From Practice: Economical and Safe Vacuum Clamping for Production of Seating Furniture

From Practice: Economical and Safe Vacuum Clamping for Production of Seating Furniture

High-quality seating furniture is the main product of Kvist Industries A/S. The Danish designer furniture's wooden components are made from solid wood or moulded wood. To meet their own high quality requirements and the exclusive demands of customers, manual processing performed by experienced personnel plays an important role. In particular, great attention is paid to the perfect finish of the surfaces, which requires expert craftsmanship. Automatic fabrication of the parts is performed on six different CNC processing machines. The individual parts are clamped with a vacuum generated by Mink claw vacuum pumps from Busch.

Kvist's main markets are Germany and Scandinavia, but the significance of the Chinese market is growing as Chinese customers are also increasingly interested in Kvist products.
After 50 years, chairs and armchairs with designs from the initial years have become classics that are now experiencing a renaissance in the product range. However, newly designed seating furniture (fig. 1) is also part of Kvist's portfolio. The headquarters of Kvist Industries A/S are located in the Danish town of Arre with two production plants. One of the plants produces seating furniture, the other manufactures tables.

Fig. 1: High quality chairs and armchairs in all forms and versions are the main products of Kvist Industries A/S

Kvist operates an additional production facility in Latvia. 650 employees manufacture individual parts there. Final assembly and quality assurance of this seating furniture is then mainly performed at the headquarters in Arre.

A centralized vacuum system with three Mink claw vacuum pumps is used in each of the two Danish plants to generate the vacuum required for clamping. Both central vacuum system units have been in operation since July 2015 when they replaced the previously used dry-running rotary vane and liquid ring vacuum pumps. The rotary vane vacuum pumps required a lot of maintenance due to wear on the vanes. In addition, this vacuum generator gave off waste heat directly into the production room. Its loud operating noise was extremely uncomfortable for the employees. The water level of the liquid ring vacuum pumps had to be monitored constantly and refilled or exchanged if the water became polluted with wood dust.

Technical Director Rune Kvist was not satisfied with either type of vacuum generator since the performance was not consistent. This sometimes leads to clamping problems because the vacuum did not achieve the required level or achieved it too slowly. Rune Kvist found the optimum vacuum supply with the two central vacuum system units from Busch: "We used to have a total of six vacuum pumps installed at three CNC processing centres with two processing tables for our seated furniture production. The associated waste heat and noise problems had a negative effect on the work stations. Today we have three Mink claw vacuum pumps outside the building that go unnoticed except for the fact that we always have the vacuum we require for clamping," were the positive remarks from Rune Kvist.

In addition, operational reliability was significantly increased. The required vacuum is immediately available at any time because the pipework between the vacuum pumps and the CNC machines (fig. 2) serves as a vacuum buffer in which a constant vacuum of 200 millibars is maintained. The high level of operational reliability is also guaranteed by the demand-based control system. The only vacuum pumps in operation are the ones that are actually needed to maintain the vacuum level in the pipework system. In practice, a maximum of two of the three installed Mink vacuum pumps accomplish this. So if one of them were to fail, this would have no effect on production.

Fig. 2: Solid wood and moulded wood components are processed on several CNC machines

An additional essential benefit of the centralized vacuum system is made possible by the Mink claw vacuum pumps' demand-based control system and frequency-controlled drive: Huge energy savings can be generated. Firstly, a Mink claw vacuum pump requires less energy than comparable vacuum pumps of a similar size. This is made possible by contact-free and wear-free claw vacuum technology. Furthermore, the pumping speed and vacuum performance remain constant throughout the entire life cycle, while the pumping speed and achievable vacuum level of dry-running rotary vane vacuum pumps declines due to wear on the vanes. This is why one smaller Mink claw vacuum pump can often replace a larger dry-running rotary vane vacuum pump. In practice, this means that less motor output is required for comparable pumping speed.

As a basic principle, fewer vacuum pumps are needed for centralization of a vacuum supply than for decentralized installation directly at the CNC processing centres. That is how the previously installed six rotary vane vacuum pumps were replaced by three new Mink vacuum pumps. Six rotary vane vacuum pumps used to operate during all working hours. In practice, the centralized vacuum system unit has demonstrated that only one Mink vacuum pump operates at 50 hertz during working hours and a second is sometimes automatically turned on, but only briefly and usually at a lower frequency. This gives Kvist energy savings of 61% or 22,600 kilowatt hours per year (fig. 3) for seated furniture production alone. The second centralized vacuum system used for manufacturing tables achieves similar values.

Fig. 3: A comparison of the energy consumption of the previous decentralised vacuum generation with dry-running rotary vane vacuum pumps and the current central solution with Mink claw vacuum pumps

Rune Kvist is satisfied with the centralization of the vacuum supply. This is why he has also decided to use this technology at the new plant in Latvia and has installed two identical centralized vacuum systems with Mink claw vacuum pumps.

According to Rune Kvist, "maintenance of the Mink claw vacuum pumps amounts to practically nothing." They have a maintenance contract with Busch that includes annual inspection of all Mink claw vacuum pumps and changing the gear oil. Due to wear-free operation of the claw vacuum technology, there is no need to exchange wear parts like the vanes of rotary vane vacuum pumps, nor for tasks related to operating fluids for liquid ring vacuum pumps.

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