If you are in the habit of seeking out abandoned railways, you may have stood in the shadow of more than one Victorian iron bridge. Massive in construction, these structures have proved to be extremely robust, with many of them still in excellent condition even after years of neglect.
When you examine them closely, an immediate difference emerges between them and any modern counterparts, unlike almost all similar metalwork created today they contain no welded joints. Arc welders like reliable electrical supplies were many decades away when they were constructed, so instead they are held together with hundreds of massive rivets. They would have been prefabricated in sections and transported to the site, where they would have been assembled by a riveting gang with a portable forge.
So for an audience in 2018, what is a rivet? If you’ve immediately thought of a pop rivet then it shares the function of joining two sheets of material by pulling them tightly together, but differs completely in its construction. These rivets start life as pieces of steel bar formed into pins with one end formed into a mushroom-style dome, probably in a hot drop-forging process.
A rivet is heated to red-hot, then placed through pre-aligned holes in the sheets to be joined, and its straight end is hammered to a mushroom shape to match the domed end. The rivet then cools down and contracts, putting it under tension and drawing the two sheets together very tightly. Tightly enough in fact that it can form a seal against water or high-pressure steam, as shown by iron rivets being used in the construction of ships, or high-pressure boilers. How is this possible? Let’s take a look!
How Rivets are Formed
The above simple description of the process leaves out a few details. Rivet snap and set tools — heavy solid steel formers to fit both the straight and mushroom shaped ends of the rivet — are used on both ends of the rivet, on the mushroom end to hold it in place while it is being hammered, and at the straight end to hammer the sheets together with the snap and form a smooth new mushroom shape with the set. Thus a typical riveting gang such as that working on a railway bridge would have included two people working on the rivet itself, one on either end. Sometimes the person holding the set on the mushroom end would have to crawl into confined spaces such as inside a boiler to perform this task.
In the Victorian era the work would have been all done by hand, the rivet shaped by repeated blows to the set by a hammer, but from the early 20th century onwards a typical riveting gang would have used a hand-held pneumatic riveting gun. If you think of the tool Rosie the Riveter was often depicted as holding, you’re in the right place. However as the video below from 1949 shows, the scene would have otherwise changed little from the Victorian era, with the operator of a small forge tossing glowing rivets up to a worker who catches them in mid-air before placing them in the hole to be forged.
Swapping Rivets for Welders or Bolts
In 2018, you are only likely to encounter this type of iron rivet being used in heritage restoration work or in work that is intended to emulate it (The aviation industry uses rivets, but not quite the same as the ones you’d find in boilers or railway bridges!). Even then, as for example with the boiler on the new-build steam locomotive Tornado which was welded rather than riveted, it is by no means a given that rivets will be used. You can see a video of a modern-day riveting gang installing a boiler patch at LNWR Heritage in Crewe though, typical of this kind of riveting work. It’s ironic that even one of the most iconic riveters was pictured at a time when the practice was dying out, Rosie the Riveter’s portrait appeared while American shipyards were embracing the welder. Structural steel with mating holes is still used today, but high-strength bolts of a quality unavailable during the height of riveting have completely taken the place of rivets.
Speaking personally as a Hackaday scribe, my dad had occasional need to rivet as a blacksmith. Done by hand with the set positioned in the hardy hole of an anvil it was a more difficult job for an enthusiastic teen than you might expect, and I remember more than one attempt that emerged with distinct play in the resulting joint. I suspect I hadn’t managed to keep my rivet hot enough and thus its contraction had been less than it should have been. The rivets we were using were smaller than the ones in the videos after all. Perhaps if there had been handy online tutorials back in the day I’d have had more success.
Header image: Simon Lee [CC BY-SA 2.0].