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Armed to the teeth
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09/02/2008
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There’s a simple way of achieving massive speed reductions in a single package, as Tom Shelley has been finding out
Nasa has developed a compact, robust gear mechanism which it says offers substantial speed reductions by using planetary gears with two diameters in different sections – with different numbers of teeth.
Design engineer Dr John Vranish, who works at Nasa’s Goddard Space Flight Centre in Maryland, USA, and has his name on a whole host of patents, has developed the concept. Initially, it was aimed at space applications, but now the idea is being offered for general use and is likely to find major applications in robotics.
Vranish calls this idea a “phase-oriented gear system”. A recent Nasa article has described how it might be employed in what it described as a “relatively simple speed reducing differential planetary transmission”. This consists of a sun gear, idler gear, three identical planet gears, a ground internal ring gear, and an output internal ring gear of slightly greater diameter and number of teeth. Each planet gear consists of a top output gear – and lower ground gear – engaging section of slightly smaller diameter and number of teeth (typically one tooth fewer).
Power is input by the shaft attached to the sun gear. The ground internal ring gear is fixed. As the planet gears roll round, they cause more teeth to engage the outer internal ring gear during each rotation than the ground internal ring gear – meaning that the outer internal ring gear has to rotate backwards slightly to compensate. The term ‘phase shift’ refers to the difference in the numbers of teeth in the two sections of the planet gears and the difference in the numbers of teeth in the ring gears, so that the meshing points are initially in step and then shift until they are out of step, before going back into step – one tooth ahead or behind – as they rotate.
Many gear reduction mechanisms rely on similar principles: the Sumitomo Cyclo mechanism, and the Harmonic Drive, for example, as does the Spinea ‘TwinSpin’, described in Eureka in February 2006, and the nutational ‘SpaceGear’, which featured in the November 2003 issue. The patent literature contains a number of references to the principle being applied to two- stage planetary gearboxes, though we have not so far been able to locate this arrangement exactly. Since a patent has been applied for, Nasa seems hopeful that this is a new variation on the theme.
NASA Goddard Space Flight Centre
Pointers
* Reduction is achieved by using planetary gears made with two sections with different diameters and different numbers of teeth engaging on the insides of two ring gears.
* The arrangement is simple, rugged and achieves a very large reduction in a small space
* Many gear reduction mechanisms rely on similar principles. Nasa has applied for a patent
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Author
Tom Shelley
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