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Thread: On loose carrier key screws

  1. #141
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    Quote Originally Posted by Disciple View Post
    Slotted is the easiest to form so it will always be around.

    Phillips #2 can be forced to fit a remarkably wide range of screw sizes, whereas Torx of Allen require a set. This must appeal to manufactures who can assume that almost everyone has a #2 Phillips driver.
    At least flat head screws aren't as common as they were in the 1920s, but Phillips somehow became the de facto standard and it's just completely terrible. Everyone has a Phillips screwdriver, but that's only because Phillips screws are used in almost everything. If manufacturers just phased them out over time I believe people would adapt just like they adapted to Phillips.

    But whishing for Phillips to go away seems just as hopeless as whishing that daylight savings time would go away... Both simple things to make happen, but the fact that they won't is proof of just how messed up and f#cked this world truly is.
    Last edited by 556Cliff; 10-06-21 at 20:00.

  2. #142
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    Slot headed screws never caused me any issues outside of trying to turn them with power tools.

    Phillips heads are utter garbage only rivaled by Allen heads.

  3. #143
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    I won't hazard a guess.

    But, I can tell you that you can tell how a bolt or screws fails by looking at the fracture surface.

    An over torque will result in a 45 degree fracture surface, and side loading (shear) will result in flat break. If the failure is pure tensile overload there will be some slight necking at the break and doming.

    This screw probably failed due over-torquing during installation:



    This probably failed due to a crack under the head of the forward screw, and fatigued until overload failure, the rear screw is pure overload, note the small flat spot from 6:00 to 10:00 on the forward screw which indicates a fatigue plane. The rear screw shows tensile overload.


  4. #144
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    So, just how do threaded fasteners self loosen?

    Actually, that is a well researched question and one that is widely misunderstood by some engineers, evidenced by the common term used to describe it: vibrational loosening.

    In 1968 Gerhard Junker published ‘New Criteria for Self-Loosening of Fasteners Under Vibration’ SAE Paper 690055, which summed up his work in testing threaded fasteners in conditions that resulted in self-loosening. His findings were that the primary cause of self-loosening was horizontal motion of the bolt head or nut against the bearing surfaces. To put it in simple images:





    When the bearing surface moves relative to the head or nut, there will be a tangential frictional force 90 degrees to the direction of motion. In one direction the force will tend to tighten the fastener, in the other it will loosen it. when loaded in the tightening direction the preload will tend to resist rotation of the fastener, but in the loosening direction the preload will tend to assist in the rotation. Testing has shown that if sliding motion is reduced the likelihood of self-loosening is reduced. Vibration that does not result in relative motion between the fastener and the bearing surfaces plays almost no part in self-loosening.

    Self-Loosening of Bolts and Nuts You should read the three articles at the bottom of the page as well.

    So, how much motion does the carrier key have, and what forces are trying to move the carrier key?

    There is almost no side-to-side motion with the key as it fits very closely on the channel on top of the carrier, so any motion would be fore-and-aft. This is also the direction the gas from the gas tube pushes so just what magnitude of force are we look at?

    The maximum pressure in the gas tube is around 4,000 psi, the area inside the carrier key that mates with the tube is 0.180" diameter, so the maximum force the key will see in that direction is 102 pounds.

    The clamping force was 4,000 pounds and the frictional force resisting would be in the neighborhood of 800 pounds. There is not much incentive for a properly installed key to move, in any direction.

    Staking is more for piece-of-mind than an engineering necessity. And, remember the maximum removal torque is not supposed to exceed 100 in-lbs.
    Last edited by lysander; 10-06-21 at 22:16.

  5. #145
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    Quote Originally Posted by 556Cliff View Post
    Everyone has a Phillips screwdriver, but that's only because Phillips screws are used in almost everything. If manufacturers just phased them out over time I believe people would adapt just like they adapted to Phillips.
    What would you propose as a one-size-fits-most replacement for Phillips #2? A single driver, not a set. Consider that many people use or try to use a #2 bit on #1 and #3 Phillips fasteners. Or how many slotted fasteners end up with the middle hogged out from someone using a much too small blade on it. There are a lot of people who seem to refuse to buy more than one flat and one Phillips driver.

    Quote Originally Posted by 17K View Post
    Phillips heads are utter garbage only rivaled by Allen heads.
    What is wrong with Allen head?

  6. #146
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    Quote Originally Posted by Disciple View Post
    What would you propose as a one-size-fits-most replacement for Phillips #2? A single driver, not a set. Consider that many people use or try to use a #2 bit on #1 and #3 Phillips fasteners. Or how many slotted fasteners end up with the middle hogged out from someone using a much too small blade on it. There are a lot of people who seem to refuse to buy more than one flat and one Phillips driver.
    A single driver, as in a replacement for the most prolific Philips #2? I'm not really sure I'd feel comfortable with that approach in these modern times... I think everyone should get the whole set of bit's and find the appropriate one for every different size drive slot and there should be no "one bit size fits most" standard like we have with the Phillips #2 today. But I think this modern set of drive slots and bits should be TORX Plus.

    I do see that the first generation of TORX drive screws are catching on in all types of construction and assembly more and more which is a good step in the right direction, but they still haven't overthrown King Phillips.
    Last edited by 556Cliff; 10-07-21 at 11:08.

  7. #147
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    Quote Originally Posted by Disciple View Post
    What would you propose as a one-size-fits-most replacement for Phillips #2? A single driver, not a set. Consider that many people use or try to use a #2 bit on #1 and #3 Phillips fasteners. Or how many slotted fasteners end up with the middle hogged out from someone using a much too small blade on it. There are a lot of people who seem to refuse to buy more than one flat and one Phillips driver.



    What is wrong with Allen head?
    A nearly round hole and a nearly round wrench to turn it has caused me a lot of grief over the years. Not to mention that most of the ones on guns have the wrench cavity the same size as the shaft and cut deep enough that when removing it you’re just about as likely to twist the head off as you are to round out the hole.

    Slotted screws are a generally tougher design.

  8. #148
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    Quote Originally Posted by 17K View Post
    A nearly round hole and a nearly round wrench to turn it has caused me a lot of grief over the years. Not to mention that most of the ones on guns have the wrench cavity the same size as the shaft and cut deep enough that when removing it you’re just about as likely to twist the head off as you are to round out the hole.

    Slotted screws are a generally tougher design.
    Agree. Although you should have a good set of hollow ground screwdriver bits to work with slotted screws to lessen the chance of slipping out and damaging the slot... I can't stand common (tapered) flat head screwdrivers.
    Last edited by 556Cliff; 10-07-21 at 11:15.

  9. #149
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    I do. I despise chisel tip screwdrivers.

  10. #150
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    Quote Originally Posted by lysander View Post
    It all depends on how you see them fail.

    - Screw head breaks just under the head. These are usually the result of over aggressive staking that winds up side loading the screw head (FM 1). This is why the diameter of the head needs to be controlled very tightly so there is little play between the head in the counter bore. The other possibility is the screw was over-torqued during assembly (FM 2). If you look at the fracture surface, you can tell which failure happened, An over-torque will result in the 45 degree shear plane, side loading in a 90 degree plane.

    - Screw is loose. This can have several causes: (FM 3) Insufficient torque to start with. (FM 4) Dirty mating surfaces during assembly giving a false clamping at specified torque. (FM 5) Over-torque, but instead of cracking the screw, the threads yields. (FM 6) threads yield under service loads (in this application, highly unlikely as the forces in the axial direction are rather modest). (FM 7) Screw un-screws itself under service conditions. (FM 7) Tampering, or unauthorized removal of the screws.

    In the AR design, only the last the last two failure modes are mitigated by staking, so the argument to not stake does have merit. The primary reason the Army wanted staked screws was to prevent Private Snuffy from thinking it was okay to remove and clean under the carrier key (or, maybe Sergeant White-Gloves, and Lieutenant Inspection).

    From what I have seen in broken screw heads: side loading is the biggest problem.

    The biggest advantage I see with Ned's OCKS knurling, is that it gives excessive material displacement most people seem to feel is necessary a place to go before it starts to bear in the head of the screw and side-load it. (And, keeping the head dimension consistent, that itself is worth a good bit.)

    If you read the drawing requirements, staking is only supposed to add 5 - 45 in-lbs to the removal torque. That does not require much in the way of material displacement.

    Oh, and if you use a MOACKS, or similar tool - Tighten opposing screws at the same time, do not tighten down one side, then tighten down the other.
    THIS /\ /\

    The TDP even states to do this. Stake both sides of the screw simultaneously. Of all the videos I have seen of people staking, they do the opposite, they do one side then the other.
    Last edited by the AR-15 Junkie; 10-22-21 at 07:47.

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