Broken Gear Teeth

[Cokeman]

I had purchased a complete version 2 mechbox to install into an Aftermath Knight M4 gun. For two purposes, as a learning experience and also to have a backup gun that shoots faster than the stock 290 fps.

I also got a G&P M120 High speed motor and a 9.6 v 1600 mAh battery. The orginal install seemed to be fine and I shot about 100 BBs through it at home. Then when I took it to a game (loan to a friend), it jammed after about 30 shots when we were just trying to set the hopup. When you pulled the trigger, you can hear a little click (sounded like something was trying to move).

When I got home, I took the motor and grip off, exposing the lower end of the mech box, I unjammed it by releasing the anti-reversal latch. Re-assembled and dry-fired the gun and it worked. Put it on auto and gave it a good burst. It jammed after about 3 seconds. I unjammed it again. Tried firing again. This time, I think it fired twice on full auto (two short bursts, less than a second each time), then the gears just spun freely - well, it made a funny sound - but didn't pull back the spring.

When I took the mechbox apart, I found that these two gears had broken teeth (see attached picture). Does anybody have any ideas why these teeth broke?

I'm planning on getting a new set of gears/shims and fixing the mechbox. I'd like to prevent the same mistake if possible.

P.S. I did a search on this website for broken gears, stripped gears and didn't anything helpful.

[Ross]

maybe a bad shim job?

[Boyso]

Or could be anything else. It's probably firing full auto after a jam that caused the teeth to break.

[kalnaren]

A few thoughts:

-Poor shimming
-Mismatched spring. If you're going for a high speed setup you need to match the spring strength.
-Did you re-grease the mechbox? Stock aftermath guns are greased with a terrible goop that probably causes more resistance -and thus stress- than actual anti-friction function.
-Shitty gears. Aftermath guns don't have the highest quality components. You're gears simply may have sucked.

I still think bad shimming -combined with shitty gears and poor greasing- is the most likely culprit.

[Cokeman]

The mechbox was new - it came with a M120 spring (I think).

I had taken it apart a few times - so, maybe I didn't put it back together with the shims in the proper place. However, even when new, it jammed (spring partially compressed) and I had to take it apart to unjam it.

So, it seems that that everybody thinks it is because of poor shimming. I will pay attention to shimming when I attempt my next repair with a new set of gears and shims.

I found this website for how to shim
http://pageproducer.arczip.com/daedalus03/shim.html
Is this OK?

[Cokeman]

The mechbox in question is a J.G. Complete Version 2 Mechbox (Rear Wired for M16s)

Product Description: Brand new Jing Gong Version 2 mechbox fitted with metal bushings and an upgraded spring. These are rated to shoot between 390-400fps but have been known to vary from 370-430fps on the rare occasion.

If this mechbox is paired with a G&P M120 High speed motor, would it be a problem?

Originally Posted by kalnaren

A few thoughts:

-Poor shimming
-Mismatched spring. If you're going for a high speed setup you need to match the spring strength.
-Did you re-grease the mechbox? Stock aftermath guns are greased with a terrible goop that probably causes more resistance -and thus stress- than actual anti-friction function.
-Shitty gears. Aftermath guns don't have the highest quality components. You're gears simply may have sucked.

I still think bad shimming -combined with shitty gears and poor greasing- is the most likely culprit.

[kalnaren]

Sorry, I missed that it was a JG mechbox, though what I said about shimming/greasing still generally applies.

I don't know enough about building higher speed setups to say for certain that the combination of components was likely to cause damage. JG mechboxes are pretty good (for the price), but their stock components can be weak. I would suspect upgrading the motor and running a 9.6v could lead to premature failure without any other part replacement. If you redid the exact same setup you might get 40,000 rounds out of it before failure... or you might get 100. Such is life with ACM components.

[MaciekA]

Shimming a gearbox well enough that it can avoid catastrophe from bad shimming is not too hard, however, as some of the other guys here have suggested, there are a multitude of ways a gearbox can fail.

Some general tips:

1. Look for guides on how to shim starting from the bevel gear. Pinion to bevel alignment is important and you can really introduce a lot of wear and tear and axle stress at the bevel simply from bad motor height.

2. Take as much time as you need to shim those gears. Buy fine (thin) shims, I personally like the Modify-brand "Advanced" set from airsoft parts.

3. Learn to verify your shimming as you go. The gears should spin freely but have no play. You can verify your steps by rebuilding your gun with only the lower receiver, and taking some shots with the compression parts left out of the mechbox (i.e. no spring, no cylinder, no piston, etc... just the gears, and no ARL). You can learn a lot about how a gearbox should sound just from testing various shimmings and motor heights this way. Don't attach your upper receiver or the hopup until you're pleased with the sound.

4. Try out some good grease, but note that this won't make up for bad shimming. I've used the Modify gear grease for a while (contains PTFE, i.e. Teflon), I've also tried the Madbull stuff. Those are all good. I'm in the process of testing out Superlube (PTFE).

5. Definitely learn to test your compression for a good seal. If you don't know how, just post questions here and we'll try to help. Remember, there's no rush, and when you take your time, you can get really awesome results even from inexpensive parts.

6. When you get your hopup and upper attached and are ready for firing tests, pay special attention to backpressure and jamming. Turn your hopup all the way off so that you don't accidentally jam a BB and send all the pressure back into your mechbox, thus grinding the piston to a pulp. This is super important.

7. Related to the above tip, take out your barrel + hopup (i.e. they should be attached) and try to manually feed a BB into the chamber. The bucking lips should hold the BB in place without it going into the tube. Now take a needle or pen or something that can fit into the chamber, and GENTLY push the BB. If it takes too much pressure to push the BB through, that can create jams and back-pressure. Watch out for this.

8. If you're just experimenting and getting your bearings in the mechbox world and want to reduce damage from mistakes, try picking up a M90 or M100 spring. Your velocity will only be around 300fps, but you'll be at least limiting your damage and getting everything else right while you prepare yourself to go to the higher velocities (and higher stresses).

9. If you're looking for decent gears that can take a beating, try the SHS gears. They're inexpensive and are used in lots and lots of high RoF, high fps setups.

10. Know and learn that due to the tiny tolerances and differences in every single part of your mechbox, you will need to re-shim from scratch every time you get new gears, or a new motor, or a new pistol grip, new bearings/bushings, and in some cases, even from a new piston.

11. If you're looking for reading material, look up "angle of engagement" and try to soak up why that's important -- it will help you get a more sturdy setup that can take higher RoF and strong springs.

If you have trouble please post here and badger us about it, it can take a few tries before you get it right and you haven't even hit the tricky parts yet -- gears are the EASY part

[MaciekA]

Also, make sure you do a sanity check on your gearbox after you have put it back together.

1. Check the nozzle is attached to tappet plate by pushing on it a couple times. It should spring back forward.

2. Check the trigger springs are doing their thing by pulling the trigger.

3. Check the selector plate spring.

Watch all the videos that Brian from Echo1 has done on YouTube. His channel is called "Echo1TechSupport". He does little checks like this throughout his videos and some of his habits can help build an intuition within the mechbox.

[Styrak]

Could be metal fatigue just from use, or lemon gears. It's not an uncommon issue.

[Cokeman]

I tried to follow the posted advice as much as possible. I didn't have a new set of gears on hand. So, I cannibalized the gears from the original Aftermath gearbox (I counted the teeth, they were the same ratio as the ones from the JG mechbox).

I removed everything (except for the trigger switch) from the mechbox and placed shims until the gears spun freely (giving it a good spin - it spun for about 2 seconds). I noticed if I added any more, it would bind. So, I figured that the shim was "OK". I re-assembled the gun and test on my chrono. It shot at 380-390 fps, with a 20 rps. I fired about 120 BBs through the chrono (mainly because my chrono is temperamental and sometimes doesn't register the BBs). Then I set up a target to "stress" test the gun. After about another 100 rounds (long full-auto burst, it jammed).

So, I took it apart again. Again, everything seemed fine. This time, I slowly turned the gears and watched how everything was turning. I paid careful attention to the meshing with the teeth on the piston. That's when I noticed that it wasn't really meshing very well with the last metal teeth. Just a visual inspection showed that last metal tooth was spaced a little farther than the other teeth (see attached picture). Using a ruler, the spacing between the plastic teeth was about 3 mm. The spacing between the metal teeth and the plastic teeth was about 3.5 mm. Is this normal? Can this slightly wider spacing cause my gun to jam?

I took the piston from my Aftermath mechbox. The spacing between the metal teeth and the plastic teeth was about 3 mm. So, I used it instead. It worked fine (I was shooting full auto bursts for about 5 seconds at a time) until something broke. But, it kept working, but at a lower fps. When I took it apart, I noticed that the metal tooth had broken (bent over towards the plastic teeth), causing the piston to release prematurely stripping all of the plastic teeth (second attachment). So, now I guessI need to buy a new piston. Any recommendations for an inexpensive one?

[MaciekA]

Originally Posted by Cokeman

So, now I guessI need to buy a new piston. Any recommendations for an inexpensive one?

The SHS blue piston works pretty well.

Don't you just love learning by breaking stuff ?

[ThunderCactus]

That metal tooth broke, to me that indicates the piston is hitting the back of the mechbox, or seizing, while the sector gear is still engaged.
Do what you did before, cycle the gun by hand through it's motion, but remove the spring and leave the spring guide and piston in there. See if the piston hits the spring guide.
If it's a bearing spring guide, the back of the piston MAY be jamming on the bearings.
I've seen pistons get jammed on mechbox windows as well, the piston rails near the back of the mechbox may be poorly cast and causing the piston to seize up.
How you can test for that, is by assembling your mechbox without the spring guide or spring, and pushing the piston to the back of the mechbox with an allen key through the air nozzle. Ideally, gravity should be enough force to have the piston fall forward.

[Cokeman]

ThunderCactus,

I think your diagnosis is right about the broken metal tooth on the piston. I was using a bearing spring guide. See the three attached photos. In these pictures, I am using the piston that came with the J.G. mechbox. The spacing on that last metal tooth is about 0.3 mm wider than on the Aftermath piston (which I had stripped and broken the metal tooth).


First photo: Shows the bearing spring guide and piston in the starting position.

Second photo: The piston has moved to almost line up with the bearing on the spring guide. Even when I cycle it by hand, the piston sometimes hits the bearing.

Third photo: The piston is all the way back, just about to be released. However, it doesn't seem to make contact with the back of the mechbox.

I also tried your second suggestion - pistons jamming on mechbox window/piston rails near back of mechbox. I re-assembled without the spring and spring guide and verified that the piston could slide smoothly to the back of the mechbox.

So, I guess the broken teeth was caused by the piston striking the front edge of the bearing on the spring guide. If so, why are bearing piston guides used/sold?

Originally Posted by ThunderCactus

That metal tooth broke, to me that indicates the piston is hitting the back of the mechbox, or seizing, while the sector gear is still engaged.
Do what you did before, cycle the gun by hand through it's motion, but remove the spring and leave the spring guide and piston in there. See if the piston hits the spring guide.
If it's a bearing spring guide, the back of the piston MAY be jamming on the bearings.
I've seen pistons get jammed on mechbox windows as well, the piston rails near the back of the mechbox may be poorly cast and causing the piston to seize up.
How you can test for that, is by assembling your mechbox without the spring guide or spring, and pushing the piston to the back of the mechbox with an allen key through the air nozzle. Ideally, gravity should be enough force to have the piston fall forward.

[Cokeman]

Originally Posted by MaciekA

The SHS blue piston works pretty well.

Don't you just love learning by breaking stuff ?

Yep - that's what I'm using this Aftermath gun for (I actually have two of them - my brother gave me his). So, the cost of this training is at $325 and rising