1. Markku Tahkappa, Finland,1957 Nordic
World Champion model
SURVEY OF RUBBER MOTOR LUBRICANTS
INTRODUCTION
Lubrication of rubber motors is critical
for sport and competition flyers
alike. Maximum turns, energy return,
and motor life are all increased by
suitable lubricants at the cost of a
small increase in motor weight. This
short article will present results of a
preliminary survey of energy return
efficiency for a number of currently
available candidate lubricants. The
results favor Dow Corning Molykote®
33 grease but the statistical power of
the data is low.
Many lubricants have been tried
over the years. Early lubricants were
typically water-soluble soap and
glycerine and tended to migrate from
the motor to the fuselage, gradually
increasing the weight of the model.
Recently, modelers have focused
on chemically inert polysiloxane
oils. There are many potential
silicone-based lubricants and some
comparisons have been published.
For example, Paul Rossiter, in Free
Flight Quarterly No. 39, concluded that
the energy return performances of
equivalently wound motors lublicated
with ArmorAll, an aqueous emulsion
of silicone oil and other ingredients,
and neat high-viscosity silicone oil
are not significantly different. Earlier,
Dan Driscoll, in the January/February
2007 issue of MaxFax, the newsletter
of the D.C. Maxecuters, had reported
that Dow Corning 33, a silicone-
based grease containing lithium soap,
allowed significantly (8%) higher
turns to break than either Armor All
car cleaner or the nearly equivalent
product, Son of a Gun.
Both solid greases and liquid
oils are used in the lubrication of
machinery. Greases usually exhibit
shear-thinning behavior such that the
resistance to flow decreases as shear
rate increases. This allows grease to
stay in place between static bearing
surfaces under normal load whereas
oil would flow away from the contact
region. Generally, oils tend to be used
in low load applications while greases
are used where loads are high. It is
interesting to inquire whether highly-
wound rubber motors have normal
stresses between strands sufficient to
extrude liquid oils away from strand-
to-strand contacts thereby justifying
use of grease lubrication.
10 Free Flight www.freeflight.org
Figure 1 Arduino-controller rubber tester
PHOTOGRAPHY:BOBMORRIS

2. EXPERIMENTAL
The test rig, shown in Fig. 1, combines
an Arduino RedBoard, a 12-volt
stepping motor, a SparkFun Big Easy
stepper motor drive, a 12-Volt supply,
two load cells salvaged from Cen-Tech
500 gm digital scales, and two Burr-
Brown INA 125 instrument amplifier
ICs. It can accurately measure the
torque and pull force developed during
winding and unwinding 4 strands
of 1/8" Tan Super Sport rubber. The
Arduino serial port communicates
with an iMac desktop via a USB cable
(red).
Samples for the survey were ~0.8
gram loops of 1/8" June 2009 Super
Sport, tied using a water knot, a
variant of the overhand bend which
I have found to rarely break during
rubber motor testing or in the field.
The loops were doubled to 4 strands
which would be expected to break at
about 150 turns.
A typical Arduino sketch is shown
below:
This sketch outputs turns, torque and
pull force values once per revolution
up to 100 turns, then reverses stepper
motor direction and continues to
report data back to zero turns where I
stop it manually.
DATA AND DISCUSSION
The data are sent to the iMac as a
long series of numbers representing
Figure 2 Torque and pull force vs turns for a 12,000 cP silicone oil-lubricated double loop
of 06/09 Super Sport rubber.
Figure 3: Torque versus pull force plot of the data in Figure 2.
www.freeflight.org Free Flight 11

3. sequentially number of turns, pull
force and torque. The values are then
pasted into a Numbers spreadsheet
column. Numbers sorts them into
other columns representing the three
variables using an index column
which reads 1,2,3,1,2, etc.. If index =
1 then the corresponding value in the
data column represents number of
turns; 2 corresponds to torque etc.
Fig. 2 shows torque versus pull
force for 4 strands of 1/8" rubber,
about 2" long, lubricated with 12,000
cP silicone oil. The plot shows the
characteristic oscillations of pull force
and torque which accompany writhe
loop transformations, first reported
by Rene Bahout in the July 2002 Free
Flight Quarterly. The energy return
efficiency is the ratio of the areas
under unwinding (lower) and winding
(upper) torque curves.
Fig. 3 shows pull force vs torque for
the data of Fig. 2. This plot, especially
the unwinding branch (upper), shows
distinctive humps corresponding to
the comings and goings of successive
rows of writhe knots.
The measured energy return
efficiencies for motors using eight
candidate lubricants are shown in
Table 1. The values are all significantly
less than the typical 79% efficiency
reported by Carrol Allen for simple
stretch tests. The difference can be
attributed to strand-on-strand sliding
friction loss in wound motors.
While there is considerable scatter
in the data, the highest efficiency
lubricants are either greases or
very-high-viscosity silicone oil. This
tends to support the hypothesis that
extrusion of lubricants away from
strand-to-strand contact areas is
important in highly wound rubber
motors.
Figure 4 Energy return efficiency vs maximum torque for three lubricants; each point repre-
sents a complete wind-unwind cycle.
Table 1 Measured energy return efficiencies for eight lubricants: Dow 33 light is a lithium-soap-thickened silicone grease; the 12,000
centiPoise silicone oil was provided by Carrol Allen; Ultimox and Krytox are perfluoropolyether oil-Teflon™ greases; Magic Lube PTFE and
Aladin MagicLube II are silicone oil-Teflon™ greases; ArmorAll is a proprietary aqueous suspension of silicone oil.
12 Free Flight www.freeflight.org

4. A related and interesting observation, illustrated in Fig.
4, is that the energy return efficiency drops significantly as
maximum torque is increased. The dependence appears to
vary somewhat between lubricants but more data would be
needed to confirm this.
CONCLUSIONS
• The data suggest that grease-type lubricants provide at
least equivalent energy return efficiency compared to high
viscosity silicone oils.
• The ability of grease lubricants to resist squeeze out might
be especially advantageous where a motor is expected to
power multiple flights.
• Dow Corning Molykote® 33 exhibited the highest energy
efficiency. Dan Driscoll’s earlier finding that Dow 33
allowed significantly higher max turns strengthens the
case.
• The perfluoropolyether PTFE greases, Krytox and
especially Ultimox, offer good lubricant properties but they
are expensive and also have density values roughly twice
that of silicone oil, a disadvantage pointed out by Carrol
Allen.
• Energy return efficiency decreases significantly as torque
increases, suggesting that strand-on-strand sliding is a
significant loss mechanism.
Those last few turns may offer diminishing returns.
ACKNOWLEDGEMENTS
Son-in-law Mike Nutt got me interested in Arduino. Fellow
Brooklyn Skyscrapers Tom Vaccaro and Carrol Allen
provided advice on test rig components, programming and
wiring. Aram Schlosberg encouraged me to write up the
results and offered valuable editorial comments.
Bob Morris, Flanders, N.J.
morrisresearch@gmail.com
E-36
Ask about free postage on 2 kit orders
BMJR Models
P.O.Box 1210
Sharpes, FL. 32959-1210
321-537-1159 www.bmjrmodels.com
Starduster-E-36
By
Harry Grogan
Satellite-E-36
By
Dale Elder
$48.00
$40.25
www.freeflight.org Free Flight 13
The “water knot”