Reinforcing The STU Sector Gear

Sector Gear
 

Parts manufacturers usually don’t recommend the use of super torque up (STU) gears with spring ratings higher than M/SP140. For those who tried usually resulted to shorter (than normal) gear lifespan. The truth is STU’s (Super Torque Up) can wind up spring ratings up to SP/M150 and can be reinforced for a more dependable set-up (been using mine for more than a year of regular use without any problems). This procedure requires access to an acetylene welding torch and some welding rods (ordinary welding rods will wear out quickly based on experience and would require you to use “Hard Facing” type welding rods which have a stronger/harder metal property).

Gear Sets: The Sector Gear And its Weak Point

The sector gear is the top most gear on an assembled AEG (air electric gun) gear set. It is the gear that meshes and winds the piston (or piston body). This gear usually fails when the last tooth wears out (the last tooth takes the most stress because this is the part that carries the load of a fully compressed spring (the last tooth of the piston which is the inserted metal tooth on most polycarbonate/plastic piston body, mates to this part). So by theory and logic…. The last tooth of the sector gear is the part that will surely wear out or break off first (either with a stock or a heavy rated spring). With this in mind we tried our hand in strengthening/reinforcing the last tooth to see how long will it hold out on a basic skirmish airsoft gun (aeg). An airsoft rifle that sees regular weekend usage and fires an average on 2 high capacity magazines, roughly around 850 rounds of BBs per gaming day.

Tool Requirements:

1. Acetylene Torch

2.”Hard Facing” Welding Rod

3. File Set

4. Table Vise Clamp

 5. Grinder with a thin grinding stone/blade

Procedure:

Given that you have the tools, the capability and the “know how” to use them….

Using a Vise Clamp, Securely position the sector gear so that you can “build up” the metal on the last tooth by welding the metal of the “Hard Facing” welding rod (be sure to remove the combustible coating of the electric welding rod by hammering it first).

You can “build up” the last tooth up to a 2 gear teeth span (from the last tooth).

After the welding phase dip the red hot part of the gear in mineral oil to temper it. Dousing it with water will make it brittle and we don’t want that happening….while letting it cool down slowly will make it soft… we also don’t want that to happen.

When the welded sector gear is cool enough to be handled. Securely grip the sector gear into the table vise clamp and start grinding (be sure to wear safety goggles when doing this) the welded last tooth of the gear into shape, using a hand grinder. Carefully grind the welded last tooth removing the excess metal and blend it with the original gear thickness and roughly the same height.

After achieving the proper thickness and height. Carefully shape the welded part into the proper gear height by using a file. This takes up some patience and skill, and I never said that this procedure is going to be easy; you got to either have the skill or had a lot of practice. What we are trying to achieve is here is to shape the welded last gear tooth into something that resembles a shark’s fin...

Note:

While hand filing, be sure to get the proper “space/clearance” between the last and second to the last tooth. To check the gear tooth spacing, set the gears into the gearbox/mechbox to see if it meshes properly with the piston body and to see if none of the welded part touches any part of the gearbox/mechbox by spinning it manually (I also have a piston body close while filing to check the gear tooth spacing while filing it into shape). I everything meshes like it should and it spins freely with the other gears (spur, bevel gear) then you got the clearances right.

Check the reinforced sector gear tooth if it meshes properly to the piston body teeth. If everything looks good and it meshes properly then it’s done and ready to be tempered and cleaned before installation.

Temper the welded last tooth by carefully heating it with the acetylene torch till its red hot, then dipping it in mineral oil or used motor oil. After this is done, clean the carbon of using detergent and fine copper wire brush. After this it is ready to be installed.

Shim and lubricate as you would shim any gear set.

The added/extra material on the last tooth on the sector will provide added support when it is under stress, preventing it from breaking off. While the “hard facing” welding rod material and tempering will provide material hardness preventing it from wearing out prematurely.

About The Modification

Been using this procedure with worn-out sector gears with success. Actually my TM (Tokyo marui) M14 uses a sector gear with this modification and has seen its fair share of use and abuse regularly for more than a year without encountering any problem. Will post some updates about the condition of the reinforced sector gear when I open up the gearbox/mechbox for its routine lubrication and maintenance. With the introduction of low priced infinite torque ratio gear sets. This airsoft modification has been considered unnecessary, though I still found it really useful for TM (Tokyo marui) version 7 gearbox/mechbox and found the fabrication of a smaller version 7 spur gear to meash with the infinite torque ratio gears a bit of an impossibility…. Thus rendering this modification a requirement for airsoft guns (particularly the TM M14) using the version 7 gearbox/mechbox for SP/M150 rated spring applications. In order to achieve the desired and required power rating (FPS) for a marksman/sniper airsoft rifle and still get a decent (if not reliable) lifespan out of the gear set.