Ilh structural inspection of overhead cranes (gary j davis, p e )

Product Focus

Inspection of Overhead Crane Structures Failure to complete overhead crane and hoist inspections and proper equipment maintenance could lead to serious injury, death or destruction of property.

By Gary Davis, P.E.

Gary Davis, P.E., is director of consulting services, Integrated Machinery Solutions (IMS), Fort Worth, Texas.

H

ow can you make the best use of inspection time that is limited by an aggressive production schedule? Hard-worked cranes have to keep running, but these are the cranes that need the most attention. It is not practical to inspect everywhere, and if the surface is covered by debris, you cannot inspect anywhere. Fortunately, structures respond to loads in predictable ways, which helps identify generic locations most likely to have damage. This article is an overview of a guide that can be downloaded at the address listed below.

Ref. 1: The Crane, Hoist and Monorail Alliance, an

Only properly trained and qualified personnel should perform inspections. A formal training and certification program is recommended to ensure that inspectors are qualified and remain qualified. For guidance on the minimum requirements for crane inspectors, refer to CMAA Specification No. 78-2002, Standards and Guidelines for Professional Services Performed on Overhead and Traveling Cranes and Associated Hoisting Equipment.

Fig. 1: Beam bending diagram

OSHA Cooperative Program, Fact Sheet No. 1, Proper Inspection and Maintenance of Overhead Cranes. http://www.mhia.org/industrygroups/osha The structure is part of the crane “machinery” and deserves similar attention. The scope of crane structure inspection includes: Bridge girders; Trolley frames; End trucks, end ties, equalizer saddles, sill beams, and gantry legs; Platforms, ladders, stairs, cabs and cab hangers; and Bolted and welded connections.

Adhere to a formal inspection program

Failure to complete overhead crane and hoist inspections and proper equipment maintenance could lead to serious injury, death or destruction of property. A proper inspection program has the following benefits: Reduces chance of fatality or injury to personnel; Demonstrates commitment to safety as a high priority item; Ensures compliance with laws and standards including: OSHA, ANSI, and ASME; Provides accurate records for maintenance personnel; Reduces liability exposure; and Prolongs equipment service life.

The structural inspection task should be maintained separately from other disciplines. A manual should be created for work instructions, procedures, and customized inspection checklists. Results from each inspection should be reviewed and approved by a qualified engineer.

Periodically check the quality of your in-house inspection program by using an outside expert to inspect one of your most important cranes.

The selected firm should specialize in inspection and design of overhead material handling equipment.

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Inspection guidelines

Since overhead cranes have the same basic configurations, a generic list of bridge and trolley inspection points can be created. For tabulations of recommended inspection points, refer to the website link in Ref. 1. A custom list of inspection points should be developed for each of your cranes.

Ideally, the structure should be clean before starting the inspection.

Areas covered by multiple layers of undisturbed debris, indicate a longterm neglect of inspection. A viewing distance of not more than two feet is recommended for routine visual inspections. When a crack is discovered, all other locations with the same detail must then be carefully inspected.

The following tools are useful for routine structural inspections: Putty knife: For removing debris from areas of interest; Small wire brush: For detailed, local cleaning; Camera: Use a compact, weatherproof, high resolution camera, suitable for sharp close-ups. With average lighting, most pictures will have better quality without use of the flash; Mirrors: A small inspection mirror with an extension handle, and a 4x5inch acrylic locker mirror; and Flexible Borescope: For inspection of inaccessible areas such as the interior of box girders, end trucks, and end ties. Borescopes are available with optical or video displays.

industrialliftandhoist.com • November 2011

Overhead Cranes What to look for

Inspectors must be able to identify the primary load path of the structure; single-failure points; and areas where tension stress occurs, including: Locations where stress fluctuates between tension and compression; Areas that have been repaired or modified; and Locations where flexing of thin-walled elements may occur.

To identify single failure points, look for members in the main load path that have no redundancy. Think of a link in a chain; any one chain link is a single-failure point. If failure of one element does not cause the entire structure to fail, then that element is not a single failure point.

When structural damage is found, do not release the crane for service until it has been evaluated by the responsible engineer. The engineer should determine if non-destructive testing, repairs, or load testing are required before releasing the crane.

When major damage is discovered, consider hiring an engineering firm that specializes in engineered repair procedures for overhead cranes. Hasty, non-engineered repairs are prone to repeated failures. Makeshift repairs can end up costing many times more than a properly engineered repair.

“Patch plate” repairs

Fig. 2 shows a patch plate repair. Patch plates usually indicate an improper repair. The repaired area cannot be inspected because it is covered by the patch plate. Patch plated areas should be evaluated by the engineer and a formal repair should be completed.

When to inspect

Check the manufacturer’s manual for inspection instructions. If the manual is no longer available, refer to these documents: ASME B30.2-2005, Overhead and Gantry Cranes (Top Running Bridge, Single or Multiple Girder, Top Running Trolley Hoist); OSHA 29 CFR 1910.179(j) for Overhead and Gantry Cranes; and CMAA Specification No. 78-2002, Standards and Guidelines for Professional Services Performed on Overhead and Traveling Cranes and Associated Hoisting Equipment

Welding, stress concentration, and fatigue

For hard-worked cranes (CMAA class “E” and “F”), most of the structure design is based on fatigue stress limits due to welded joints.

Cracking will occur if these cranes are not properly engineered, detailed, and fabricated. In order of importance, look for: Poor quality/defective welds; Abrupt changes in structure geometry; Sharp inside corners, also known as “re-entrant corners”; Gouges, notches, and un-ground flame cut edges; Welded attachments to tension members; and Welds that meet in corners.

High-quality welding is important for dynamically loaded crane structures. Stress concentrations are created by geometric imperfections such as undercut, porosity, slag inclusions, incomplete fusion, craters, wrapping welds around edges, and hydrogen entrapment.

Internal forces are produced when welds shrink. These forces are created when the base metal outside of the heat affected zone resists the weld shrinkage. Weld shrinkage creates a local stress field called “residual stress.” When the structure is loaded during normal service, the residual stress adds to the design stress.

Since these stresses act in all directions, their

Tension, compression, and bending

For beam members, look for cracks in the tension zone and end connections. Fig. 1 illustrates simple beam bending. Most bending members on cranes behave as simple beams.

When loaded from the top, the bottom surface is always in tension. At least the middle two thirds of the tension zone should be inspected.

Material at the top of the beam is in compression.

If there are cracks in the compression zone, they will not grow. Cracks in the tension zone will grow, but the speed of growth depends on the magnitude of the loading, and the number of loading cycles.

Fig. 2: Patch plate repair

November 2011 • industrialliftandhoist.com

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Product Focus Fig. 3 shows the bottom edge of an end truck at the bridge girder connection.

summation creates multi-axial stress. Steel is very ductile; it can sustain large deformation before failing. Ductility makes steel a forgiving material.

When one area is overstressed, the material deforms and automatically redistributes the stress to make it more uniform. But when steel is loaded simultaneously in multiple directions it behaves like a brittle material. A simple example of this is when three welds meet at a common corner.

Each line of weld shrinks in its long direction. The point at the corner is subjected to tension forces in three perpendicular directions. Look for cracks at the corner points where welds meet.

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Example of stress concentration

Fig. 3 shows the bottom edge of an end truck at the bridge girder connection. The pink arrow points to a tack weld at the bottom of the girder connection.

Tack welds cool rapidly, making the adjacent base metal brittle and prone to cracking. The blue arrows point to a crack through the bottom flange of the end truck. This crack was probably caused by the combined effect of the tack weld and the stress concentration created by the girder connection plate.

industrialliftandhoist.com • November 2011

Overhead Cranes Fig. 4 is a computer model of this connection showing stress intensity.

Red indicates high stress, and blue shows low stress.

As suspected, the stress is concentrated in the bottom flange of the end truck exactly where the crack was found. Abrupt geometry changes, inclusions, gouges, and notches create small areas of high stress that are prone to cracking.

Conclusion

The ability to identify critical inspection points cannot be over emphasized. The inspector must have a basic understanding of how structures respond to loads.

The success of the overhead crane and hoist inspection program depends on oversight by a competent engineer. If this expertise is not available in-house, consult with an engineering firm that specializes in structural inspections and engineered repair procedures for overhead material handling equipment.

November 2011 • industrialliftandhoist.com

Fig. 4 is a computer model of the bridge girder connection showing stress intensity.

References 1. Gary Davis, Guidelines for Inspecting Overhead Crane Structures, Integrated Machinery Solutions, Fort Worth, Texas, July, 2011. Download the full text at: http://www.team-ims.com/documents/IMS%20 Crane%20Inspection%20Guide%201.pdf.

2. Omer W. Blodgett, Design of Welded Structures, James F. Lincoln Arc Welding Foundation, Cleveland, Ohio, 1976.

3. American Society of Mechanical Engineers, Overhead and Gantry Cranes (Top Running Bridge, Single or Multiple Girder, Top Running Trolley Hoist), ASME B30.2-2005, New York, NY, 2005.

4. Crane Manufacturers Association of America, Inc., CMAA Specification No. 78-2002, Standards and Guidelines for Professional Services Performed on Overhead and Traveling Cranes and Associated Hoisting Equipment, Charlotte, NC, 2002.

5. S. Timoshenko, Strength of Materials, Part II, 2nd edition, D. Van Nostrand Co., Inc., New York, 1941.

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