Broken Springs and Air Rifles

It could. I would try to cut it without generating much heat, perhaps with coolant.

I would suspect it has more to do with the closed ends on the springs, and their being work hardend more so than their centrally coiled counterparts.

The Russian DshK 12.7mm heavy machine gun had a interesting recoil spring to counter this problem, along with nullifying any resonance. Three wires were braided into one "cable" for lack of a better term, and this cable was wound into a coil, forming the recoil spring.

Right ingenious, I'd think.

Allen

When you pull a spring, the tension is not even across the spring's length. It's much higher at one end than the other. I suspect that this is why your springs break near the end.

Do you have a picture? Spring failure can occur for a variety of reasons, including but not limited to corrosion, surface defects, inclusions or other defects in the spring material, excessive local stress, and defects in manufacturing. If you really want to know why your springs are breaking, the Spring Manufacturers' Institute provides a Failure Analysis Service.

http://www.smihq.org/public/services/failureanalysis.html

i might suspect the one or two " extra " pumps each round might be affecting spring life.....what shape is the spring..? conical base ? or ( i forget the proper nomenclature right now } like in a ball point pen?

how much " gap " between cylinder walls and spring? could the spring be galling the cylinder walls?

Is it the end that moves suddenly, or the end that stays relatively still or can it be either end?

Only answer exactly, if you are 100% sure exactly.....anything else maybe misleading....

I could well imagine that its the "moving" end of the spring breaking, due to stresses generated by coming to a sudden stop.

If I am right and you know roughly how many shots you have fired, or the length of time its in use before breaking, then you should turn it "end for end" at about 75% of its "life". Assuming of course that both ends are identical!

This may extend its life up to 1.5 times or so of the present......

This is just an idea based on some thoughts generated......not a factual knowledge bank......I used to play with air rifles many years ago, hunting for the pot as I had a huge garden and a lot of pheasants, pigeons, partridges and sometimes rabbits.

Tuning them was an (illegal) hobby of mine.....

Houses were too close for a cartridge rifle or even a shotgun (which I had but I hate biting on fur/feather covered pellets that the cook missed!).

The rifles concerned are slowly rusting away in the cellar nowadays.......sadly!!

I am interested to see what the final conclusions are.......

"...hunting for the pot as I had a huge garden...an (illegal) hobby of mine....."

Well, I never...

LOL! A friend of mine did the same thing to his eyebrow with my gun! I told him the scope had about 3" of eye relief and not to put his face too close. Did he listen....No! He ended up with blood gushing from his brow!

I've heard that the best way to port a gun is to step the hole so that it creates turbulence which allows the air to move faster! I've not tried it, but it would be interesting to get views on this!

As to the idea of the spring breaking partly due to the shock waves traveling up and down the spring after it has been fired causing nodes and anti.nodes, that sounds quite possible! Maybe a thicker tar would help to dampen the vibrations down a bit!

My scope had also several inches of eye relief, it was quite easy to sort out! provided you at least knew what you were doing!!!

Police marksman have no idea........!!!

The idea of the porting with steps could be a good idea, I actually polished mine.........a golf ball finish might have been even better.......but I never thought of anything like that.......

I was planning eventually to fit a cone shaped plug to the end of the piston, to mostly fit the "cone" I had cut for a transfer port, thereby reducing the amount of air remaining even further. But I never did get around to it........I wanted to put a couple of "O" rings (or something similar) on it to cushion it a bit......and stop any metal to metal contact.

The pressures after my adaption were actually probably less as it stopped the gun "Dieseling" on oil completely.....

I think mainly because the pressure was now acting on the pellet and not just building up inside the cylinder, with only a tiny original transfer port of less that 0.1" (mine was 0.2")......my cone plug would have caused dieseling again I am sure after freh oiling.......it would have also increased the weight of the piston dramatically......also increasing energy in the pellet I feel.......and the recoil of course!

After re-reading, it looks like I was trying to get a diesel action, no I was not, but after a fresh oiling, you need to "shoot off" the excess oil each time......I just noticed that it did not happen any more after opening the transfer port......

The shock waves to the spring are only to be expected otherwise.....but how to erase them without slowing things down!!! That is the problem......

Ok, to answer a few questions! This spring is from a break barrel, ie one pump, one shot! My last spring that broke, broke at each end! The spring is 210mm long by 18mm dia with the breaks at 25mm at 20mm. The wire is 3mm in dia. When the spring was in the cylinder, there was about a 1mm gap between it and the wall all the way round! This has caused cant issues on this spring. Looking at the general wear pattern on the outside of the spring, it has very slight rubbing along it's outside surface! I've had about 5000 shots with it so it's done well, maybe it's just a repetitive strain issue but I was wondering why break always in this position, close to it's ends and not in the middle! I will try to get a photo but I'm off shooting my PCP now as it's a nice day! Cheers all for the input.

it might be that the spring is the " weak link" in the chain of events...easier to replace than say .. the pump seal?...or the piston arm?... just thoughts on the subject is all

Special Interest Model Books (UK), Workshop Practice series have an excellent book on spring design and spring manufacture.

From it you will work out why springs fail in the turns at one or both ends. Quite simply the turns at fixed and moving ends are the first to transmit force upon compression or extension and then the force moves to the turns closer to the centre. In equilibruim the the force equalises but while being charged (and possibly discharged) the spring is moving and the force is not constant.

The book mentioned above explains overloading, fatigue and a host of other parameters. Mine cost me about $20 from Australian Model Engineering. But they are available through a large variety of book shops.

So many uncertainties in this problem, it will be hard to say if any one single factor causes the location of the failure. While the OP of a cutting grind seems plausible, I would wonder why a spring manufacturer would cut or shape a spring after hardening. One possibility not mentioned, in the compression of the spring or in it's free release there maybe some minor pinching or whiplash impact damage that repeatedly occurs at this location on the spring. Over a few thousand pumps this might fatigue the location sufficiently.

Another idea comes to my mind now, this might actually be a planned failure point. Not in the purely greedy planned obsolescence, buy a new air rifle sense. But, a break at this point causes a safe failure for the user. Then maybe a manufacturer does deliberately effect this location on the spring.

(This should be a fun discussion. )

I'm thinking it might be due to compression waves (i.e. resonance or non-resonance). Just a thought.

I think standardsguy is on the right track, but a little more needs to be added. Nodes and anti-nodes. They go hand in hand with resonance.

When the compression is released the spring vibrates a frequency. (resonance) the wave will run back and forth (end to end) until the energy dissipates with friction. The anti-nodes that this energy dissipates will causes a rise in temperature. This will cause a fatigue point in your spring, similar to bending a piece of wire back and forth until it breaks. I'm thinking.

A way to get around this would be to apply a damping device somehow to the spring. This will change the resonance frequency and anti-node position. Another way, buy a box of springs and enjoy.

Cryogenically treat those springs. Stress relief, the spring wire is stressed in the manufacturing of the spring, that's why you see the failure towards the ends and not the middle(not wholly the reason but a big darn part of it.). Retained Austinite is converted to martinsite, the spring is strengthened. Basically you will get a tougher piece of steel, springs will retain their "memory" longer. Look into it. dkimmel@300below.com.

Thank you Dave, I will ask google if I can do that in house as there is very little engineering down here in Southern Spain! If I can do it with a blow torch and some bricks or a bread oven then I may be in luck!!

OK, Google says I need a seriously cold freezer! I'll buy a few more springs and change them if and when!

You are more than welcome. It's just a suggestion. By the way, how is the great nation of Spain doing today? One of the great things about this internet is getting to know and talk with folks like yourself. If you or anybody there needs some more information or has questions, please get ahold of me. Thank you! dkimmel@300below.com

I'm just sayin' when a spring is stretched, the force on it is not even, and increases toward one end - it's one of the first "real-world" examples that they teach you in calculus class--stress is calculated as a derivative. All springs (and cables) usually, and naturally, break toward the ends.

I'll give you a GA for this. The simple concise explanation and reference to a basic education course makes it worthy in my book. Possibly whatever treatments done to mitigate this effect on the hook end migrates the break from the tip of the spring.

Hum, I think these are compression type springs used in this type of air gun.

Could this be due to end effects?

When the spring is tensioned, the end where the load is applied (each end) has metal in a combination of torsion and bending.

This settles down after a turn or two but at some point in between, the combined stresses are at a maximum.

Springs are highly stressed to start with, and the effect of this is to speed up fatigue failure.

I'm going entirely from memory here of machine design lectures over 40 years ago, so if the explanation is fuzzy, so is my memory, as I've never used that part of the course.