Snap!

[Colin Allen]

Dear Hive Mind,

I managed to do something today that I have never done or seen before.

I recently built a gearbox for a boneyard rifle that I bought.  It is a ZCI gearbox shell with the internals being pretty much what came out of the old gearbox, including the King Arms 18:1 gears.  The shimming was very nice and the gun sounded rather good.  AOE is also good.

Today, I took it along to a site to test it; at first, all was well.  Then, nasty going noises started, ending with the whirring sound of death.

On getting it home, I discovered what is seen in the photo: the spindle of the KA spur gear has sheared.

The gun was running on a 1450mAh 25c 11.1V LiPo driving an M105 spring.

So, Hive Mind, what went wrong?

 

View attachment 112226

 

[DerDer]

Any clue as to the specs of the motor? Might be an avenue worth exploring...

 

[gavinkempsell]

Well as someone who knows feck all about the subject my tupence worth is... It's been unbalanced for a while and the vibration finally got the better of the axle/gear/ shaft.

 

[Colin Allen]

Any clue as to the specs of the motor? Might be an avenue worth exploring...

G&G Afrit 25K.  Further investigation reveals two teeth missing off the lower (complete 360 degree) section of the sector gear.

I think I have found the problem; the spur gear bearing on the left hand side of the gearbox does not run anything like as smoothly as any of the others; it is pretty awful.

 

[DerDer]

That could certainly be the culprit. Had a V2 gearbox, where it was quite gritty sounding.  Ended up having to knock the gears out with a pin punch. Opted to replace the bearings with bushings and seems to be holding up nicely.

 

[Colin Allen]

That could certainly be the culprit. Had a V2 gearbox, where it was quite gritty sounding.  Ended up having to knock the gears out with a pin punch. Opted to replace the bearings with bushings and seems to be holding up nicely.

Yes, that is what I did this morning; bushings have been pressed in and new gears are on their way.

 

[Leo Greer]

Most likely the bearing issues are a symptom of the problem, not the cause. Issues like this are usually related to the spring bending, or the piston wobbling around while cycling and causing uneven force on the spur gear, which translates to the bearings beneath the spur gear failing significantly more than other positions.

We see this a lot more in America where spring power is typically much higher, and most American techs use bushings, or J-caged bearings for that reason, since regular bearings aren't equipped to handle that force. I'm actually surprised this happened with your build, as lower power builds are most often fine with bearings, in my experience.

In your particular case, it sounds to me like you had a bad gear combined with another separate issue, as the gear should always be stronger than the bearing. Higher RPM also exacerbates the issue (shorter cycle time increases the force by quite a bit), but it sounds like your build is rather moderate.

What spring are you using? Is the piston a good fit with the gearbox shell?