The mainspring - why it can fail, and how to pre­vent it

Airgun World - - Technical Airgun -

With one ex­cep­tion, spring break­ages are very rare, so let’s get that ex­cep­tion out of the way first; it oc­curs when a spring coil is de­lib­er­ately heated and col­lapsed – or the end cut and flat­tened – and ei­ther over­heated or, sin of sins, quenched, ei­ther of which al­ters the spring’s met­al­lurgy and makes it brit­tle and al­most guar­an­teed to break at some point. Other than that, I can­not re­mem­ber the last time I had a mainspring break, and that’s in 50 years of shoot­ing air­guns, un­til a few days ago.

Around eight years ago, Mike Wright and I com­mis­sioned a small batch of main­springs in­tended pri­mar­ily for our HW95s, and they were the po­lar op­po­site of the springs that have been fash­ion­able among tuners since the 1980s, when lead­ing ri­fles like the Fein­werk­bau Sport and Weihrauch HW77 had their orig­i­nal long, thin springs swapped for short stiff springs by tuners. Al­though not as soft, and nowhere near as long as the springs fit­ted by Fein­werk­bau and Weihrauch, our batch of 12 main­springs were a sort of half­way house be­tween the man­u­fac­tur­ers’ and tuners’ springs.

This was not our first batch of springs; we had started by blindly fol­low­ing fash­ion and order­ing springs wound from 3.25mm di­am­e­ter wire, which pro­duced a stiff spring and a ‘quick’ shot sen­sa­tion, but when I took my first ten­ta­tive steps to­wards record­ing the re­coil cy­cle, we re­alised that they wasted a lot of en­ergy, man­i­fested as pis­ton bounce and con­se­quent re­coil surge. The key to in­creas­ing en­ergy ef­fi­ciency and re­duc­ing pis­ton bounce and surge was to go for thin­ner sec­tion wire, and more length of wire in the form of more coils.

Our sec­ond batch of springs were 19.1mm coil di­am­e­ter, 2.9mm wire di­am­e­ter, and had 36 coils, which was longer than we thought we needed, but which al­lowed us to cut the springs to a suit­able length for each cal­i­bre. At the full 36 coils, the springs gave around 63mm of preload in the HW95, and within a

whisker of the le­gal limit in my .177 HW95 – and would have been over in larger cal­i­bres – so the springs were short­ened by de­gree un­til we were safely le­gal at just over 11 ft.lbs., which was with 34 coils in .177, and 33 in .20 and .22.


My six springs went into a va­ri­ety of ri­fles to as­sess their suit­abil­ity, all bar two be­ing fur­ther re­duced in length un­til they be­came too short to be of use; they were the lucky ones, be­cause the two 34 coil springs were put through spring hell, be­ing sub­jected to dry-fir­ing, det­o­na­tion and diesel­ing, all de­lib­er­ate, and all in the name of science. Fol­low­ing a cou­ple of years of such abuse, one was re­tired to my .177 HW95, in which it pow­ered at least 18,000 shots, prob­a­bly many more, and was then re-de­ployed, along with a light­weight pis­ton and a pis­ton ex­ten­sion that took the stroke down to 77.5mm, in my HW98, which was then loaned to a friend for use in HFT.

Fol­low­ing the re­turn of the HW98 a year or so later, the pis­ton and spring found their way back into my .177 HW95. Muz­zle en­ergy was down from that in the HW98, thanks to the 95’s shorter bar­rel, but at 10.6 ft.lbs., it was not re­ally worth both­er­ing about, so the ri­fle was put into com­mis­sion.

Fol­low­ing a zero ses­sion ne­ces­si­tated by a change of scope, the pel­let POI dropped over the course of per­haps two dozen shots by an inch or so at 25 yards, so I put the ri­fle over the chrono and found 50fps had been wiped

“us­ing a low-stress spring means few if any war­ranty re­pairs”

off the muz­zle ve­loc­ity, and the most likely rea­son for such a sud­den big drop in ve­loc­ity was a bro­ken mainspring.


On strip­ping the ri­fle, I found that preload had re­duced to just 34mm, so I knew there was a spring break­age which, un­usu­ally, turned out to be near the front end of the spring. Break­ages are more usu­ally found about an inch from the rear end, and are caused by re­peated wave dis­place­ment col­li­sions, in which the rear of the spring is still ex­pand­ing as a wave of com­pressed coils hits from the front, greatly stress­ing the point at which the wave fronts col­lide. That ob­vi­ously was not the cause of this front end break­age, so what was?


When fit­ting the spring in my HW95, I’d mea­sured the preload at 53mm, yet the other 34 coil spring, which has seen rather less use, gave 55mm, so the for­mer ap­pears to have lost 2mm in length through creep, com­pared to even a well-used spring, which from ex­tended com­pres­sion tests with other springs from the same batch, sug­gests the frac­ture could be due to a com­bi­na­tion of ex­treme old age and a very hard life.

An­other pos­si­bil­ity is that, as the spring lost length through creep, it reached a point at which the com­pres­sion stroke al­tered the tim­ing of wave dis­place­ment col­li­sions to co­in­cide with the point of break­age, pos­si­bly in con­cert with the spring’s age and health. Let’s look at the mech­a­nism that causes the wire to frac­ture.


What I think hap­pens is this: When a coil spring is com­pressed, the en­tire length of the wire comes un­der tor­sional shear stress, and when the pis­ton bounces and the ex­panded front end of the spring starts to com­press, it also is sub­jected to tor­sional stress, which means it’s ef­fec­tively in tor­sion in the op­po­site direc­tion to that of the still ex­pand­ing rear of the spring. This is fine un­til a wave from the ex­pand­ing rear end of the wire and a wave from the com­press­ing front end meet, and where the two waves meet, the two op­pos­ing forces place huge strain on the wire, and it can frac­ture.

So why doesn’t this hap­pen ev­ery shot, why is it such rare oc­cur­rence, and why are some springs in some ri­fles and tunes more likely to frac­ture than oth­ers.

The springs in most fac­tory air ri­fles are spec­i­fied not to be highly stressed dur­ing the shot cy­cle, and that’s in­ten­tional be­cause, just as the man­u­fac­turer uses enough grease in­ter­nally for the ri­fle to last many years with­out at­ten­tion – which most tuners deem to be way too much – us­ing a low-stress spring means few if any war­ranty re­pairs, which all man­u­fac­tur­ers wish to avoid at all costs, be­cause war­ranty re­pairs eat into prof­its. In all but very aged fac­tory stan­dard spring air­guns, spring frac­tures are very rare, and on the very few oc­ca­sions they do oc­cur, it’s nor­mally due to a spring man­u­fac­tur­ing fault.

“If you no­tice any of the symp­toms of a bro­ken mainspring, don’t use the ri­fle”


There are other pos­si­ble causes of spring frac­ture with fac­tory springs, but they need some­thing dras­tic to go wrong, such as a det­o­na­tion that causes fe­ro­cious pis­ton bounce, or per­haps one too many dry fires. Short of pour­ing a highly volatile liq­uid down the trans­fer port, or se­rial dry fir­ing, spring frac­tures in fac­tory stan­dard springers are ex­tremely rare.

The springs used in mod­i­fied air­guns are an­other mat­ter, be­cause tuners, both am­a­teur and pro­fes­sional, seem more con­cerned with ef­fect­ing a change in the shot cy­cle that’s per­ceived as ben­e­fi­cial, or sim­ply rais­ing the muz­zle en­ergy, than they are with whether or not the spring, or any other mod­i­fied com­po­nent, might fail two, three or more years down the line. It’s worth not­ing that the mantra amongst many tuners is that ‘less is more’ when it comes to lu­bri­ca­tion; again, the fo­cus is more on what the newly mod­i­fied ri­fles does now, rather than in the fu­ture when the grease runs out.


There’s no hard ev­i­dence to sup­port this, but my gut feel­ing is that the more ex­treme the tune, the greater the like­li­hood of spring frac­ture. Many tuners to­day seem ob­sessed with cre­at­ing min­i­mal sight pic­ture dis­rup­tion, so they re­duce re­coil by short­en­ing the com­pres­sion stroke and/or re­duc­ing cylin­der di­am­e­ter, in ei­ther case ne­ces­si­tat­ing higher peak cylin­der tem­per­a­tures and pres­sures, and ask­ing ever more from the mainspring – some­thing has to give, and it seems that some­thing is of­ten the mainspring.

An­other tun­ing mod­i­fi­ca­tion that seems to in­crease the risk of spring break­age is to overdo light­en­ing the pis­ton, be­cause the lost mass has to be re­placed by spring force, and the mass of the pis­ton helps to slow pis­ton bounce ac­cel­er­a­tion, which oth­er­wise cre­ates a fierce rear­ward dis­place­ment wave in the spring.


A few highly mod­i­fied re­duced cylin­der bore ri­fles have re­port­edly suf­fered mul­ti­ple spring break­ages, ac­com­pa­nied by rear spring guide break­ages, in­cred­i­bly, even part­ing two-piece steel guides’ tubes from their bases, and the con­sen­sus seems to be that too lit­tle preload is al­low­ing the rear end of the mainspring to lift away from the guide base, then ham­mer­ing it back on the back block and caus­ing the break­ages. If that rea­son­ing is cor­rect, it’s be­cause re­duced cylin­der bores mean re­duced pis­ton bores, which means re­duced di­am­e­ter main­springs, and re­duced di­am­e­ter main­springs are stiffer main­springs, so you can­not use much in the way of preload.


The first sign that a mainspring has frac­tured of­ten comes when you break the bar­rel or un­latch the un­der­lever and they drop fur­ther than nor­mal be­fore there is any spring pres­sure. If you try to cock the ri­fle, the chances are that it will feel in­cred­i­bly rough as the frac­tured ends grate against the pis­ton side or spring guide, though that does not al­ways hap­pen, if the break is in the sec­tions of spring sur­round­ing the spring guide or top hat, and the two ends screw into one an­other, as hap­pened with my spring break­age.

If you load the ri­fle with­out notic­ing any­thing amiss be­cause the cut ends are in­ter­twined, the next symp­toms are a dras­tic soft­en­ing of the shot cy­cle, pos­si­bly ac­com­pa­nied by a lot of spring noise, and a large down­ward shift in pel­let point of im­pact.

If you no­tice any of the symp­toms of a bro­ken mainspring, don’t use the ri­fle un­til it has been stripped and checked, be­cause the ends of a bro­ken spring can cause very nasty in­ter­nal dam­age.


My HW95 now has the re­main­ing 34-coil spring in­side, and is shoot­ing very nicely in­deed at 10.6 ft.lbs. with H&N Field Tar­get Tro­phy, with the op­tion to in­crease that, whilst re­duc­ing pis­ton bounce, by adding preload, if I feel in­clined. For the present, I think I’ll do some ac­cu­racy test­ing to make sure all is well, and then just en­joy shoot­ing it. I

My next move will be to as­sess ac­cu­racy from the bench.

The springs were short­ened to 34 coils for .177 (top) and 33 coils for .20 and .22 (below).

I can­not ever re­mem­ber a spring break­ing at the front end.

The frac­tured spring was the one I’d left coil bound for 1,000 hours back in 2012. It’s had a hard life, all right!

The wire frac­tures at the point it is sub­ject to tor­sional shear in op­pos­ing di­rec­tions.

The short­ened pis­ton stroke might have con­trib­uted to the spring fail­ure.

With a static spring force on the pis­ton of just 132 lbf, the cock­ing ef­fort is low.

The surge amounts to 20% of the re­coil; more preload would re­duce that and in­crease muz­zle en­ergy.

A de­lib­er­ate det­o­na­tion com­pressed the spring by 50mm in seven mil­lisec­onds, easily enough to frac­ture a spring.

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