Spring talk: Some facts about mainsprings
by B.B. Pelletier
Somebody somewhere asked whether the HW97 has a square-section mainspring. I believe it was CF-X guy, but I'm not sure. I checked around and learned that this is apparently either an urban myth or there may be some aftermarket hobbyists in England installing these springs in their guns. The U.S. versions that I was able to check have standard round-section wire springs.
I thought I'd use this as an opportunity to explore coiled wire mainsprings and perhaps learn a fact or two along the way.
The mainspring in most modern spring guns is a compression spring. When it compresses, the area of greatest stress is around the surface of the wire. Therefore, a thicker wire has a greater surface area and can tolerate greater stress. However, the larger the wire, the longer it takes for it to relax (spring back to rest), so a thicker wire can also slow you down. The thickness of the wire has to be balanced against the load (stress) it's required to handle as well as the number of times it will be expected to be stressed without failure.
Do square-sectioned wire mainsprings really exist?
Yes, they do. Back in the 1980s, when airgunners were experimenting with lots of different things to boost the power in spring guns, the square-sectioned wire mainspring was made and sold as an aftermarket mainspring for a variety of different guns. The wire from which these springs were made had a square cross-section instead of round. The theory was that the wire could be made thicker if it was flat when the coils collapsed and touched each other. Instead of two circles touching (like two beach balls) the coils would collapse to the point that two flat surfaces touched each other (like two boxes). You can get more mass into a smaller space this way. However, there was a problem. The square corners of the wire were points of extreme stress that began breaking down almost immediately when the spring was compressed. You might have gotten 500 powerful shots before your gun's mainspring started to fail. After that, the gun began to buzz and vibrate and power went straight down.
Webley air pistol springs
The older Webley pistol mainsprings were made from wire that was flat on two sides. Imagine a full circle that has been ground flat at 12 and 6 o'clock. When these springs were wound, the flat sections ended up opposite each other so the spring could compress closer than if the wire had been left round. It was a similar concept to the square-sectioned wire spring, except that it used round wire to begin with. These springs last longer than the square-sectioned springs, but not as long as springs with round wire. They don't seem to be any more powerful, because the Weihrauch HW 45, which has a round-sectioned wire spring, has a lot more power than an old Webley Senior with a flat-sectioned spring.
Spring wire material and treatment
The steel from which coiled mainsprings are made plays a big part in how long the spring lasts, as well as how powerful it can be.
Music wire is the carbon-steel wire used to make musical strings for stringed instruments, including pianos and harps. It is held to rigid tolerances and is very free from inclusions, as it must be to resonate correctly. It happens to make great coiled steel mainsprings, too, so many airgun springs are made from it. They are made from other types of steel, too, and I'm not going into the subject that deeply, but you can do a lot of research on compression springs on the web.
Heat treatment or some other kind of stress-relief is necessary after a spring is wound. What was a straight piece of wire or a wire wound from a large spool is now wound into uniform coils. That introduced all sorts of stresses into the surface of the wire. So, you do something to relieve those stresses. Shot-peening is a common stress-reliever, and you can tell when a spring has been shot-peened by the rough surface on the wire. If you don't do a proper stress-relief, the spring will fail earlier.
Spring ends should be ground flat
For the spring to work best in the confines of your airgun's powerplant, the ends should be ground flat. I've seen a number of foreign springs where the wire was left full-sized on the ends. When the spring decompresses during firing, it will vibrate more violently if the ends are not square to the axis of expansion.
Springs will take a "set" after manufacture
The spring becomes shorter after a full compression. This is sometimes called scragging and is part of the spring manufacturing process. Before scragging a spring may be too long to fit inside a gun, while afterwards it fits just fine.
Once it has been scragged, it will remain the same length for many thousands of shots if it was made properly to begin with. Then, in its old age, it will begin to break down and start to bend inside your airgun. You can feel when this happens by the increased vibration when you shoot. Some spring guns vibrate so much that they're painful to the shooter. You nearly always have a bent spring when that happens.
A little-known fact
When a mainspring decompresses, it doesn't just spring to its relaxed length and remain there. Instead, it opens and closes like an accordion for a few milliseconds. There are moments when neither end of the spring is touching the end of the gun it normally presses against!
There is a lot more I could do on mainsprings, but I don't want to bore you. Let me know if you want to know more.