RubberLubeNFFSDigVol50No1

Markku Tahkappa, Finland,1957 Nordic World Champion model

SURVEY OF RUBBER MOTOR LUBRICANTS

INTRODUCTIONLubrication of rubber motors is critical for sport and competition flyers alike. Maximum turns, energy return, and motor life are all increased bysuitable 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

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EXPERIMENTALThe 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 DISCUSSIONThe 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.

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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

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.

ACKNOWLEDGEMENTSSon-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, [email protected]

E-36

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The “water knot”