Bad motor?

Discussion in 'Gun Building, Modifications & Repairs' started by Gray., Oct 8, 2020.

  1. Gray.

    Gray. New Member

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    I've been working on a broken AK47 a bit, and I suspect the motor's bad. It's a Chaoli, pretty sure it's unmodified. However, before I buy a new one or look into it more I was wanting to ask you guys.

    So, I was doing some research which is what led me to believe the motor's the problem.

    ISSUE: The piston pulls back about 1/2 to 2/3 the way but won't go any further, hence staying in that position.

    I know it's not a gearbox issue, all of the parts in there look good, plenty of lubrication, and it all works right. (I think I messed up the shims though, oops!)

    When I turn the motor shaft with my fingers, it does NOT turn smoothly. It feels rough and kind of sticky.
    The motor does, however, spin when I give it power. I'm pretty sure it can't handle the stress of compressing the spring all of the way.

    Any thoughts? I'd be happy to give more information or pictures if requested.

    (Also feel free to give me suggestions on better formatting this post)
     
  2. Ben3721

    Ben3721 Active Member

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    Well, dc motors can have many types of failures.
    it simply could have thrown a wire off the armature and now its just shorting out and grinding.
    It could have a wire short to the armature, which is important to test for any new motor.
    A magnet could has dismounted and is dragging on the armature.
    The brushes could have a defect.

    I would take the motor apart, but most likely its done for. Luckily motors aren't very expensive.
     

  3. Gray.

    Gray. New Member

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    Ok, thanks for the reply. I'll take it apart and look, most likely I'll buy a new motor. Even if I don't need it, I guess it's always good to have an extra motor around.
     
  4. aotsukisho

    aotsukisho Well-Known Member

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    Messing up shims might result in you locking up the gearbox if it is shimmed too tight.

    Depending on the strength of the magnets, the armature will feel 'ratchety' or like a tactile feedback volume knob clicking while turning. This is due to the magnets interacting with the poles on the motor as they will 'find' an equilibrium multiple times during rotation.

    Stronger magnet motors might make this difficult to use as a gauge, as the strength of the ratcheting will make rotations feel rough when done by hand. If the unloaded running motor doesn't sound screechy or grainy it's probably fine

    This probably would've manifested as the battery heating up quick or fuse blowing, and a loose magnet would be immediately obvious I think just handling the motor.

    Wire short in the armature, I think you meant to say, shorted coils due to damage or insulation failure.

    Possible, brush-commutator contact needs to be clean, brushes should not bind in the brush hood, and brush springs should be equal in tension and moving freely. Need to also make sure the brush tail is pinched by the spring into the hood for best electrical contact.


    Gray, there is some critical information missing in order to diagnose your situation.

    1. Motor spec (turn count, magnet type)
    2. Battery spec (voltage, discharge rating, connector type)
    3. Gearbox spec (main spring, gear ratio, wire/connector condition, MOSFET controller/trigger switch condition)

    https://play.google.com/store/apps/details?id=jp.nas.giri&hl=en_US
    Another test you might want to run is see if your phone can detect the ballpark RPM with an app like this and match it up to your motor's datasheet. If the RPM is significantly lower than spec then something with the motor needs to be fixed.
     
    Gray. likes this.
  5. Gray.

    Gray. New Member

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    Messing up shims might result in you locking up the gearbox if it is shimmed too tight.
    Ok I'll make sure to fix that later.

    Depending on the strength of the magnets, the armature will feel 'ratchety' or like a tactile feedback volume knob clicking while turning. This is due to the magnets interacting with the poles on the motor as they will 'find' an equilibrium multiple times during rotation.

    So, I turned the shaft again with this in mind, and the 'sticky' feeling I had must have just been from the magnets. It had six distinct spots where it was harder to turn, with six spots in between them where it turned smoothly. Motor seems less likely now. Thank you.

    Possible, brush-commutator contact needs to be clean, brushes should not bind in the brush hood, and brush springs should be equal in tension and moving freely. Need to also make sure the brush tail is pinched by the spring into the hood for best electrical contact.
    I'll check this out if I can get the motor apart.

    Gray, there is some critical information missing in order to diagnose your situation.

    1. Motor spec (turn count, magnet type)
    2. Battery spec (voltage, discharge rating, connector type)
    3. Gearbox spec (main spring, gear ratio, wire/connector condition, MOSFET controller/trigger switch condition)


    1. Turn count: i think it's around 18,000-22,000 rpm?
    Magnet type: looks like neodymium

    2. Voltage: 8.4V
    Discharge rating: 650mAh
    Connector type: looks like a B3

    upload_2020-10-9_16-29-44.png <-this kind
    (Battery is Ni-MH)

    3. Main spring: Muzzle velocity on the website says new is 360-380 fps, so I assume it would be a M110 spring?
    Gear ratio: 18.72:1 (you'd just say 18:1, right?)
    Wire/connecter condition: wires and connectors all look good, pretty sure there's no shorts anywhere
    Trigger switch: just a simple metal contact, it's simple and works well
    (Version 3 Metal Gearbox)

    Gun model is A47-0509!

    Need any pictures?

    Edit: I just used your RPM measurement app, and it said my motor was running at around 20,000 RPM, but then my battery died... recharging it now. Unloaded motor sounded a little grainy.
     
    Last edited: Oct 9, 2020
  6. aotsukisho

    aotsukisho Well-Known Member

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    RPM works as well, as a general rule turn count and RPM are inversely proportional. Lower turn count and higher RPM means higher power consumption.

    Neodymium magnets tend to be shiny and silver, due to the protective plating on them. Ferrous magnets are usually a dull grey or black. Neo magnets have different strengths depending on quality, but can always pick up larger tools like screwdrivers and such with magnetic flux alone. Ferrous magnets usually can cause tools to stick, but cannot hold the motor's weight unsupported.

    Leave the endbell connected to the can, you can just pry out the brush springs with a small screwdriver and visually inspect the contact surface of the brush and the comm through the brush hood. If there is a buildup of carbon, a pencil eraser can clean it off. Be gentle with the brushes, they are relatively brittle and can crumble.

    This battery does not seem like it is large enough to provide the motor with enough power to cycle, 650mAh is below even AAA battery capacity.

    Yes that's correct, likely M110 or M115
    Yep stock 18:1 ratio
    I was mainly asking if the gun has seen some wear and tear, pitted trigger contacts can also be a failure point

    If the motor is running fine unloaded close to spec, then that is fine. 22krpm is likely taken at 7.2-7.4v, and if your battery is as small as you say then voltage drop would've sagged it to 6v or lower so I think the motor is not in question.

    See if you can find a larger battery around the 1000+mah range in order to provide the necessary current to the motor in order to get the entire cycle completed. Most AK style NiMH that I've seen are around the 1400-1600mAh range, lipo batteries can be smaller capacity due to their ability to deliver more current (minimum 850mAh 30C, 1000mAh 25C).
     
    Gray. likes this.
  7. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    650mah NIMH? I don't think they ever made a pack with cells that low.

    It's not a green cell is it? Says Sanyo on it?

    Plus...not all motors "purr" when under use. Back 10 years ago I had a motor dyno for Airsoft and here is the link.

    http://forum.mnairsoft.org/viewtopic.php?f=59&t=28181&p=305096&hilit=Motor+dyno#p305096

    Some motors were very scretchy...


    ICS Infinite - Long Type


    RPM - 25400
    Amp Draw - 2.12 Amps
    CONS - 17
    Noise - High, screetchy whiney motor
    Heat - Very warm to the touch.

    All motors were tested at 9V and notice the RPM's...it's all over the board.

    TM EG-3000HC (High Cycle)

    RPM - 32800
    Amp Draw - 2.92 Amps
    CONS - 3.5
    Noise - Low, it hummed with a slight pitch that reminded me of a dentist drill.
    Heat - barely warm to the touch.

    Result the Current King for Speed!

    And now the motor you are waiting for....

    G&P M170 Devil Jet

    RPM - 0
    Amp Draw - 18.7 AMPS
    CONS - 0.0
    Noise - None
    Heat - None, slight vibration at 1.4V @ 17amps like the motor was trying to spin up.

    Results - DOA like a cheap ACM motor. Note: It was very hard to turn even by hand. Magnet was really powerful, like Angelina Jolie to Brad Pitt on the set of Mr. and Mrs. Smith. When 4" away it latched onto the EG3000 like...well AJ on BP....

    I will be contacting my distributor about the bad G&P M170.

    Test with replacement motor!
    G&P M170 Devil Jet
    RPM - 34000
    Amp Draw - 4.7 AMPS
    CONS - 50.0 (Holy Moly!)
    Noise - High, shrieky high pitch
    Heat - Moderate, was getting warm to the touch quickly.
     
    -Spitfire- likes this.
  8. Gray.

    Gray. New Member

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    Ok, in that case it's ferrous magnets.

    The internals of the gun all look great, except some minor things that won't make a difference.
    The outside of the gun, however, looks a bit beat up. Luckily I only care about the inside.

    Ok, gonna borrow a friend's battery and test if that's good enough. The ak had been running off a NICD 1500mAh 8.4v that was old and done for, so I was testing with my cheap battery from my cheap gun...

    So I guess I'll re shim the gearbox and buy the proper battery.

    Again, I appreciate the help. Now I have some more knowledge at my disposal as well!
     
    aotsukisho likes this.
  9. Gray.

    Gray. New Member

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    Here's an update. (Little bit late)

    The gearbox worked great with a higher power battery. Notice how I used the past tense... After testing the gearbox to make sure it cycled fine, this happened:
    IMG_20201024_151213403.jpg IMG_20201024_151232493.jpg
    The... front of the gearbox cracked...

    I suspect the gearbox shell was made of cheap pot metal, and it was weaker after being stored without use for a long time.

    Maybe it's my fault, I'm not really sure.
     
  10. Ben3721

    Ben3721 Active Member

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    That sucks. Typically version 3 shells are a bit more durable than v2. But a radius of the shell window would have likely prevented this from happening long ago
     
  11. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    Storage would not have weakened it. However, if you used it in cold weather and the spring was too heavy or overloaded, it can contribute to that type of failure.

    Most mechboxes are zinc allow pot metal btw...
     
    -Spitfire- likes this.