I have a medical device I am designing which will lie under the patient bed.

I need to add a "connector/joint" on this device to permit the attachment of some consumable.

Here is a quick overview of the process:

basic assembly schema

  • Blue part is low cost injected plastic disposable part that will be assembled and disassembled only 2-3 times before being disposed.
  • Orange part is the device, can contain more complex parts (springs...).
  • Orange part must last thousands of assembly/disassembly.
  • Orange part is not movable, only blue part can be moved when assembling.
  • When assembled, it must resist to at least 100N force in all direction, including rotation around the "snap axis".
  • Orange part must be around 3cm which is a good side to apply pressure with two fingers.
  • Disassembly pressure force should be low as only two fingers will be use, other hand will retain blue part.
  • It should be easy to assemble/disassemble with "closed eyes" as the device will be under a bed and there might be no visibility on the device itself.

As my drawing might be a bit confusing, here is a common device that is similar (aka a buckle).


The main problem with buckle design is the require pressure force for disassembly which is way too high, it's also too hard to assemble and not robust enough. But it is a good example for the "general principle".

What I am looking for is not for someone to design the assembly for me, but I am looking for exiting systems that are an alternative to a snap in joint and any pointer that would help me design it.

  • $\begingroup$ "The main problem with buckle design is the require pressure force for disassembly which is way too high, it's also too hard to assemble and not robust enough." That's a bit eyebrow raising for a requirement. Perhaps a spring loaded lever would be a better choice instead of a snap fit? And maybe consider a swivel as part of the attachment point in order to simplify the connector but also allow for the required rotation. $\endgroup$ – user16 Sep 7 '18 at 23:01
  • $\begingroup$ I guess I wasn't clear enough. The whole point of my question is to find other designs (spring lever...) to overcome the snap in limitations. $\endgroup$ – Nicolas Goy Sep 7 '18 at 23:35
  • $\begingroup$ I slightly modified your title and explanation to make that more clear. I hope it helps. $\endgroup$ – user16 Sep 8 '18 at 0:12
  • $\begingroup$ What torque does it need to resist? You say a “buckle” is too difficult to release - can you quantify this? What force is acceptable? What size constraints are you working with? Can the user see it as they do it? Would two smaller joints be OK or does it need to be a single connection taking the 100N? These are just a few of the questions that spring to mind - there’s just not enough information here to provide a useful targeted answer $\endgroup$ – Jonathan R Swift Sep 9 '18 at 14:07

I would use a collar lock with a slide action locking gate. Solves the durability issues and can be used thousands of times. The collar can be square and slide into a dual keyway to support the blue item while a cylindrical gate (spring loaded or not) shuts behind it. Collar thickness determines load bearing capacity.

  • $\begingroup$ I ended up making something similar with a spring loaded collar and a lever to unlock it. $\endgroup$ – Nicolas Goy Jan 25 '19 at 21:29

For a circular attachment, then a groove and matching balls held with a spring loaded collar. Used on brake lines for trailer and also much higher pressure/flow rates in hydraulic fittings.

Image from https://hydraulicmegastore.com/product-category/hydraulic-products/quick-release/

enter image description here

  • $\begingroup$ Do you have an example illustration or photo? $\endgroup$ – Nicolas Goy Sep 8 '18 at 20:52
  • $\begingroup$ A google search on hydraulic fittings shows many - the image is just one... $\endgroup$ – Solar Mike Sep 8 '18 at 20:56
  • $\begingroup$ Thanks, I didn't know which term to use. $\endgroup$ – Nicolas Goy Sep 8 '18 at 21:46
  • $\begingroup$ I looked a bit in the hydraulic fittings I could find, and they have two problems: 1) You need to move the collar for assembly, which I'd like to avoid. 2) My experience with those is that they require a lot of force to uncouple. (but some with smaller force might exists) $\endgroup$ – Nicolas Goy Sep 8 '18 at 22:03
  • $\begingroup$ Re your point 1 - you failed to mention that and re point 2, the only time these are difficult, in my experience (and having used many hundreds when working as an agricultural contractor), is when they are still under pressure - which is a good thing so one does not get a face full of hot hydraulic fluid. $\endgroup$ – Solar Mike Sep 8 '18 at 22:08

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