Low Backlash Planetary Gears with SMAs

This paper investigates applications of nitinol shape memory alloys in the field of planetary gears. Using superelastic austenitic components consistent backlash can be bridged by pretensioning the gears. Furthermore, thermally triggered, martensitic actors can also be applied in order to minimize the backlash of planetary gears. There is a strong and increasing demand of low backlash planetary gears for different applications. Accordingly, the conceptual design of the prototypes considers two kinds of appliance for the respective requirements. First, there are planetary control gears of a smaller scale. These are fitted with superelastic components, which include an adequate pretension that bridges the gear’s backlash. Control gears are in use in considerable quantities in different industrial sectors, e.g. as recliner, as drive unit for landing airbrakes or as robot gears et cetera. Characteristically short operating time during the life cycle does not cause thermal inhomogeneities during operation. Therefore, adequate pretensioning is an effective mean to achieve low backlash. Concepts including a thermally triggered mechanism are designed to balance changes of the gear’s backlash due to thermal expansion of the gear’s components. This function is required in planetary gears that provide a superior power density. It is essential to identify constructive solutions that permit an optimal functional and geometric integration of the mechanism. The mechanism’s functionality enables a static adjustment of the transmission to realize its low backlash operation at environmental temperature. The mechanism will offer a definable adjustment travel corresponding to the extrinsic two-way effect, having a martensitic actuator being prestressed by an antagonist spring. The dimensioning of the mechanism provides a balancing adjustment travel according to the backlash decrease due to thermal effects. Thermally caused teeth clamping and in consequence a total failure of the transmission are prevented by the smart solution for the transmission’s operation at low backlash.