Gearbox Basics

Leftyflip

Gearbox Basics

Right so, where do we start? Ok, got it. I’m Ger, otherwise known as Leftyflip or AEG Monkey, for the past year and a bit I’ve been tinkering with guns and fixing/upgrading them, infact I’ve been fixing my own since before I ever started skirmishing, but enough about me.
“How do you get that range? It’s ridiculous!” or “How do I build a good reliable gearbox?”, these are questions I’ve found myself getting asked quite a bit and I usually end up going into a very in-depth discussion about it, sorry guys, I do apologise!

In this article or guide even, I’ll go through the basics of gearboxes and internal parts, from air nozzles to sector chips, what does what, what’s necessary and how they work with the other parts to make your AEG shoot BB’s out of the other end. And at the same time, I’ll try and answer any questions you have.

The Gearbox Shell
Face it, with out this fairly vital piece of metal (and very occasionally, plastic), we wouldn’t get very far, would we? So what types or versions are there, and what guns do they fit into?

Version 1: Tokyo Marui’s first gearbox for their FAMAS, it’s a weird looking thing isn’t it? But hey, it’s been around for years and it works!


Version 2: The gearbox we all know and love, it's the one in your Armalites, MP5s(except the "K" variant), The new G&P Stoner LMG, SCARS, etc. Some people think it's great, other's do not. But this is what it looks like:

ICS also have a split design V2.

Version 3: This version can be found in AKs, G36s, the MP5K, AUGs and some others. It's argued as one of the best designs out there, for being robust and easy to work on. V3 gearboxes use a motor mount, not shown here. There are different types, for G36's and AKs.


Version 4: The only gun that uses this is the PSG-1, unless I've missed something... I've seen very little in the way of parts being advertised, but hey, I've yet to see one across my desk, that's something!


Version 5: This one is only found in the TM Uzi and I cannot find a decent picture of it (I'll come back to this one)

Version 6: The epic wincake gearbox in my opinion, it's a brilliant design, which G&G made a little better with the quick change spring funtion. unlike the other models the switch assembly is located on the outside, which makes it that bit easier to work on. This is found in the P90, F2000 and the Thompson M1A1.


Version 7: Found in the M14, and an absolute *bleeeeeep* to work on, these use specific V7 gears and pistons. Like the V6 the wiring and switch is located on the outside.


Version 8: This one is found in the TM Type 89 and has a mechanical burst feature in it, it's somewhat similar to the Version 2.
Image link won't work, will sort it ASAP.


Right that's it for now, next time I'll start to discuss gears, shims, air nozzles and all that rubbish.

Ger:)

Leftyflip

Well know we know what gearbox goes in what shooty stick, what makes everything happen? Even though it doesn't go whirrh, we'll start from the barrel end and work backwards!

The Inner Barrel: It's a shiny pipe thing right? Well, close, but there's more to the barrel than some people know. At the very basic end of things, in airsoft, we're dealing with what are essentially muskets. Okay, they won't burn your face off, but the principle is the same, smooth bore and a round ball (wahey!). The barrels bore (inner diameter) has a relation to fps, range and accuracy, i.e, the larger the inner diameter or bore, the more the BB is going to bounce around in the barrel and the more air escapes around it. So if you remove some of that space you can reduce the BB bouncing around and gain some more FPS, accuracy and range.

6.05, 6.04, 6.03 or 6.01, what do I go for? Something that comes up every so often. I'm a huge fan of 6.03mm TBBs (TightBore Barrels), I find they offer excellent accuracy, a good FPS boost and a nice range increase, I personally wouldn't see the need to go any lower, however, there's not reason to not try (just use good BBs). But it's not just about the bore either. There is another factor there too, internal finish, having a 6.01mm tightbore is all fine and dandy, unless the inner surface has more scratches than you can count. Small discrepancies like that can affect accuracy and range, as the air flow isn't consistent.

Then there's the Tanio Koba Twist barrels, to quote Evilrobotshane (I think), the best thing about them is that you can do the Bond opening credits with them. Well maybe, I'm not a fan, not for the price anyway. The thinking behind them is that the "rifling" creates a consistent cushion of air around the BB, stabilising it and stopping it from bouncing off the sides, although you get a big fps drop when you use them, as the internal diameter is quite large.

In short, a good tightbore (Madbull, Systema, Prommy, Vanaras), that is well cleaned and coupled with good BBs means you're on to a winner. But remember the tighter it is, the more cleaning it will require and higher quality BBs too.

TK Twist barrel theory: "Rifling" creates consistent air cushion to stop BB bouncing off the wall of barrel, increasing range and accuracy. However, when used with full auto, the cushion is compromised and it's just a normal boring barrel, from what I've seen in some cases.

Hop-up Rubber and Nub: Remember kids, always use a good quality rubber! No really, those stock black rubber hop-ups aren't exactly the best, they're generally designed for higher fps, ya know like crazy American 450fps and stuff (they're mad!). Anyway, what is the hop-up and how does it work?

The hop-up is the the mechanism that applies back-spin, which gives the BB a longer flight path and gets you some range. By the way kids - the hop-up decreases fps, so do us a favour don't lie to the marshall when we ask if it's off, cos we'll catch you out!

The hop-up rubber and nub is what you need to get the back spin. It also stops your mags contents spewing out of the barrel after you put a mag in. The rubber has a small nub on the inside of it, this goes over the slot on the barrel and protrudes down through it to get to the BB. The nub (a small cylinder) pushes down on the rubber to impart more (or less back) spin to the BB, giving you more or less range, and in doing so decreasing fps.

Different materials (silicone, rubber) will give different results, for example, the stock rubber hop-up in a CYMA AK will give you a decent range, a silicone hop-up in the same, will vastly improve that range.

Green thingy: hop-up rubber.
Black thingy: nub.
Brass ring: barrel centrey thing.


Hop Chambers: Plastic or metal, what's the advantage? Plastic hop-up chambers help the airseal remain consistent by added a bit of flex around the air nozzle (more on that during the week), the metal ones are more durable, but can wear out the air nozzle sometimes (some cheapy one's are a bit too coarse on the inside surfaces for my liking, and it can be quite difficult to fit a hop and barrel in sometimes, but the plastic ones make it a little easier.

Leftyflip

"310, 310, 315, 360, 310, 346"
"Errrr, that's a little odd..."
"The cylinder's been drilled"
"Aaaaaarggghhh!!!"
Yes it's time for all these bits and what they do, grab a cuppa and relax!

The Air nozzle: This lovely little plastic tube is what channels air from the cylinder into the hop chamber and makes the BB come out of the barrel at some sort of speed. So what makes a good air nozzle?

The first thing is to make sure you have the right nozzle for your AEG, a P90 air nozzle won't work in a FAL, etc. Some AEGs use the same air nozzles, ie Tavors use M4 air nozzles, as do a few others. If you don't have the right one, you'll get misfeeds (if it even feeds) and a low fps, as the seal isn't completed properly.

Bore-up parts are great, they vastly improve your air seal and consistency, which you can't argue with really can you?

Cylinder Head: It goes on the front end of the cylinder (figure that out with a non-ported cylinder :P) and channels the air into the air nozzle and into the hop, etc,etc. But what's better? Nylon or Metal? One o-ring or two?

Metal cylinder heads have an obviously harder surface than nylon, which means that impact is transfeered much more effectively and at the end of that impact is the gearbox shell. Nylon cylinder heads help to dissipate the impact and prolong the life of the gearbox and piston/piston head. This is because of the softer surface. Nylon is also quieter than metal when impact is involved, so by using nylon cylinder heads and piston heads you can get a slightly quieter AEG.

I personally prefer nylon or polymide (G&P) parts and once again when it comes to these parts, bore-up is king.

One o-ring or two? Once o-ring I've found is quite enough for the Irish limit, the two o-ring cylinder heads are usually found in guns with higher a higher FPS to help keep the seal and focus the air out of the hole, not around the side. But at the same time, I've toyed aroound with the two o-ring heads to see how consistent I can get an AEG, the answer is very - if you do it right!

Piston Head: This lovely little disc is what gives you the FPS, without it the piston itself would be off centre, and would just pointless slap around the cylinder - a bit of a fail!

Metal or plastic, well as per the above for the cylinder heads, I prefer plastic ones, for the same reasons.

Bearing piston heads are great, they stop the spring twisting and snapping and can improve consistency through the same. There's some debate over running both a bearing spring guide and piston head, or just one, I run both when I can which means I can use a weaker spring and improve ROF.

Leftyflip

Spring Guide: This has several purposes, it keeps the spring in one place, hence the name. On some gearboxes (Version 2) it allows the stock to be attached to the reciever and it helps boost FPS.

Bearing or plain Jane, plastic or metal? I tend to use metal bearing one's when I can, as I like the fact that the spring will have an awful hard time catching and twisting, which eventually leads to breaking. And metal because it'll stand up to a lot more abuse than the plastic one's, which I've seen snap where the main guide part flares out and locates in the gearbox.

An aftermarket spring guide can increase your FPS by twenty or so depending, especially bearing ones as they reduce the amount of space between the spring and piston. Unlike some of the other internals, these are gearbox specific, ie, you must use a V2 guide in a V2 box, etc, etc.

The Spring: Well lets face it, ye'd be fecked without one, it's the most important part (well almost), as you well know, it's what gives you the FPS and affects ROF and wear on parts (the shell, piston and gears). Why are some springs better than others? What's a linear and non-linear spring?

Some springs are better than others due to the use of better material and manufacturing standards, along with strict quality control and testing. I've found that with cheaper springs (Element in particular) the output you get is far from what it should. I refer to the last time I used one, I had been asked by a mate to take a look at his KWA M4A1, it wouldn't fire and was blowing fuses, sorted that out easily enough and realised that the cylinder was drilled. I replaced it and put in an Element M90. Chrono readings: 390, 400, 395, 405, 360 (ish). Put in a Guarder M85 and got 310, 311, 315, 310, 310. As you can see the slightly more expensive spring not only put out what it should have, but was far more consistent.

For the sake of €5 (if even in some cases) you can see the difference. The cheaper spring was all over the place and nowhere near where it should've been. The more expensive, well that speaks for itself.

Linear Vs. Non-linear
In short, a non-linear spring is easier for the gearbox to draw back, than a linear one, it's to do with Hooke's Law.
There's a very good write-up here, which can explain it far better than I can

Leftyflip

Oh dear, I think I'm long overdue an expansion to this article...
Where was I? Something about springs and pistons and things...

The Piston: The piston itself is what makes everything work the way it should, no piston means no air being pushed through the cylinder, and just lets the gears spin wildly. I'm a big fan of polycarbonate pistons, for the reason that I'd rather that went instead of a €50-€60 gearset, but after having a chat with Thermo on Sunday, there is also another side to it, especially when using high ROF set-ups, pistons will get a lot more wear and a €30 metal piston will last longer than the €10 polycarbonate counterpart, so if you've to replace a piston every month and a gearset every 6 months, maybe it's cheaper to replace a metal piston every 3 or 4 months and a gearset every 6 months, but hey, to each their own.

The piston is engaged by the sector gear and drawn towards the rear of the gearbox - compressing the spring, when it reaches it's rearward limit, it is released and it shoots forward, pushing air out of the nozzle proppelling the BB along with it. There are a number of mods you can do to a piston to increase ROF. These are the AOE (Angle Of Engagement) Mod, Short stroking (removing teeth from the piston and sector gear) and swiss-cheesing it - drilling a load of holes in it). Lefty reccomends only the AOE mod, as it increases ROF, but that's it. Short-stroking means you've got to use a stronger spring to counteract the FPS drop that is involved with it. Swiss-Cheesing the piston in my opinion makes the structure weaker and more prone to breakages.

Bearings/Bushings/Shims: Ah yes, bearings and bushings, they keep the gears in place and stop the gearbox going KABOOM! 6mm, 7mm, 8mm and 9mm are the usual sizes, with 9mm being less common than the rest.
I'm personally a fan of 7mm as they're big enough to not collapse in high ROF set-ups (for a decent amount of time) but not big enough to let dirt and grit into the gaps between the ball bearings and causing them to lock up and collapse. 6mm bushings can't really withstand high ROF or high-torque, but for casual "I just want it to work" they're not too bad.
When a bearing collapses you'll start hearing grinding noises, followed by a "whirrrrr!!!" usually associated with a stripped gear. No fun!

Shims help keep the gear teeth level and to mesh properly, making the gearbox quieter and reducing wear. They're usually between 0.1mm and 0.5mm thick and made of copper or steel. They drop onto the gear axles and take up any space left between the gear and bearing/bushing, stopping any lateral movement. However if you shim a gearbox too tight, it will either lock up, reduce ROF or cause your motor to burn out, etc. So make sure you do it right.