The manual assembly of a new Siemens medium-voltage switch took more than an hour during the pilot phase. Also, the work was physically demanding and required much attention. Since processes and the workstation have been ergonomically optimised, the component is ready after about 25 minutes. At the same time, the physical strain on employees was reduced and sources of danger were eliminated.
The Energy Management Division of Siemens AG developed a new medium-voltage switch for use in industry as well as in power stations and changeover stations, which was first assembled at a provisional manual workstation in 2012. After a successful market launch, the sales figures rose so that the quantity had to be increased and the production at the Berlin location had to be expanded. The Medium Voltage & Systems department therefore asked Bohnert Systemtechnik for support in optimising the assembly workstation. Petra Stielow, a designer at the engineering office in Rödermark near Frankfurt, which also designs handling systems, remembers: “The work was carried out manually at a normal table, and the subassembly weighing around sixty kilograms had to be rotated and turned by hand. There was a constant danger of fingers being squeezed or jammed. Tools and small parts were not at hand.”
The ergonomic assembly workstation developed by Stielow and her colleague Dirk Siemko, technical manager at Bohnert, was installed at Siemens in Berlin in spring 2013 and initially shortened the throughput time by half an hour to around 35 minutes. The concept envisages that the assembly working surface can be rotated horizontally via a plain bearing and mechanically locked in the desired position. With the aid of two rotating modules, it can also be tilted towards and away from the worker. Thus, the medium-voltage switches can be easily and safely swivelled and assembled from all sides. Indexing ensures engagement at the correct angle; the rotating modules are hydraulically unlocked via a foot pedal so that the operator's hands remain free at all times. The height of the workstation can be adjusted to the height of the employee and the assembly height of the subassembly using two electric lifting columns of the type “Shop-Floor”. They are lifted and lowered using a manual control switch. A supply unit with synchronous control controls the components.
“The components we use come from ROEMHELD’s modulog program,” explains Stielow. The specialist for assembly and handling technology from Laubach near Gießen in Hesse has been a supplier and partner of Bohnert system technology since 2012. The designer explains why: “All components are of high quality, very reliable, almost maintenance-free and service-friendly. For us as a development service provider, it is also important that the advice and the support in our planning is extremely competent, prompt and helpful. Also, the modulog range is a modular system that can be used very flexibly; the individual components can be easily combined with each other.”
ROEMHELD’s modulog program includes numerous modules for horizontal and vertical rotation, tilting, lifting, positioning and moving. The standard components are designed for different loads between 100 kg and 1,000 kg. They can either be operated by hand directly on the module or by hand or foot using a lever or push-button. One of ROEMHELD's latest developments is a rotating module with media feed-through. Fixtures that are clamped with zero-point clamping systems, for example, can be operated hydraulically, electrically or pneumatically without complex piping. The Shop-Floor lifting column used by Bohnert is available with a stroke between 200 and 600 mm, in three variants for loads up to 600 kg and can be operated by a hand switch, foot lever or foot switch. The horizontal rotating modules used are designed for a load of 200 kg each. One of them is equipped with an indexing of the rotating position in steps of 60 degrees, and the other one functions as a counter bearing without position markings.
Siemens was delighted with the assembly workstation, and in the same year, ordered a second version for a production site in China. Shortly afterwards, an order was placed for a production line for the medium-voltage switch with three workstations.
“For this line, we divided the assembly work into three steps and designed the corresponding manual workstations,” remembers Petra Stielow. First, the basic components of the support plate, pole plate and the poles are assembled, the pole plate is fixed by hand on the side of a stop. “This prevents it from slipping and at the same time the subassembly is optimally positioned at the centre of gravity,” emphasises the designer.
Gravity roller belts are used to move the various parts, components and the assembled subassembly to the various workstations arranged next to each other. A pre-assembled shaft is mounted at assembly station two, then the cable harness at the third station. In the end, the throughput time could be reduced by another ten minutes, says Stielow: “In 25 minutes, the medium-voltage switch is ready for the final inspection.”
The time saving is only one plus. She lists further advantages: “Thanks to the adjustable working height and the reduction of the force required from 500 N to 50 N, employees tire more slowly and can concentrate longer. The improved and safe handling and the simple and logical operation also increase the assembly quality. At the same time, the risk of accidents has decreased, so that long-term workplace-related absences and absenteeism are reduced.”
Thanks to the improved working conditions, the company is also efficiently supported in the implementation of the requirements of the professional associations for hazard analysis for the reduction of psychological stress at the workplace, mandatory from 2015. When designing the production line, Siemko and Stielow also made sure that Siemens could use the workstations flexibly: If only small quantities have to be assembled, this work can be carried out completely at the second assembly workstation; the remaining stations can be activated via the control panel. If required, the system could also be used for the assembly of other switch types after an appropriate modification. Bohnert Systemtechnik also convinced its customer with the production line: in the meantime, the customer considers using another line at a location in China.