Resizing a mainspring
by B.B. Pelletier
I recently started writing more reports about the fundamental things that many airgunners would like to know. These are things you don’t have to know to shoot an air gun; but if you do know them, they can often help you recognize why your gun does what it does. You also don’t have to know what makes an automobile work; but when it stops working, sometimes knowing how it’s supposed to work can help you find the problem and fix it.
Today, I’m going to show you how a brand new mainspring is set to its operational length before installing it into a spring-piston airgun. It’s not necessary to do this; because, over time, the mainspring will automatically reduce in length after being in the spring gun under some tension and being compressed many times. If you do this first, it makes the job of spring installation easier in some cases.
When a coiled steel spring is wound, it ends up being longer than it will after some use. Robert Beeman wrote that the surface of the wire in the spring is oriented like strands that run along the axis of the spring wire and twisting the wire into a coil twists those strands unevenly.
As the spring is used (compressed and released), it shortens (the coils get closer together); but until they do, the spring coils will be separated a little farther than they should be. If the spring has many coils and each is separated a little more than it should be, the overall length of the spring can be up to several inches longer than it should be. Once it assumes its final length, it will still have the same strength as when it was longer. It will just be easier to work with.
Not all scragging is done as I’m describing it, so please focus on what I’m presenting here because it applies to spring-piston mainsprings. Scragging is the English term for what many spring makers call “set removal.” Set removal, or scragging, means compressing the spring to its solid length (i.e., all coils are touching, and the spring cannot be further compressed). Once it’s done, it allows the spring to operate at a higher load capacity without overstressing.
Spring-piston mainsprings are almost coil-bound (fully compressed) every time the gun is cocked. In fact, measuring the potential overall length of the compressed spring and determining whether it can be further compressed is an aspect of spring-gun tuning. Tuners often install washers that decrease the length that a mainspring has to compress, just to get the spring closer to its coil-bound state when the gun is cocked. They’re trying to wring out all the power the spring has to give when they do this.
This practice will put the operational length of the spring very close to the stress limit of the spring. If the spring is scragged, the stress limit is set at the spring’s fully compressed length; so if the spring never gets compressed quite that far during operation, it stays within the bounds for which it was made. It will, therefore, last a long time. While it’s impossible to compress a spring that has been scragged past its operational limit, you can compress it right up to the limit. I personally like staying on the safe side of that limit.
We learned how to measure the length of a compressed mainspring in the recent report titled The spring-piston power plant. You learned a technique that tuners use to determine the compressed length of the spring by measuring the wire diameter and counting the number of coils. The method I showed is not absolutely precise, but it will get you within a few hundredths of an inch, which is usually close enough. However, if you want to go all the way to coil-bound, you may have to adjust the length of the spring by adding washers to take up some of the space in which the spring exists. You can then fine-tune this length by removing some washers or even by thinning one of them, if you choose to go all the way to the coil-bound limit. If you stop just shy of that limit, your mainsprings will always fit inside your gun.
A scragging tool
In industry, scragging is usually done with a hydraulic press, but there’s an inexpensive way to do it. Just use a long piece of threaded rod with two washers and two nuts, and tighten the spring between the two washers. I showed that tool in the blog last week.
A simple tool for scragging coiled steel mainsprings.
When you tighten the nuts, you’ll notice that the spring really wants to twist. You have to hold both nuts with wrenches, and they will try to get away from you as you try to tighten them. This is spring torque, and sometimes it can be felt when certain spring-piston guns fire.
This is important!
Look at the photo below of the spring-piston powerplant. The compressed mainspring has to fit inside the piston and between the end of the piston and the base of the spring guide. That is the entire length in which the compressed mainspring must fit.
The mainspring fits inside the piston and goes back to where the trigger begins — minus the base thickness of the spring guide that isn’t visible in this picture.
What you cannot appreciate from the picture above is what the mainspring looks like when it’s compressed. But when you scrag the spring, the shape jumps out at you. The spring tries to squirm away from the force that compresses it.
Here is the mainspring before it’s in the scragging tool.
The spring has been installed.
The spring is compressed almost as far as it’ll go. Notice how it wants to curve away from the in-line force that compresses it? That is what happens inside your spring-piston airgun. Only the piston and spring guide prevent the spring from looking like this.
The mainspring is completely compressed. There’s still a slight curve to the spring, which the inside of the piston and the outside of the spring guide will remove. Leave the spring in this tool fully compressed for four hours and it will be scragged.
What’s happening inside your spring-piston gun is that the mainspring is being held compressed between the piston and the spring guide. The more room there is, the kinkier the spring will be when compressed, and the more room it will have to shudder and shake when it expands. You will feel that as vibration. Remove the extra room, and you remove the vibration.
A word on springs
There have been comments on this blog in the past about stretching springs to increase their power, and I want to address that now. You can’t stretch a spring and increase its power for more than a couple of uses. What will happen if you try is that you’ll cause the premature failure of the spring. One or more of the coils will collapse, resulting in a canted spring. The results will be greater vibration and less power.
The metallurgy of a coiled spring is very precise and has not been covered in this report. There are things like forming techniques, heat-treatments and stress relief that are part of what make a spring capable of doing what it does; and while it’s possible to change the characteristics of hot-formed springs, we use springs that are wound cold and cannot be changed. When they reach the end of their life, they’re done and are not suited for reclamation except through scrapping.
It’s important to get all the performance a spring has to offer since there’s no way of adding life at the end. Scragging normally does that; however, in the case of the spring inside a spring-piston air rifle, the scragging process can occur naturally during operation since the spring is nearly coil-bound every time it’s cocked. So, scragging doesn’t really add life to an airgun spring.
What it does is make the spring shorter so it’s easier to stuff inside the powerplant. But if you don’t want to do it and can get the spring into the gun anyway, you can either cock the rifle and leave it cocked for about four hours or else you can just use the gun normally and the scragging will take care of itself over time.
One final thought
Manufacturing being the imprecise process that it is, every airgun powerplant will have slightly different dimensions — even within identical models. So, the length of the spring that fits inside the powerplant will change from one gun to the next. That’s why manufactured airguns cannot be held to tolerances as close as those that are tuned by hand. Some of the “slop” we see in off-the-shelf spring-piston guns is there to account for these small differences. That’s why an individually tuned gun can usually be made smoother and more powerful than one made by a factory process.