What are the chain grades and how are they derived ?

Chain grades refer to the strength and quality of a chain. They are typically used in applications such as lifting, rigging, and load-bearing.

The history of chain grades is rather complex. They are not in fact material grades but rather product grades. The origin is that the grade is the breaking strength of the chain expressed as a grade x chain diameter squared. It only works in imperial units, so a 1” grade 40 chain breaks at 40 x 12 = 40 Tons. A ½” grade 80 breaks at 80 x ½ 2  = 20 Tons.

For metric chain sizes many companies started using grades denoted by a letter to make the distinction. Others used an abbreviated number such as 4 instead of 40.

Coincidentally the mean stress at failure when expressed in N/mm2 is almost 10 x the breaking strength in tonnes derived from the above formula. So imperial grade 80 chain has a mean stress at failure of approximately 800 N/mm2. The mean stress at breaking force is now used to define the grade. Grade 40 then became M or 4; grade 60 became S or 6; grade 80 became T or 8, and grade 100 being V or 10.

There were a few variations along the way. The original BS grade 40 chain 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.

As all grades of chain became hardened and tempered, the letters and numbers became interchangeable and expressed as M(4), S(6) and T(8). However, when the European standards programme started in the late 1980s, it was agreed to use number designations  for medium tolerance chain for chain slings and letter designations for fine tolerance chain for hoists. At the same time the terms ‘medium tolerance’ and ‘fine tolerance’ were introduced where previously chain for hoists was termed ‘calibrated’. In practice, all machine made chain is calibrated as part of the manufacturing process, the distinction is one of accuracy.

The European method of separating number and letter grades was then adopted in ISO standards. There are also some other variations. Chain for use in hand operated hoists is through hardened but for power operation it is surface hardened to give improved wear resistance. So we have grade T (Types T, DAT and DT) the types DAT and DT being surface hardened, and in ISO standards we have grades TH and VH, these being through hardened grades T and V for hand operated hoists.

For components for use with chain, the component grades are not defined strictly by stress levels, but rather by being compatible with the same grade and size of chain. 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 use with grades 8 and 10 chain.