Re: Aluminum vs. Steel Scope Mounts
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">In a situation on an aircraft with an AL material with a steel fastener through it, almost without fail we design the part so that a freeze fit bushing goes through each individual piece of the stackup first to increase the bearing area and thus reduce the bearing stress. AL has little tolerance for bearing stress.</div></div>
That's not quite accurate. While yes, there are joints like that, they are the minority. On a 747 for example, there are tens of thousands of steel and titanium fasteners that go through aluminum structure without any sort of bushings. And I'm talking primary structure here--fuselage and wings. Of course it has lower bearing strength than steel, but you simply take that into account when designing the joint by using the correct number of fasteners to get the strength you need.
Anyway, back to the subject. I don't know the details on the extrusion EGW uses or the heat treatment processing, but there's more to it than that.
What people notice most is surface hardness--how easily the part scratches or dents. While 7075-T6 is certainly harder than 6061-T6, the difference isn't that dramatic. A bigger difference is made by the type of coating--Type III hard anodizing adds a bunch of surface hardness to either alloy. Hard anodized 6061 will be harder than 7075 without and will "seem" like the tougher material.
While I agree resistance to scratches and dents is desirable for parts like these, don't assume surface hardness equates proportionally to ultimate strength. Virtually none of the aluminum parts on planes are Type III hardcoat anodized because it drastically reduces their fatigue life, and doesn't increase ultimate strength. So their parts are easy to scratch and dent, but they're strong and will last a long time.
Another thing people don't consider is the design of the joint itself. One reason steel rings are more likely to damage an aluminum rail during recoil, is most of them use a crossbolt machined flat which does not extend very far into the cross slot of the base. This means there is not much bearing area between the two so the lug on the base is more likely to be dented. If the lug on the base is not full width, obviously this reduces the bearing area even more and makes matters worse.
That's the main reason I wouldn't use most steel rings on bases such as EGW's extruded ones. Seekins rings, for example, will have less tendency to damage the rail not because they're aluminum, but because the nicely machined lug on the bottom extends deeply into the cross slot, spreading the load over a larger bearing area. Of course as mentioned, they can certainly make marks in the clamp area on a softer rail. Any sort of lack of perfect fit will cause the harder rings to mark up the softer rail. What most don't realize is that with a 7075 rail without hard anodize and rings made from 6061 with hard anodizing, the results wouldn't be much different.