BB’s ‘speriment: Part 1
This report covers:
- A coiled steel mainspring CANNOT be compressed farther than coil bound!
- Where the mainspring lives
- Removing vibration from a spring-piston airgun
- But wait…
- Some more on mainsprings
- One final thought
I have wanted to write this report for over a month. I have been quiet too long about what I believe is a major error that’s been allowed to stand without opposition. GunFun1 — today I’m picking on you. But not to worry — you come out the winner in the end.
Oh, and reader Not Rocket Surgery, in March of 2012, you asked me a question I said I couldn’t answer.
Once a spring has been scragged is there any reduction in cocking effort?
I responded by saying that I hadn’t scragged a mainspring in 10 years and I wasn’t about to, just to answer the question. Well, I’m not doing that, exactly, in this report, but what I am doing is closely associated. Today I start to answer your question, NRS. You are probably fighting battle bots by now, but for the record (not Rekord) BB does get around to it — sometimes.
And now for my pronouncement.
A coiled steel mainspring CANNOT be compressed farther than coil bound!
There, I have said it and now I will defend it. When a mainspring becomes coil bound, the steel coils are in contact with one another. For it to get any smaller, the steel in those coils would have to compress. Let’s look at some pictures.
This is as simple as it gets. A scragging tool is a length of threaded rod, two large washers and two nuts.
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.
Where the mainspring lives
Now let’s look at where the mainspring lives. That plays a big part in this discussion, as well.
This piston has been “caught” by the sear and is in the cocked position. The mainspring in this gun (there is no spring in there now) has this much space to occupy. Please notice that the outside of the spring is contained inside the piston and the inside of the spring is contained by the spring guide (removed here for a better view) and the piston rod. This is from an older Diana — a 34, as I recall.
A mainspring next to the scragging tool.
What you cannot appreciate from the pictures 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. Let’s see.
And here is the spring in the tool.
This next picture is very important. Look at it and then look back at the picture of the cocked powerplant above.
See how squiggly the mainspring gets when compressed? But when it’s inside the piston, as shown above, it can’t do this. There is nowhere for it to go! This spring is nearly compressed, but not quite.
And here is the mainspring completely compressed. If you want it to get any shorter it will take many tons of pressure to deform the steel spring coils. This 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 you have just seen is proof of why it is impossible to over-compress a coiled steel mainspring — just as a spring-driven clock can’t be overwound. Check it out online if you don’t believe me.
Removing vibration from a spring-piston airgun
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. That, my friends, is a big “secret” to tuning a spring-piston powerplant.
Here is where BB turns the tables and GunFun1 comes out on top! We have all read about GunFun1’s experiments with spring guns. Some time ago ( a few years?) he experimented with removing the mainspring preload from a springer and testing its velocity. And he took out a significant amount of spring without loosing anything. See, GF1? Old BB does pay attention!
Well, here is my idea. I have this mainspring that just came out of my Marksman model 70 breakbarrel. You will remember that it looked straight until I removed the spring guide and then the back of the spring got all hinky. Let’s look.
Well, looky, looky, who’s got the cookie?
What we have here, dear friends, is an opportunity. BB told you that the end cap unit came out of the rifle about two inches when all tension was off the spring. And here we see that the bent end of the mainspring is also about two inches long. It’s not exact, to be sure, but it’s close enough for BB’s ‘speriment. Can you guess what B.B. Pelletier is proposing to do?
That’s right! He wants to whack off the last two inches of this mainspring, then dress the end and stick it back into this rifle. Then he plans to test the velocity again.
Is the old mainspring good enough for another 20 years? Probably not. It is straight — er, but it’s not perfectly straight. So old BB will put her back in the rifle but then take her out after 10,000 shots, or so. Yeah — like that is ever going to happen! When this air rifle has 10,000 shots on it old BB Pelletier will be a footnote in the history of airguns. And FWB 124s will sell for more than a thousand dollars! But perhaps we will get to see a little of what GunFun1 was telling us about mainspring preload.
Some more on mainsprings
There have been comments on this blog in the past about stretching springs to increase their power, and I want to address that. You can’t stretch a spring and increase its power for more than a few 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. Maybe! But if BB’s ‘speriment does work we may all learn something more about coiled steel mainsprings.
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.
It’s important to get all the performance a spring has to offer since there’s no way of adding life when it reaches the end of its life — maybe. 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. However, since the spring we are talking about today has failed at just one place, I hope to determine whether there is some merit to what GunFun1 has told us.