This article originally appeared in the August print issue of IMPO Magazine. To view the digital version, click here.
In-depth alloy chain sling inspection requires attention to detail, but the results are well worth it — a safer, more efficient lifting process. Let’s kick off discussing the effect of twisting and bending.
Consider that chain is evaluated by applying loads in a pure tensile link end-to-link-end fashion and rated accordingly. Rigging chain around edges or corners alters the normal loading pattern significantly. A lack of proper padding or consideration of the D/d ratio for chain can result in twisted and bent links. Once a chain is twisted or bent it will alter inner link stresses which can result in failure. For this reason all chain containing twisted or bent links must be removed from service immediately.
National Association of Chain Manufacturers (NACM), representing domestic manufacturers of welded and weldless chain since 1933, has conducted D/d testing on alloy chain. As a result of this testing, the NACM came out with the chart below which shows reductions in working load limits based on D/d ratio of alloy chain rigged around an edge or a corner. Consult the manufacturer for any D/d below 2. The latest revision ASME B30.9 2014 released for sale this month has adopted this chart into the new standard. Using proper sling protection, following the D/d capacity reductions and exercising proper rigging practices will eliminate damage to your alloy chain slings.
Nicks and Gouges
When chain is used to lift, pull or secure materials, the outside surface of the links can come in contact with foreign objects that can cause damage. Nicks and gouges frequently occur on the sides of a chain link, which are under compressive stress, reducing their potentially harmful effects.
The unique geometry of a chain link tends to protect tensile stress areas against damage from external causes. Often these tensile stress areas are on the outside of the link body at the link ends where they are shielded against most damage by the presence of interconnected links. Tensile stress areas are also located on the insides of the straight barrels, but these surfaces are similarly sheltered by their location. However, gouges can cause localized increases in the link stress and can be harmful if they are located in areas of tensile stress, especially if they are perpendicular to the direction of stress.
Figure 1 shows nicks of varying degrees of severity. Reading clockwise, at three o’clock there is a longitudinal mark in a compressive stress area. Since it is longitudinal and located in a compressive stress area, its effect is mitigated, but good workmanship calls for it to be filed out by hand.
At about five o’clock there is a deep transverse nick in an area of high shear stress. A similar nick is located at six o’clock in the zone of maximum tensile stress. Both of these nicks can create a potentially dangerous escalation of the local stress and must be filed out with careful attention to not damage other parts of the chain link or chain. A nick that was located at eight o’clock has been filed out properly. Although the final cross section is smaller, the link is stronger because the stress riser effect of the notch has been removed. The remaining cross section can now be evaluated for acceptablity by measuring it and applying the criterion for worn chain. See the “Wear Allowances Table” below:
Wear and Corrosion
When used in rigorous material handling applications, chain can easily become worn or corroded. It is important to inspect chain for defects on a regular basis to avoid an unsafe lifting condition or even operator injury. When corrosion and wear occur, it results in a reduction of link cross-section which can lead to decreased strength of the chain.
Corrosion can occur anywhere chain comes in contact with harsh chemicals, water or when it is used in tough environments.
Wear can occur in any portion of a link that is subject to contact with another surface. The natural shape of chain confines wear, for the most part, to only two areas. These are, in order of importance, (a) at the bearing points of interlink contact, and (b) on the outsides of the straight side barrels that may be scraped from dragging chains along hard surfaces or out from under loads.
When wear or corrosion is observed, the next step is to determine how severe the damage is and if the chain can still be safely used. General surface corrosion can be removed by cleaning and oiling the chain. If pitting is observed after cleaning and oiling, remove from service. Next, the operator should take a caliper measurement across the worn section of chain and compare it to the minimum allowable dimension for that chain.
Stretch and Chain Elongation
A visual link-by-link inspection is the best way to detect dangerously stretched alloy chain links. Reach should also be measured from the upper bearing point on the master link to the bearing point on the lower hook. The smallest sign of binding or loss of clearance at the juncture points of a link indicates a collapse in the links’ sides due to stretch. Any amount of stretch indicates overloading, and the chain should be removed from service.
Note that a significant degree of stretch in a few individual links may be hidden by the apparent acceptable length gauge of the overall chain. This highlights the importance of link-by-link inspection.
Alloy steel sling chain typically exhibits well over 20% elongation before rupture. The combination of elongation and high strength provides energy absorption capacity. However, high elongation or stretch, by itself, is not an adequate indicator of shock resistance or general chain quality and should not be relied upon by riggers to provide advance warning of serious overloading and impending failure. Overloading must be prevented before it happens by selection of the proper type and size of slings. Again, any amount of stretch is overloading and the chain should be removed from service.
There is no short-cut method that will disclose all types of chain damage. Safety can only be achieved through proper inspection procedures. There is no adequate substitute for careful link-by-link scrutiny.
OSHA Chain Sling Inspection
Since first published on July 27, 1975, the OSHA Chain Sling Inspection section has undergone very few changes. These regulations have and continue to serve as a comprehensive guide for those responsible for chain sling inspection. For a full review of the applicable sections of the Code of Federal Regulations (29 CFR 1910.184), visit www. cmworks.com.
Peter Cooke is a Training Manager specializing in Rigging & Load Securement for Columbus McKinnon Corporation.