Swaged Metal Band Shear Force

[Slugworth]

I am looking for information on how to calculate the holding force of a metal ring crimped (or swaged) around a tube.

That is, I am looking to correlate a reduction in ring diameter around the tube to a shear force (the force required to slip the ring off of the tube).

I would guess this is similar to calculating for a press fit of a collar on a shaft. The problem is, I am dealing with a Platinum band (Pt 90%, Ir 10%) and a plastic tubing (PEBAX), so I'm not sure how much information I can get on the material. I realize that the actual force required can probably only be obtained empirically, due to the crushing of the plastic tubing, etc.

Thanks in advance for any help you can provide.

 

[MintJulep]

Interesting problem.

PEBAX looks like slippery stuff, so I suspect that classic press-fit equations won't be applicable. Or maybe more correctly, whatever holding force you develop won't be generated from friction at the interface.

I would guess that the holding force would be developed mostly by the resistance of the tube to radial compression as the ring tries to slide axially. This would probably have a lot to do with the geometry of the ring.

The equations for tube drawing might be applicable.

You might find this interesting:

http://www.minitubes.com/images/article_palengat.pdf

 

[unclesyd]

Are you crimping against something like a terminal fitting?


If I recall correctly that reduction of OD is the guide for crimping. Just looked at some hose with crimped fittings and it looks like the hose OD is reduced to something less than 20%.

One of the hose catalogs has a section on what is required on crimping, with either brass or steel, a fitting on different type hoses.

In the lab we used a crimping tool with parallel jaws with slots to accommodate different size rings and hoses and gives enough compression to match the hose pressure.

 

[Slugworth]

MintJulep: Thanks for the link to the paper. Interesting stuff. I think you are right about the holding force on the tube being developed by the compression on the PEBAX itself. This is why I'm not sure I can just calculate it without doing some experimentation.

unclesyd: This is swaging a Platinum/Iridium marker band onto some plastic tubing (slipped onto a removable mandrel) using a rotary swager. The plumbing hose fittings were something I was thinking about as well, since I just re-plumbed my entire house with PEX--using copper crimp rings. The swaging operation, however, reduces the diameter of the ring the full 360 degrees. I do think that the reduction of OD is the right thing to measure to determine if the swage (or crimp) is good. I just don't know how to theoretically determine how much holding force I can get by reducing the ring diameter by X amount.

 

[MechDp]

Well your materials really narrow down the application
I'm not sure I'd trust any of the textbook methods with the material thickness that I think your dealing with (.001"-.002"). I ended up encapsulating the marker band under another layer to reduce the risk rating, on a project of mine. The mfg had no problem doing it.

Even if your band is thick enough to compress and analyze analytically. I'd still encapsulate it to eliminate any chance of it shifting or falling off if this is a medical app.

 

[Slugworth]

MechDp: Good guess on the application. The part is actually encapsulated in its final form. Where I am going with the whole correlation of an OD to a holding force is I am trying to come up with a non-contact test method for checking that the swage is done properly, and I wanted to make sure that the idea was sound in theory. That is, that I could do a calculation based on OD that would closely approximate the values I would get in actual destructive testing. I think your point about the thickness of the band is a good one.

 

[foglights]

PEBAX looks very brittle. Is the tubing deformed in the swaging process?

This is an amazing question. So the ring is compressed onto the tubing but you you don't know the psi used to press. And according to the question, the ring will break free from the tube before the tubing breaks. Without an intense knowlege of metallurgy I don't think you would be able to calculate the force required to press the guage metal ring onto a given diameter. Even if you did, I don't think the nature of metalurgy data is such that you could calculate some sort of psi.
This sounds like an impossible question to solve without experimentation.