Exploration of the planets and moons of the solar system will require several types of vehicles, including manned and unmanned rovers, construction vehicles, and mining vehicles. These vehicles will need mechanical drives that are lightweight, efficient, reliable, and long-lived, even as they operate under harsh conditions involving temperature extremes, rough terrain, hard vacuum, and abrasive dust.
Space mechanisms design requirements are strongly application-specific. Factors that will influence rover vehicle drive design decisions include the speed, torque, weight and space allowances, temperatures, lubrication, challenges of the environment, and requirements for efficiency, life, and reliability. In addition, heat dissipation can be a problem for high-power drives, especially if solid lubricants are used.
This study at the NASA Glenn Research Center considered classical gear designs, including simple spur and helical stages and combinations of simple stages such as epicyclic (planetary) transmissions, including compound planetary and differential transmissions and harmonic drives. Each of these types has specific advantages and challenges.
The study also considered novel concepts, such as traction drive transmissions and hybrid traction-gear transmissions. Novel concepts need additional development work to establish their reliability and suitability for particular missions.
Very high gear ratios are available by cascading transmission stages. The stages need not be of the same type. For example, the 1500:1 ratio used on the Mars Exploration Rover was obtained with an 18.8:1 planetary first stage and an 80:1 harmonic-drive second stage. Staging can optimize the performance of a transmission. The Mars Exploration Rover benefits from the high torque to weight of the harmonic-drive output stage while avoiding problems from the limited life of the input bearing of the harmonic drive.
Some of the general characteristics are summarized in the table.
| Advantages | Challenges |
|---|---|
| Spur and helical gear pairs | |
| Highest mechanical efficiency (>98 percent) | Offers only a moderate speed ratio per stage |
| Relatively inexpensive | Requires many stages for high ratios |
| Compatible with dry lubricants | Helical gears have thrust load |
| Requires shaft offset (except reverted train) | |
| Harmonic drives | |
| Space qualified | Limited life of wave generator bearing |
| High reduction in one stage | Moderate efficiency (<80 percent) |
| Can be hermetically sealed | Not very suitable for dry lubrication |
| Near-zero backlash | |
| Simple planetary gear train | |
| High efficiency (<90 percent) | Complex (many components) |
| Compact, close-coupled design | Requires multiple stages for high ratio |
| Much experience in aeronautics | |
| Distributed loading of gears and bearings | |
| Compatible with dry lubricants | |
| Compound planetary gear train | |
| High efficiency (<90 percent) | Complex (many components) |
| Distributed loading of gears and bearings | Moment loads on planet bearings |
| Compatible with dry lubricants | |
| Differential planetary gear train | |
| High ratio (500:1) possible in a single stage | Complex (many components) |
| Distributed loading of gears and bearings | Low efficiency, especially for high ratio |
| Compatible with dry lubricants | |
| Traction drive | |
| High efficiency (<94 percent) | Complex (many components) |
| High ratio (250:1) | High weight (30-percent higher than planetary) |
| Smooth operation (low noise/ripple) | Possible surface durability problems |
| Compatible with dry lubricants | Needs technology development |
The graph relates the mass to the output torque of high-ratio, space-rated drives from four commercial vendors. The drives include 80:1 and 160:1 harmonic drives, two-stage planetary units with ratios from 61:1 to 114:1, and miscellaneous drives with ratios up to 17933:1.
Mass of high-ratio transmissions from four commercial vendors as a function of the output torque. Curve-fitting mass M versus torque T yields M = 0.0678T0.7782, where M is in kilograms and T is in newton-meters.
Long description of figure.
Find out more about this research:
Space Mechanisms Project:
http://www.grc.nasa.gov/WWW/spacemech/
Glenn’s Mechanical Components Branch:
http://www.grc.nasa.gov/WWW/5900/5950/
Glenn contact:
Fred B. Oswald, 216-433-3957, Fred.B.Oswald@nasa.gov
Author:
Fred B. Oswald
Headquarters program office:
Exploration Systems
Programs/Projects:
MTLAMPS
Last updated: October 16, 2006
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216-433-8258
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216-433-2912
SGT, Inc.
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