Speed Up Production With Automated Cranes
A component part supplier to the automotive industry was
looking for a way to increase productivity and reduce unit
costs. Their process involved taking various size steel plates
at one end of the production line, plasma cutting the blank
shapes, separating the cut shapes from the scrap via robots,
and finishing the part through a shot blast. The over riding
design consideration was that the system should be as automated
as possible to reduce the manpower involved, and that it should
be operated within given time cycles to meet production schedules.
The customer requested radio remote control and a physical
hard-wired pendant for emergency back up.
After exploring other crane manufacturers and receiving a
tepid response the customer contacted SOS Customer Services
and invited them to submit a quotation for a suitable crane
for the lifting and transportation of the raw plates and subsequent
cut blanks between the different stations.
There were two aspects to consider in the design of the crane,
mechanical and electrical.
The crane was designed as a double girder, top-running bridge
with a span of approximately 60 feet equipped with a custom,
dual-rail running trolley with a 360º - degree rotation
unit to carry the hoist and a mast. The bridge girders are
of a conventional box girder design with a large trolley gauge
to accommodate the large trolley diameter.
The runway with A.S.C.E 40lbs. rail is 185 feet long. Suitable
columns were anchored with base plates to the floor and tied
into the building columns for lateral support. Power supply
to the crane was provided by a four conductor, 110 amps, insulated
The end trucks were designed with one side having guide rollers
set tight to the runway rail to assure squareness of the crane
at all times. Transverse and rotation trolleys were also designed
on the same principle i.e. one side tight, the other free.
A high degree of accuracy of positioning to all motions,
particularly in rotation, had to be maintained at all times,
so it was decided to design the crane with a rigid telescoping
mast, with a 2 point pick up. Initially, consideration was
given to using a standard stacker crane design with a slewing
bearing for rotation, but the small diameter bearings available
made it difficult to position a large physical plate load
accurately at its extremities. A custom 360º degree rotation
trolley was designed using rail rolled accurately to a 13ft.
diameter circle, and welded to a support structure. This large
diameter trolley provides additional stability for the mast
and contribute to a higher stopping accuracy. The mast is
driven by two DEMAG wheel blocks with guide rollers (as shown
above) and additionally supported by two idler wheel blocks.
The telescoping unit of the mast incorporated a neoprene guide
roller arrangement to allow for uneven rope lay-off and prevent
jamming of the mast.
To raise and lower the telescopic mast a DEMAG DH1025 two-speed
wire rope hoist was chosen. The hoist features a 10:1 ratio
gearbox, creep speed motor and an 8/2 rope reeving.
The hoist was equipped with a special broken wire rope detection
device. This is to prevent the telescoping mast from dropping
more than approximately six inches in the event a hoist cable
Cycle time and the need for accurate positioning were the
determining factors in the electrical design of the crane.
A detailed cycle time study resulted in a total time requirement
of 421 seconds for the worst condition i.e. furthest travel
distances. This provided only 24 seconds of spare time from
the specified 450-second (7.5 minutes) cycle. This study did
not include any overlapping motions, and simultaneous bridge
and trolley travel. This would further decrease the overall
cycle times. While an accuracy of ±1/8” is achievable
at slower speeds, a ±1/4” accuracy was chosen
to reach the desired time cycle.
The production cycle for the crane is: From home position,
travel to pick up plate from stack, transport to squaring
table for alignment of plate, then on to plasma cutter. The
cut plate complete with the skeleton is then transported onto
the robot, which picks the cut blank from the skeleton and
places it on a conveyer to send to the shot blast. The crane
then picks up the skeleton, deposits at scrap and returns
to home position.
Communication interface from crane to robot, plasma cutter,
electro-permanent magnet and squaring table, was achieved
through the use of PLC controls. The PLC has a screen for
man to machine interface, so that the operator can program
parts, diagnose service problems, and can step by step through
the automatic sequence if required.
In order to achieve final accurate positioning we initially
planned to use an encoder with a closed loop frequency drive
and sensors to confirm location, but opted to go instead with
the Stahltronic code rail and reading heads because it is
an absolute linear positioning system. This means that regardless
of wheel slip, the crane still knows its exact location.
Radio communication modems were chosen to communicate between
the ground and crane mounted controls. Initially, we considered
a hard wired DeviceNet however we chose the radio modems for
Multiple sensors act as target points to confirm the actual
positions of the individual crane components. All motions
have acceleration and deceleration built in to provide smooth
starts and stops. Manual overrides of all motions are provided
via a Telemotive radio control system. A telephone line modem
was established for access to the crane for off-site trouble
The possibility that the magnet may pick up two plates instead
of one existed, so a weight detection device had to be designed
into the system. Induction sensors were discarded in favor
of strain gauges on the hoist wire rope dead ends for weight
calculations. The PLC determines the weight for each programmed
part. If the weight does not match to within 15 lbs. (i.e.
more than one sheet), the cycle is stopped and an audible
The crane has now been in trouble free service for approximately
five years, and for the majority of that time on a three-shift
rotation. Most of the initial service calls were related to
the cycle being interrupted by problems in other components
of the production chain rather than the crane itself (e.g.
stop crane signals from the squaring table or robot). The
telephone modem link for off-site trouble shooting has proven
invaluable in speedy diagnosis and resolution of these problems.
The harsh environment created some initial problems with
the code rail and reading heads so sweeper brushes were added
to keep the code rail clean and free from foreign matter.
The system has been reprogrammed several times to accommodate
additional stations and sequence of operation of the work
cycle, and continues to work trouble free.
The customer recently ordered a second automatic crane for their new production facility.
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