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Leistungsstarkes Werkzeugspannen durch Aktorik aus Formgedächtnislegierungen

Powerful mold clamping through actuators made of shape memory alloys

© IFW
Overview project goals "FGL-Spann"

Tool clamping systems have a major influence on the precision and economy of machining processes. In conventional systems, disk spring assemblies are used to apply the clamping force and additional hydraulic cylinders are used to release the clamping force. Although this is an inexpensive way of implementing reliable tool clamping, the spring assemblies are subject to wear and the hydraulic cylinder requires a relatively large installation space. Furthermore, the clamping force of the spring assemblies cannot be adjusted without greater effort. For this reason, the IFW is developing and researching a new type of adaptively adjustable clamping mechanism based on shape memory alloy actuators in the "FGL-Spann" cooperation project with Ott-Jakob Spanntechnik GmbH. These are intended to replace the conventional disc springs and the hydraulic cylinder and enable a significant reduction in installation space with simultaneous adjustability of the clamping force.

In conventional clamping mechanisms, disk spring assemblies are used to apply the required clamping force. These hold back the tie rod without an additional energy supply, so that the tool holder is securely clamped by the clamping system. An additional hydraulic cylinder and a hydraulic unit are required to realize the release or clamping. Pressurization of the hydraulic cylinder compresses the spring assemblies and disengages the tie rod. The stroke of the tie rod opens the collet and the clamping mechanism is released. When the pressure on the hydraulic cylinder is reduced, the spring force of the spring assemblies pushes the tie rod back to its original position and the system is clamped.

The use of standardized standard parts in the form of disk springs provides an inexpensive and reliable way of implementing a tool clamping system. Nevertheless, the clamping force is not adaptively adjustable and an additional hydraulic unit is required. The clamping force can only be changed by varying the number of spring assemblies used.  In addition, the spring assemblies are subject to wear through material fatigue and abrasion due to the many load changes and the friction between them. An alternative approach to realizing a tool clamping mechanism is therefore the use of a bidirectionally acting actuator. For safe clamping of a tool holder by a bidirectionally acting actuator, a high clamping force of at least FS = 10,000 N and clamping travels of Δs = 10 mm are required. Furthermore, there is a requirement for the smallest possible installation space so that the system can be retrofitted in conventional clamping systems. This is not feasible with already known actuator solutions (e.g. piezo actuators) in a small installation space.

In the "FGL-Spann" project, an actuator system based on shape memory alloys (FGL) is therefore being developed and researched to replace the disk spring assemblies and the hydraulic unit, which enables a bidirectional action for clamping and releasing the tool holder, as well as an adaptively adjustable clamping force. The new development builds on findings from preliminary investigations and the "Hybrid Spindle" research project also conducted at IFW. In the aforementioned project, an adaptive bearing preload element made of a Ni-Ti shape memory alloy was successfully developed and tested. Actuators made of shape memory alloys can be activated magnetically or thermally. Actuation is achieved by a phase transformation between the austenitic and martensitic microstructural components of the alloys, resulting in a change in the shape of the actuator geometry. Due to their high energy density, it is possible to generate comparably high compressive and tensile forces, as in hydraulic actuators, in a very small installation space. By using FGL technology, an innovative new development of a tool clamping system with an overall actuator is to be implemented, consisting of individual actuators made of shape memory alloys connected in parallel and in series. The interconnection of the actuators to form an overall actuator enables the required high clamping forces and clamping paths to be realized. In addition to the FGL actuators, research is also being carried out into a clamping force measurement system integrated into the clamping system to monitor the effective clamping force.

The aim of the project is to implement a powerful and adaptively controllable clamping mechanism with a small installation space. At the same time, an increase in the wear resistance and energy efficiency of tool clamping systems is to be achieved through the use of FGL actuator technology. Furthermore, the project will generate an in-depth understanding of the possibilities and limitations of using innovative FGL technology.