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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.

Compression springs
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.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

16 thoughts on “Spring talk: Some facts about mainsprings”

  1. BB,

    Yes,it was me who asked for this post.This music wire must be strong since it is used also for shark fishing.This is a great post and I dont think is borring since it is stuff that many of us havent seen.I we knew that then it would be borring but not now.So please continue.And also,read that you were going to make a post of the hw97 again,looking forward to it.

    CF-X guy

  2. BB,

    Glad to know that the hw97k spring in not square main spring.One question,
    If the US hw97k doent have a silencer,is it loud because of the shorter barrel?

    CF-X guy

  3. CF=X guy,

    It’s not really any louder. The noise a spring gun makes is in its powerplant – not in its muzzle blast, which is really very small. Barrel length doesn’t matter much to springers – as long as they are the minimum length to develop adequate power.

    The 97 will so8und like your CF-X.


  4. I think the use of flat springs is not all that far in the past. I have a Beeman Tempest which had a flat spring when I bought it(used). Although it performed well enough, 15 years ago I replaced the spring with a new one from Beeman, which was also flat. Its been through dozens of cans of ammo and still performs almost like new. I think flat springs are a practical solution when there is little space and the most spring possible is wanted.



  5. BB Pelletier,

    I waned to tell you something.When they told me my grandpa died I was in venezuela in a school trip.They gave me the suprise that he is alive and gave me the remington genesis as a gift.I know the joke was a bad one but im glad I still have my grandpa!!!!!!!!!!

    Mr.Hunidr’s grandson(Michael Hunidr)

  6. as there seem to many educated people here can anybody help with info on a spring ? i have a bsa meteor that was bought for my 14 birthday (now 32), i had a new spring fitted a few years back but have never used it since. Anyway at 16meters i am getting 2 1/2 inch grouping which is really bad. i have tried the iron sight , redfield scope and a pecar berlin scope all made no difference. i have cleaned the barrel which was fine anyway i she was put to bed oiled. i have allso tried various pellets without any success. so the only thing i can think of is if the spring is moving to change the recoil. is this possible or am i overlooking something

  7. matt,

    Is your airgun a smoothbore or is it rifled? Meteors came both ways. If it’s a smoothbore, you may not get much more accuracy from it. If it’s rifled, it can do better.

    I want you to try something for me. First, drop three drops of silicone oil down the transfer port. That’s the hole behinds the barrel when you break the gun open. Then stand the gun on its butt for several hours to let the oil seep into the piston seal.

    You say you have all kinds of pellets. Here are some brands to try. RWS, H&N, Gamo and Crosman. The other brands, especially those made by Camil, are often not real precision.

    When you shoot, hold your gun as loose as you can. DO NOT grasp the forearm, buy lay it on the flat of your open palm. Don’t hold the butt tight to your shoulder and don’t grasp the pistol grip tight. The goal is to let your gun move as much as it wants to when it fires.

    Give that a try with any sights and see if it doesn’t improve things.


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