The start of a new

movement A NASA invention is one of several poised on the brink of commercialization. But it needs Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/124/08/47/6355223/me-2002-aug3.pdf by guest on 01 October 2021 outside help to get there. By Paul Sharke, Associate Editor

T ISN'T OFTEN THAT engineers who design machine components get to speak about I new architectures. That's something left to the designers of sexy computer chips, not for practical nuts-and-bolts folks . But "architecture" is exactly the word John Vranish sprinkles into the conversation as he's describing his latest invention. The NASA engineer has found a novel way of combining two fundamental machine ele­ ments-gears and bearings. With it, he may indeed be looking at the construction of a new way of moving things. Four years ago, Vranish's bosses at the Goddard Space Flight Center in Greenbelt, Md., asked him to come up with an "over­ achieving" planetary speed reducer, some­ thing that could step down a motor output dramatically in a simple and lightweight form. It seemed they had many small mir­ rors they wished to move on the next space telescope, but they weren't necessarily will­ ing to pay the weight penalty that using many conventional reducers would invoke. By giving up its need for rolling-element bearings. a gear-bearing planetary transmission What Vranish came up with doesn't look achieves a light. compact speed reducer. Rollers handle radial loads while tooth ends take thrust. much different than any ordinary planetary transmission When the teeth of one gear bearing roller are meshed at first inspection. It has planets and rings and a sun gear. with the teeth of another, their respective bearing surfaces But there is a difference. It has no bearings-not in the touch also. The gear teeth transmit mechanical force while conventional sense of pairs of rolling-element compo­ countering thrust. The roller takes the radial loads. nents supporting every gear. Vranish displayed two of his planetary reducer proto­ Instead, Vranish has combined gear and bearing into types at the recent National Design Engineering Show a single machine part he calls a "gear bearing." In its in Chicago. With the first, he achieved a 70: 1 reduction simplest form, a gear bearing sandwiches a crowned in a single stage. With the second prototype, Vranish spur or helical gear between two cylindrical rollers. employed "phase tuning" to build a 325:1 reduction. The gear and the rollers share a centerline. The radii of More about phase tuning in a moment. the rollers equal the pitch radius of the gear. If only spur gear teeth make up the gears in mesh, the

MECHANICAL ENG INEER ING AUGUST 2002 47 between individual bearing-element speeds and their fixe d spacing can make the larger balls or rollers overdrive the smaller ones, ca using skidding, or lnicro-chatter, along the raceway. If no carrier is used, faster bearings collide with slower ones, making matters even worse. By eliminating balls, rollers, and carriers, gear bearings sup­ press micro-chatter. According to Vra nish , gear bearings self-adjust to elim­ inate the slight asse mbly misalignments that come

w ith installing bea rings Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/124/08/47/6355223/me-2002-aug3.pdf by guest on 01 October 2021 Exploded view of the planetary transmission depicts two ring rollers. The colors here between shafts and hous- correspond to those in the front view, ings. Bearings are install ed shown on previous page. within fairly close tolerances: thrust-carrying capacity is limited, Vranish explained. In too tight, and they'll squeeze that scenario, the top surfaces of the teeth carry any thrust the balls into premature wear; loads as they roll along the bottom of the roller. Where too loose, and the component will thrust loads are large, gear bearing architecture lends itself perform poorly at reducing friction. Sometimes, the bear­ to the use of herringbone, or double helical, gears to take ing cocks slightly during installation, leading to shaft wob­ adva ntage of their superior thrust capacity. ble. Gear bearings, by avoiding any secondary asse mbly, Although NASA 's telescope application needed a light­ reduce chances for nusaligmuent. weight, compac t, super-reducing planetary gear train, the But these are traits that Vranish called " nuances" of basic elements that make up the transmission are incorpo­ the new design. His enthusiasm built as he spoke about rated readily into a number of other motion devices, in­ several high-performance techniques, such as phase cluding slides, reversers, linear- to- rotary converters, and tuning and rifle true anti-backlash, which work w ith eve n speed increasers. The sam e approac h can handle a gear bearings. doubling of speed or a 2,000:1 reduction in a single stage, along with everything in between. Vranish said that the gear bearing asse mblies eliminate sliding fricti on even under load . D evices constructed from gear bearings encounter rolling friction only. An­ other advantage gear bea rings have over conventional roller bearings is their elimination of carriers normally employed to keep friction- redu cing elements apart. Obvious differences crop up immediately upon placing a gear bearing differential transmission alongside a con­ ve ntional planetary transmission . First, of course, the new reducer eliminates eight ball bearings needed to support the planets and suns in a three-planet reducer. Two different load paths distinguish the reducers as well. Loads travel through the conventional reducer by way of the relatively weak bearings . In the gear bearing reducer version, the load goes from output ring to upper planet to lower planet to ground ring, a much stronger path, according to Vranish.

NUANCE AND PERFORMANCE Turning a prototype by hand produced a smooth output and a great de­ In addition to producing huge speed reductions in small crease in speed. spaces, gear bearings list sm oothness and precision mo­ In explaining phase tuning, Vranish said that a typical ti on control among their attributes. Smoothness, because planetary reducer with three planets would require at they reduce micro-chatter of ordinary gear and bearing least a difference of three teeth between the ground and combinations. Precision, because they eliminate a ro ta­ output rings. That's because the planets would be spaced tional wobble built into many transmissions. equally around the ring in 120-degree increments. Each Elem ents in the mechanism underlying micro-chatter tooth on the planet ordinarily has to engage a full tooth are the small size and loading differences that distinguish between input and output rings, leading to the mini­ one ball or roller from another. Another contributor to mum required three-tooth difference. th e phenomenon is the carrier that spaces the rolling bear­ Phase tuning changes that. To enable their asse mbly, ings evenly around the race. Together, the inconsistencies the input and output sides of the planet gears are cut as

48 AUGUST 2002 MEC H ANICAL ENG I NEER ING separate elements. Because of this, the gear sides can be rotated fractionally with respect to each other, then fas­ tened together. The possibility of creating a difference of only one tooth between the ground and output rings thus opens. Vranish has proved the concept with a rapid prototype model. Rifle true anti-backlash produces a planetary transmis­ sion with zero backlash. To visualize it, imagine the top and bottom halves of a planet gear bearing. The top one consists of a shallow helical gear and bearing machined from a single length of stock with an internal hex run­

ning through it. The bottom half also has a gear and Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/124/08/47/6355223/me-2002-aug3.pdf by guest on 01 October 2021 bearing, as well as a mating hexagonal stem from which the points have been lopped off. Enough clearance exists for the upper gear bearing to slide over the stem. First, though, drop a pair of back-to- back Bell eville springs over the stem. Now, nest this gear bearing pair, along with two other identical pairs, between the rings and sun of a planetary transmission. With the transmission unloaded, note that the spring pushes the upper gear and bearing into the helix Through phase tuning, a one-tooth difference between ground and output rings makes extraordinary reduction ratios possible. of the ring until it can go no further. It's taking up the backlash. Under load, the hexagonal stem and its mating the rings of Saturn, he explained. The public expects hexagonal hole generate ample frictional locking force to tangible benefits. hold the gear teeth against counter torques. If the teeth Perhaps the greatest success for the program was the wear, the Belleville springs take up any additional backlash commercialization of Nastran, the NASA structural as soon as the load is removed. analysis system whose development began in the 1960s. Today, MSC Software of Santa Ana, Calif., sells a propri­ TECHNOLOGY TRANSFER etary version of NASA's finite element analysis software. Of course, gear bearings don't interest everyone. They NASA technologies enter the commercial sector by were merely one example of technologies unveiled at one of two main routes, M itchell said. The simplest, the Chicago show by various NASA agencies. One in­ straight licensing, is more or less a matter of qualified ventor from the Kennedy Space Center discussed new companies filling o ut paperwork, he explained. More data acquisition devices and instruments; another, from complex collaborative arrangements are hammered Marshall Space Flight Center, discussed a quick-con­ out with "Space Act" agreements, which define the nect ball and swivel joint that had been used for satel­ product to be commercialized, divide responsibilities lite docking. among parties, and decide on the handling of intellec­ Many NASA inventions provide opportunities for com­ tual property. mercial developers to work with the various agencies (The National Aeronautics and Space Act of 1958, through licensing or cooperative agreements. According amended Oct. 30, 2000, formed NASA and encour­ to Darryl Mitchell, commercial technology manager for aged commercial development of space, while devot­ Goddard, the idealized version of technology transfer ing space activities to the peaceful benefit of mankind.) puts a private sector firm to work furthering a NASA in­ As an example, Goddard is actively seeking partners vention. Be it through licensing or collaborative agree­ to share in the commercialization of gear bearings, ment, the commercialization firm develops a product Mitchell said. that not only benefits the private sector, but returns Meanwhile, Vranish has taken the invention about as tech nology improvements to the agency. Such "dual far as he needs to for meeting the agency's requirements use" technologies payoff in many ways: first, by solving on a future space telescope. Now is the time to bring in a particular NASA problem; then by advancing a tech­ outside experts who understand manufacturing, or mar­ nology in the commercial domain; and, finally, by pro­ keting, or any of the many steps that must be negotiated viding NASA access to innovations that stem fro 111. the on the way to commercial product success. original invention. As for new architectures, you get the idea that a hands­ Although NASA has been in the technology transfer on guy like Vranish might be a little less comfortable business for many years, the program, of late, has gained using words like "vision" than he is with terms such as more emphasis in the light of shrinking budgets. In­ " reduction ratio." But he admits that, after spending as creasingly, NASA sees the value inherent in its technolo­ much time as he has with the little gear bearings, he's gies. At the same time, "The public expects more bang begun to see a range of motion problems where they for its buck," Mitchell said. For instance, it's not enough might work just fine-and perhaps work even better anymore for NASA to merely acquire scientific data on than what's being built today. •

M EC H AN ICA L ENG IN EERI NG AUGUST 2002 49