I have always installed thick barrel bushings and plugs on my guns usually the extra thick ones. I use both EGW and Kart bushings for this. I don't use high rated recoil springs any more so as not to batter the lower feet of barrel lugs and slide stop pin... BUT here's my question, where is the stress on the bushing? Is it at the lower feet "where it curves to the plug"? Or is it at the little notch that goes in the slide "slotted" for it? Or when bushing is inserted, does the outer diameter of plug, if mated well to slide take the stress? So as not on the notch? Basically I am curiuos where the weakest point is for breakage on a standard bushing and plug, and where they usually fail, say if someone is running a 22 pound spring or something and shooting high powered rounds? Where is the fail likely to be? The lower feet, or the slooted notch etc etc?
There are two points of stress - where the recoil spring plug transfers the load into the bushing, and the slide tab where the bushing transfers the load out of the bushing to the slide. The amount of force to be transferred is a constant, equal to the strength of the spring when compressed to recoil. For a standard Government model, that's 16 pounds. What ammo is being shot doesn't affect this. If the recoil spring is a 22-pound spring, then that's the maximum and the choice of ammo won't change it.
An improperly-fitted spring that binds before the slide stops will exponentially increase the force, however.
Stress is force over area. The parts of the bushing are close enough together that we're really concerned with shear, not bending. The first load is resisted by an area that's the thickness of the flange times the circumference of the hole for the barrel. The second load is resisted by the area that's the thickness of the tab times the length of the point at which it makes contact with the slide.
Measure up your bushing and calculate those two numbers. Divide each area into 16 and whichever gives you the higher stress is the one.
Keeping in mind that even mild steel has a strength of about 22,000 psi, you can see that it does not require a very large area to resist a load of even 22 pounds. In fact, you only need an area of .001 square inches. For a 16-pound spring, you only need .00072 square inches. A standard barrel bushing easily provides that much area in both stress locations.
thanks... very informative... understanding the stress on lower tabs is easy enough... however when hand fit a bushing to slide so as tolerances are very snug with the slide and bushing, it is possible that the resistance is then just transferred to the "entire" bushing outer diameter and slide? As the spring and plug will be also be having pressure on bushing into slide metal also... so as "shear" force will not just be translated to bushing tab and slot... granted I could see the force all being to the tab with a loose poorly fitted bushing, but when you hand fit one so as metal is essentiall in full contact with bushing to slide, will this "essentially" relieve the stress off the "tab" with a end result of a stronger more dureable set up... or will it always be tab no matter how tight or loose the bushing to slide fit...
I think you're correct, with a tightly-fitted bushing (one that's bordering on a press fit), there are two things in operation that will relieve stress at least on the tab, and/or shift stress elsewhere. First, in a directly linear pull there would be frictional resistance. But since the force input is NOT axial to the bore, the force is applied eccentrically. The recoil spring pushes the plug, which in turn pushes against the flange below the bore centerline. The tab is more or less at the bore centerline (vertically), so as the spring pushes the bushing is going to want to rotate around the bore centerline (as viewed from the side of the slide, like in the STI animated view). So the bushing is going to be trying to get wedged/cocked in the slide, which would likely cause a significant amount of resistance to allowing it to move forward out of the slide.
However, I don't see any way this geometry would reduce the stress at the flange-to-bushing interface. The flange is outside of the slide, so regardless of how the "bore" portion of the bushing is being retained in/by the slide, the flange will always be subject to pretty much the same force trying to separate it from the actual bushing part.
No matter how you alanyze it, unless there's a spring bind issue I just can't see a problem. Even 22 pounds is NOT a lot of force to apply to steel. The neat thing about this is the way any structural element works. Going back to my first-year structural engineering class, the professor expressed it very succinctly. (You have to envision this being said by a very short Scotsman with a definite "Napolenonic" complex and a [i]heavy[i] Scottish burr. Think Scotty on the original Star Trek.)
"Strrrrrress always goes where there's mateeeeerrrial to rrrrrreeeesist it."
I just get to thinking, on things to try to perfect for the perfect carry piece... so far I have come up with Norinco, with Kart easy fit barrel and Kart thick bushing and thick plug... with close attention to all parts working correct and mating finely, building mine now, takes a little while... I also change out the disconnector, as many just seem a wee bit too short on the lower paddle end... now I use "Nighthawk" disconnectors, as they are machined from barstock, hardened and coated, and are a little longer than most stock ones, but they have worked perfectly and have "better fit" than all the originals I have replaced... so far anyway... on 2 Norinco's and a Para LTC Commander, oddly enough the commander was the exact same dimensions as the Nighthawk disconnector, but just wanted to replace it with the barstock one, as essentially everything else was changed out already... the Norinco's were both shorter on the paddle end, and neither was ground true or square, so out they came... good parts if I can grind the paddle square on the lower end, will use them as spares for guns they fit...