Over 7 m long and weighing around 24 t - and yet the huge component rotates and turns smoothly, precisely and without jerking. The new robot welding system with integrated rotation and turning positioner in Severt's production hall pushes forward into new dimensions for robot welding: The component that has just been processed - it will later be the swivel arm of a mining excavator - is not even the maximum: up to 45 tons can be moved on the system; a total of twelve drive axes ensure an optimum welding position. "The weld seam should always be in the trough position if possible, hence the many degrees of freedom," explains Frank Gevers, Head of Development and Sales at Severt. All axes - apart from the six of the robot arm that guides the torch neck - are moved by SEW servomotors.

Wilhelm Severt Maschinenbau, founded in 1964, employs around 400 people at its sites in Vreden in Münsterland and Gorlice in southern Poland. The demand for ever larger assemblies has been increasing for years, for example in the energy sector, as ever more powerful wind turbines require correspondingly larger mobile cranes for their assembly. "Previously, such large assemblies were welded by hand," says Frank Gevers. "But now the shortage of skilled workers in the welding sector is having a full impact, and there is an increasing lack of capacity for manual welding." This was a key reason why Severt decided to develop another fully automated robot welding system with a positioner for correspondingly large parts. It makes the work of the welding specialists easier and enables them to use their capacities efficiently.

Severt manufactures welded assemblies with a unit weight of 500 kg to currently 80 tons. The production facility has 60 modern manual welding workstations and 15 robot systems for welding and cutting. Customers include manufacturers of agricultural machinery and heavy construction machinery, as well as others from the vehicle construction and energy technology sectors. The development of positioners was initially intended to relieve the workload of the company's own employees, but also met with interest from other companies. Today, the construction of manually controlled welding turntables and complete robot welding systems is an important pillar of the company. The new 45 t system for large components basically consists of two work stations; they stand next to each other and are connected via a traversing track. Each station is equipped with a rotary and turning positioner, which enables flexible positioning of the components to be welded across three axes.

Low backlash, high positioning accuracy

The system is equipped with two six-axis welding robots, each mounted on a gantry. The gantry lifts and swivels the robot and also moves along the work stations via a traversing carriage. The maximum travel distance of this system is 40 m. The linear axis of the trolley and the swivel axis of the tower arm are driven by backlash-free precision servo geared motors from the ZN series. They consist of a cycloidal gearbox and a synchronous servomotor. Backlash and hysteresis loss are well below one angular minute due to their function, meaning that the ZN gearboxes work with positioning accuracy. Both stations are linked - each robot can work at both stations. This means that either two components can be processed in parallel on the welding system, or both robots can weld together on one component at one work station.

Robot controller coordinates all drive axes

The drive technology had to overcome a number of challenges for this system concept. In order to keep the component in the best possible position at all times during the welding process, all twelve axes of a station must work together in a coordinated manner. The robot controller therefore simultaneously controls both the axes of the robot arm and the external axes of the positioning system, which are implemented with SEW servomotors. Each work station has its own control unit, meaning that the entire system has two robot controllers that work together. In automatic mode, the robot controller also controls the axes of the positioning system. In manual mode, control is via Movi-C controllers from SEW-Eurodrive. It is possible to switch between manual and robot mode at any time and continue working seamlessly. Due to the high torques that occur during parts handling, the required drive power was divided between two motors. This requires both drives to work absolutely synchronously, which SEW solves with Movikit multi-axis controllers: the software module for Movi-C controllers implements mechanically coupled drives so that a load can be driven together. The drives are synchronized to prevent vibrations from occurring.

This article appeared in issue 10/23