Innovative materials with high strengths are increasingly being used in many areas of mechanical engineering. The use of such sheet materials helps, for example, to reduce vehicle weight and increase crash safety.
A particularly noise-intensive process step in sheet metal processing is punching. In this process, the cutting punch is placed on the sheet and force is built up. Once the maximum cutting force is reached, the sheet breaks off – and the energy that was stored in the press during springing is abruptly converted into kinetic energy (cutting impact). As a result, intense mechanical vibrations occur in the press components. These excite the surrounding air so that sound waves are emitted into the machine environment.
The noise level can be well over 100 dB, which is roughly equivalent to the volume of a jackhammer. The intensity of the noise generated depends on both the process excitation and the characteristics of the machine.
Problematic machine noise, expensive protective measures
Machine noise can cause long-term physical and psychological damage to the people involved. In addition, it leads to a reduction in the ability of machine operators to concentrate and to an overall reduction in productivity because, for example, production times are restricted for noise protection reasons. Noise pollution is a particular challenge for small and medium-sized enterprises in growing regions with a high population density – due to the short distance to adjacent residential areas. According to the law, the continuous noise limit of 85 dB must not be exceeded.
Developing and equipping plants with soundproof booths often involves high costs and effort. While these booths are effective, they increase space requirements and limit accessibility for adjustment, maintenance and repair work. Solutions to reduce excitation, such as cutting shock absorbers, are again usually expensive as well as high-maintenance and are not suitable for applications on high-speed presses with high stroke rates, for example.
Seeing sound with an acoustic camera
At the Institute of Forming Technology and Machines (IFUM), research is currently being carried out into solutions for noise reduction that can circumvent the disadvantages described as far as possible. The focus here is on both passive and active measures directly on the machine structure, with which the acoustic machine properties are to be optimized.
Targeted noise reduction measures require a deep understanding of the acoustic machine behavior. The aim is not merely to identify the noise sources, but to gain knowledge of the component-related frequency ranges excited during a stamping process and the resulting frequency-dependent sound pressure level fields on and around the press. For this purpose, measurement and simulation-based investigations are carried out at the IFUM.
Various measuring devices are used for the experimental recording and analysis of the sound emissions. The acoustic camera from CAE Software & Systems used at the institute is a microphone array consisting of 112 microphones surrounding an optical camera (Figure 1). The measurable frequency range is 10 – 24,000 Hz and the operating range is up to 120 dB.
With the aid of the camera and evaluation software, it is possible to record and analyze the sound emissions. Frequency spectra and sound pressure levels are recorded, which can be clearly assigned to the individual machine components (Figure 2).
Models for mapping machine acoustics
Since the design measures for noise reduction in particular cannot be implemented and tested on existing presses, or only at great expense, they are derived and evaluated in the simulation. Acoustic machine models are developed at IFUM for this purpose. The models are based on rigid CAD data of the machines, from which a dynamic multi-body simulation model (MBS model) is first created – by means of defining degrees of freedom of movement of individual press components and the contact conditions for force transmission between them. In the next step, the elastic properties of essential components are taken into account by mapping them using the finite element method (FEM) and integrating them into the rigid model.
With such elastic MBS models, the machine vibrations describing structure-borne sound can be represented virtually (Figure 3). To represent the sound radiation into the press environment, the models are extended by acoustic properties. This is done by modeling acoustic layers, which are used to convert the structural vibrations of the press into the acoustic quantities such as sound pressure or sound power. For example, the simulation enables a visual illustration of the sound pressure level distribution on the press as well as in its surroundings.
Noise reduction with the help of simulation
Acoustic machine models are used to research noise reduction measures. These can be systematically derived and virtually tested in the simulation. The geometric and material design of the machine components significantly determines the vibration behavior of the machine and thus the sound transmission between the process and the environment.
Within the scope of simulation studies, the virtual machines are modified constructively and the respective effects on the machine acoustics are analyzed. For example, the edges of the machine housing, which represents the largest radiating surface of the machine, prove to be unfavorable from an acoustic point of view. By suitably rounding off the edges, a local reduction of 5 dB in the sound pressure levels at a short distance can be achieved (Figure 4).
Benefits: Design quieter cutting presses and save costs
The research results can be used during the design phase of cutting presses. By means of constructive reduction of noise emission, high investments can be saved – in procurement or in the development and manufacture of noise protection booths, but also of active solutions such as cutting impact dampers.
In the long term, the results contribute to the further acoustic optimization of forming machines. This not only improves noise protection, but also increases the cost-effectiveness and thus the competitiveness of machine manufacturers and users.