Airsoft M4 gearbox seems to be very tight?

[r2d2]

Is it normal for airsoft gun gearbox to be very tight and hard to turn?

 

Got my first airsoft gun from boot sale. Owner had no idea where it came from and if it is working. It was without battery, so I got new battery (8.4V 1400mAh with 2.5A discharge) and charged it with working charger (I use it for RC planes). It makes me think battery is ok. But gun wasn't working.

 

At first I thought it had something to do with electricity. Battery and leads were heating fast. Disassembled the gun and tested everything with multimeter. Wiring is in good shape. By the way I have discovered motor was working fine out of the gearbox.

 

I have disassembled gearbox, cleaned and oiled it. Everything seems fine inside. Checked few YouTube moves to make sure I assemble it back correctly.

 

But it is still not working. It looks like motor can't turn it at all and starts to heat itself plus heating wires and battery. And when I try to turn gearbox manually with screwdriver it turns and works correctly (pump goes all the way and triggers on the end pushing air), but it is very hard to turn. Is it normal? I would suppose that gearbox input gear should be easy to turn. Maybe even with nail.

 

Or it could be half dead motor not capable of turning gearbox and running fine outside it? 2.5A battery should be more that enough to power it. I was thinking about putting more current from static power supply, but not sure if it won't burn.

 

Any advise please?

[RR01]

It sounds like the gearbox has too many shims on the gears, making them hard to turn. First, strip the gearbox again of everything but the trigger mechanism (so gears, spring, piston & cylinder). Now put the Spur gear (the one with the large cog at the bottom which sits top left in the gearbox if you have the case open in front of you with the nozzle end pointing to the right) back in but with one thin shim on the bottom.

 

Now spin the spur gear in the bearing / bushing to make sure it moves freely but without any side to side play/wobble. If it wobbles too much then you may need to buy a new set of bushes. Next put the other gearbox half on top and make sure the spur gear spins freely in the gearbox. Check how much play there is between the two sides of the gearbox (i.e. push the gear shaft in alternately from each side to see how far it moves). It should be about 0.5mm at most. If it's more than that then add shims to the top of the gear shaft until you have the correct clearance. Now try spinning it again within the complete gearbox shell held tightly together. It should spin freely but with minimal movement side to side.

 

Now repeat that process, but with the Sector gear (the one that pulls back the piston). First set it in with the Spur gear installed and put a shim on the bottom so that the two mesh together but don't rub on the flat surface of the Spur gear. Now remove the Spur gear and go through the shimming process with the Sector gear. Again, make sure it spins freely but with little side to side play between the gearbox shells.

 

Once you have that sorted, put both Spur and Sector gears in and check they spin freely. Now you can add the motor gear. You don't need to heavily shim that one as it will move slightly with the motor insertion anyway. Just make sure that all three now spin freely together, then add the anti-reversal latch and put the rest of the gearbox back together, refit the pistol grip and the motor, connect up a battery and see if it all works before you fit it back into the receiver (saves a lot of hassle if it doesn't work).

 

If it all work OK out of the receiver then rebuild the gun and try again, but make sure the motorf isn't set in too deep, as that can cause it to jam up and not work. When it is working you should hear the gun cycle smoothly. If the motor sounds harsh then it's in too deep. If it spins but the gun doesn't fire then it's not in deep enough.

 

One other thing. You said you were using a 1400mAh 8.4V 2.5Amp discharge battery. Did you mean a 25Amp discharge battery, as a 2.5Amp wouldn't move the gearbox at all?

 

Hope this helps.

 

Mike

[Airsoft-Ed]

It could be something as simple as the motor being wired in the wrong way around.

 

If the motor is trying to turn the gears the wrong way then everything will just lock up as it only works one way around.

 

Try swapping around the wires plugged into the motor.

[r2d2]

Mike, thank you so much. I'll follow your advise! Probably will have few hours after work today. Sorry for wrong battery spec. It is 25Amp.

Btw thanks for hint about motor deep control. I wasn't realising it was present. I'll check that. Maybe it is set to deep and can't turn.

 

Airsoft-Ed, I thought about polarity mismatched, but it is wired correctly. Thanks for mentioning something simple can be wrong. Now I am suspecting it could be battery after all. As I wrote it is new and never proven to work. Maybe it's faulty. Chinese factories aren't perfect.

 

I have few 7.2V batteries. None of them will fit into the gun, but they should be ok just to try.

[Hibernator]

If you're turning it manually with the spring etc still in there, it's bound to be hard to turn. The spring needs a lot of pressure to compress.

 

If you assemble the gearbox without the spring/piston fitted, then the gears should spin freely like the wheels on a hot wheels car. Too many shims in that situation makes them harder to turn.