A Calibrated Pull Test Tool

[rgsparber]

I will be tapping 1/4-20 holes into what I think is bronze. It is essential that I do not strip out the threads as I pull on the eye bolt secured into this hole. My goal is to perform a pull test with a known force and then inspect the threads for damage.

I don't have a scale able to measure 250 pounds, so I decided to make a set of sacrificial bolts that will break at a known force. Here is the story.

Click here.

Your comments and questions are welcome. All of us are smarter than any one of us.

Rick

[DIYer]

Thanks rgsparber! We've added your Calibrated Pull Test Tool to our Fastening category,
as well as to your builder page: rgsparber's Homemade Tools. Your receipt:

Calibrated Pull Test Tool by rgsparber

tags: fasteners, tester

[Frank S]

Rick, I can understand your concerns about pulling the threads out of the bronze frames. Your method of testing the strength of the bolts will tell you if they are strong enough but you haven't mentioned the one critical factor regarding the strength of the material they will be screwed into. Which in your case happens to be your primary concern, of not pulling the threads out of the frame when dealing with similar materials such as a bolt and a nut it is generally thought that a nut of 80% thickness relative to the diameter of the bolt the male threads will fail before the female threads. If a bolt is threaded into cast iron or brass the machinery's handbook recommends 1.5 times depth or length of thread per diameter. Aluminum and tough plastics at 2 times. But when it pertains to castings a lot depends on the quality of the pour, but 2x generally as a rule of thumb will suffice if in doubt err on more depth of thread engagement, if at all possible, possibly 2.5 times if feasible.
With bronze depending on the age, you could be looking at as low as 12 to 15 KSI up to 28KSI on the upper end of the scale.
Would it be possible to drill and tap a hole somewhere that won't be seen or used to a depth that will allow thread length of 2x the diameter then screw a bolt in a torque to the specs of the bolt if the thread does not pull out at the max recommended torque, then you should be golden.

[rgsparber]

Rick, I can understand your concerns about pulling the threads out of the bronze frames. Your method of testing the strength of the bolts will tell you if they are strong enough but you haven't mentioned the one critical factor regarding the strength of the material they will be screwed into. Which in your case happens to be your primary concern, of not pulling the threads out of the frame when dealing with similar materials such as a bolt and a nut it is generally thought that a nut of 80% thickness relative to the diameter of the bolt the male threads will fail before the female threads. If a bolt is threaded into cast iron or brass the machinery's handbook recommends 1.5 times depth or length of thread per diameter. Aluminum and tough plastics at 2 times. But when it pertains to castings a lot depends on the quality of the pour, but 2x generally as a rule of thumb will suffice if in doubt err on more depth of thread engagement, if at all possible, possibly 2.5 times if feasible.
With bronze depending on the age, you could be looking at as low as 12 to 15 KSI up to 28KSI on the upper end of the scale.
Would it be possible to drill and tap a hole somewhere that won't be seen or used to a depth that will allow thread length of 2x the diameter then screw a bolt in a torque to the specs of the bolt if the thread does not pull out at the max recommended torque, then you should be golden.

Frank,

Thanks for your help.

I'm not sure we're on the same page here. My plan is to screw a calibrated bolt into the threaded hole in the bronze frame, which I will later use to lift the frame. Then do the pull test.

All other hardware associated with the lift is rated at 2 to 3 times the expected load.

By depth of thread, I assume you mean the number of threads cut into the bronze. This is part of my concern. That is why I want to test the individual threaded holes.

Where possible, I will be drilling a 1/2" hole and using SNAPTOGGLE anchors, which will spread the load on the back side of the bronze. In 1/2" thick steel, these anchors are rated at over 1200 pounds.

Isn't a torque test partially dependent on the friction between the nut and the threads? What is assumed? Such a test would let me avoid machining the necked-down section of these bolts, which is very attractive.

Thanks,

Rick