In-depth alloy chain sling inspection requires attention to detail, but the results are well worth it — a safer, more fficient 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
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.
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 poten-
tially 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 sec-
tion can now be evaluated for acceptablity by measuring it
and applying the criterion for worn chain. See the “Wear
Allowances Table” on page 17.
In-Depth Alloy Chain Sling
Understanding the major elements that professional riggers should
consider when performing an in-depth alloy chain sling inspection.