How To: Shim the Bevel Gear

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IsparkTheLa
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How To: Shim the Bevel Gear

Post by IsparkTheLa » Fri Mar 11, 2011 3:47 pm

I have read a lot of information regarding the “how to’s” of shimming, but the importance of shimming the bevel gear is often overlooked. Many of these guides tell you to begin the shimming process with the spur gear and work your way to the bevel gear. I am going to do my best to describe a process that is exactly the opposite; to begin with the bevel gear.

The engagement between the motor pinion and bevel gear is a crucial interaction of your gear boxes system. When done correctly, you will reduce mechanical noise (gear whine) and ultimately increase gear efficiency. Often time’s people will shim the bevel gear so it has clearance from the spur gear, they then assemble the gear box and blindly adjust the motor set screw until the pinion meshes with the bevel gear. This method works, but also has a couple disadvantages. Here is why…

The motor has one axis of movement: the “X” axis…left & right. It can only be adjusted towards the center of the bevel gear or away from it. It cannot be adjusted up or down. If the bevel gear is shimmed to low and you adjust the set screw, the pinion will attempt to force the bevel gear upwards. Since the bevel gear is secured between the bushings, it will have no where to go. This causes stress on your gears and results in a high pitch whiny noise when the gears cycle. This is an exaggerated picture of what happens when the bevel gear is shimmed too low:
Image

In the same respect, if the bevel gear is shimmed to high, you will still hear a decent amount of mechanical whine. This is because there is too much slop or slippage between the gear faces on your bevel in pinion gears. Ideally, you want about a .1 - .2mm gap between the peak of the pinion gear and valley of the bevel gear. This distance may differ between gear sets, but I have found those measurements to work well for me. This picture refers to the spacing between the “peak” of the pinion and “valley” of the bevel gear:
Image


To gain the most transmission from your pinion, you want the back edge of the pinion teeth to be equal to or just outside the radius of your bevel gear, like this:
Image

Here is the method I use to shim a version 2 gearbox. Remove all shims from the top of your bevel gear. I will call this the top side of my gear:
Image

Put your bevel gear in the right side of the mechbox and install the grip:
Image

Now put the motor into the grip and fasten the motor plate:
Image

This is where you adjust the set screw and align the pinion so it is equal to or just outside the bevel radius, like pictured below. Some MB’s actually have a window cut into the side allowing you to easily look into the mechbox.
Image
Image

Now push on the axel of the bevel gear to see how much slop or distance there is between the valley and peak. This will give you an idea of how many shims to add to the top of the bevel gear. Again, you don’t want the bevel tight to the pinion, but you don’t want it to loose either…..just have to experiment.
Image

Then disassemble the grip from the mechbox and add all necessary shims to the top of the bevel. It is a good idea to check your work by reassembling the grip and observe. If you are satisfied you can now continue to shim the remaining gears…

Before using this method, my gun had a very high pitch whine that sounded like a sewing machine and adjusting the set screw had no effect. This method of shimming the bevel gear drastically reduced noise and the the hope is that it will also help reduce stress and add longevity to your mechbox.

Hope it helps!
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Master TCAA
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Re: How To: Shim the Bevel Gear

Post by Master TCAA » Sat Mar 12, 2011 6:15 pm

Wow Matt. Once again I'm impressed! Great pics
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Swasin
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Re: How To: Shim the Bevel Gear

Post by Swasin » Sat Apr 23, 2011 10:39 pm

Matt, Im assuming you did this to your M4? I will try and do this from now on (If possible with my V3 gearbox's...Ill make it work), I too sure noticed a huge change in the noise in your M4. It was sounding great today: quiet, soft, and smooth :D Great guide!

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Re: How To: Shim the Bevel Gear

Post by brendizzle » Wed Aug 24, 2011 1:15 pm

wow thats cool ive never heard of that before, i always just shim it so it has a little bit of wiggle space and then adjust the motor height accordingly. this method looks way more precise though so ill definitely have to give it a try.
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