Tethered cycloid varies its ratios

Tom Shelley reports on some novel adaptations of a most interesting mechanism

John Hammerbeck, who invented the SCRAM compliant variable ratio coiled spring drive described in Eureka's September 2000 edition, has come up with an even more intriguing mechanism based on hypocycloidal reducers. Hypocycloidal reducers have an internal ring gear that walks round an inner output gear to produce contra rotation at a high reduction ratio. Hammerbeck's innovation is to tether the ring element in such a way that it can still move from side to side and up and down, but cannot rotate. If the elements are gears, the ring gear is acted on by a cam follower running round its outer periphery. As the non rotating ring gear is made to walk round the internal gear, the latter is forced to turn in the opposite direction. Speed reduction arises from the differences in circumference between the inside of the ring gear and the outside of the internal gear. If the gears are replaced by smooth wheels, the same effect is achieved. The only frictional interface is the one between the inside of the ring and the internal element. In a further development, in which the elements have variable circumferences, the ring may be replaced by a circular tube, inflated either hydraulically or pneumatically. This would change its effective internal circumference, and thus the transmission ratio. The mechanism could be constructed so as to pass rotation without a sealing gland. This would enable a reducer constructed with the high speed input side running dry in a separate compartment from the low speed output side, running in a lubricated compartment. The inventions exist in the form of working models. The fixed ratio mechanism model runs very smoothly, and achieves a 4:1 reduction ratio. The variable ratio mechanism model runs less smoothly, because the variable circumference is achieved by varying the circumference of a circlip. The model is nonetheless good enough to demonstrate the principle. It is also possible to use the mechanism as a pump or pulse driven turbine, making use of the fact that the space between the walking ring and an outer fixed ring varies. As a pump, drive is to the inner wheel and the ring walks around the inside of a circular chamber. It is necessary to have an element between the inner and output ports that can either be squashed flat by the ring, or pushed into the chamber wall, when the ring is at top dead centre. The arrangement has the advantage of no sliding seals. All three inventions are covered by patent applications. Scram Gears John Hammerbeck Pointers * It is possible to make a hypocycloidal speed reducer with a tethered transfer ring. * By using an inflatable transfer ring, it is possible to vary the reduction ratio * It is also possible to adapt the same mechanism to a pump or pulse driven turbine, which has no sliding seals