Recommendation on thread type for split-ring collar

[Ingenuity]

I have to load test existing unbonded post-tensioning tendons to an existing buildings floor system that was built 50 years ago. I do this regularly - usually to 1/2" diameter 270 ksi 7-wire stranded tendons - but this one was built using 1/4" dia high-tensile wires with "upset" (button-heads) ends. An older, now antiquated PT system, that was once patented back in the 50's.

For this specific building, the tendon is made up of 12 individual wires connect to a circular 'washer' that bears on shims plates to a cast-in steel bearing plate. The 'washer' has a recess where a split-ring collar was attached and connected to a 70 ton stressing ram via a pull bar.

I have a (for me) unusual situation where I have get manufactured a custom lipped-and-threaded split-ring and a threaded collar as per the attached preliminary schematic:

Since the split ring is in two-pieces I will have an alignment pin/hole on each half so they marry-up, and also a nib (on top) that prevents the rings from turning when the collar in installed.

Previously I have had 4" diameter wedge collets manufactured (for a different project, but similar application) and they used a UNC thread that was tough to use in the field because of slight thread damage after use.

Is there a thread type (rolled, roped, square etc) that will work better in this application?

I only plan on getting two sets manufactured. I need to reuse the collar and rings several times, typically stressing to 100 kips.

Thanks in advance.

 

[hydtools]

Buttress thread like that used on drill steel.

Ted

 

[Ingenuity]

Thanks hydtools. I will check with my machine shop to see if they can do such a thread type.

 

[TugboatEng]

This application is going to require a thread that tolerates a wide variance major/minor diameters as there is going to be some variance in how the collar fits on the rod. Whatever thread tolerates that would be best. I would personally think a bolt on split collar, think Victaulic grooved piping, would be best.

 

[Ingenuity]

I did find the original US patent from 1958.

Looks like a trapezoidal thread (ACME?). Not that the patent sketch is a drawing for a machinist to use.

 

[dhengr]

Ingenuity:
Note that your second picture is for a patent application, and can almost never be used for fab. or machining purpose. It is something of an artistic representation, sometimes intended to obfuscate as much as possible, while still telling its pictorial story for patent purposes. Any fairly course power thread should work, an acme thread would probably be fine. You want a strong thread which is much less susceptible to minor damage from reuse.

I would make your blue pulling cap piece thicker on the top, still with the sloped top shape, so as to minimize any deflections due to the load transfer for the center to the circumference. I would ask the machinist which thread he can most easily make. You will have to design these pieces for strength and for sufficient thread lengths for your 100k pulling force. I would likely have him make your green split ring as a single, doughnut shaped, piece to fit/mate with the pulling cap and the bearing pl. groove. When everything fits well, he can cut the split ring in half and provide a bunch of small shim pieces the thickness of his hacksaw kerf, and recheck the mating fit-up and thread play.