Gearbox locks after putting new spring and bushing

[Souske]

Hello,

I installed a KM 100M (1.00 j) spring into my stock TM gearbox, and also added new Systema metal bushings. I made sure everything was placed correctly (including the shimmings and bushings), and also importantly, the anti-reversal latch (I learned how to put this in correctly from my previous experiences). I have opened up the gearbox many times up to this point, but I cannot still figure out what's wrong.

 

When I close everything back up (the spring is hard to put in, as it's longer than the stock spring) and try to run it, the gear box locks up. The piston travels to the back (as it should), but locks there and stops, leaving the spring completely compressed. When I continue to press the trigger (tap once or twice after), it sounds like the gears are trying to move, but they sound jammed. Because the piston is locked to the back with the spring fully compressed, opening the gearbox has been very very difficult everytime I have to re-open it..

 

I'm guessing because the spring is really hard/long when I push it into the piston, the spring probably bends out to the bottom when the gear box runs (when the piston moves back and compresses the spring), and pushes on the sector gear, and pushes the gear out of alignment- thus causing the jam. I'm guessing the spring does this because it's longer? I also checked all the teeths of all the gears, and they're all clear of any debris. Once I realign everything (including the bushings, etc) and make sure the gears are rotating smoothly with each other, I put it back together...only to get the same result. Can anyone tell me what needs to be done?

 

Also, I'm running a charged 9.6v 1100 mah NiMH battery. Thanks in advance.

[Catman]

I had something like this before: can't remember what the internals the gun has at the time but I figured it was the spring locking up inside the piston when it was compressed, of course whenever I tried to opened the gearbox like this it'd try kill me - I bought an upgrade piston and it worked fine.

 

I take it if you put your stock spring back in it works fine?

[Souske]

Thanks for the response.

I haven't tried that yet, but I'll go ahead and put the stock spring back in. I thought stock TM pistons worked fine with upgraded springs?

[juicymeat]

When you said you put your shims back in, did you mean you just put the original shims without altering them, or did you mean you reshimmed your gearbox...my bet is that you didnt reshim.

 

I had this problem when I first upgraded my gun, I redid my shim job and everything was fine, you cant just change bushings and not check your shim job, because even the slightest deviation of about 0.2mm (every bushing/gearbox/gear will have their own slight deviations) will offset the shim job and cause your gearbox to lock up.

 

EDIT: Your stock piston will do fine against a 1J spring, also if you did reshim, try using a more powerful battery, but I'm pretty sure its the shim job.

[Souske]

Thanks for the response.

I have a charged 9.6v battery, so the battery isn't the problem.

When you mean "reshimming," do you mean by taking the shims out and lubracting them and placing preciesly them at the original places (where they're supposed to be at)? If so, I did that as well. When I put my old spring back in, everything is working flawlessly. If it was a shimming problem, wouldn't I also encounter problems with my stock spring installed?

Please keep the suggestions coming, as this will eventually lead to a formulation of a solution.

[juicymeat]

If it is a shim problem, then yes you can possibly have a problem even with the stock spring, because the shims are not allowing the gears to move freely.

 

What you mentioned, is not reshimming, thats just sorta like, relubing your gearbox for more smooth operation, and regular maintenence. Reshimming is changing your shims to match the newly put in metal bushings, and in this cause, the metal bushings are thicker than your stock nylon bushings, causing your gears to lock up because they are bounded too tight on the axles by the gearbox.

 

Heres a guide that I made for davedawg123 on his M14 post:

 

When youre testing for smooth gear movement, I would try spinning from the sector gear, rather than the bevel gear...as for your shimming try this method...

 

First, put in the spur gear in with a 0.3 shim on its left (bottom) side (its important to try and not let the spur gear touch the left side of the gearbox), then close up the gearbox. Then, try spinning it from where the cylinder should be, if it spins easily, youre fine, next check to see if it wobbles too much by lifting the gearbox on its side, and poking it (so it would be like lifting the gearbox up above your head with your left hand, and letting gravity do its work, you poke the right side of the gear axle through its bushing, if theres too much wobble, add the smallest shim you have to the top of the spur gear, close it back up, test, and repeat if necessary.

 

Secondly, put the sector gear in, most likely no shims on the bottom wont work, so youre going to have to opt for a 0.2 or a 0.3 and see if either works, what youre looking for is that the bottom of the sector gear is not touching the spur gear, but has the smallest gap that you can get between them for smooth meshing. Once you find that point, close the gearbox back up and test again for smooth gear movement, and side to side movement by lifting the gearbox up and pushing the gear axle. Again, if theres too much movement, then add the smallest shim you can get away with to the top to minimize side to side movement, but still allow free movement.

 

Lastly is the bevel gear, personally I think this is the easiest one to shim...because mine only needed a 0.2 shim on the top. Most importantly for this gear is to check that the bevel gear isnt riding on top of the spur gear like I mentioned with the sector gear, most likely though you wont need a shim on the bottom side, but then again gearboxes vary.

 

As for your problem, I think I have an idea of whats wrong, and its just overshimming your gears.

 

BTW sorry, but I just opened up my gearbox yesterday, and I gave you the wrong shim configuration on my previous post. Heres the actual configuration:

 

Spur: 0.3 bottom, .15 + .20 on top

Sector: .3 bottom

Bevel: .2 on top

 

Also, as for minimizing side to side movement, its OK to have a little bit of movement, just so long as it is reasonably small, maybe like .1mm but nothing overly large. If youre unsure, add the smallest shim (like switch maybe a .2 to a .3 and see if you can still freely spin the gear while reducing side to side movement). A general rule of thumb that I got off some guide is that the ideal shim job for each gear will not allow .1 mm more shimming, as in if a .1mm shim is added, it would be too tight.

 

If youre looking for additional shims, I'd recommend Prometheus shims as I HEARD that they come in .1mm sizes for the more precision jobs.

 

I know shimming is a headache but yeah...have patience, and good luck, hope that helped.

 

EDIT: OH I almost forgot, but you dont have to screw the screws down in the gearbox when youre testing your shim job, but it would be a good idea at the very end, that when you think youve got it all down, to screw in all the screws to check and see how well they run in a tightened gearbox.

[Boba_Fett]

Metal bearings and bushings are thicker than TM standard ones. So you have to use less shims.

 

Spur: 0.3 bottom, .15 + .20 on top

Sector: .3 bottom

Bevel: .2 on top

 

This looks like the standard shimming but it will only work in TM standard gearboxes with TM standard bushings.... no other manufacturer is macking such identical parts over years...and this is no discussion what parts of which manufacturer are sturdier

[juicymeat]

That is less shims...TM standard shimming is a hell of a lot more shims:

 

Spur: 0.5 top, 0.3 x 2 bottom

Sector: 0.3 x 2 bottom

Bevel: 0.15 top, 0.3 bottom

 

Taken off the back of my Guarder FTK shim map, which is the same as the TM shim map, because I took off all the shims, compared them, and even used some of the TM shims to complete my shim job.

 

EDIT: Whats this about the sturdiness of different parts?

[Boba_Fett]

Ah, then this shim plan might help

 

EDIT: Whats this about the sturdiness of different parts?

 

*waving the hand like a jedi* There is no sturdiness.....

[Souske]

Thanks for the response everyone.

I can see how the shimming width can effect the gears' movement, but if the stock TM shimmings were too thick, wouldn't I get jams also with my stock TM spring? Because at the moment, I put the old spring back in, and it's working flawlessly with the new bushings and old shimmings.

 

Edit: Boba, you mentioned putting less shims on. Did you mean by taking some shims out?

[Atomic]

The stock spring is lighter and putting less lateral stress on the gear axles. The stronger spring is putting so much force on them, they are locking up.

 

Shimming "loose" will allow your gearbox to fire, but will also slowly tear up your components. Follow the guide and you'll be fine. Sometimes, it takes a try or two, becuase when you tighten all the screws, there's no longer any play. A literal "hair" of side to side play is what I prefer to aim for.

[Souske]

That is a big possibility...

But I have also heard people say if the compressed length of the new spring is longer than the compressed length of the stock spring, that would cause the lock up. They say the solution is to remove the first teeth of the piston at the front. Can anyone confirm this? Or this this a pure shimmings issue? There doesn't seem to be a set uniform answer for this.

Thanks in advance.

[TriChrome]

Before I answer your latest question let me say: every shim job is different. When you install new bushings and/or new gears you have to start from scratch with shimming.

 

You can have 2 TM gearboxes, with 2 sets of Systema bushings, and 2 sets of Prometheus gears and the ideal shimming on both of those gearboxes will be different from each other.

 

So since it seems like you're using the stock shim placement I would take the gears out and take off all the shims, throw them away, and start from scratch (I really hope you purchased a new set of shims for this project because it's not good to use the stock TM ones; you can't as easily tell what thickness they are, and you may need extras).

 

 

 

But I have also heard people say if the compressed length of the new spring is longer than the compressed length of the stock spring, that would cause the lock up.  They say the solution is to remove the first teeth of the piston at the front.  Can anyone confirm this?  Or this this a pure shimmings issue? 

<{POST_SNAPBACK}>

You don't want to cut the first tooth off your piston. That is reserved for ultra-high-speed set-ups (I'm talking 30-35+ RPS).

 

The only way a thicker spring would make your gearbox lock up is if the spring is so much thicker that the piston can't travel all the way back to be released; I've only heard of this happening on 600+ FPS springs.

 

As long as you're installing everthing else correctly the shimming is most likely at fault. .....also make sure your motor isn't too tightly adjusted.

[Souske]

I didn't get a set of new shims for this project. The spring is only supposed to chrono at 340-350 FPS.. so it's definately not a 600 FPS spring.

As for the motor being too tightly adjusted, do you mean the motor height being far too high up?

 

Also, isn't most of the force pushing the piston back forward from the spring itself?

[juicymeat]

What Trichrome meant by that 600 fps comment is that only a spring of that tension would cause your gearbox to lock up, provided that you installed everything correctly.

 

So basically all that I and Trichrome mean to say is, its most likely your shim job (sorry if I misinterpret your words Trichrome). Just look at my short shim guide, and you should be good to go.

 

Yes, a motor adjusted too high would mean that a motor is too tightly adjusted, and yes, most of the force pushing the piston back forward is from the spring tension that the piston holds back.

 

Now come to think of it though, it could be your motor adjustment, because screwing in the base plate, but not the motor adjustment plate, also attributes to motor height alignment, and you might have tightened the plate more in one case than another.

[Souske]

So it could be the height adjustment after all? One quick question, If the gearbox locks up again, is it ok to adjust the motor height adjustment and try to fire it again in hopes of "unlocking" it? Or must I re-open the gearbox when the piston is locked back and have the spring try to kill me again?

[juicymeat]

If it is the motor, then yes, it is ok to readjust and try to fire again without any need to open the gearbox.

[Souske]

Alright, thanks for the suggestion. I'm going to put the upgrade spring back in, and give it a try. I'm awaiting a 2nd pair of hands to help me just in case I have to re-open it with the spring fully compressed. I'll post how it goes after I try this out.

[Souske]

I finally got it to work. I removed the first teeth of the piston (the metal one that is removable on the stock TM piston). Once that was done, everything works fine now.

 

One thing I noticed is that my 8.4v battery cannot pull the spring, only my 9.6v. Since I installed a KM 100M 1j spring, I figured I should be getting around 350 FPS. I did the coke can test, and found that I can't quite cleanly puncture the can. It does break the can's surface, but it won't cleanly go through.

[Rapier]

NO NO NO! That will cause your piston to blow out now!

 

I had the same problem when I put a Gaurder M110 spring in my CA G36K. He spring was about 1.5" longer than the stock spring. I checked it and when it was totaly compressed it was actualy too long fro the piston to clear the sector gear (I actualy blew out 2 systema polycarb pistons trying to get it to work). I cut 3 coils off the spring, ground it flat again and put it back it. It's worked like a champ ever since.

[Souske]

Oh ok, so I should leave the teeth back in then? So for cutting the coil, how much would you suggest for cutting a KM 100M spring? Thanks in advance

[Rapier]

Like I said, mine was 1.5" longer so I cut 3 coils off the end that was wound tighter (variable pitch spring)

[Souske]

Oh ok, I think I may try that... as my 8.4v battery should be able to run it as well. When you mean the tighter wound end, I'm assuming you're referring to the back end of it (the end that does NOT go into the piston).

[Rapier]

Technicly either end of the spring can go into the piston. I'm refering to the end where the coils are closer togeather when not under tension.

 

Did you end up re-shiming your gearbox properly?

[Souske]

Not really, as I didn't buy another set of shimmings... so I'm using my current stock TM set. They worked well with the old spring and systema bushings.