Guidelines for overhead crane safety
Published in: Plant Engineering
Date: 4/4/1991
By: Toth, A.R.
Guidelines for Overhead Crane Safety
Crane operators who anticipate overhead load movements are one of the key factors in a safe installation. Qualified operators plan moves in advance, using a careful blend of intelligence, common sense, and experience. However, the operator represents only the human part of the crane safety equation.
Mechanical elements are the other side of the factor. Specifying the proper equipment, and then using it correctly, are equally important.
A detailed inspection program is essential to ensure that the overhead crane and its many components are in good working order. An inspection program is easily established without additional staff. Cost of the procedures is recovered over the long run through reduced capital, maintenance, and breakdown expenses.
Four steps are involved in organizing the overhead crane inspection system: equipment identity, recordkeeping, use, and classification.
Equipment Identity
Too often, cranes are purchased and no one bothers to record the information required for repair or maintenance. Equipment is occasionally moved from one location to another, and the operational and service records do not follow.
Users should identify overhead cranes by manufacturer, serial number, facility number, name, color, or location. These data form the basis for inspection frequency, subsequent repairs, and equipment or parts replacement.
Service people require accurate equipment information before making evaluations and ordering parts. There is no room for guesswork when replacing worn components. Equipment identity should be accurately tied to parts books as well as care and operation manuals.
Recordkeeping
There is no single correct method for keeping accurate records, but a set of formal procedures should be adopted. Some managers use a color coding system where a dot of paint or marker is put on the equipment to indicate what was inspected and when. Other managers prefer a manual entry charting system, chronologically oriented and devoted to a single crane, identical pieces, or manufacturer. A third approach uses a double-entry method in which the system is programmed into a computer file which does the sorting, assimilation, and recollection of information.
Records should identify the equipment, what specifically was inspected, and who did the work. Records should also include work recommendations as a result of the inspection, and when the action was taken.
Accurate records become particularly useful when evaluating the need for replacing or adding equipment. Records allow the purchaser to more closely specify the exact overhead crane for the job.
Use
Overhead cranes can be over or under-applied. Inspection programs should match what the particular piece does relative to its capabilities.
The best way to determine an overhead crane’s level of use is to assign someone to observe the routine load handling operation over a representative period of time. Recording the number of movements and operational or actual running time determines the percent of load operational time. Overhead cranes are then classified as normal, heavy, or severe service.
Normal service involves three possibilities: * Less than 85% of the rated load and not more than 10 lift cycles per hour * Randomly distributed loads within the rated load limit * Uniform loads less than 65% of the rated load for not more than 15% of the time on manually-operated equipment, and 25% of the time for electric or air-powered equipment.
Service is considered heavy when the overhead crane performs within its rated load, but the use exceeds the parameters of normal service.
Severe service involves normal or heavy service in an environment that is unfavorable, harmful, or detrimental to the crane.
Classification
There are three types of inspections for in-service overhead cranes: functional test, frequent, and periodic. Service classification determines how often each inspection is conducted.
Functional test and frequent inspections require no recordkeeping. Checks are usually conducted by the operator because of his intimate knowledge of the crane. Functional test inspections are performed at the beginning of each shift. Frequent inspections should be made monthly for overhead cranes in normal service, weekly to monthly for heavy service, and daily to weekly for severe service.
Periodic inspections are usually conducted by a person with greater technical knowledge, experience, and training. Careful records should be kept of each periodic inspection. Checks should be done yearly for overhead cranes in normal service, semiannually for heavy service, and quarterly for severe service.
These three inspection procedures assume that operations do not exceed the crane class-of-service rating. If conditions are beyond the scope, replace the crane with one that has a rating at least equal to the work, or assume a severe environment and increase the inspection frequency.
Inspection Actions
Operators should conduct functional test inspections at the beginning of each shift as a means of putting the equipment through its paces before evaluation under a load. This inspection is comparable to an airplane pilot walking around his craft to make a visual inspection, and then revving the engines and testing the flaps before takeoff.
Pushbutton action of the controls should be inspected by depressing and releasing each button. For multispeed buttons, there should be a noticeable detent or detent travel distance for each speed step. A more detailed check should be made if the buttons do not function properly, or if speed steps are not discernible for each motion.
Upper travel limit devices for the hoist motion should be given a functional test. If the system includes a backup travel limit, manually test the device by tripping the backup or by providing a bypass for the primary limit stop.
Items under scrutiny during a frequent inspection are the same as those checked during periodic examinations. The difference between the two are who does the inspecting and the extent to which the items are examined.
Eleven components of the crane system should be looked at during frequent and periodic inspections.
Below the hook device inspections should include any conceivable means of attaching the load to the hook. For example, on chain slings look for wear, nicks, cracks, breaks, gouges, stretches, bends, weld spatter, discoloration from excessive temperature, and throat opening of hooks. Any abnormality is reason to discontinue use of the sling until a more detailed inspection is made.
Load hooks should be inspected for deformation, cracks, or chemical damage. Ensure that throat openings do not exceed more than 15% of the normal, or more than a 10-deg twist from the plane of the unbent hook. Also check the hook thrust bearing, hook retaining nut or collar, and fasteners used to retain the nut.
Make sure swivel hooks freely rotate 360 deg under a load. Hook latches should bridge the throat opening to retain slings and attachments in their unloaded condition.
Hook or load blocks should be inspected for freely rotating reeving sheaves or sprockets, sheave guard wear, corrosion, and free entry and exit of the wire rope or chain.
Periodic checks should evaluate the extent to which items found during frequent inspections have progressed. Determine whether repair or replacement is necessary.
Wire ropes should be inspected for kinking, crushing, unstranding, birdcaging, strand displacement, core protrusion, corrosion, heat discoloration, presence or absence of lubricant, broken or cut strands, broken wires in the strands, and reduction in diameter in a localized area. Rope and connections, particularly clamps, clips, and wedge-type sockets, should also be examined.
Load chain inspection includes operational observation for smooth feed through at the load and idler sprockets in the raise and lower directions. Surface wear of welded link chains indicates binding in either the drive or idler sprocket area.
Normal wear of welded link chains takes place on the contact surface between the links. Interlink wear should be inspected by slackening the chain, displacing the two adjacent links, and examining the inside end surfaces where the links are normally in contact under load.
Periodic examination of wire rope and load chain includes a detailed evaluation of the items found during the frequent inspection. In addition, count the number of broken wires, confirm the loss of core support or damage internal to the rope, and conduct a link-by-link inspection to determine whether the chain can continue to be used or should be discarded. This inspection method establishes a predictable service life of wire rope and chain for future replacements.
Upper reeving components should be inspected while operating the hoist. Wire ropes should run over sheaves, not next to them. Ropes should enter and exit the sheaves without rubbing the sides of the groove.
Wire rope drums should be checked during normal operation of the hoist to ensure that the rope enters and exits the grooves in the drum freely, without rubbing the sides. Verify that the wire rope is wound in the grooves, not crossed over the top.
Periodic inspection of rope drums includes inspecting the grooves for irregular wear that conforms to the contour of the rope strands. Check the size of the grooves with a gauge. The drum should be inspected for signs that the rope crossed over grooves, scuffed drum flanges, or came in contact with the housing surrounding the drum.
Load chain pocket wheel and guide should also be inspected during the normal hoist operation to ensure that the chain enters and exits the wheel and guide freely. Jerky or pulsating chain motion indicates a worn chain or pocket wheel and guide.
Structural components require a simple visual inspection. Unusual deflection, loose fasteners, cracks, and corrosion are potential problems.
Wheel inspection applies more to underhung cranes than top-running types. Unusual noise and skewing or binding are signs of wear applying to both systems.
Periodic inspection of the structural components and wheels involves a detailed investigation of the items observed during frequent inspections, including measurement of wheels, tracks, and rails.
Controls and limit switch frequent and periodic inspections should include functional test checks of pushbuttons and testing upper travel limit devices. In addition, periodic inspections include checks for pitting and deterioration of the electrical contacts in controllers, master switches, pushbuttons, and limit switches.
PHOTO : Specifying the proper equipment, putting it in the hands of a qualified operator, and establishing a detailed inspection program based on use and duty keep overhead cranes safe while performing at peak efficiency.
PHOTO : Several components of the crane system should be looked at during frequent and periodic inspections. For example, operators should check load chain for interlink wear, and examine wire rope for unstranding, strand displacement, core protrusion, and broken wires.
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