Long-Life, Lightweight, Multi-Roller Traction Drives for Planetary Vehicle Surface Exploration
- Created: Tuesday, 01 May 2012
These drives can be used for Earth-based applications where extreme temperatures are involved.
NASA’s initiative for Lunar and Martian exploration will require long lived, robust drive systems for manned vehicles that must operate in hostile environments. The operation of these mechanical drives will pose a problem because of the existing extreme operating conditions. Some of these extreme conditions include operating at a very high or very cold temperature, operating over a wide range of temperatures, operating in very dusty environments, operating in a very high radiation environment, and operating in possibly corrosive environments.Current drive systems use gears with various configurations of “teeth.” These gears must be lubricated with oil (or grease) and must have some sort of a lubricant resupply system. For drive systems, oil poses problems such as evaporation, becoming too viscous and eventually freezing at cold temperatures, being too thin to lubricate at high temperatures, being degraded by the radiation environment, being contaminated by the regolith (soil), and if vaporized (and not sealed), it will contaminate the regolith. Thus, it may not be advisable or even possible to use oil because of these limitations.
An oil-less, compact traction vehicle drive is a drive designed for use in hostile environments like those that will be encountered on planetary surfaces. Initially, traction roller tests in vacuum were conducted to obtain traction and endurance data needed for designing the drives. From that data, a traction drive was designed that would fit into a prototype lunar rover vehicle, and this design data was used to construct several traction drives. These drives were then tested in air to determine their performance characteristics, and if any final corrections to the designs were necessary.
A limitation with current speed reducer systems such as planetary gears and harmonic drives is the high-contact stresses that occur at tooth engagement and in the harmonic drive wave generator interface. These high stresses induce high wear of solid lubricant coatings, thus necessitating the use of liquid lubricants for long life.
Because of their near-pure rolling contact, traction drives can operate unlubricated at very cold temperatures or at high temperatures by using low-wear, high-traction materials or coatings. Oil-less traction drives will not encounter the temperature swing problems of other proposed planetary vehicle drives. Traction drives also will be less sensitive to dusty conditions if sealed properly, and will also not contaminate a planetary environment because there is no liquid lubricant used.
The oil-free traction drive is a “toothless” drive system that is capable of dry operation using low-wear, high-friction materials and coatings. Multi-roller traction drive configurations offer high reduction ratios (>30 to 1) in a single stage, reducing motor size and providing a lightweight wheel drive system.
A traction drive with nearly pure rolling action provides much longer life than could be achieved with gear or harmonic drive systems in applications where liquid lubricants could not be used. Use of ceramic-coated titanium or polymer rollers will reduce the weight of the drives and also reduce the cost to launch.
This work was done by Richard C. Klein, Robert L. Fusaro, and Florin Dimofte of NASTEC, Inc. for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. LEW-18826-1