Thread: Replaceable shear pins

Hi Guys,
Interesting problem.....

This is the part on my prosthetic leg that should shear if/when something nasty happens. I have been informed that they are supposed to shear at 10 Nm, although I don't know if that is true or not ,as have fallen a few times and all that has happened is that my leg has twisted my hip to buggery....and did not shear....anyway.

This a new part....It has just cost me $600. The last one was free but only lasted 6 months. I'm guessing there are no more 'free ones'......
No NDIS in WA yet.....Don't know when its getting here.....
My pins appear to be wearing away over time.....The bushing I am using now only has one pin left. If they both go, my alignment is lost and my gait goes to the pack.....

These pins are part of the whole bushing...not separate. The whole thing is CNC machined out of one piece of s/steel.

I was wondering if if it may be possible to modify said part to use 'replaceable' shear pins...something like roll pins...although I think they would be too hard and would not shear very easily.Temper the pins before I use them?
If one shears, I could drift it out and fit a new one......sounds easy....
I would mill off whats left and drill 2off 2.5mm holes
Pins are 2.5 mm in dia, 3.6 mm high and bushing is 10 mm thick. OD of bushing is 30 mm.
Brass pins? Aluminium pins? Can you buy them? Is there a way to calculate how much torque would shear the new pins?
Thoughts?
Steve

20171223_192338 (Medium).jpg

I would think maybe 2.5mm aluminium tig filler wire might make a good source of cheap pins?

Yes every material will have a known shear strength and that can be calculated for the cross section of your pin. BUT in my experience the real world tends not to be so predictable, and things will shear when they shouldn't, and not shear when they should

If you were designing this I'd say get some pins that were likely candidates, set it up in a jig, and then put a torque wrench on them. However I understand you're a user and it probably doesn't bother you what the number is, you're more interested in it shearing when it should.

I'm not an engineer and I'm not providing advice. I presume if these don't shear somebody could get injured. However if it were me, I would look at silver steel, it's available in 2.5 mm diameters and is sold soft. I would harden it (heat to cherry red and quench it in water), then leave it fully hard for use. It would then be wear resistant, but brittle so if it did fail it would be a catastrophic failure and shatter, rather than yielding, which is the whole point in annealing. I'd then set up a dummy piece in a vice and try to replicate something like a real world failure just to satisfy myself that the pins would fail at a guestimate of the force. Then really nothing for it but to try them. If they fail early then try annealing them to provide greater ductility and yield strength.

Having said that, I won't ever try to reinvent the wheel, if somebody else has gone to the trouble of doing that for me. The pins you have should be tested to be approximately correct. It's likely the manufacturer won't be terribly forthcoming with information, but you never know your luck, and you could ask them what the pin material is. It's only the cost of a phone call or email. Failing that, given you're going to destroy the existing pins anyway you can do some investigation of the material in doing so ie how does it file, spark test for carbon etc etc

As Pete says, you could make up a test rig and try things out. I think the standard TIG wire size around there is 2.4mm
If you are having issues with it not shearing then perhaps something softer may be worth thinking about. If you need help then contacting TAD in WA (TADWA – Enriching lives through solutions for independence for people with disabilities, older people and carers in Western Australia.) and they might have someone nearby who could make up a test jig or even help with the mod.

Michael

Hi Steve,
I am only guessing here but I think the high price of the bush and shear pin assembly is an indication that there were some higher-level machining and product materials sciences applied in manufacturing your part.

Certainly it would be a stainless steel.Given that 10 Newton metres calcs out to only 7.3 ft-lbs, it doesn't sound like a particularly tough grade needs to be used. Possibly Not 316L which is oft referred to as surgical stainless steel and used in medical implants.

I am no engineer either,but can see clearly that the existing pin material is not performing as it should and a tougher grade of stainless possibly a higher carbon grade, like 420C (also heat treatable) may be required.

Is your bush shown the bottom half of an assembly with a shoulder screw pulling the two together?

Grahame

Originally Posted by Michael G

As Pete says, you could make up a test rig and try things out. I think the standard TIG wire size around there is 2.4mm.Michael

3.2 mm was also a standard size Michael, a bit closer to those 3mm diameter pins.

Stereob , if you need some 316Lx 3mm wire I probably have some somewhere. If you are going to do a trial I'll hunt the wire and send you some.

Grahame

Thank you all! Some good info there.

Yes, pins need to be brittle to shear cleanly. Silver steel may be a good start.

Bushing is held to the main body via a LH thread M6 ( I think ) S/S CSK Cap screw. ( did I get it all?..lol )
( God knows why they used LH thread...lol ...to make it harder to find a replacement )

I have never heard of TADWA.....I'm not in Perth. I might give them a call.

I think the design is flawed anyway( IMHO ) as the retaining screw creates friction between the two parts and takes some of the torque load off the pins. This depends on the tightening torque of the retaining screw.
Not easy to measure or maintain as one needs 3 hands to do it. Two to hold a T Spanner on the main body ( so it doesn't twist ones leg off ) and a third to operate the torque wrench.....

I did even consider working out how to thread the pins in, so they easily replaceable but not sure if that would work.
Even if I only get 1 or 2 refurbs out of each Bushing,before they flog out , it will save me a significant amount of money.

I was also wondering if a ball detent system could be used. Something that locked in position but 'gave out' when enough torque was applied?
It would be alot more complicated , less robust to wear and more expensive to manufacture, I think.....anyway....Thanks again.
Merry Xmas guys!

Steve

Small brass rounds are available in imperial sizes, 3/32", 1/8", 5/32" from George Weston in Qld and presumably from suppliers in WA. I have 5/32" (lathe lead screw shear pin) and will post some if you want to try it.

Grahame I'm curious as to how you came to some of those conclusions?

I'd suggest the price reflects the fact it's a piece of, essentially, medical equipment. Made in low quantities to basically a captive market. I think Steve is fortunate the price doesn't have another zero on the end of it! Steve's testimony that the design has obvious flaws only adds weight to belief in my opinion. I've spoken with a number of people with prosthetics over the years and it seems, while there have been massive advances, getting the whole lot right can be "a challenge" to say the least. That's especially the case with athletes who may have very specific requirements and place high demands on the equipment.

Those pins MAY be stainless, and I agree it would be a good idea, on the other hand I don't see any reason they would have to be. I dare say their life expectancy is such that they would be expected to be replaced before any corrosion became a significant consideration. The pin size was quoted as 2.5 mm diameter. If the pins are not shearing I'm not sure I understand how you're concluding the replacement should be tougher grade? In my opinion the replacement would need to be either a lower tensile strength, or reduced ductility to induce a shear failure. Which of the two would be difficult to say without a more thorough understanding of how/why those pins are designed to shear. A material that needs to able to withstand a relatively gradual loading before failing may be quite different to one that's designed to fail with sudden shock loading. How and why it's designed to fail I have no clue based on what's been said so far.

Steve I think you would be on a hiding to nowhere going down that path of a ball detent or similar overload system. 2.5 mm is pretty small when it comes down to it, and I think coming up with a design like you're thinking and could be successfully and easily manufactured at that scale would be quite a challenge. Unless you had some specific need to "reset" you leg mid stride, I feel you'd be better to focus on improving the existing design by making the shear pins replaceable. From that single photo you've provided so far, it looks to me like that "bushing" has been ground and then the pins inserted. Do you know how they are fixed? Unless they're specifically designed to be replaceable, I doubt they're threaded and my guess they're Loctited in, that's how I'd do it anyway. I doubt the pins and body would be machined in one piece as you suggested. I also doubt they're shrunk in, as getting a good shrink fit on a 2.5 mm pin wouldn't be a walk in the park.

Originally Posted by Pete F

From that single photo you've provided so far, it looks to me like that "bushing" has been ground and then the pins inserted. Do you know how they are fixed? Unless they're specifically designed to be replaceable, I doubt they're threaded and my guess they're Loctited in, that's how I'd do it anyway. I doubt the pins and body would be machined in one piece as you suggested. I also doubt they're shrunk in, as getting a good shrink fit on a 2.5 mm pin wouldn't be a walk in the park.

No Pete, the Bushing is definitely machined from a single piece of s/steel

Originally Posted by Sterob

No Pete, the Bushing is definitely machined from a single piece of s/steel

So you're saying those pins are created in situ from the parent stock? How are you so certain of that?

Sorry if I find that difficult to believe, but it would be a completely pointless, expensive, and relatively difficult to do accurately, when instead the pins could be inserted after the rest is machined. To give an example of difficulty, that chamfer on the larger bore? From the photo you provided it would appear the normal tool to create that chamfer would interfere with the pins if they were there when machining. I have no doubt the rest of the "bushing" is machined as one piece, but if that was to also include the pins for no good reason it would be one of the worst examples of engineering I've seen in a long time.

Sorry...you are correct. I just had a closer look.My eyes aren't as good as they used to be.
I feel like knob....

The pins must be pressed in. It is almost impossible to see a seam where the pin meets the Bushing.

Don’t feel bad, a well made fit can be very difficult to see as two different parts, it’s often the overall geometry that gives the game away. If I had more time, to make you feel better I would ream a hole in some steel and press in a pin, then surface grind that surface so it was all flush. I can pretty much guarantee you would not be able to tell me where that pin was inserted, or at the very least wouldn’t have noticed if you hadn’t been told.

The point of establishing how it’s manufactured is if those pins were pushed in, you may be able to remove them, and then your job is half way done in making the whole lot remanufacturable. They may however be tricky to remove, but I think a good place to start would be some general heat on the “bushing” eg in an oven. Remove it and hit the pins with some electronics freeze spray from Jaycar etc using foil to keep the spray only on the pins. You said one of the pins is already missing, and that may provide more clues to replacing it.

By starting gently like that it’s unlikely you will damage anything and find yourself in a situation where you go past the point of no return. If it doesn’t work you can then try more aggressive techniques to remove them.

Originally Posted by Pete F

Grahame I'm curious as to how you came to some of those conclusions?
.

I spent too much time on a site where the prosthetic was attached by osteointegration.
The attachment was bedded into the living bone, so all the fittings had to be stainless or titanium. My thinking and resultant comments were guided by that.

This was in the wee hours of the morning mind you, some of us don't seep that well.
I was not until the narrative had progressed, that I twigged we not talking about a need for stainless and titanium.

For external uses, the bush price was a rip-off and should be on NDIS.

It is a given that any low volume manufactured problem is always passed onto the unfortunate consumer. I have a CPAP and spares like harness and seals are outrageous.I am sure it is the same for all special medical equipment. I have friend with a wheelchair and what he pays is unbelievable.

Originally Posted by Grahame Collins

I spent too much time on a site where the prosthetic was attached by osteointegration..

You win the Chocolate Frog Grahame! Well Done....lol
It is Osseointegration. I didn't mention that as I thought it would cloud the issue, as it is an external part.

Steve