NASA Goddard Space Flight Center (GSFC) has unveiled a mechanical architecture breakthrough by combining gear and bearing functions into a single unit that significantly improves gear drives across the board for electrical Applications, internal combustion, and turbine motors.
The gear bearing design incorporates rifle-true anti-backlash, improved thrust bearing performance, and phase-tuning techniques for superior low-speed reduction. Because it combines gear and bearing functions, it reduces weight, number of parts, size, and cost, while also increasing load capacity and performance. (Figure 1)
By incorporating helical gear teeth forms (including herringbone), gear bearings provide outstanding thrust bearing performance. They also provide unprecedented high- and low-speed reduction through the incorporation of phase tuning. Phase tuning allows differentiation in the number of teeth that must be engaged between input and output rings in a planetary gearset, enabling successful reduction ratios of 2:1 to 2,000:1. They provide smooth and accurate control with staggered teeth anti-backlash. Gear sections of a planet are staggered in rotation, so the teeth of one section and the teeth of another section make contact on opposite sides of the backlash gap. This, effectively, eliminates the backlash and allows for smooth and accurate control even under load. The inner portion of a gear spins (like a rifle bullet) as it moves axially via spring action. The spinning motion continues until the backlash is taken out. The rifling angle is a mechanical locking angle. This produces a planetary transmission with zero backlash. NASA inventor John Vranish explains how it came to pass:
“It all started when we were trying to get superior resolution for telescopes. We were looking at ways to put motors with high speed reduction capabilities on some of the new telescopes. We wanted them compact, with a lot of power density—you can’t have a lot of mass when you’re lifting things into Earth’s orbit. We looked into a number of approaches to this problem. In the end, we believed you could do it with basic gearing without going into exotic materials, focusing on existing technology and materials to get the extra speed reduction.
“The search for these basic gearing solutions led to epicyclical planetary transmissions, which provide very high speed reduction along with heavy power and torque densities. However, these require lots of bearings in order to stabilize the sun gear, the planets (both in orbit and rotation), and the output ring gear. We started eliminating these bearings by incorporating rollers into the planets, sun, and idler sun with roll races incorporated into the ring gears. The rollers took care of the lateral loads, stabilizing where everything went, and they assembled easily—at least, easier than all the other bearings we were dealing with before. Plus, the entire structure was now very compact.
“This left me with the issue of axial bearing. I had to find a way to get the tops of the gear teeth to interact with the bottom of the rollers to produce an axial bearing function. That’s pretty much where it all started. The initial prototypes of this were giving us speed reduction rates of 70:1 to 300:1. We were struggling to get them higher when we discovered that using phase shifting in the planets led to speed reduction into the thousands. This was done by phase tuning the output stage of the planet, as opposed to the input stage of the planet, a revelation to us. Upon hearing this, management at NASA headquarters asked, ‘Can you get the equipment of less than a single tooth in difference?’
“And we found that, with the correct timing, we could. The tooth count comes in digits, but the arrival comes in fractions of a tooth. That’s done by phase tuning the tops of the output stages of the planets. We’ve found a one-tooth difference between ground and output rings is possible, creating opportunities for significant reduction ratios at both low and high speeds (from 0.5:1 to >2,000:1). Gear bearings provide superior speed reduction in a small package. They form rolling friction systems that function both as gears and bearings and are compatible with most gear types, including spur, helical, elliptical, and bevel gears. These self-synchronized components can be in the form of planets, sun, rings, racks, and segments thereof. (Figure 2)
“From there, we were ready to take this technology to the outside world. The initial feedback from potential customers wasn’t positive, however. We found that, in general, the customers weren’t as interested in speed reduction. These customers were working in factories, not on space vehicles. They wanted improved production rates, high throughput, and more powerful motors for power presses, lathes and grinders, slitting and rolling equipment, construction equipment, lifting and handling equipment. That led to the development of the outrunner gear bearing. The outrunner approach achieves lower speed reductions by decreasing the angular distance the input must move, while increasing the angular distance the output moves in response. Input angular distance is decreased by having the input operate on the input ring gear bearing. The output is increased by the planets operating on the output sun gear bearing. Outrunner gear bearings achieve speed reductions from 70:1 down to 2:1 just by working the gear sizes and tooth count—the gear teeth design gives superior thrust bearing performance, and more evenly distributed planet loading reduces cyclic loading and rough spots, reducing noise and vibration. Gear bearings can be applied to many types of motions including linear, rotary, or motion hybrids.
“The versatility of gear bearing technology led to our involvement with the F-35 joint strike fighter jet. At the time, the F-35 had an air brake problem. The plane must be able to slow drastically in the air, which personnel addressed by throwing up an air brake. This brake would absorb a massive amount of air drag, and could rip right out of the airplane. I was tasked with inventing a high speed reduction ‘gadget’ with a lot of power density to take that kind of air pressure. Speed reduction, however great or little, always comes with back-drive. So they needed a magnetic brake in there to stop it. I made the argument that this one would not back-drive. (Figure 3)
“And it didn’t. We have designed epicyclical gear bearing transmissions where, as the tooth tries to back-drive, it begins to roll backwards. As it starts to roll backwards, the contact point on the upper stage trying to roll backwards moves along the surface of the gear tooth, along its involute. The involute of the ground stage opposing it moves along its involute also. The one on the ground stage is moving outboard, opposite to the one on the output stage, and they’ll reach a point where the action lines cross. At that point, it rolls into equilibrium and it will not roll back. However, the rifle-true anti-backlash prototypes did not perform with sufficient smoothness, and this approach was abandoned. A new stagger tooth approach, eliminated backlash while preserving smooth running.
“You can do this for outrunners, too. It’s all in how you set up the gearing. Gear bearings are more structurally rigid and provide higher overall load capacity compared to fixed planetary designs. Also, reduced cyclic loading decreases susceptibility to fatigue failures.
“Gear bearings combine gear and bearing functions to reduce materials and cost, while also reducing weight and simplifying the design. I’ve always argued that ‘simple isn’t simple. We start with a lot of ideas, and make them simpler and simpler. People look at the end result and think, “I could have thought of that!” But if someone is going to use it, it has to be simple. NASA’s gear bearing technology is based on two key concepts: the roller gear bearing and the phase-shifted gear bearing. A new modular gear bearing is also offered with a simplified manufacturing technique. All designs are capable of efficiently carrying large thrust loads. (Figure 4)
“Existing gear systems have drawbacks including weak structures, large size, and poor reliability, as well as high cost for some types (e.g., harmonic drives). Gear bearings solve these problems with simpler construction, fewer parts, and superior strength. In planetary gearsets, gear bearings can eliminate planet carriers or planet bearings, substantially reducing parts count and cost. The ring gear can be mated with a motor housing to eliminate the motor’s front bearing and further reduce cost. In addition, gear bearings eliminate concerns that the center of the carrier is coincident with the center of the sun gear and equalizes the loading for the planet gears. Gear bearings also eliminate separate bearings, inner races, and carriers, as well as intermediate members between gears and bearings. Load paths go directly from one gear bearing component to another and then to ground.”
By selecting the appropriate manufacturing method, gear bearings can be tailored to benefit any application from toys to aircraft. For tight-tolerance applications, such as in helicopters, machined gear bearings could provide very high performance over conventional gearsets. For medium-range applications, such as in power tools, cast gear bearings could reduce cost and size. For low-tolerance applications, such as toys, injection molded plastic gear bearings could substantially reduce cost. Beyond reducing parts, the high-load capacity of gear bearings can further reduce cost by enabling the use of less expensive, lower strength materials.
For More Information:
NASA’s Goddard Space Flight Center (GSFC) is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe. A byproduct of their mission-focused work is a vast portfolio innovative technologies and intellectual property discovered and developed to enable and monitor space missions. Many of these technologies also have utility for industrial applications here on earth.
Through its Innovative Technology Partnerships Office (IPTO), NASA-GSFC seeks commercialization partners for this innovative technology and others in its intellectual property portfolio. NASA has the authority to grant licenses on its domestic and foreign patents and patent applications pursuant to 35 USC §§207-209. NASA follows the regulations set forth in 37 CFR §404. For more information, contact NASA-GSFC’s IPTO at 301-286-5810.
