March 23, 1998 Design News
Multiple body panels? Expensive
jigs and fixtures? Computer-controlled weld stations? Not necessary. The Jumbo transfer
press stamps a car's entire side from one punched blank.
by Roy O'Connor, European Bureau Chief
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 Müller 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
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 Müller 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
Lubricating the giant system is a challenge. Two oil
reservoirs supplied by Baier & Köppel 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.
Transfer press control system
consists of a visualization level with industrial
PCs and a machine automation system using PLCs.
Consequently, torsionally stiff but flexible MODUFLEXr
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
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
"The trend is towards ever larger presses,"
adds Ulrich Frank, Müller 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 fiber-optic
Daimler-Benz would also like to expand its large-press
facilities and is seriously considering whether Jumbo
is to have a sibling!
An army of sensors
Seven Siemens S5 PLCs monitor all activities within
Müller Weingarten's transfer press. This information
comes from the system's many data-gathering components.
For example, Mannesmann Rexroth's HED 8 series of pressure
switches, which operates in five ranges up to a maximum
pressure of 650 bar, monitors hydraulic pressure. VADM
and VADMI vacuum suction nozzles and solenoid valves
from Festo AG transport freshly minted car body parts
from one press station to the next--free from the risk
of nicks, bending, or scratches. Non-contacting inductive
proximity sensors from Balluff Inc. notify the vacuum
system of a part's presence. All told, the Jumbo transfer
press employs more than 6,000 actuators and sensors.