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This message edited by FA-Chris M (UBCS). 2010-07-23 22:30:00
I had someone ask me by email for help with shimming, one of the most tedious aspects of gun tech work.
I figured I'd share "my way" to everyone in hopes it helps out future or possibly current gun techs. If you feel like copying and posting this elsewhere, please include my name at the bottom.
"For shimming I always use a shim set with .1 size shims. I like the sets that come with only .1 and .2 shims as you have more freedom to customize over those that come with .1, .2, .3 and .5. Sometimes you don't even get .1 or sometimes you get .15 which I feel are useless unless you're putting two together to make .3.
Also note, if you are putting in new bushings, test fit each gear with their bushings and make sure the bushings are not too tight. The inner hole of bushings and the shaft size of gears is not always standard. Sometimes you must use a small round file to widen the bushing hole to fit you gears. Make sure the bushing and gears fit right (no slop but free spinning) before you begin shimming or you'll be getting a false idea of your shimming from the tightness of the bushings.
The way I do it is to put .3 worth of shims on the bottom of the spur gear (middle gear) and then put the gearbox shell on and a few of the middle screws near the gear to get it all as tight as it will be fully closed. I take my finger or a little tool and check for slop in the gear. I also spin the gear to get an idea of how free it should be turning as the shimming should not slow the gear down.
From there I add shims to the top, .2-.3 at a time and screw it all together and re-test for slop. I do the spin test and keep adding until either the gear locks (will not spin) or spins noticeably slower.
From there I remove .1 if the gear was just slow, .2 if the gear was locked. Screw the gearbox together and retest. If it spins as fast as it did before I began to shim and has a good amount of rotations then I leave it. If it is still a little tight I'll take another .1 away.
Remember, the bottom always gets its .3, the top is where you add or remove shims.
Once that's all set I grab the sector gear (by piston) and put .3 on the bottom and test it in the gearbox for meshing with the spur gear. If it spins well with it, does not grind and does not seem too far away (visually) I'm good. If it was too far away, I may remove .1 off the bottom of the sector gear and test it again with the gearbox screwed together for how it meshes with the spur gear. If it meshes better I go with .2 on the bottom of the sector gear, if it grinds I add .1 until it does not. Rarely do I have to use more than .3 under the sector gear though. Rarely have I had .1 or nothing under the sector gear either...
Once I have the spur and sector gear meshing correctly, I shim the top of the sector gear in the same manner as the spur (adding .2-.3 until it locks or slows and then removing .1-.2 until it spins freely but has little slop)
Once the sector gear is shimmed, I'll test fit it with the spur gear again with the gearbox screwed shut.
If it all fits well as it should I remove the sector gear and move on to the bevel gear (by motor). If it does not seem to move right I'll check the spacing on the bottom again. If the gears are now grinding, I'll take .1 off the top of the sector gear and put it on the bottom of the sector gear and test again. Sometimes grinding does not really occur until the top is properly shimmed...
With the bevel gear I usually put nothing on the bottom. I shim the top like the other two gears. I have had a few guns where I needed to shim the bottom of the bevel gear but it's rare and when I do it's been .1-.2 at the most. You'll know if they grind once you've got the top all shimmed. I'll shim the bevel gear with the other two gears out or sometimes with the spur gear in so I can test spin by pushing the spur gear in gearboxes without the "window" by the bevel gear.
Once all gears are finished I put them all into the gearbox and test spin them. They will not spin as fast as they did by themselves but they should spin nicely and without much noise or hesitation. If you find they do not, go back and test gear pairs (sector/spur, spur/bevel) to locate the problem.
That's it! A proper shim job can take you an hour or more to do so be patient. Doing it right will add longer life to your gears and allow you to feel more confident at high speed or torque."
FA-Chris M / Fireman 7/23/10 |
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Cpl Pwn-90 |
Posted: 15:27:00 on 07-23-2010
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Nice info |
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Yeah, took so long to write with my one good hand that I felt it needed to be posted. :-) It's exactly how I do it in all my 30+ per second guns and I hardly ever have gear issues, even with stock gears. |
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LCpl Priest |
Posted: 18:06:00 on 07-23-2010
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This message edited by IRA_Priest. 2010-07-23 18:06:00
Very Nise page, Chris. Can I see a "Chris's Tech Talk" section in the near future? lol |
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Yeah you were typing with your "one good hand." And where was your other hand Chris? Perhaps in your lap? hmmmm? |
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You know I'm a righty for that...
My LEFT hand was wrapped up and in a brace and the only thing in my lap that was swelled. |
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Oh wow, I handed that one to you... |
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Thank you FAChris for the shim guide,It's perfect for an FNG like myself. I was infact curious about this proceedure and was about to ask.Do you think an "out of box" AEG could use a shim job, or at least a check? Or has it only been needed after upgrades, and factory shim settings with a "fresh out of box" are usually good? |
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I've yet to see a stock gun that didn't need some tweaking. A lot of companies use a shim "recipe" that's pretty generic and usually more on the loose end to give maximum rate of fire and the least wear.
I know that that sounds good to most but it also allows a little more slop then you may like which will lead to faster wear and a higher chance of breaking due to less surface area being used on each gear.
Do note, once in a while you'll find a gun that needs little to no shim change but it's somewhat rare. I'd not stress over it right away on a stock gun unless you plan to run 11.1 lipos or such. If you're using a 9.6 or lower the stock shimming won't be all that bad. Heck, you may never have an issue. |
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sounds good to me, thanks. How do you feel about brass bushings in a stock to slightly warm M4 G&G/CM ,it has G&G metal gear set.I was thinking of the ball bearing type but read a pretty bad reveiw on them, but the source is unknown.The way the guy talked he might have been 15yrs old and screwed his gun up and blamed the bearings. |
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dude please make a video. words can only get me so far lol |
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I've had brass bushings in a few guns and in one they were never a problem and in another they wore out a little faster then steel bushings.
As for bearing bushings, I'm also done with them on high speed builds. Even the high end brands have blown apart on me at 25+ rates of fire. When going from bearing to solid I've only really noticed a 1-2 drop in ROF so long as the gears were shimmed right and the bushing holes were not too tight on the gears. A few sets of element bushings I've bought had to be filed down a little on the inside to take most of my gears without any restriction. I assume this is just bad craftsmanship from element though since all three sets I bought had some (but not all) bushings with the same issue. |
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And as for a video... I'll let Shwell cover that. I'm not really the video sort of guy. :-P |
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I work on that soon.. I will pretty much Read what he wrote, and demo it on vid.. be advised, it will be a long vid... |
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thats cool the more in depth the better ,i learn more from watching someone do it then from reading...
but great job on the guide chris it has helped alot. |
