Chain Slings
by Francis Hardy, P.Eng. Project Coordinator, CSAO
Only one type should be
used for overhead lifting.
Basics
Chain comes in many sizes and grades. But only one type--alloy
steel chain, grade 80 or grade 100--is suitable for rigging and
overhead hoisting in construction.
Alloy chain possesses excellent ductile properties typical
of most rigging hardware. The links actually stretch a little
when overloaded. This deformation is a warning sign, cautioning
the user that the chain has been overloaded and its capacity
reduced. Deformed chains must be removed from service.
Chains that are not made of alloy steel--for instance, those
used to tie down equipment and material on flatbed trucks--don't
have good ductile properties. They can fail without warning.
For this reason, non-alloy chains are prohibited for overhead
hoisting.
Many properties make chain popular in construction and other
industries. Chain is
The disadvantage of chain is that it's only as strong as its
weakest link. When wires break in a wire rope sling under load,
it doesn't necessarily translate into an accident. But when one
link in a chain fails, the load will come down. While regular
inspection is essential to all rigging operations it is especially
critical where chain slings are being used.
Chain Marking
Alloy steel chain grade 80 is marked with an 8, 80, or 800;
grade 100 is marked with a 10, 100, or 1000. The letter T is
referred to in the American Society for Testing and Materials
(ASTM) standard as one acceptable marking for alloy chain. However,
not all manufacturers fabricate to this standard.
Although some manufacturers emboss their identification on
each chain link, the identifier is only required for every 3
feet or 20 links, whichever is shorter. At least one manufacturer
has started painting chain for purposes of sorting and to distinguish
it from other makes of chain. A distinctive colour code prevents
confusion by ensuring that the proper chain is selected for a
particular situation. It's important, however, that paint or
paint layering doesn't prevent users from identifying surface
defects.
Sling Marking
Before using any chain sling, check to make sure that it's
the correct grade. Check the chain and the sling tag. Construction
regulations in Ontario require that all chain slings come with
a tag. The tag identifies size, reach, working load limit (WLL),
serial number, manufacturer's name or symbol, and sling type
(number of legs). In addition, the manufacturer should be able
to provide the user, at time of purchase, with a certification
of proof test. Chains missing a tag should be either discarded
or properly retagged.
Safety Factor
Most chains are fabricated with a 4-to-1 safety factor. For
chain manufactured in sling form, the safety factor ranges between
3.5-to-1 and 5-to-1. Ontario regulations require that all slings
incorporate a 5-to-1 safety factor. Make sure that all slings
meet this criterion. For slings fabricated with less than a 5-to-1
safety factor, the working load for the sling must be decreased,
that is, derated. The derating formula is
|
WLL x (manufacturer's safety
factor)/5 |
For example, if a chain manufactured with a 4-to-1
safety factor
has a WWL of 1000 pounds, the derating calculation is
|
WLL x manufacturer's sf/5 =
1000 x 4/5 = 800 pounds |
The working load limit for this sling is 800 pounds.
Inspection
Chain should be inspected according to two schedules: daily
(or before use) and periodically.
DAILY
It's good practice to inspect chain slings daily or before
use.
PERIODIC INSPECTION
Chain slings should be thoroughly inspected in the shop. This
inspection should be done at least annually and more often if
the chain sling is heavily used. When possible, each chain sling
should be catalogued with records kept of inspection and maintenance.
It's a good idea to measure overall length when the chain
is first purchased. This measurement can be used as a reference
later when checking for stretch wear during the service life
of the chain.
Periodic inspection should check for the following conditions:
