Gearbox making twanging noise

[alex90]

so iv built gearbox myself and shimmed and got the engament angle so picks up piston in the 12oclock position

and fitted it fired few test shots and making a funny noise from what sounds like the main spring after the piston have slammed forward

this is the spec

18.1 gears and shimmed perfectly

g&g etu

dream army high torque motor

tomtac metal rack piston

pom piston head

WII tech cylinder

SHS cylinder head and nozzle

ASG ultimate tappet

ASG ultimate gearbox shell

modify 8mm ceramic bearings

standard spring guide (non bearing)

guarder SP 95 spring    

and im using 11.1 25c lipo

any idea what the noise could be or if I need to worry about it the gun cycles fine and sound good until it twangs

thanks alex

 

[Sitting Duck]

the twang can sometimes be down to the spring & the metal spring guides

the coils of the spring resonate against the metal spring guide

doesn't happen it every build but this twang happens usually now & then on some builds

saw a post on airsoft mechanics and they suggested if it drove you nutz a heatshrink sleeve over the guide

though this can start to fray or break up in use - however somebody posted about a special h/s tube that was self adhesive

I have lived with it on the odd gun myself as it happens say once in 3 or 4 shots only on the odd build

to avoid this - just SHOOT MORE

Some - very few guns come with a robust plastic bearing spring guide - D-Boys do and the guide is nice

(box is ok but spring guide & even stock piston head is quite good)

Suppose you might be able to find a very very thin walled pipe and epoxy/glue a short length on

that might be a better option but would really need to thin walled for it to function with spring slipping past on compression

There was a post on a Krytac doing this but we tend to think it might also be the AR latch not functioning 101%

Meaning it didn't catch correctly everytime on the 4 lugs of the bevel gear - quite lame having just 4 lugs

Plus this is a Krytac so you'd expect it to be 101% (ish) pukka for the price

I mean check the AR latch & its spring is operating correctly

but I feel it might be the metal spring guide & spring causing the twanging

 

[alex90]

was playing around last night and it definitely coming from spring/guide area and its almost very shot/cycle of the box it happens

but this is running without the top receiver on so might try it with that on or maybe just live with it and the AR latch is a 5 lug style but the spring not 100% but does function but not to worried about that as the motor is pretty torquey and the etu SHOULD finish every rotation

 

[alex90]

do you think a bearing spring guide would have any effect on the sound  

 

[Sitting Duck]

do you think a bearing spring guide would have any effect on the sound  

only to change it to more a TWOOOONNNNNNGGGG than a TWAAAANNNNNGGGGG - lol

Seriously - it "might" but same time it "might not"

However depending on what your gun shoots at a bearing spring guide will compress spring a smidge more

The bearings will add about 4mm compression - or just over a tooth - this will be about 15fps

So DON'T just chuck in bearing spring guide coz it could place you over site limit if you are close to it already

if you are at 320fps & site is 350 yeah nowt to lose if you feel it is driving you nutz.

The sound in my mind is the metal on metal that might resonate a little as spring settles

Think also we in UK might suffer this more than US as they have a higher fps so stronger spring may settle quickly

but this resonate theory crap of mine is just that.....

You could try a bearing spring guide if fps is a bit low & safe to proceed but could still do it in the end

However with old spring guide you could look into plastic or heatshrink or tape to just take off the metal resonance sound

Sorry I can't give you a full final solution, I haven't found a complete cure/answer to this myself if I'm honest...

Oooh edit - just had a thought in desperation...

Could also try turning the spring round the other way too - might help or make it worse

Yes irregular springs usually go with tighter coils at the back

have found that if you fit an irregular spring it can vary by about 10fps - depending on the direction

(some have said the difference is greater but I have chucked in various springs

in a quick change APS UAR gun and these are my general findings on many occasions)

this might also alter the resonance risk factor too

some guns use regular springs - spring looks the exact same at both ends

Irregular springs are supposed to be fitted tight coils on spring guide to assist it easier piston retraction

But tbh a m100 or so spring isn't going to bust a gut/gun if fitted the other way around

Might be worth a try perhaps either before or after if twang/twong still exists

 

[alex90]

was never sure what way the spring went round so could well be the wrong way round and im unsure of fps yet as not tested

but I think in this case might be worth just sucking it up and leave alone for a bit and maybe in time might settle down once everything had a chance to bed in

but thanks for help

 

[Sitting Duck]

was never sure what way the spring went round so could well be the wrong way round and im unsure of fps yet as not tested

but I think in this case might be worth just sucking it up and leave alone for a bit and maybe in time might settle down once everything had a chance to bed in

but thanks for help

see how it goes, maybe order a bearing spring guide as/when the fps drops a bit

swap the spring and bearing spring guide around if it still drives you nutz in a while

obviously if the fps really drops over time sub 300fps then you need a new spring

The bearing spring guide Vs std guide is only about 15-20fps max

I'm still kind of stabbing at stuff - if I was that good I would know for sure how to eliminate it completely in every build

I'm 90% certain twanging is from the spring/guide resonance and it "might" be more common

on diameter of spring/guide clearance and/or the orientation or how the damn spring is fitted plus other crap

BUT - this is still me guessing/stabbing at crap if I'm honest

Springs do fade a bit over time but so do the seals - so yup best live with it for a while if possible

then have another go at it when it really needs it re-doing spring/guide seals n stuff like AoE

Leave it a while - read up some stuff especially the horror stories & disasters/mistakes others make

Then hopefully after that you will have gained more insight from other people's failures

Your own work benefits from gaining wisdom from others screw ups than your own

(wish I had done this a little bit more. Our screw ups are the best way but the most costly)

Hey - at least your box went back together - my first one didn't

well in my defense it was a crap plastic box with metal gears (SRC Dragon- horrible thing)

But didn't help it exploding across the room and never found the AR spring - WTF was that thing ???

plastic threads all stripped - yup learned you DO need the AR latch that I couldn't find the spring to

sounded $hite and ran hot blowing fuse from my first attempt at shimming - yeah that will do I thought

A real work of art - yeah think we will buy a new box son...

That iffy gun that was playing up - yeah it is totally f*cked now boy

I'll shut up & yeah - just see how it goes for now sounds about right