Stress Testing a fastener

I needed to add a coupler to a forged eye bolt rated at a pull force of 500 pounds. The eye bolt has a 1/2" long 1/4-20 section, and I needed 2".

I used my lathe to drill a #7 hole and tap it 1/4-20 through a piece of 1/2" diameter CRS 1" long. Using a #7 drill means 75% thread engagement.

Then I cut a bolt to an OAL of 2.5". Screwed together, I needed a way to test its pull force.

My solution was to drill and tap a 1/4-20 hole in a 1/2"- thick aluminum plate. This was clamped in my heavy-duty vise. I screwed in the fastener under test and pulled on it horizontally with a 1/4-ton chain hoist attached to a 1"- wide strap. The strap was anchored to the legs of my workbench, and a small stack of 2-by-4s raised the strap so the chain was level.

I then made the chain tight. Since the chain has little ability to stretch, it is nearly impossible to make it tighter with the hoist's ratchet. Instead, I ran a second 1" strap around the table so it went over the chain and under the table. The strap is perpendicular to the chain as viewed from above. This strap deflects the chain from horizontal. It was then easy to tighten this second strap until it was as tight as I could make it.

Thinking of the force diagram, the base of my right triangle is the chain's tension, F1, before the second strap is applied. The downward force from the second strap is F3. The resulting tension on the chain due to the downward force is the hypotenuse of my right triangle, F2. Before tightening the second strap, I was unable to tighten the first strap any further. When I tightened the second strap, I advanced the ratchet until I could no longer tighten it. My guess is these are about the same tension. This means that F1 is approximately equal to F3. From the force diagram, I see that F2 is about 1.4 X F1.

I don't know what F1 is, but I do know I was able to stress the fastener about 40% beyond the maximum I could apply with just the strap's ratchet.

It would be great to know what this force is, but I can't think of an easy, inexpensive way to measure it. Any ideas?

Rick