Humble Inventions #3: Springs

In Nuts and Bolts, Roma Agrawal takes a look at the good old spring:

“Springs are things that store energy when their shape is changed by a force. When the force is removed, they ping back to their original shape and release that energy – and this energy is made to do something useful.”

The more they can be made to curve, the more energy they can store. Think of how a bow works – pull (store energy), release (energy transferred to arrow).

 

Springs can store energy in multiple configurations – “when squashed” (e.g. ballpoint pen), “when stretched” (e.g. trampoline), or “when twisted”. One of the easiest laws in physics was Hooke’s Law, the one that describes the relation among force, energy and stretching/compression of a spring. It was an easy equation, no calculus, perfectly linear. And thus, one never give any thought to – there are so many harder equations… Except, as Agrawal tells us:

“This law was far-reaching. Beyond springs, Hooke’s Law allowed engineers to predict how much an elastic system expand, contract, or move under particular forces. This could be how high a ball would bounce, how a material might absorb sound, or even how much a tower might sway under wind and earthquake forces.”

 

Mechanical wristwatches are an intricate system of springs and gears:

Though this pic seems to be all gears, there are lots of springs too, starting from the one that stores energy when you wind the wristwatch. (If you’re interested, check out this highly interactive, step by step explanation of how a wristwatch works. Trust me, it is an awesome explanation of a great piece of engineering).

 

Accurate timekeeping became super-important for two major activities – first, for ensuring the railways were “at the right place at the right time”; and second, for knowing one’s longitude at sea (timekeeping has an indirect connection to longitude detection, and beyond the scope of this blog, but you can trust it is true because the British Navy issued massive rewards for the most accurate time pieces that could work even on the rocking seas). And springs, as we saw, have a major role in their accuracy.

“Chronometers led to a marked improvement in the accuracy of maps… reduced the number of shipwrecks… increased trade.”

 

As buildings got taller and bridges longer, the impact of high winds had to be factored into their design:

“This is where springs spring into action.”

The springs absorb some of the energy; and also allow the structure to sway within safe limits (and thus allow part of the wind/earthquake energy to be dispensed without damaging the structure itself).

 

Another major use of springs is wrt sound absorption, i.e., helping make places quieter despite all the surrounding noise. How’s that? Sound, remember, needs a medium to be transmitted (solids, air); and sound travels better in some medium than others. The trick then is to separate two rooms/floors by not just walls, but also by air gap between them. Why? Because sound travels worse through air than solids, and thus gets dampened more if it passes through air. But the rooms and floors still need to be joined together, of course. Joined how? You can’t use solid materials, because sound would be transmitted via the solid. It is springs to the rescue as the connector. Less solid, and the spring can also absorb some of the sound’s energy and reduce the parts of it that get transmitted.

 

It is hard to imagine how much sound absorption could be happening because of such spring-based setups. Agrawal proves it is enormous by pointing out that the best music halls in the world use springs to isolate themselves from the noisy streets, and also to isolate different rooms in the music hall from each other. If that still felt too niche an application, then she reminds us:

“The use of springs to mitigate sound in structures has changed the way we design and plan our cities.”

Huh? Without springs, she says there were two possibilities. Cities would have been designed the same as currently – buildings close to each other, near metro lines – but a lot more noisy and experiencing greater vibrations, with greater stress and sleep deprivation. Or people would have started to live further and further away from each other to have some silence, thus increasing commute times even more and reducing the benefits of cities (bringing large groups closer, with all the attendant benefits of diversity of skills and capabilities).

 

Who’d have thought the spring had such significance? As Agrawal says:

“Springs, for me, are the epitome of versatile engineering.”