Rings, links and chain

For centuries the commonest forge weld in the world was the weld that closes a chain link. Anchor cables, crane and hoist chains, farm chains, trace chains for horses and the chains of mine winders were all made link by link at the forge, each link a short bar bent into an oval and its ends scarfed and welded. Rings for harness, wheel tyres for carts and wagons, hoops for barrels and bands for timber were made the same way. Hart’s book of 1910 recorded that chain links a foot or more long could be forge welded, and that gas and oil fires were often used for chain welding and similar repeated work.

This chapter works through the arithmetic of rings and links: the stock for an oval link, the weight of chain per metre, and the shrinkage that makes a welded wheel tyre grip its wheel. It describes how links were made and closed, and why chain, once the forge welder’s great product, is now made by electric welding to standards that a forge cannot meet.

The stock for a link

A chain link is an oval, or more exactly two semicircles joined by two straight sides. As with the ring of Chapter 4, its stock must match the length of its middle line, the line through the centre of the bar, plus an allowance for the weld.

Figure 1Stock length for an oval chain link

middle-line length = 2 × (inside length − inside width) + π × (inside width + stock diameter)

Stock

10 mm round

Inside size

40 mm long, 20 mm wide

Weld allowance

about 10 mm

1. straight sides: 2 × (40 − 20) = 40 mm
2. ends: π × (20 + 10) ≈ 94 mm
3. stock to cut ≈ 40 + 94 + 10 ≈ 144 mm

The semicircular ends have a middle-line diameter equal to the inside width plus one stock diameter, just as a ring’s middle line does, and the straight sides are the inside length less the inside width, the part not taken up by the curved ends. Every link in a chain is cut to the same length from a measured bar, usually with a gauge or stop on the shears, so that the links come out the same size and the chain runs true over a sprocket or sheave.

Weight of chain

Chain sells and is lifted by weight, and its weight per metre follows from the link.

Each link of the chain above has about 144 mm of 10 mm round bar in it, about 0.089 kg. Links interlock, so each one advances the chain by its inside length, its pitch, which here is 40 mm. A metre of chain has 1,000 ÷ 40 = 25 links, weighing about 25 × 0.089 ≈ 2.2 kg. The weld adds a little metal at each link, and a heavier link pattern, with a stud across the middle as used in anchor chain, adds more. The calculation also shows how much stock and how many welds a long chain needs: a 30 m chain of this size has 750 links and 750 welds.

Making a link

Making a forge welded link was a sequence learned to the point of habit. The cut bar was heated and its ends scarfed, often both at once, then bent round a horn or mandrel into an open link with the scarfs overlapping. The link was threaded through the last link of the chain, closed so that the scarfs lay together, fluxed, brought to welding heat in the fire, and welded on the anvil’s horn or a special anvil shaped to the link, with a few quick blows on each side. The weld was then dressed round so that the link’s section was even all the way round. A skilled chain maker could make links of small sizes rapidly, and the trade was organised for speed, with the bar, the fire, the anvil and the growing chain arranged so that no movement was wasted.

The difficulty was that every link had to be good. A chain is only as strong as its weakest link, and in a chain of 750 links the weakest weld, not the average, sets the chain’s strength. Chapter 7 showed how wide the spread of forge weld strength can be. Even a chain maker whose welds were 99.9% sound would expect, on average, to make one poor weld in a thousand, and a long chain might contain it.

Proof loading and its limits

Forge welded chain was therefore proof loaded after making: stretched to a load well above its working load, and inspected for links that had opened, stretched or cracked. Proof loading caught the worst welds but not every weak one, as Chapter 7 explained, and a link that passed its proof could still fail later under shock or fatigue. Inspection in service, looking for stretched, worn or cracked links, and withdrawal of any chain that showed them, was the other safeguard.

Electric welding takes over

Electrically welded chain, made by resistance butt or flash welding on automatic machines, replaced forge welded chain for nearly all purposes during the twentieth century. The machine bends each link from bar of controlled steel, clamps it, passes a heavy current through the joint to heat it, and pushes the ends together with a controlled force, expelling the oxide and forming a solid-state weld in a second or two. Every link gets the same heat, force and time, and the welds are far more consistent than hand welds. The finished chain is heat treated, proof loaded and marked to the standard it is made to. Lifting chain today is made and certified that way, and Resistance Welding by the Numbers describes the butt and flash processes.

Figure 2Forge welded and electrically welded chain

	Forge welded	Electrically welded
Heating	the link's ends in a fire	the joint by electric current
Force	hammer blows	controlled upsetting force
Oxide removal	flux and hammering	expelled by flash or upset
Consistency	depends on each weld by hand	the same heat, force and time for every link
Steel	wrought iron or mild steel	controlled grades, often heat treated
Testing	proof load, inspection	proof load, inspection, certified to a standard
Use today	ornamental, heritage and craft chain	all lifting and load-bearing chain

Ornamental and heritage chain

Forge welded chain is still made for gates, fences, fireplace furniture, church fittings and heritage restorations, where it must match old chain or carry no load that could hurt anyone. The shop makes it for those uses and marks it clearly as not for lifting. The arithmetic of link stock and weight applies unchanged.

Rings

Rings, of every size from harness rings to large hoops, are made like links: bar cut to the middle-line length plus a weld allowance, scarfed, bent round, closed and welded, then trued on a tapered mandrel, as Cravens mentioned in the 1921 list of the smith’s tools. The mandrel lets the smith stretch a ring that has come out slightly small by driving it down the taper, and true its roundness. A ring that comes out large cannot be shrunk easily, so smiths err slightly on the short side and stretch to size.

Wheel tyres

The iron tyre of a wooden cart or wagon wheel was the most demanding ring a country smith made. A long bar was bent into a hoop, welded, and made slightly smaller than the wooden wheel it was to fit. The tyre was then heated in a large fire until it had expanded enough to slip over the wheel, dropped on, and quenched with water. As it cooled it shrank, gripping the wheel and drawing the felloes and spokes tight together. Dunham’s book of 1917 used exactly that example, the smith who heats a tyre so that it will shrink to fit, to make a point about expansion in welding.

Figure 3Heating a wheel tyre to fit by expansion

temperature rise needed = shortfall in circumference ÷ (circumference × coefficient of expansion)

Wheel circumference

π × 1,200 ≈ 3,770 mm

Tyre made short by

10 mm

1. expansion per °C = 3,770 × 0.000012 ≈ 0.045 mm
2. temperature rise to close the shortfall = 10 ÷ 0.045 ≈ 220 °C
3. heated to about 300 °C above the room, the tyre is about 3.5 mm larger than the wheel, room to drop it on

The arithmetic shows why the tyre needed only a moderate heat, well below red, to fit, and why the fit was so tight when it cooled: the 10 mm shortfall became a strong grip that compressed the whole wheel. Too much shrinkage crushed the wheel’s timbers or dished it too far; too little left the tyre loose. The weld in the tyre had to be sound, because the shrinkage put it in tension from the moment the tyre cooled, and a cold shut in a tyre would open on the road.

Bands and hoops

Hoops for barrels and bands for timber, pulleys and hubs were made the same way: welded rings made slightly small and shrunk on, hot. The principle survives in engineering as the shrink fit, used to fix gears, bearings and rings onto shafts, where the interference is calculated rather than judged. The forge weld is no longer needed for most of them, but the expansion arithmetic is identical.

Welding rings and links in the fire

Small rings and links can be welded in a single heat, because they are thin and the scarfs are on the same piece. Larger rings need more care: the weld must come to heat while the rest of the ring stays cooler, so that the ring does not sag or distort, and the ring must be supported so that the weld lies in the fire’s reducing zone. Hart noted that gas and oil fires were often used for chain welding, because they could be arranged to heat just the weld area of each link, and the same idea is behind the localised heating of electric chain machines.

Pricing chain by the link

Because each link is cut, scarfed, bent, threaded, welded and dressed, the time of a chain is set by its links, not its length or weight. The shop times its first few links on each job and prices from the time per link. This is an illustration: if a practised smith makes a 12 mm decorative link in about six minutes, a 40-link chain takes about four hours of forge time, and at a shop rate of A$95 an hour excluding GST its labour is about A$380, several times the cost of the bar. A customer who expects to pay by the metre, as for machine-made chain, is shown the arithmetic, which usually settles the question of why hand-made chain costs what it does.

Inspecting old chain

Old forge welded chain still turns up on farms, boats and heritage sites, and owners ask whether it is safe. The shop’s answer is that its condition can be inspected but its welds cannot be proved. Links can be checked for wear at the points where they bear on each other, for stretching, for cracks and for corrosion, and any link with a visible defect condemns the chain; but a link with a hidden unbonded weld looks the same as a sound one. Chain of unknown history is therefore treated as decorative only, whatever its condition.

The running case: chain for the church gate

This is an illustration. A parish asked the shop for a length of chain to match an old one on its churchyard gate, a decorative chain from which the gate hung open. The shop measured the old links, about 12 mm stock with an inside size of 50 by 25 mm, calculated the stock length, cut a first link and adjusted the length by 2 mm after checking it, and then made 40 links. Each weld was dressed to match the old links’ slightly lumpy profile. The chain was supplied with a written note that it was decorative and not for lifting or restraint, and the shop recorded the stock length and link count on the job card for future orders.

A failure: the farm chain

This is an illustration. A farmer once brought the shop an old forge welded chain that had broken while pulling a stump with a tractor. The broken link showed a flat, grey face over most of its weld, with a thin rim of bonded metal round the outside; the link had probably been weak since the day it was made, and had finally let go under the tractor’s pull.

The shop did not repair the chain. It explained that forge welded chain of unknown history should not be used for pulling or lifting, and that a rated, electrically welded chain of the right grade, with its tag, was the right replacement. The old chain went back to the farm as a fence ornament.

Exercise: links and weight

A smith is to make a decorative chain of 16 mm round bar with links 60 mm long and 30 mm wide inside. Using the oval link rule with a weld allowance of one stock diameter, what length of stock is needed for each link? How much does a metre of the chain weigh, and how many links and welds does a 12 m length need?

The middle line is 2 × (60 − 30) + π × (30 + 16) ≈ 60 + 145 ≈ 205 mm, so with a 16 mm allowance each link needs about 221 mm of stock.

Each link contains about 221 × π ÷ 4 × 16² ≈ 44,400 mm³ of steel, about 0.35 kg. With a pitch of 60 mm, a metre has about 1,000 ÷ 60 ≈ 16.7 links, about 5.8 kg per metre. A 12 m length needs about 12,000 ÷ 60 = 200 links and 200 welds, each of which must be dressed and inspected, which is why the job’s time is set by the number of links rather than the length of chain.