Modified gearbox dramas

So just finished a build with Aus gel 1.38 spring, SHS 16:1 metal gears, metal nozzle, chi hai purple motor, 11v, plunger head and rack. Shimming seemed pretty good when I put it together. Anyway after two mags it ran super hot and the trigger melted!! It didn’t sound great initially but I played with the motor height and it sounded ok. Rpm wasn’t super high but I put that down to the larger spring.

I’ve done the same build in another HK with 18:1 and 1.28 and it runs fine. Would the 16:1 and 1.38 be the issue or is it likely to be a shimming issue? After looking at the shell it looks like there is some wear around where the pinion would be. Teeth looked fine. Maybe bevel height is wrong and pressing down on the pinion/motor shaft??? The sleeve on the motor is the one that came with it but I changed it in the other build. Could this make a difference??


did you notice any melting or damage on the wiring?
also after you shim the box, did you test for gear spring, was it smooth?
coz the only thing that melted seems to be around the trigger metal contact area,
so that’s where the heat is building up, which is an electrical issue rather than mechanical O_O?

maybe the spring is too much for the motor to pull, hence the “not super high” rof,
or maybe the shimming is too tight?
something is causing the current draw to be too high and heat up the contact then melt the material

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Wires, motor and battery were hot but no melting. Will go back to a 1.28 spring I think and trying shimming the bevel again.

A trigger contact seems hard to find on initial googling. Thinking of maybe going for a mosfet that replace the trigger contact like a Perun v2. Thought??

Hi Mate. I’m also relatively new to this. But 16:1 is a high speed gear. Purple chai hai is also a high speed motor. 1.38 is a heavy spring. The problem is your setup has no torque. So essentially you are “stalling” your motor causing excessive amp draw and the weak point is your trigger contacts. If you drop your spring rate you might run into pre-engagement. If you add a mosfet your weak point will be your wiring. I would do a bit of research regarding high ROF setups. Your ROF is 39,000/60=(650RevsPS) 650/16.75(gear ratio)=38 10% efficiency loss = 34gels per second. With that setup.

To me that photo of the damage to the gearbox looks like it has melted from motor heat.

Good luck. Hope this helped.

:thinking: good point on the pre-engagement. Will see how it goes.

Any ideas on where to get a new trigger contact??

pretty sure the v2 trigger blocks will screw straight in. I’ve done the reverse… used a war interest trigger block in an MK V2 and H92, simply because of the quality wiring. That motor (high speed) will be drawing a fair bit of current as opposed to using a high torque with the exact same build.

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I know it seems excessive but when i shim a nylon box as well as putting the screws in i put it in the receiver. Then i attach the grip, put the mag release in and put the upper on and front pin and check the shimming through the ejection port for tight spots.

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Thanks for the link. That’s exactly what I need! :ok_hand: