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 revolutionised 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 factors
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
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. The area should be clear of personnel during the test. Under no circumstances should anyone be permitted to ride on the hook or load. Before commencing work, the operator should perform a walk-around check to look for loose or damaged parts, or a visual inspection should be made from a catwalk or other safe observation point. Table I indicates a number of daily visual checks that should be made on the crane system. In general, a crane operator should not close the main switch or emergency contactor until checking to ensure that no one is on the crane and there are no warning signs. When the lift approaches the rated load, the operator should test the hoisting brakes by raising the load a few inches and applying the brakes limit switch on a crane or hoist should not be used as an operator control unless a backup switch is available. A load should not be lowered below the point where less than two full wraps of wire rope remain on the hoist drum. The operator should land any attached load and place the controllers in the "off" position when leaving a crane unattended. The sling should always be removed from the hook and stored where it is protected from damage. The operator should make sure all controllers are in the "off" position before closing a main switch. All controls should be placed in the "off" position in case of a power failure. Loads should never be carried over workers heads. The crane should not be used for side pulling. This movement creates dangerous load swings, and creates potential damage and excessive wear to wire ropes, rope guides, and limit switches; and results in equipment deterioration. Wire rope hoists are designed for lifting loads in straight vertical paths; not for winching operations. Wire rope strength is calculated by straight-line pull. Only a cradle or enclosed platform should lift compressed gases.
Hand signals should be clearly agreed upon and understood before moving a load. The operator or hooker should make sure of several conditions when attaching or moving a load:
Hoisting rope or chain is free of kinks or twists and not wrapped around the load
Load is attached to the load block hook by a sling or other approved device
Sling and load will clear all obstacles or obstructions
Load is balanced and secured before lifting more than a few inches
Multiple lines are not twisted around each other
Hook is brought over the load in a manner to prevent swinging
There is no sudden acceleration or deceleration of the moving load.
The operator should never lift two separately rigged loads at the same time. Whenever a movement is made, the operator should pay constant attention to controlling the load, never allowing the control pendant to swing freely. When possible, traveling and lifting movements should be carried out simultaneously to reduce operating time.
The operator should approach the desired position as far as possible at the main speed and use creeping for final positioning to avoid unnecessary inching, which negatively affects the service life of certain components. Pendulation of the load is avoided by moving the crane/hoist or trolley in the same direction as the load is swinging. The operator should not drive the crane against endstops and avoid bumper contact.
Maintenance
A preventive maintenance program should be based on the crane manufacturers recommendations. The crane should be taken out of service to a safe location before adjustments or repairs are begun. The crane should not be put back into operation until all guards are reinstalled, safety devices reactivated, and maintenance equipment removed. All inspection reports and repairs should be entered into a logbook with the service technicians signature.
Mechanical connections should be checked, particularly the bolts between the main girder and endtruck. Nuts, welding, and riveting, including rivets on the electric hoist casings rope anchorage, should be checked to ensure they are secure. Examine the rail endstops and seating of the bumpers and endstops. Crane wheels are clearly visible when the bumper plates are removed from the endtruck. Also inspect the flat cable used to supply current to the trolley, hoist, or crane. No visible damage or sharp bends should be present. Collector heads of the downshop power supply should be free from oil, grease, paint, and dirt. Badly bent busbars must be straightened or replaced immediately. Carbon sliders should be regularly checked for wear and replaced immediately when worn to prevent damage to the busbars or short circuit conditions. When heavy wear is found on wheel flanges, the crane runway should be examined to determine whether it still conforms to tolerances and requirements. Damaged cables, in particular the round cable to the push button pendant in the floor control must be replaced immediately.
Inspection
A daily visual inspection is important to safe crane operation. In addition, a complete inspection at 1-to-12-month intervals is recommended. A crane idle for more than a month should be inspected thoroughly. Several deficiencies should be looked for during this inspection:
Deformed, cracked, or corroded members
Loose bolts or rivets
Cracked or worn sheaves or drums
Worn, cracked, or distorted parts, such as pins, bearings, shafts, gears, rollers, and locking and clamping devices
Excessive wear of chain drive sprockets and excessive chain stretch
Cracks on crane hooks using magnetic particle or other suitable methods
Pitting or any deterioration of controller contractors, limit switches, pushbutton stations, or other electrical apparatus.
Periodic inspections of wire ropes are not only necessary, but also required by OSHA regulation, and, as a result, must be properly documented. A thorough inspection should be performed at least monthly.
The reason for the critical nature of the inspection is that the wire rope itself is literally "consumed" during use. The rope gradually loses strength during its life and must be carefully checked to assure that it retains sufficient strength to perform the required work during the period before the next inspection.
Wire ropes should be relaxed and at rest when inspected. Each rope must be treated individually. There are several possible areas of deterioration:
- Reduction of rope size below nominal diameter due to loss of core support, internal or external corrosion, or wear of the outside wires
· Number of broken outside wires and degree of distribution or concentration of these wires
· Worn outside wires
· Corroded or broken wires at end connections
· Corroded, cracked, bent, worn, or improperly applied end connections
· Severe kinking, crushing, cutting, or unstranding.
Critical points along any given rope should receive more attention than others, because some areas are usually subjected to greater stresses, forces, and hazards. Critical areas include pickup points, or sections of rope which are repeatedly placed under stress when the initial load of each lift is applied, such as those sections in contact with sheaves. Other critical points include end attachments; equalizing sheaves or sections of a rope in contact with and adjacent to sheaves, as on boom hoist lines; drums; and sheaves. The inspection should look for signs of heat exposure and "bright spots" which indicate abuse, such as contact with cross members of a trolley frame or load. Regular lubrication extends the life and properties of wire rope. If in doubt as to safe working strength or condition, do not hesitate to install a new wire rope. Make sure the equipment is right for the application when new purchases are required. Qualified technicians should carry out planned or preventive maintenance contracts or repair service. Employee safety and productivity both benefit with an awareness and concern for safe working practices.
Typical crane inspection checklist
Endtruck
· Check wheels for wear
· Girder bolts (410 ft-lb)
· Drive unit bolts
· Check wheel bearings for lubrication if not lifetime sealed
· Grease open gearing
· Adjust brakes
· Power pick up arm installation
· Check endtruck bumper
· Check oil level in gearboxes
Girder
- Check tagline and trolley endstop installation
· Check trolley rail for wear
- Inspect condition of walkway/platform
Electrical
· Check contactor points
· Tighten electrical connections on all motors and terminals
· Examine pendant and taglines
· Verify proper function of master switches and electrical accessories
· Ensure proper fitting and condition of all cables
· Ensure supply voltage within ï% when hoist is operating
- Ensure amperage on all motors equal on all phases
Hoist/Trolley
· Torque drive unit bolts (see endtruck)
· Check oil level (grease) in drive gear
· Check oil level in hoist gear if not lifetime sealed
· Check bottom block for deformation/ cracks and free sheave movement
· Adjust hoist, trolley, and bridge travel brakes
· Check trolley bumper
· Lubricate and inspect rope guide, pressure ring, and wire rope
· Limit switch bar movement (if applicable)
· Check limit switch up and down
· Check sheaves for wear
· Check trolley wheels for wear
- Check bridge wheels for wear (endtruck)
General
· Check power feed and collectors
- Examine runway endstops, rail alignment, and splices
Functions
· Test hoist brakes with full load
· Run trolley in low and high speed full length of girder
· Test accessories for proper function
- Run crane in low and high speed full length of runway
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