Plant IV of the Linde group of plants responsible for industrial trucks and hydraulics is located in the town of Weilbach in Lower Franconia, Germany. With about 160 employees, this plant manufactures 48 different counterweight variants for electric, diesel and LP gas forklift trucks. The plant’s foundry operates in two shifts, while machining is carried out in three shifts. The finished counterweights are shipped to the assembly shop at Plant II in Aschaffenburg.

“Our products consist of so-called structural counterweights. They have to be suitable for higher loads, since other components are screwed to them,” explains Dieter Holzmann, manager of Plant IV. “We cast about 36,000 tonnes of GG 15 gray iron annually in Weilbach; that is enough for more than 26,000 forklifts.”

The production sequence

Linde manufactures the counterweights using the sand casting process. This is done by pouring molten iron into a molding box made of quartz sand mixed with resin and hardener to achieve the necessary strength. The required protective layer between the sand and the molten iron is applied in the form of an alcohol-based wash; since September 2001 this has been done by a KR 125 six-axis jointed-arm robot from KUKA Roboter GmbH, Augsburg.

The shape of the molding box depends on the particular pattern used; the box and pattern are placed by the user on a roller conveyor and filled with a mixture of molding sand, resin and hardener. The roller conveyor then transports the combination of pattern and molding box to a turnover station. There the robot controller reads in the code of the box and thus recognizes which box types are to be processed in the cell, and in what order. In the turnover station, the molding box is separated from the pattern. An operator then carries out a visual inspection of the box, and if there are no defects releases it for transfer to the robotic cell.

As soon as the conveyor system receives the appropriate signal from the robot controller, the box is indexed forward on the roller conveyor until it stops underneath a manipulator. The manipulator picks up the box and swivels it into a position where the robot can blow out of the mold the loose sand not bound to resin and hardener. After that the manipulator lifts the molding box over the wash basin. The KR 125 then picks up the flood lance and applies the wash to the mold with great precision. Subsequently the manipulator rotates the box to allow wash residues to drip into the flood basin. The cycle time of the cell is six minutes.

Once the manipulator has placed the box back on the roller conveyor, the mold is moved along the conveyor to a position where the alcohol-based wash is flamed off, and then to a coring-up station, where operators insert cores made of quartz sand. Lastly the matching cope and drag boxes are placed one atop the other, clamped, and taken on a transport trolley for casting.

Robot taught by user’s personnel

The company Boll Handhabungssysteme GmbH from Elsenfeld, Lower Franconia was contracted to install the robot. This KUKA systems partner first checked on the basis of the Cartesian coordinates whether the robot under consideration, the KR 125, could in fact reach the furthest corners of the mold. Since the results of these investigations were positive, Linde decided to use the KR 125, its reach being optimally suited to the requirements of this application. Another decisive factor was the robot’s attractive price/performance ratio.

The package supplied by Boll encompassed the robot, including its controller, the engineering for the application program, and the interfaces to the periphery. Also included was the safety fencing, which was constructed in cooperation with the safety departments responsible at Linde.

In addition, Boll created the basic handling program for the KR 125 and trained the user’s operating personnel in Weilbach. The Linde employees are now able to teach the robots themselves. Of the 31 programs required for the various molding boxes, so far Linde has implemented 25. This is where the advantages of the KUKA Control Panel come into play, with its familiar, easy to use Windows man-machine interface.

Linde also concluded a maintenance agreement with the KUKA systems partner, guaranteeing 24-hour service when needed. Both companies benefit in this regard from their geographic proximity and the resulting short travel times.

The teams from Linde and Boll got together regularly for working meetings during the introduction phase, which began in August 2001 with the beginning of assembly. The main topic of these discussions was development of the interfaces between the new system and existing systems. Other priority topics were on-site programming, safety issues and adapting the robot gripper system to the flooding lance.

More cost-effective manufacturing

In an initial summary, Dieter Holzmann notes the following results: “Since we have saved one operator in each shift, we now produce the counterweights much more cost-effectively. As for the payback period, we expect it to be two years for the entire system, including the software. Furthermore the operating personnel replaced by the KUKA robot have been given more worker-friendly jobs in other departments. And the vapors arising from the alcohol-based wash don’t bother the robot at all.”

Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld, Germany