Gear Shimming, Question

[nic_s]

Here's what I'm wondering, and I'm hoping you guys can help.

Sometimes I have to choose between zero sideways movement of the gears in the mechbox, and zero resistance when I spin the gears by hand. It seems that even if I use the thinnest shim possible, I can't have both 100%. It will either have no sideways movement and offer slight resistance, or there is some movement (less than 1mm) and the gears spin with no resistance.

Note: This is on a Ver.3 mechbox using Systema oilless bushings and Systema shims. Stock gears.

Any advice is appreciated.

Nic

[mcguyver]

1mm is alot of movement. Your shims should be 0.1, 0.2, 0.5 mm respectively. you should be using the 0.1mm shims as a baseline and go from there. You may have to allow some movement (less than what you can compensate for with the 0.1mm shims) to get "low rolling resistance" from the gears. I've yet to run across a situation that allowed too much lateral gear play that could not be addressed with the above mentioned shims. Just keep at it as it may take a good deal of time to get it just right.

And make sure your bushings are not loose in the mechbox as they should fit tight and not fall out. And make sure if they are tight that they are driven in fully. Then your shimming should be straight forward. If the bushings are loose and fall out, either try different bushings or replace the mechbox. Don't think this doesn't happen all the time. Mechboxes can be toast even though they "look fine". If the bushings aren't a snug fit along the outer race, shimming will be difficult to impossible and you now will start to significantly increase wear to the "bushing holes" and increase their diameter. This leads to even more gear play and eventually failure. The looser the bushings are, the faster this will occur.

[nic_s]

OK, they are quite loose and they do fall out on their own. But the mechbox itself is almost new, so that puzzles me. However the owner of the gun fired it for about forty seconds non-stop, and as a result the bevel gear was sheared. Could this cause the premature wear on the mechbox?

[mcguyver]

Who knows exactly when or what caused the damage to your box for absolute certainty. But it's fair to say that the previous owner was no help. Remember that the bushings MUST provide a secure and stable platform for your gear train or the whole thing is doomed to fail. Your mechbox may "look good" but may not be worth a hoot. Just try different bushings if you have some available, and if not get yourself another mechbox. I have at least 4 or 5 Marui and G&G mechboxes that this happens to and I won't use them again. It's not uncommon for this to occur, but some guys will disregard it and use them anyway. Just think of bushings from a mechbox exactly the same as any bushing or bearing in any other application, like the transmission in your car. They must be secure in the mechbox to do their job so that the gears and everything else can do their job.

[CDN_Stalker]

I recall reading a tip on one of the forums regarding loose fitting bushings. Said to use a (early now, can't think, the pointed punch thing you use to dent metal before you start drilling the pilot hole....... spring loaded center punch? :-? ) and dimple the metal on 4 sides (like 12, 3, 6, and 9 O'Clock) of the bushing hole about a 1mm away from the edge. That'll reduce the hole size by a slight bit, enough to tighten up the bushing. I haven't tried this yet, so I can't confirm, but it makes perfect sense.

[Mantelope]

Since the bushings and the holes that they're placed in take a large amount of stress, I wouldn't trust that punch method to keep the bushings tight. Essentially, only that small, small chunk of metal distorted by the punch is keeping the bushing tight, which means it will probably bend back into being loose again pretty soon.

Anyway, in my experience, I've found that when you have to choose either lateral movement, or friction, lateral movement is the preferred option.

[nic_s]

I also just shimmed up a brand-new M14 with Area 1000 bushings, and some of the bushings do not sit tightly at all. Can it really be that after only a few games the mechbox is worn?

[Blastyman]

I have had many gearboxs with the loose bushing syndrome and never noticed any ill effect generally.

Cranked really high FPS and poor shimming may cause the bushings to turn in the hole but in my experiences even when I bushing was loose in the hole it didn't turn in the hole with the gears providing the gearbox wasn't shimmed too tight.

So if its not turning in the hole its not going to make the hole bigger. But like I said you have to ensure you don't make it too tight with the shims or it might.

[nic_s]

Gotcha! So from this info then I'd say that the M14 is OK. It's a PDI 150% spring, so the fps wont be anything too crazy. But then again I'm using a 9.6V....
Only time will tell.