The Art of Shimming and The Secret of Gear Ratios?

[AG1212]

I need help with shimming gears and getting the right ratio.

 

For shimming I really don't know what do and it's my first time really working on a GB. I understand too tight and too loose are bad, but on which gears do I put the shims and on what side of the GB?

 

And for Ratio the setup I'm going for is 1joule high speed with fast trigger response, while using mostly semi. So that would be a highish speed motor ( its only a light spring ) and 11.1v battery if that makes any difference at all.

 

Thanks.

[Docv400]

Have a quick search AG, shimming is one of those things that has been 'done to death' on here

 

Too tight is indeed bad, but a bit loose is OK.

I start with the left gearbox shell, and shim the spur (middle) gear just enough so the outer edge of the larger gear doesn't touch the bushes for the sector or bevel gears.

Then the sector gear, shim it so it just clears the spur gear where they overlap, ditto the bevel gear.

Then it's just a case of shimming each one to the r/h shell.

A spot of silicone grease on the axles will hold the shims in place on the gears, they're easily dropped off while you're putting the gears in and out.

 

Have a look here as well, before you go and buy Airsoft branded shims...

Shims...don't get ripped off!

 

[AG1212]

Right thanks. At all sounds easy in theory but in practice not so much Thanks for the heads up on the shims btw!

[infected]

First, let's say this is my method, but not the only one. Okay here goes.

 

Shimming:

*Press your bushings into place. If they are a tight fit, a small NYLON tap hammer will nudge them in to place. If they are lose, put a few drops of superglue around the bushing hole just to make sure they don't fall out.

*With the bushings in place, start with the spur gear & the left half of the gearbox. Set the spur gear in and see how freely it spins. Usually there is a a bit of friction so I put a .2-.3 shim in there under the spur gear. Then put both sides of the gearbox together and put a few screws in the gearbox to hold it closed. Do this because often the gearbox might close tighter than you'd press it closed by hand. Then reach in with a thin screwdriver and try to spin the spur gear. It should spin freely. There will be some slack. That's when you start putting shims on top. Estimate the amount of shiming needed, put the gearbox shell back together and test with the screw driver again. It should spin freely, and have little to no movement along the gear axis. Remember, it's better to have a little slack than be too tight.

*With the spur gear done, next is usually the sector gear. You need to put just enough shims between it and the left side of the gearbox shell to keep it from rubbing with the spur gear and the cut off lever. Test with the gearbox shell closed and screwed together. You can hold the gearbox so that it's left side down and shine a light in there and see the gap between the gears. You want a little, but not too much (.2-.3mm should be fine). Now again add shims to the top of the sector gear and test with the gearbox closed and screwed together. Repeat till that gear is correct. When testing, make sure the cut off lever moves freely and that the spur gear engages the cut off lever as it should. Push the cut off lever out of the way and the gears should spin very easily.

*The bevel gear is tricky. This is where a lot of gearbox noise is made. If you're not picky, just put a .2 shim between it and the left side of the gearbox and then shim above the bevel gear until it's correct (as you did the other gears). If you are picky, then put your motor on the gearbox with the bevel gear in place with a .2mm shim. Look at how the motor engages it. The pinion should be 3/4 - 4/5 of the way engaged with the bevel gear. Adjust motor height and shimming as needed for them to engage properly. Then take the motor off and shim the top side of the bevel gear as you did before with the other gears.

* Enjoy a well shimmed gearbox.

 

As for the gear ratios. Stock gear ratio is about 18.9:1 if I remember right. So, going to a lower gearing like 16:1 will cause you to lose torque and increase rate of fire. When torque drops you lose trigger response. When torque increases you lose ROF. That's a generalized statement and really just deals with ratios not motors. If you're going for a high rof on a 1 joule spring you don't need to swap out your gears. With a 1 joule spring and stock gear ratios with a properly tuned gearbox and a 20C or greater 11.1v 2000mah lipo (or larger), combined with a high rof motor (matrix turbo or some such), then you should be about 30 rps. Put some 14:1 ratio gears and you'll start getting near 40 rps.

 

Hope that helps a bit.