Gears Stop in Wrong Position, and Spring Guide Moves - V2 gearbox

[Clanktank]

Building a DMR from a V2 gearbox, but I have two issues:

1. When firing the gears seem to spin too far until they meet the piston again, so the resting position after a shot is the sector gear partially engaged with the piston and the tappet plate pulling the nozzle back.
Has a high torque motor, SHS piston & head, SHS cylinder & head, and SHS 13:1 gears. Running 11.1v motor with 110 spring, but it's an interim spring as a bigger one will likely be needed to get to DMR power.
Cutoff lever changed for a steel one, but is exactly the same shape as the original.
Same happens with original gears and motor as well. Both sector gear cams only seems to move the cut-off lever a small amount, but it must be working otherwise it would go full-auto.
Tappet plate is moving properly until the first shot, so the issue is not that, it must be the gears rotating too far.
Picture shows what I see immediately on opening the gearbox.
Ignore the bent tappet, it's just because it's now not in the guide, and the anti-reversal was temporarily taken out as well.

 

2. The spring guide moves out of alignment (tilts) every few shots and blocks the piston travel. It's a quick change guide and has the proper seating slots. Anyone had this issue?

[Adolf Hamster]

so issue number 1 is very common, and not really an issue per-say. essentially the motor and geartrain still has momentum when the cutoff lever trips and will roll round until stopped. it's basically free precocking, as all precocking does is intentionally run the motor a little longer so it stops near the end of travel. a mosfet with active braking will remedy this (although sometimes can go too far and stop the system too quickly giving lockups on semi), a better solution would be a mosfet with precocking which will let you intentionally control where the box stops.

 

issue number 2 i'd say is happening because you're testing it outside of the reciever (going by the picture), assuming this is going to an ar derivative (the sr-15 in your profile) then when you put the gearbox into the reciever and bolt on the buffer tube/stock, the bolt that threads into the back of the box will go into the spring guide and hold it tight to the back of the box, stopping it from tilting.

[Asomodai]

Issue 1 is not really a problem and will probably be non existent once you put the heavier spring in. It really helps with trigger response without paying for a pre cocking mosfet if you want to look at it that way! 

 

Issue 2, as above. You need to put it in the receiver and install the stock tube. 

[Clanktank]

59 minutes ago, Adolf Hamster said:

so issue number 1 is very common, and not really an issue per-say. essentially the motor and geartrain still has momentum when the cutoff lever trips and will roll round until stopped. it's basically free precocking, as all precocking does is intentionally run the motor a little longer so it stops near the end of travel. a mosfet with active braking will remedy this (although sometimes can go too far and stop the system too quickly giving lockups on semi), a better solution would be a mosfet with precocking which will let you intentionally control where the box stops.

 

issue number 2 i'd say is happening because you're testing it outside of the reciever (going by the picture), assuming this is going to an ar derivative (the sr-15 in your profile) then when you put the gearbox into the reciever and bolt on the buffer tube/stock, the bolt that threads into the back of the box will go into the spring guide and hold it tight to the back of the box, stopping it from tilting.

Okay, that all makes sense, but it sounds pretty awful.
This also means I have other issues with air seal as the fps was jumping between 170 and 230 when first test firing, so I think the nozzle may be the wrong length.

[Adolf Hamster]

2 minutes ago, Clanktank said:

Okay, that all makes sense, but it sounds pretty awful.

 

you mean the gearbox noise?

 

if it's gear noise then shimming and particularly motor engagement sounds like the culprit.

 

4 minutes ago, Clanktank said:

This also means I have other issues with air seal as the fps was jumping between 170 and 230 when first test firing, so I think the nozzle may be the wrong length.

 

assuming you've done the usual static checks to ensure the piston/piston head/nozzle is sealing good then nozzle-hop is usually then the culprit.

 

check your hop unit positioning, typical for AR pattern guns to have a little play between the gearbox and outer barrel when assembled, a spring or o-rings around the outer barrel in front of the hop unit will push it tight to the gearbox and ensure it's in the right place, after that then looking at the nozzle length/hop rubber feedlips would be the usual suspects.

[Clanktank]

54 minutes ago, Adolf Hamster said:

 

you mean the gearbox noise?

 

The gearbox was perfectly shimmed, it just clunks and isn't smooth firing, but part of that may be the piston guide needing bracing with the stock screw.

[ak2m4]

 

11 hours ago, Clanktank said:

This also means I have other issues with air seal as the fps was jumping between 170 and 230 when first test firing, so I think the nozzle may be the wrong length.

 

Looks like that's either a mp5k or AK long nozzle ?  Grab yourself a M4 nozzle.  Give it a measure first though, get yourself a set of digital calipers.  Spring guide tilt is a pita when testing, this is why I love the e&c shells so much, they use 4 lugs and prevent this.

[Clanktank]

52 minutes ago, ak2m4 said:

 

 

Looks like that's either a mp5k or AK long nozzle ?  Grab yourself a M4 nozzle.  Give it a measure first though, get yourself a set of digital calipers.  Spring guide tilt is a pita when testing, this is why I love the e&c shells so much, they use 4 lugs and prevent this.

It is an M5 clone but has an M4 nozzle, so the new nozzle  is the exact length as the original. The cylinder head has changed so could this make a difference?

 

13 hours ago, Adolf Hamster said:

so issue number 1 is very common, and not really an issue per-say. essentially the motor and geartrain still has momentum when the cutoff lever trips and will roll round until stopped. it's basically free precocking, as all precocking does is intentionally run the motor a little longer so it stops near the end of travel. a mosfet with active braking will remedy this (although sometimes can go too far and stop the system too quickly giving lockups on semi), a better solution would be a mosfet with precocking which will let you intentionally control where the box stops.

 
I've been thinking about it over night but still have concerns. If this is common then what is it called? Whenever I search for over-spin then always diagnosed by having a double-tap, but this is just the sector gear basically hitting the piston again with no further travel, but why does it happen on both old and new gears and parts?

Edited January 23, 2022 by Clanktank

[colinjallen]

15 minutes ago, Clanktank said:

It is an M5 clone but has an M4 nozzle, so the new nozzle  is the exact length as the original. The cylinder head has changed so could this make a difference?

 
I've been thinking about it over night but still have concerns. If this is common then what is it called? Whenever I search for over-spin then always diagnosed by having a double-tap, but this is just the sector gear basically hitting the piston again with no further travel, but why does it happen on both old and new gears and parts?

It is called momentum.  A moving object will not stop dead unless a brake is applied to it; it will come to a halt once it has been slowed sufficiently by friction or another form of resistance.

It does not have a name in airsoft; it is just something that happens.

[Adolf Hamster]

15 minutes ago, Clanktank said:

I've been thinking about it over night but still have concerns. If this is common then what is it called? Whenever I search for over-spin then always diagnosed by having a double-tap, but this is just the sector gear basically hitting the piston again with no further travel, but why does it happen on both old and new gears and parts?

 

don't really think there's a particular term for it, it's just a partial overspin.

 

when the cutoff lever trips, and assuming there's no active braking from a mosfet, the motor just shuts off but the mass of the rotating parts, the gears etc are still spinning so they just keep rolling until something stops them, which will be a combination of natural friction, the strength of the magnets in the motor (eg a strong neodynium motor will slow down quicker) and, when it hits the piston again, the energy to pull back the spring.

 

overspin usually isn't commented on unless you have a fast enough setup that it can not only pull the piston back a little, but all the way to roll over to a second shot (which is the double tap). as mentioned it's the same effect as you're intentionally trying to acheive if you're using a mosfet with precocking, which will run the motor intentionally a little longer so when it stops it's nearly all the way back, idea being the time between pulling the trigger and the shot actually firing is much shorter without the need to have a really fast motor/gear combo.

 

it's genuinely nothing to worry about, indeed depending on the setup you actually want it to happen a little bit, as if you stopped the motor too quickly (eg by using active brake) then it can stop quick enough to have the cutoff lever still engaged and lockup the trigger.