Clamping system made from shape memory alloys

Design and function of spring-based tool clamping systems © IFW

SMAs are characterized by the fact that a change in length occurs when energy is introduced, for example in the form of heat. This effect can be used to realize forces and actuating movements. Once the actuator has cooled down, it automatically returns to its original state. This bidirectional change in length between the low and high temperature phase is based on the flipping of the molecular lattice structure between the twinned and untwinned lattice state or the change between the martensite (cold state) and austenite phase (hot state). The effect described is known as the two-way effect. The actuator can be used bidirectionally by switching between the low and high temperature phases. Both functions of conventional systems - i.e. clamping and unclamping of the tool holder - can therefore be combined in the actuator.

Design of a thermal SMA actuator for a milling spindle

Based on the two-way effect, the bidirectional SMA actuator should be able to be integrated into a clamping system available on the market as a retrofit solution. Therefore, the same requirements apply to the SMA actuator as to conventional systems. The aim is to achieve a clamping force of at least 5.8 kN and a release force of at least 3.0 kN. At the same time, the required stroke is defined as 7.4 mm. To implement the desired retrofit solution, the size is limited to a maximum outer diameter of 34 mm and a maximum length of 405 mm. Furthermore, the shortest possible switching time is required in order to enable the necessary dynamics for the tool change.

In accordance with the requirements, various alloy types and actuator geometries were analyzed with regard to their potential performance. The alloy NiTiCu7.5 was identified as a potentially suitable alloy due to the high achievable elongation of 8 % and the high maximum permissible mechanical stress of 300 MPa. At the same time, the material has high cycle stability, which can be expected to have a positive effect on the reproducibility of the adjustability of the actuator force. With regard to the geometric actuator shape, actuator wires made from the previously selected alloy were identified as a suitable concept.

Parameters give a clamping stroke of 7.52 millimeters

In the high-temperature phase, the clamping force is applied by heating the wires. When cooling down with the aid of an air flow, the release force is realized when switching to the low-temperature phase. This operating principle was chosen because the two-way effect exhibits anisotropic behavior when forces are applied. Due to the anisotropy, the forces resulting from the phase transformation during heating are higher than the forces of the phase transformation during cooling. The thermal SMA actuator wires are actuated by electrical resistance heating. The selected wire diameter of 1.5 mm specifically favors high dynamics during switching, as the low thermal mass per wire has a positive effect on the duration of the heating and cooling processes.

Concept of the thermal SMA actuator © IFW

The actuator wires are fixed in the wire holder and the wire clamping ring using grub screws. To prevent the wires from kinking while the release force is being applied, three wire guides are provided between the wire holder and the wire clamping ring. The maximum realizable force per wire is 530.14 N with a permissible tension of 300 MPa (manufacturer's specification) and a wire cross-section of 1.77 ^mm2. With the current actuator concept, a parallel connection of 20 wires is planned, so that a maximum total force of 10.6 kN is potentially possible. By utilizing the entire available installation space length - i.e. a SMA actuator with a total length of 405 mm and a free working length of 376 mm - a maximum clamping stroke of 7.52 mm can potentially be achieved.

By utilizing 2 % elongation, the requirement of the necessary tensioning stroke of 7.4 mm is thus fulfilled and the wire is not stressed in the limit range. Furthermore, higher elongations would lead to a reduction in the maximum achievable service life of the actuator wires. With the parameter combination used here, a service life of more than two million cycles is expected.

Summary and outlook: more energy-efficient clamping system in a reduced installation space

The innovative SMA actuator concept uses the shape memory effect to perform the necessary positioning movements and realize the required forces. The aim is to reduce the energy required for the clamping cycle. The SMA actuator is developed as a bidirectional system so that the separate clamping and unclamping functions of conventional clamping systems are combined in one component. This makes it possible to replace the hydraulic release unit and thus reduce the energy requirement. The innovative design of the SMA actuator thus creates a potentially more energy-efficient clamping system in a smaller installation space.

Funded research project

The "FGL-Spann" research project is funded by the Federal Ministry for Economic Affairs and Climate Protection (BMWK) on the basis of a decision by the German Bundestag as part of the Central Innovation Program for SMEs (ZIM) and is supervised by the German Federation of Industrial Research Associations (AiF).

Prof. Dr.-Ing. Berend Denkena (1959), Director of the Institute for Production Engineering and Machine Tools (IFW), Leibniz Universität Hannover

Dr.-Ing. Heinrich Klemme (1987), Head of the Machines and Controls Department at IFW, Leibniz Universität Hannover

M.Sc. Jannes Vornkahl (1997), Research Assistant in the Machine Components Department at IFW, Leibniz Universität Hannover