed a design for a lab- 
oratory and ultimately a space telerobot system (Kuban and Williams, 1987). 
Critical to its performance is a differential pitchlyaw joint, having high 
torque in a compact volume, low loss, ability to operate in space, and zero 
backlash. A roller drive design was selected to meet these needs. High roller 
load capacity and reasonably high traction coefficient are both desirable to 
minimize the size of the joint while still carrying the required torque. Low 
wear is important for long life. Ion-plated gold on hardened steel was 
selected as the initial material combination. Data from the Roller Drive 
Material Performance Program are essential to this and other advanced roller 
drive mechanism designs. 
DESIGN 
REQUIREMENTS 
HIGH TORQUE/ 
LOW LOSS 
DRYNACUUM 
OPERATION 
ZERO 
BACKLASH 
CD-88-32530 
1-215 
SUMMARY
Roller drives offer several beneficial characteristics in servomechanism
applications. The best use of these inherent qualities in a given design is
often dependent on the performance of the materials chosen for traction
rollers. Application of roller drives to space mechanisms requires this
performance data under typical traction conditions in suitable environments.
An experimental program is under way to develop this information. Parametric
conditions and specimen materials have been chosen so that the resulting data
and understanding will be valuable to the design and development of advanced,
roller space mechanisms, from precision positioning devices to telerobot
joints.
REFERENCES
Fusaro, R.L., 1987, "Tribological Properties of Polymer Films and Solid
Bodies in a Vacuum Environment," NASA TM-88966.
Kuban, D.P., and Williams, D.M., 1987, "Traction-Drive, Seven-Degree-of-
Freedom Telerobot Arm: A Concept for Manipulation in Space," 21st
Aerospace Mechanisms Symposium, NASA CP-2470, pp. 111-130.
Loewenthal, S.H., Rohn, D.A. and Anderson, N.E., 1983, "Advances in
Traction Drive Technology," SAE Trans., Vol. 92, Sect. 3.
Loewenthal, S.H., and Zaretsky, E.V., 1985, "Traction Drives," Section 34,
Mechanical Design and Systems Handbook, H. Rothbart, ed., McGraw-Hill
Book Co., New York.
Loewenthal, S.H., and Schuller, F.T., 1986, "Feasibility Study of a Discrete
Bearing/Roller Drive Rotary Joint for the Space Station," NASA TM-88800.
Spalvins, T., 1985, "The Structure of lon Plated Films in Relation to Coating
Properties," NASA TM-87055.
Tevaarwerk, J.L., 1985, "Rolling, Slip and Endurance Traction Measurements on
Low Modules Materials," NASA CR-174909.
1-216


Roller drive materials performance

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