It is clear that my tempering of the car spring material left the steel in our limb irons a tad brittle.

I remain convinced that this design for the limb is basically sound.  It was the execution that was at fault.  The next set will not have that ridiculous cutout in the rear and will be made from some brand new 4140 rather than the salvaged car springs I have been using.  I am also thinking it might be a good idea to send the irons out and have them professionally  heat treated as we have clearly reached the limit of my abilities in this area.

The video in the last posting showing the limb rupture is quite instructive.  With an inswinger style machine all of the heavy fragments are  directed in towards the stock in what is a relatively safe direction.  Not that I will allow this observation to diminish my prudence, but it is heartening to see this worst case scenario prove relatively benign.

For now my mind clings to the memory of that one good shot we managed when the news seemed at its best about this project.  The chronograph was not set up as the light was not right for it to work properly.   What I remember of the event strikes me still as remarkable for its velocity and flat trajectory.  I will not hazard a guess at a numerical value for what I saw.  Suffice it to say, the witnessing of that one uniquely powerful shot has infused new momentum into this quest.

4 Responses to “Post mortem”

  1. Pat B says:

    I’ve got to say I’m quite puzzled why it broke off where it did. Isn’t that the bit protruding beyond the bundles? That’s not a point of major stress. Is there any chance that it caught on anything during its journey?

  2. nick says:

    The point where it broke is perhaps 1/2″ back from the point of maximum stress at the fulcrum. The hook on the end of limb iron doesn’t really protrude from the bundle as much as it is embedded into the bundle by the forces driving the limb forward. There is zero chance it caught on anything while being drawn back and no witness marks that would indicate collision with the stanchion after the break.

    I was somewhat obsessed with trying to keep the weight down on the irons and had included that swooping cutout on the side plates beneath the hook. That, and the faulty temper, is all that it took.

  3. Pat B says:

    Hmm…strange. But the forces that are, as you say, “driving the limb forward” don’t exist yet. During cocking the net force is the other way, at least as long as the angle is still < 90 deg. Your winch is actually pushing the arm back *into* the bundles, and thus possibly making it tend to protrude from the other side. Maybe you could try cocking the unbroken arm by itself, if that's possible, and just see if there is any tendency for the hook to move back and away from the bundle?

  4. Nick says:

    I was able to have a good view of the hook as the limbs were being cocked and it showed no tendency to move back out of position. Also keep in mind that the bundle opposite the bundle that bears on the hook is up against that long sweeping surface on the belly side of the limb. This effectively locks the limb and stops any rearward movement. I have never observed rearward movement of the limb with any of the limbs I’ve tested. Just not an issue.

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