Guess I've been away too long.
Lysander, whence the broken screw pic?
|
||||||||||||||||||||||||||||||||||||||
Industry Professional
Guess I've been away too long.
Lysander, whence the broken screw pic?
Senior Member
Without holding the key in a fixture I don’t see any way to really stake both sides simultaneously. Just holding it in the clamp and tightening both screws at the same time would still cause the side with least resistance to deform first and then the other side deforms with the force being held by the screw head on the other side.Originally Posted by the AR-15 Junkie
Senior Member
He's got some pics posted in post #143 a few posts up.
Senior Member
whence
adverb
from what place?: Whence comest thou?
from what source, origin, or cause?
Senior Member
Ah, missed that detail.
“You have made us for yourself, O Lord, and our heart is restless until it rests in you.” -Augustine
Senior Member
Googling "broken gas key screw"
The diagnoses on the cause is my opinion, based on what you can see of the fracture.
Last edited by lysander; 10-28-21 at 19:05.
Member
PM sent.
Industry Professional
I totally agree with your opinion. I was concerned many years ago when I brought out the first MOACKS but I could not make it happen in testing, and all these years later I have never had a complaint about it. Doesn't mean it never happens, but if it has, it didn't get reported to me. I have not tested it further due to no incidents reported.... makes me wonder now and I ask what you think, could it be that with the screw torqued down, the clamp friction on the underside of the head is sufficient to resist any side loading from staking, keeping the head from moving laterally, and then when the other side is staked, any or most of the residual side load is balanced out?
On my industrial stakers, both sides, all four stakes occur simultaneously so there it should be no issue.
Senior Member
With the screw torqued to 58 in-lbs, the clamp load is around 1800 pounds, so the friction force available to resist side loading is around 350-400 pounds. A 10-28 UNC thread torqued to 20 in-lbs* generates a clamp load of 550 pounds, so a MOACKSwill generate a much larger load, obviously, as it deforms the metal of the carrier key. So, what happens:
1. The carrier key will slide over until the threads of the screw, the screw cap, or the far side of the slot in the carrier stop it. Which it hits first depends on the tolerances, if the screw threads or cap stop it, you'll have trouble, because you are side loading the screw.
2. In the nominal conditions the carrier can allow up to .009" side-to-side movement of the key, and the clearance between the screw threads and cap and the holes are .003". There are 64 cases for the tolerance stacks between the three pieces, (big cap, big screw cap, big key, big carrier/big screw, big screw cap, big key, small carrier/ . . . etc) a third of them have the the threads or cap stopping the side ways movement, and side loading the screw. The other two-thirds have the side loads taken up by the carrier slot.
3. The range of clearances range from 0.0005" to 0.018", and the stresses on the screw can be quite high.
4. Unfortunately, when you go to stake the other side, you do not relieve any stress put on the screw, if there was any. The reason is: if the screw was displaced a few thousandths of an inch after the first staking, it is now deformed and unless sufficient load is applied on the opposite side to not only equal the initial load but push the screw into the displaced staking, the screw will remain deformed, and under stress.
__________________________
* 20 to 25 in-lbs is the torque a person with average hand strength can generate with a 1 inch diameter knurled knob
Last edited by lysander; 11-17-21 at 08:27.
Senior Member
When I say "over-torque", I mean the torque applied was more than what the screw could stand, not necessarily that you applied a torque in excess of what the book recommends.
If you only applied 58 in-lbs, the screw was weak.
Posting Permissions
Reply With Quote
Bookmarks