A high-pressure environment

Flat clinching with pneumatic impact cylinders

Pneumatic cylinders able to withstand high amounts of kinetic energy are called impact cylinders. In these cylinders, impact energy from compressed air causes the piston rapidly accelerate to a high speed within a defined stroke. Its kinetic energy is then transferred to other processes. The non-profit innovation center at the Steinbeis Transfer Center for Drive Engineering and Robotics in Engineering has developed an impact cylinder with reduced rebound which can be used for flat clinching, an innovative new joining method. The project formed part of an SME innovation and development scheme funded by the German Federal Ministry of Economics and Technology.

The difference between impact cylinders and conventional pneumatic cylinders is that in an impact cylinder, the piston does not use throttled exhaust gas and is not brought to the stop position via open-loop control. Instead, it is accelerated until shortly before it reaches the stop position. It therefore has greater kinetic energy. But the use of impact cylinders in industry has considerable disadvantages – especially due to the rebound forces upon starting the piston and the major mechanical stress that the housing is subject to upon impact.

Together with the company Lehmann-UMT, based in Pöhl in the German state of Saxony, the Steinbeis team in Chemnitz ran an experimental study to develop a technical solution to reduce rebound forces in the impact cylinder when the piston is started. This was done by adding a balancing weight positioned axially opposed to the impact piston. As the piston and the weight are activated at the same time by the compressed air but travel in opposite directions, their momentums compensate each other, thereby reducing the overall mechanical impact on peripheral housing and drive components.

The impact cylinder is suited to a range of applications, primarily in the area of separating and joining via forming processes such as stamping, riveting and clinching. These require a large amount of force and a high forming energy applied at the joining point.

In partnership with the Department of Virtual Production Engineering at Chemnitz University of Technology, the Steinbeis team in Chemnitz managed to fit an impact cylinder with innovative flat-clinching technology. Developed at Chemnitz University of Technology, this punctiform, single-step joining technique results in joins that are flat on one side. This means that mechanically joined, friction-locked and form-locked joins can be used in visible areas or in functional surfaces. The technique is also ideal for joining different types of materials with each other, such as plastic and metal. As such, flat clinching is ideal for multi-material design with a short, effective process chain and for intelligent lightweight construction using different materials.

For the first series of tests, the team designed and built a prototype impact cylinder for an energy of 250 J. Using the flat-clinching method at operating pressures of up to 8 bar and a piston stroke of 100 mm, it was possible to join aluminum sheet of the types AlMg99.5 and AlMg3 up to a total sheet thickness of 4 mm, as well as DC04 steel plate up to a thickness of 2 mm. The result: a component surface ready for painting, without the need for any costly extra finishing beforehand.

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