The decision to incorporate automation into overhead cranes and hoists in manufacturing applications is often intimidating. However, the task is simplified by devising a plan that identifies the needs of the handling system, as dictated by the manufacturing operations it serves.
Engineering personnel involved in the decision must weigh the benefits against the drawbacks of applying automation to overhead lifting operations. Benefits include increased productivity, reduced damaged goods, better quality control, smoother flow of work-in-progress, and reduced labour costs. Drawbacks include longer "debugging" and setup periods, increased capital expenditures, extended lead times on equipment acquisition, and longer operator training periods. In general, automated overhead material handling equipment is justifiable in plant applications where it supports operations critical to the main assembly line, or where manufacturing operations within the work cell are labour intensive. One fact is obvious, however. Enough decision makers have seen automation as an overall benefit to make the industry a booming one. It is interesting to note that automation of heavier systems (10 tonnes or larger) has been commonplace for over 10 yr, while 1/4 to 10-tonne automation has exploded in the last couple of years.
To evaluate the benefits of automation, first determine system parameters. Plot the "automation area" as a three-axis cube. Determine the number and approximate location of each stop point along each axis. The grid is expressed in X (elevation), Y (east/west), and Z (north/south) coordinates. Determine the acceptable range of positioning for each point within the cube (usually ±1 to 3 in) and frequency with which that locale must be used. As the number of points goes up, and acceptable range of positioning gets smaller, system cost increases. Make a flow chart for the automated sequencing operations in order to calculate a cycle time. One full cycle includes all travel required to pick up a load at point A and deliver it to point B. Note that there may be more than one cycle if there are multiple pickup and delivery points. Rigging and unrigging times must be included and need to be estimated based on existing information. Generally, horizontal travel speeds to 120 fpm and lifting speeds to 35 fpm are practical, but faster traversing speeds can increase load swing. Divide travel distance by travel speed to calculate time for each individual motion. The sum of all individual motion times is the total cycle time. Where time is critical, consider simultaneous motion of two or more axes, such as crane (north/south) and trolley (east/west) movement. Typical manufacturing applications require 2-3 elevation points, 2-4 east/west points, and 2-10 north/south points. Keeping requirements near this range ensures a practical and cost-effective system. Slower speeds are easier and more cost efficient to automate than faster movements. Also, an object in motion is better controlled at a slower speed. Therefore, where possible, lay out related equipment in a manner that complements an automatic cranes inherent capabilities.
Typical system layout
Assume an engineering team is considering an automated crane system in an automobile assembly plant. The cranes function is to deliver engines from pallet racks to a conveyor spur off the main assembly line. Workers must perform some basic operations on the engine before it is discharged from the "work cell" to the main line. The engineering team has decided that four different engine pallet racks must be randomly accessed to allow for various engine types for each specific car "work order." Therefore, this system requires five bridge position points (pallets 1, 2, 3, and 4, plus the conveyor spur), two trolley position points (pallet and conveyor spur sides of work cell), and three elevation position points (ready to pick the engine from the pallet, fully lifted engine before crane/trolley movement, and lowered engine ready to position into the conveyor spur fixture). The operator performs some final positioning and rigging operations. While the worker discharges either the empty or loaded crane to its next stop point, the operator is performing the basic operations on the engine presently in the conveyor spur fixture.
Technological advancements Flexibility and networking capabilities of programmable logic controllers (PLCs) allow easy integration of related systems. Safety conditions are usually required before an automatic operation is performed, or queuing can be setup conditional on the upstream or downstream situation. Automatic inventory control can be integrated into the system.
Technology changes have allowed the incorporation of standard crane and hoist components into an automated system. The ability to integrate standard components with electronic control functions has revolutionized the lighter end of the industry and driven system costs down. Electric chain hoists and single girder cranes running on either standard structural steel beams (underhung) or ASCE rail (top running) are commonly used. Multiple hoists can be integrated as required for longer or multiple loads. Variable frequency drives provide speed control and aid in positioning, proximity and electromechanical limit switches provide accuracy and repeatability, and PLCs control every-thing and make it "future flexible." Proximity and limit switches are used as positioning sensors along a specific axis. Note that two switches may be required for each position point -one for travel slowdown near the position point and one for travel stop at the position point. Limit switches are mounted onboard the crane, while trip levers are mounted on runway beams. This arrangement allows minor field adjustment at the time of installation. Control stations are suspended from and travel with the trolley hoist, remotely mounted, or a combination of the two. Stations have traditional crane functions (power on/off, crane forward/reverse, trolley left/right, and hoist up/down), and automated functions (auto/manual mode, one button for each of the automatic delivery points, and selector switch for control of hoist 1, 2, or both).
Safety considerations for automated systems include all the factors of traditional overhead cranes, and more. Normally, audible buzzers and beacon lights warn of automatic startup and travel modes. PLCs may interface with other equipment to prohibit startup in an unsafe situation. Also, consideration must be given to clear travel paths for a suspended load. Never lift a load over personnel, and avoid physical interference problems. A series of fail-safe conditions should be incorporated into the safety plan. Extreme ends of travel for each axis should be limit switch protected to shut down automatic travel outside the normal operation range. PLC software may prohibit automatic startup except from predetermined points. The PLC must have position memory software that tracks the cranes location at all times to prevent automatic startup. Mushroom-head type, emergency-stop push buttons should be provided in the operators area.
Developing a crane safety program
The overhead crane is one of the most common types of heavy equipment found in industrial plants. As a result, safe crane operation is an issue of paramount importance, and with good reason. Given work safety and liability issues, and all the regulations set by OSHA, AISE, CMMA, and other organizations, everyone responsible for operating, maintaining, purchasing, or reconditioning crane systems must put safety first. Fortunately, reputable crane manufacturers think likewise in both engineering and support activities. Components such as redundant brake concepts for hoists, more reliable controls, and economical overload detection systems are engineered into the system. Standard diagnostic devices are available in conjunction with detailed preventive maintenance and safety training programs.
Safety starts with the operator. Whenever there is doubt as to safety, the operator should stop the crane, report the problem to his supervisor, and not operate the equipment until satisfied it is safe to do so; or is directed to proceed by a supervisor, who then assumes all responsibility for the safety of the lift. Operators should be familiar with the principal parts of the crane. Employees should receive hands-on training, read all instruction materials, and have a thorough knowledge of crane control functions and movements. Both the operator and person hitching or rigging the load should be required to know the location and proper operation of the main runway conductor disconnect for all cranes in the area. The operator should test all crane controls, such as limit switches, brakes, ropes, and hooks, to ensure proper functioning at the beginning of each shift.