Vehicles to be designed for exploration of planets and moons of the solar system will require reliable mechanical drives to operate efficiently.  Long-term operation of these drives will be challenging because of extreme operating conditions.  These extreme conditions include:  very high and/or very cold temperatures, wide temperature ranges, dust, vacuum or low-pressure atmospheres, and corrosive environments.

Most drives used on Earth involve oil-lubricated gears.  However, due to the extreme conditions on planetary surfaces, it may not be advisable or even possible to use oil lubrication.  Unfortunately, solid lubricants do not work well when applied to gears because of the high contact stress conditions and large sliding motion between the teeth, which cause wear and limit life.  We believe traction drives will provide an attractive alternative to gear drives.  Traction drives are composed of rollers that provide geometry more conducive to solid lubrication.  Minimal slip occurs in this contact geometry and thus there is very low wear to the solid lubricant.

The challenge for these solid-lubricated drives is finding materials or coatings that provide the required long-life while also providing high traction.  We seek materials that provide low wear with high friction.

A coating that has structural strength but still has the ability to flow at the interface can support the load and the wear process is one of gradual wear through the coating (left).  Coatings without sufficient structural strength can still lubricate by forming a very thin film at the metallic surface.  The life of this lubrication mechanism is strongly dependent on the topography of the metallic surface.

Photomicrograph showing the thin film lubricating mechanism for a polyimide coating that was unable to support the load.  A thin film of material at the metallic surface has formed and the roughness (scratches) in the surface helps hold the material in place to provide a long endurance life.  Most soft lamellar solid lubricants lubricate by this mechanism.  Proper substrate surface preparation is important for obtaining a long endurance life.

This table shows the friction and wear of various sliding couples illustrating that low friction and low wear do not always occur at the same time.  For traction drives we want high friction and low wear.  One should not assume that just because you have high friction you will also have high wear.

This table shows the friction and wear of some commercially available composite materials sliding against various counterface materials in dry air.  The table illustrates how the counterface can markedly affect the tribological properties of a composite.  Thus it may be possible to develop better traction drive rollers by considering materials that have higher friction when sliding against low wear composites or coatings.

This table compares the friction and wear properties in air and vacuum to illustrate how oxygen and water vapor can affect tribological properties.  The results presented here show that the PMDA polyimide or the Graphite Fiber Reinforce Polyimide (GFRPI) have potential for traction drive rollers in a non-oxygen environment.

Plasma Sprayed (PS) coatings were developed for high temperature lubrication applications.  This figure illustrates that in oscillating journal bearing tests the friction remains relatively high over a range of temperatures from -107º to +870º C. This high friction characteristic makes these materials candidates for traction drives for space applications on cold planetary surfaces.

The loss in radial clearance (wear) for the oscillating journal bearing tests indicates that wear is relatively low at -107ºC when compared to room temperature (25ºC) indicating that for cold planetary surfaces this could be a good traction drive material.

Picture depicts Mars Athena '03 rover. Sketch shows a Boeing concept for a pressurized Lunar rover vehicle.

For a traction drive, we need high friction (traction) with low wear. The 100% PMDA polyimide solid (#5) and film (#8) materials show promise.

This simple device can test traction roller materials in vacuum. It includes provision to cool the rollers through hollow shafts. With minor modification, it can also test gears.

Partly completed traction drive tester at left.  Project awaits restored funding.

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Last modified 31 Oct 2006