In most fields of engineering design, reducing the number of components in a product is usually desirable. It's certainly true in car-body design, where all the major auto builders are producing increasingly larger pressed body parts. Besides improving a vehicle's appearance, large parts save on jigs and fixtures, make quality control easier, and reduce inventory.
Big parts, however, need big presses. Such systems may easily occupy the space of a good-size apartment building, dwarfing all else on the factory floor.
One of the biggest can be found at the Daimler-Benz Sindelfingen (Germany) plant. It stands more than 17m high, weighs 5,000 metric tons, and covers almost 2,000 sq m of floorspace. It can stamp blanks up to 4.2m long and 2.0m wide.
Built by Muller Weingarten and affectionately known as Jumbo, the S7300 transfer press comprises six press stages and produces body panels for a number of E-, C-, and A-class Mercedes Benz cars. Three main subsystems describe its operation:
Jumbo shapes and cuts steel blanks to form complete side panels ready for assembly. Its first stage is a powerful drawing press which defines the body part's main shape. Following stages cut away excess material and add shaped features. Operation begins with 350-400 punched blanks, prestacked on one of two carriages. PLCs oversee the loading system; when one carriage empties, the other automatically moves into position.
A vacuum handling system, designed by Muller Weingarten and Arnold Maschinenfabrik (Ravensburg, Germany), transports the car body panels from one die tool, or press stage, to the next. Components include 12 sets of rubber suction grippers mounted to a common vacuum beam. A worm drive, powered from the main drive shaft, synchronizes beam motion with the press cycle.
Successive gripper sets operate simultaneously: As one set advances a panel from a press stage to an intermediate station, the next carries that station's previous occupant to the follow-on press stage. The beam then repositions itself for the next press cycle. Inductive proximity switches, tied to the overall control system, check for part presence at each suction point to activate or release the grippers.
Different body parts require different sets of press tools and vacuum transport beams. To minimize changeover time, tools and beams for the next model are held ready on nearby carriages. As some carriages move into the press, others remove the tools and beams used for the previous model. Changeover time is a mere 15 minutes.
Jumbo throws a sizable punch--closing force can reach 73,000 kN. Two 680-kW electric motors, a longitudinal shaft connecting the six press stages, and a series of reduction gear sets transfer force and torque to the various press slides.
The controlled dc motors run continuously; an eccentric crank arrangement moves the press slides from top-dead-center to bottom-dead-center and back--much like the cylinders of an internal combustion engine. A pair of heavy-duty, combined clutch-and-brake units from Ortlinghaus-Werke (Wermelskirchen, Germany) permit engagement or disengagement according to the press cycle.
Because the Mercedes Benz plant at Sindelfingen requires high reliability and minimum downtime to enable three-shift factory work, the clutch employs multiple sintered disks running in oil. This design provides not only a large frictional area but stability under high thermal loads. The clutch/brake unit also exhibits constant frictional behavior to give a uniform stroke volume and constant braking angle.
Lubricating the giant system is a challenge. Two oil reservoirs supplied by Baier & Koppel provide oil to the moving parts. One reservoir supplies the gearboxes which transfer power from the motors to the press slides, and also the overload protection system on the press slides themselves; the other reservoir, containing a higher-viscosity oil, lubricates the worm gearbox that drives the main transport system.
Maintaining high rigidity presents another challenge. Complex machines such as the Jumbo transfer press are not absolutely rigid in the true technical sense. Huge forces exerted during the pressing process can cause misalignment at critical parts of the machine. Unless compensated, these forces could damage gearbox and gears as well as bearings carrying the longitudinal shaft which drives the press.
Consequently, torsionally stiff but flexible MODUFLEX(reg) shaft couplings from Rexnord Antriebstechnik (Dortmund, Germany) are fitted to the longitudinal drive shaft. This type of coupling compensates for axial, radial, and angular offsets occurring during operation and contributes to the high percentage of uptime. The modular design, moreover, eases installation and maintenance work; fitting and removal takes place without disturbing neighboring components.
To provide efficient and reliable control of the machine's complex systems, yet give the operating team a clearly presented man-machine interface, Jumbo incorporates an electrical control system built around a machine control level and a visualization level. Seven S5 PLC systems from Siemens, linked to 6,000 sensors and actuators, control and supervise the six press stages. Fourteen industrial PCs connected via local area network make up the visualization level.
Separation of the visualization and control levels offers a number of advantages. First, the PLCs do not become burdened with visualization programs. If visualization were integrated into the PLC system, the amount of additional software required would easily be more than that needed for the actual control systems. Second, changes to the visualization software can be made without disturbing the control functions. Finally, the industrial PCs are free to carry out logging and statistical functions independent of the type of control, and they can easily be replaced, expanded, or updated.
How effective has the Jumbo been? Each hour of every day the press produces 500 to 600 body panels. Siegfried Straub, press-plant manager, lists improved quality, better dimensional uniformity, and a lower risk of corrosion due to less welding as the main advantages. Straub also points out that productivity gains more than compensate for cost.
"The trend is towards ever larger presses," adds Ulrich Frank, Mumller Weingarten sales manager for large presses. In fact, the company is currently building a 9,300-metric-ton press for BMW. Apart from the increase in total press force, the new unit will include the latest PLC and control bus technology using cable.
Daimler-Benz would also like to expand its facilities and is seriously considering whether Jumbo is to have a sibling!