A brief history of chain slings
Ben Dobbs, Head of Global Standards and Legislation at the Lifting Equipment Engineers Association (LEEA), looks at the development of chain grades.
Chain slings provide a versatile, durable and safe means of connecting loads to lifting appliances, but they must be used in the correct manner to avoid dangerous lifting practices and service damage. Unfortunately, this has not always been the case and many slings have been subjected to great abuse in service.
Chain slings manufactured from wrought iron are obsolete and no longer available. Mild steel chain slings were rendered obsolete in the early 1980s following the publication of newer standards that specifically exclude the use of this grade of chain for lifting applications. However, it is possible that examples of wrought iron and mild steel chain slings may occasionally be found in service, but their continued use is not recommended by LEEA and they are outside the scope of the Code of Practice for Safe Use of Lifting Equipment (COPSULE).
Chain grades
The history of material grades is rather complex. They are not, in fact, material grades but rather product grades. The origin is chain, and the grade is the breaking strength of the chain expressed as grade x chain diameter squared. It only works in imperial units so a 1” grade 40 chain brakes at 40 x 12 = 40 tons. A ½” grade 80 breaks at 80 x ½ 2 = 20 tons. When chains went metric some companies started using letter grades to make the distinction. Others used an abbreviated number, for example, 4 instead of 40.
Coincidentally, the mean stress at failure when expressed in N/mm2 is almost 10 x the breaking strength in tons derived from the above formula. So imperial grade 80 has a mean stress at failure of approximately 800 N/mm2. The mean stress is now used to define the grade. So grade 40 became M or 4, 60 became S or 6 and 80 became T or 8. This has continued with grade 100 being V or 10.
There were a few variations along the way. The original BS (British Standard) grade 40 was used at a factor of safety of 5:1 and because of that, it could either be in the normalised condition or hardened and tempered. To make the distinction the mark 04 was used for normalised and 40 for hardened and tempered. Later the factor of safety was reduced to 4:1 so it had to be hardened and tempered and again to make the distinction, grade M was used. So all three have the same breaking strength but the heat treatment and rating varied.
Once all grades of chain were hardened and tempered, the letters and numbers became interchangeable and expressed as M(4), S(6) and T(8). However, for the European standards in the late 1980s it was agreed to use the number grades for medium tolerance chain for chain slings and the letter grades for fine tolerance chain for hoists. At the same time the terms ‘medium tolerance’ and ‘fine tolerance’ started to be used. Previously, chain for hoists was termed ‘calibrated’ to make the distinction but in practice all machine-made chain is calibrated as part of the manufacturing process, the distinction is one of accuracy.
The European separation of number and letter grades was then adopted in ISO. There are also a few other variations. Chain for use in hand operated hoists is through hardened, which is a heat treatment process that significantly increases their hardness and durability, making them resistant to wear and tear during heavy-duty lifting operations. But for power operation, chains are surface hardened, which creates a hard outer layer on the chain to improve the wear, hence we have grade T (Types T, DAT and DT), the types DAT and DT being surface hardened. Also in ISO there are grades TH and VH these being through hardened and grades T and V for hand operated hoists.
When applied to components other than chain, the grades are not defined strictly by stress levels, rather by being compatible with the same grade of chain so whilst the maximum stress levels may be of similar order, it is the manufacturer who decides on most of the dimensions (within the confines of the dimensional envelope) and therefore the stress level. Hence some manufacturers can use the same components for grades 8 and 10.
Current grades
Currently, the majority of chain slings in service are grade 8 or grade 80 or T (Older national and international standards permitted the use of either letters or numbers to indicate the grade of chain irrespective of its intended use. However, most modern standards now reserve the use of numbers to indicate the grade of medium tolerance chain for chain slings and the use of letters to indicate the grade of fine tolerance chain for lifting appliances).
Other grades, notably 40, M, 4 and, to a lesser degree, 60, S, 6, of welded construction may also be found in service and are therefore covered by LEEA’s COPSULE. Grades 8, 80 and T have a breaking load twice that of grades 40, M, 4 and are therefore lighter for the same strength.
In many parts of the world, grade 10, or higher, chain slings are available and gaining in popularity and we are seeing some grade 10 standards across the world. On a European level, CEN TC 168 of which I am chair, is currently developing two grade 10 sling chain standards and associated grade 10 component standards. Grade 12, however, is not yet standardised.
Further guidance on chain slings is in the LEEA COPSULE which can be accessed at leeaint.com.
