By MikeG
June 9th, 2010
Ever wondered if there were new and exciting ways to make gearbox parts incompatible with each other? Well, AIM-TOP has certainly found several.
Recently in my airsoft teching adventures, I recently ran across an AIM TOP box that had suffered from catastrophic failure. The piston had lost its first steel tooth and the spur gear had lost several of its own. As for the piston, honestly, how the hell does that happen? 1: The teeth are steel, 2: The first tooth has the least stress due to spring tension. My current theory is that the gear racks are incredibly brittle and that the sector gear impact during piston pickup shattered the rack. Given the low durability here, it makes the piston inexcusably terrible. Who thought removing 55% of the piston rails was a good idea? How about deciding not to chamfer any of the piston rail edges? Brilliant! If you’re looking for places to remove material, the top of the piston is about as good as it gets. G&P did something like this with their white polyacetal pistons (which I still don’t like terribly much). But no, AIM TOP decided to be funny and remove rails so that the piston has a better chance of snagging in an M4 style gearbox.
As a replacement, I chose a KingArms POM Hard Piston. It’s a copy of the Prometheus Hard piston design, so I felt it merited a try. After installing the new piston, I was checking AOE and manually running the piston through the gun. I noticed there was some resistance at the end of the piston’s stroke. For reference, I tried the original AIM TOP piston – no resistance. I then tried a Guarder Polycarb piston – resistance. It appears that AIM TOP has made the ‘wise’ decision of oversizing the diameter of the bearings on their spring guide, and thus have been forced to make the inner diameter of their piston larger as well. This increase in bearing diameter means that the spring guide will not properly nest in a normal piston. So in my case since the piston must go, so must the spring guide. Dimensions & Illustrations follow.
- Correct Bearing OD: 15.0mm
- Correct Piston ID: >15.1mm
- AIM TOP Bearing OD: 16.0mm
- AIM TOP Piston ID: 15.8mm (not a typo, actually slightly smaller than the piece it must fit over, good going AIM TOP)
Not to be outdone, however, the spur gear had managed to loose three teeth, bravo! Since the AIM TOP gears have a proprietary tooth profile the entire set, although nearly new, needed replacement.
1000 rounds isn’t exactly all that impressive. Let me know if/when you breach 10,000+ without incident. Anyway, I don’t doubt that the gearbox I worked on probably had factory defects.
My friend has this exact same combo running his ICS M4, and it’s run fine for over 1000 rounds on an 11.1v LiPo and pulling a 430fps+ spring. The only issue we’ve run into thus far is the noise, which I seem to have fixed by reshimming the Anti-reverse latch(yeah, his ICS needs shimming of that little thing)
Outside of that, it looks like it has minimal wear on it, while it could use AOE correction, so far I haven’t had a single problem with any of them. Perhaps you got a dud set… :/
This same problem happened to me but it was my fault as I didn’t have a good AOE. The gun was working perfectly fine before I took it apart to install an M100 spring. My front tooth just snapped off but the gears were not harmed. I ordered a Guarder Piston to replace it.
FYI unless he bought a different version the spring he had was probably the M120 which shoots around 450.
The gears look nice enough, but I’m not overly convinced of their durability after this experience. I’ve also noticed that they seem to be a bit noisier than gears with a standard tooth profile. If I were buying gears on a budget, I’d go with XYTs or Guarders.
What do you think about that gear set’s quality? I have ordered that gear set (come with the same piston) but have not actually try out that gear set yet.
1 extra thing i want to mention about that piston. its steel tooth was not tapered like others piston teeth do. With my G&G gearbox, the edge of the steel teeth was stretching my cylinder. That was fixed by replacing the damaged cylinder and use dermal to taper the edge of that steel teeth.
Ascelyn,
It sounds like the situation you are describing is pre-engagement of the piston. Given the high torque motor, high rate spring and relatively low ROF of the gearbox, I highly doubt that any pre-engagement occurred. I agree it is strange that both the piston and spur gear were missing teeth, but it’s likely that the spur gear lost its teeth first but the gearbox was still able to turn and was still able to run for a short amount of time till the piston tooth broke as well. (Probably thanks to the extra dynamic loads that a broken gear would add to the system).
The spring is a very high rate one, likely in the 450 to 500 fps range.
I used that same piston in my G&G gearbox, its metal tooth is actually quite strong. If the piston tooth was really broken by just the sector gear impact force, the spur gear wouldn’t lose any teeth. My guess is that it was caused by improper AOE. The piston tooth was broken when the piston was striking back (passing the proper AOE position but hasn’t stop by the cylinder head yet) and the sector gear return at the same time. therefore both force impact head on so both the last tooth of the piston and the spur gear teeth broke. What spring was it using anyway, sounds like a real strong one to break that last piston tooth.