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A component and 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 overriding 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, hardwired pendant for emergency backup.
After exploring other crane manufacturers and receiving a tepid response, the customer contacted SOS Customer Services and invited us 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 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 baseplates 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 electrification system.
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 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 contributes 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 layoff 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 should brake.
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 lesser maintenance.
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 shooting.
The possibility existed that the magnet may pick up two plates instead of one, 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 alarm sounds.
The crane has been in trouble-free service for approximately 5 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. The customer recently ordered a second automatic crane for their new production facility.