Report: igm welding robot for preparation of parts for open-pit mining equipment Laser measurement of igm robots facilitates cost effective repair welding of open-pit mining equipment parts ((Subtitle:)) Fully automatic changeover between joint and build-up welding Vattenfall Europe AG operates four open-pit mines in the Lusatian coalfield whose brown coal is delivered straight to the power plants. About a fourth of all German electrical power is generated from brown coal. Lusatian brown coal thus provides a solid basis for energy production that will be reliable in the long run. Brown coal production from the Lusatian open-pit mines is particularly efficient due to the horizontal orientation of the Lusatian coalbed. Because of the harshness of the work the moving parts of the conveyor bridges as well as of the bucket-wheel and bucket-chain excavators are subject to enormous wear. It is more economical to refurbish some wear parts to be able to use them longer. Sprocket angles are such parts. They transmit the motive force of the bucket-chain excavators onto the bucket chain so that it can transport the excavated material to the conveyor bridge. Since 2009, the heart of the refurbishing process for these sprocket angles has been a welding robot from the Austrian company igm Robotersysteme AG. The refurbishment of wear parts in open-pit mining equipment takes place in the main workshop of Vattenfall Europe Mining AG in Schwarze Pumpe, a suburb of Spremberg, located in the southern part of the German state of Brandenburg. Among the significant processes in the maintenance of conveyor bridges, spreaders (Photo 1), the bucket-wheel excavator and the bucket-chain excavator (Photo 2) is welding work in the form of build-up and joint welding. All welding processes for the refurbishment of cost-intensive components are subject to the welding inspection of Wolfgang Wache (Photo 3). Vattenfall user report, November 2011 Version: May 2012 Page 1 of 12
Requirements to refurbish the bucket-chain drive In each of the five open-pit mines there is one conveyor bridge operating. Three or four bucketwheel or 11 bucket-chain excavators do the preliminary work for each conveyor bridge. In these special excavators, the wear pairings between the drive and bucket chain are designed in such a way that the parts that are easier to replace bear the main load of the wear. In case of the bucket-chain excavators, the sprocket angles (Photo 4), flange (tooth flank) engages in the free spaces of the chain (Photo 5+5a), transmitting the torque of the driving motor onto the bucket chain. A set of four sprocket angles form a drive group on each side of the bucket chain. For a two-sided drive, therefore, eight sprocket angles are needed. They move the 40 full buckets while simultaneously applying the breakaway force for the next free bucket. During this powerful driving of the bucket chain, primarily the flange wears, but also the flat links (the joints of the bucket chain) that slide over the side areas cause significant wear (Photo 6). The actual load can be estimated on the basis of material removal: To convey one m³ of coal, 7 m³ of overburden has to be removed. The conveyor bucket (Photo 5) has a volume of 3.5 m³. Even when conveying sand, significant forces result, but the overburden consists of very different layers of stone. It can include clay, sand, gravel, marl, brown coal and anthracite. Frost in the soil significantly accelerates the wear. Thus the planning for refurbishment requirements for the drive elements is anything but simple. Still an adequate amount of sprocket angles has to be available at any time to ensure the continuous conveying of coal. That means that it s not enough just to reserve sufficient replacement sprocket angles a sufficient refurbishment capacity must also be ensured. In fact about 300 sprocket angles must be refurbished each year. After the sprocket angles have been delivered for refurbishment, it is first estimated whether they can be refurbished at all. Every sprocket angle can be refurbished about six times, but only if the wear limit hasn t been exceeded Wolfgang Wache explains. The refurbishing of the sprocket angles takes place according to two different welding processes. The first is build-up welding, and the second is joint welding using a MAG process. Wolfgang Wache continues, First we cut the worn flange off and burn off the bulges (Photo 6). Then we attach a new flange to the body (Photo 7). This flange has to be welded to the body. Build-up welding is used to build the worn sides back up. These have historically been difficult manual welding tasks that took up to an entire shift for just one sprocket angle. Heat, dust, and noise levels were not only unhealthy but made it difficult to guarantee consistent quality in the intensive welding processes. Over the long term, automation was the goal, and in 2009 that goal was achieved. After we had examined multiple offers, we once again selected a welding robot made by igm Robotersysteme AG, located in Wiener Neudorf. We had received one robot from igm in advance with which we are very satisfied and that carries out its daily work reliably. For the call for tenders for this robot system, igm again presented the most attractive design. The igm offer provided the highest cost-effectiveness for the welding processing of the workpieces. A single robot in the robot cell is capable of switching automatically from build-up to joint welding and back. Vattenfall user report, November 2011 Version: May 2012 Page 2 of 12
Laser measurement of workpieces optimizes welding processes After cutting off the worn flange and the bulges on the sprocket angles, which have to be refurbished, the surface of the body is machined (Photo 8). The new flange is then attached by hand in preparation for robotic welding (Photo 7). Prepared in this manner, a slewing jib crane places the workpieces on the igm rotary tilting table (Photo 9). The rotary and pivot axes of this 1000 kg manipulator are integrated into the robot control system. This allows its movements to be programmed as though they were robot axes. The robot s step program is designed to allow the rotary tilting table to rotate and tilt the clamped workpiece during the process, so that all welding processes are carried out in flat position, which guaranties highest quality. (Photos 10 and 11). The preparation for robotic welding, especially after changing products, includes a determination of the position of the weld part. The robot uses the welding torch in a conventional manner to examine the position of the workpiece. igm has equipped the robot cell (Photo 12), in which a six-axis RTi 370-L robot works, with a laser sensor for the refurbishment of the sprocket angles. This laser sensor is automatically picked up as needed by the robot hand in addition to the welding torch. For this purpose the bayonet lock in the hollow shaft of the robot hand is equipped with a holding mechanism for the laser sensor. The igm laser sensor (ils) can detect deviations between the current and the original workpiece position (Photo 13). It passes the data immediately on to the robot control. Should differences to the programmed position occure, the program uses a 3D translation to adjust the movements of the torch to the actual position of the workpiece. Due to the high searching speed and the fact that the ils can find the spatial position the height and the side in only a single search pass, it is possible to save time over conventional tactile search processes. Up to 70% time savings are possible. After the position of the workpiece has exactly been determinated, the robot can automatically measure the wear by comparing the parameters with the expected condition. The position of the workpiece is known, as are the coordinates of the laser sensor in space.the laser sensor can determine the amount of wear with simple search drives over the sides of the body of the sprocket angles,. The robot control contains several programs for build-up welding assigned to different wear levels. This has the advantage that even welding technicians without specific previous knowledge of the welding refurbishment of sprocket angles can still operate the robot cell. Moreover, the automatic selection of the welding process optimizes the consumption of welding materials. Besides it is also possible for the laser sensor to check the height of the final build-up weld. One robot for two welding processes After the workpiece position and wear depth have been determined, the robot returns the laser sensor to the changing station (Photo 14). Based on the detected wear depth, the robot control determines the number of welding layers to be built up and starts the build-up welding. Within Vattenfall user report, November 2011 Version: May 2012 Page 3 of 12
the robot s working range of 3200 millimeters, the torch can reach any welding area on the workpiece clamped on the rotary tilting table. The wear depths of the individual sprocket angles varies between one to six centimeters. To build them back up, at most six times three layers are then welded on. For build-up welding, Wolfgang Wache explains, we use a two-millimeter-thick build-up welding wire that makes a firm connection to the cast steel body (Photo 4). Depending on the number of layers that have to be welded, we determine whether one side should be finished first, or for thermal reasons we have to switch between sides. But it s also possible, Toni Kilka (Photo 15), programmer and system operator, says that our operating experience allows us to determine the number of layers manually that have to be welded, and that we can control the robot accordingly by using the operating panel or the ergonomic K5 teach pendant. The igm control system is built so comfortably that program changes can be made effortlessly. That s why reprogramming is so quick and easy, for example when we change products. For build-up welding, Wolfgang Wache explains, we work with a welding current of 300 to 400 amperes. That means that the torch is exposed to a high level of thermal stress. It can withstand the high temperatures because it is cooled right in the gas nozzle. Automatic switch to joint welding After the build-up welding, the initially manually attached flange is welded to the body. Wolfgang Wache says, We use the MAG process for that, with a 1.6 millimeter thick welding wire. The second welding torch is automatically switched in. With the torch changing system, Peter Scheichenbauer (Photo 16) from igm sales and project planning in Stuttgart notes, we can use a single robot for two different welding processes with two different welding wires. To do that, we have installed the wire feed systems for both types of wire on the first robot axis (Photo 14). The torch magazine, where we store the torches safely with the hose package in two-finger grippers is placed above. To switch torches, the hollow shaft of the robot hand threads the torch from the jet to the hose package and clamps it with the bayonet lock. Thus the torch changing system permits the reliable, simple and therefore automated switching between different welding processes with one robot. This clever configuration allows us to maintain the advantages of media feed through the hollow shaft in the wrist joint. With this automation solution we have been able to increase our performance in the refurbishment of sprocket angles by 40%, Wolfgang Wache summarizes. By replacing the manual welding process with robotic welding, the working conditions for our staff has significantly improved. The robot station is currently working just one shift, but it is designed for three-shift operation. That leaves us reserve capacity that enables us in any situation to provide the open-pit mining operation with the replacement parts they need for continuous coal conveying. Vattenfall user report, November 2011 Version: May 2012 Page 4 of 12
(Boxes) Brief portrait of Vattenfall Vattenfall Europe AG is the part of the Swedish Vattenfall Group that operates in Germany. Vattenfall Europe was founded in September, 2002, as a result of the merger of the longestablished companies Bewag, HEW, LAUBAG and VEAG. Vattenfall s core business includes brown coal open-pit mining, the generation of power and heat in one of the most modern power plants in Europe, and its sales and distribution to private and business customers as well as redistributors. Furthermore the core business includes trading of electrical power products. Vattenfall obtains Lusatian brown coal in the open-pit mines in Jänschwalde/Cottbus-North, Welzow-South, and Nochten/Reichwalde. About 1.3 billion tons are currently stored in Vattenfall s overburden fields approved under mining and land planning law. The Schwarze Pumpe power station primarily uses raw brown coal from the Welzow-South open-pit mine, as well as from Nochten. At full power, 36,000 tons of brown coal are needed per day. (Data provided by Vattenfall) Vattenfall user report, November 2011 Version: May 2012 Page 5 of 12
Captions: Photo 1 (lead): Spreader for the overburden supplied by the excavators. They are connected with a belt system over 600 meters in length. Photo 2: Tenova TAKRAF offers bucket-chain excavators with conveying throughput between 1000 and 3000 m³/h and cut depths and widths between 5 and 16/19 m. Photo 3: Wolfgang Wache, EWE EWF welding foreman, Toni Kilka, programmer and system operator (left to right) Vattenfall user report, November 2011 Version: May 2012 Page 6 of 12
Photo 4: Finished sprocket angles after welding Photo 5: View of the bucket chain with a 3.5 m³ conveyor bucket Photo 5a: Position of the sprocket-angles, where flange engages in the free spaces of the chain transmitting the torque of the driving motor onto the bucket chain. Vattenfall user report, November 2011 Version: May 2012 Page 7 of 12
Photo 6: Worn sprocket angles Photo 7: New flange attached to the body Vattenfall user report, November 2011 Version: May 2012 Page 8 of 12
Photo 8: Body after surface machining Photo 9: igm RP2/1000 A.1-RCi rotary tilting table Photo 10: Joint welding in the flat position at best quality Vattenfall user report, November 2011 Version: May 2012 Page 9 of 12
Photo 11: igm robot moves the torch in the flat position even for build-up welding Photo 12: igm RTi 370-L robot with rotary tilting table Photo 13: Measuring the surface with the laser sensor Vattenfall user report, November 2011 Version: May 2012 Page 10 of 12
Photo 14: View of the changing station and the wire feed systems Photo 15: Toni Kilka: The teach pendant allows changes to the program with no difficulty. Photo 16: Peter Scheichenbauer, igm sales and project planning, Stuttgart Vattenfall user report, November 2011 Version: May 2012 Page 11 of 12
Further information: igm Robotersysteme AG Dipl.-Ing. Martin Wihsbeck IZ NÖ-Süd, Strasse 2a A-2355 Wiener Neudorf Tel.: 0043-2236 67 06-143 Fax: 0043-2236 61 576 Vattenfall Europe Mining AG Wolfgang Wache Main workshop Schwarze Pumpe An der Heide 03130 Spremberg Tel.: 03564-6-948-09 www.vattenfall.de Text: Peter Springfeld, independent technical journalist, Berlin, Email: pspringfeld.red@t-online.de Photos: igm, TAKRAF (2, 5a), Springfeld (5, 6, 7; 8, 13, 15) Vattenfall user report, November 2011 Version: May 2012 Page 12 of 12