Extending prosthetic gripper lever length for better grip strength control - TRS JAWS XFS (strong spring) [prototype]

I found that the current prosthetic split hooks and grippers all have only modest cable pull lever lengths. That means that when the lever has a length L, and the hook tip has a length M, that the cable force F(L) and the hook tip force F(M) are in relation of L/M. So if the hook is longer than the lever, one never even gets the cable force as gripper closing force, and that (as far as I think) is a pity. If ever I am the first person to think this out loud then definitely you read that here first.

Because, if the lever is longer, one may enjoy lower cable forces - no matter how many rubbers on the Hosmer hook, or how strong the spring on a hook/prehensor/gripper device. With the TRS Jaws (original version), I liked the relatively high L/M fraction, but with the new TRS Jaws XFS (very strong spring) I decided to finalize this project that was on my to-do list for a while now.

Link to TRS regarding Jaws XFS:

So here is the first user-developed hook/prehensor lever extension that I know of, all free for download and free to use, 3D-print or manufacture and distribute, if you want. By the way, I do think you read/saw that here first.

Version 3 of prototype - 15.3.2021

The reason I start with meager outfits and details for a first prototype is that I first need to see the difficult to fit parts fit well first. Here, that are two parts that need to be examined for their fit in particular:

  • The part of the lever extension that fits to the TRS Jaws, including the screw / bolt hole.
  • The part of the lever extension that fits the ball / cable for pulling the lever.

These were both perfect for the prototype. No need to perform detailed revisions there at that stage.

What I did not like too much was the pre-version-3 parallel angle of the cable insert, so I tilted that in this revision 3. Also, I felt the lever could also be offered / made longer for a more pronounced effect still, which was realized in version 4. And also, I felt it could look a bit more like the alien appendage it really is, which was done in v4, where the ridges also serve a bit of mechanical purpose.

What I really liked though in this version 3 was its sleek shape, much like the Tesla Cybertruck. So I did order one in metal just for shits and giggles - and just because I can. See how that turns out, if I need to polish it, or spray paint it.

Also, a shorter lever extension rather than a massive one has some practical advantages if one wants to wear the device while taking it through sleeves of garments, jackets, and so on. That worked here - but only because I was not trying to put on an Italian dress shirt at that moment.

Usage demo:

Photos of black matte metal version of the v3 extension.

3D-model view

Get it

Download file:

https://www.swisswuff.ch/tech/wp-content/uploads/2021/03/jaws-lever-test-3-.stl

Get 3D-printed instance/version (prices as charged by Shapeways, no overhead charges for me - but metal parts from Shapeways may not be as precise in terms of fitting as plastic parts are for some reason, so there may be surprises there still):

https://www.shapeways.com/product/N3UH73ZE6/lever-extension-for-trs-jaws-v3-15-2-2021?optionId=204668552&li=shops

How to install

You will need a screw, nail, bolt, or wire to fix this lever extension into place through the holes there. I ultimately used a 40 mm M5 screw and a rubber ring containing a safety nut.

Version 4b - 16/17.03.2021

As indicated already, I realized upon trying out the first usable version that fit onto the TRS Jaws, that the lever was still shorter than it could be. Also, I wanted this one to be more rounded with a less edgy shape. So I sat down to evolve this version 4.

[Version explained: v4 had a little glitch, that caused me to have to use a power tool to get the thingy fitted really well - 4b fixes that]

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Finite element analysis for normal load

The Young's modulus is available on the Polymaker website (check under Polymax PLA), ~1879 MPa for that material. I put in forces such as ~120 N for a ~12 kg pull, so, realistic.

The next image shows the von Mises stress as distributed over an approximated / estimated stress model of the lever in any situation where that lever is pulled regularly (i.e., -Z loading for cable bearing, +Z loading for screw mount). A color scale bar (above max color is white, below min color is black) is shown there, and one can see that the maximal forces seem to occur at the cable ball bearing (obviously), but also, along both thick ridges that I put there for added rigidity. If anywhere, I would expect the model to experience the highest forces around there.

Finite element analysis for lateral stress

If I apply lateral stress - as if hitting something sideways with the lever - then the base of the mount appears to be stressed the most, also this is just an estimate/approximation.

 

Material - Polymaker's Polymax PLA

This new version 4b was printed using 1.75 mm Polymax PLA (http://www.polymaker.com), and after setting the details (print speed, nozzle/extruder temperature: I used 225 deg C extruder temperature and not more than 40 mm/s  print speed) for that material (as indicated on the package), and after using generally sturdy/robust print options (95% infill, 5 shells, etc). The stuff then is surprisingly robust! Thank you Andrej Đukić for that tip ; )

Whether the material actually survives any type of use and abuse remains to be seen. I personally think that a metal rendition of this part should be most robust - but then, I have killed a metal/steel powder 3D print on occasion, so a minimally elastic high-grade plastic may just be better. At any rate, this is a prototype version 4b - and I do not claim anything else than this being an idea that I find very cool to look into.

Get it

Download file:

https://www.swisswuff.ch/tech/wp-content/uploads/2021/03/jaws-lever-test-4-b-.stl

Get it 3D-printed somewhere and shipped to you:

https://www.shapeways.com/product/N3A96GNJS/lever-extension-for-trs-jaws-v4-16-17-03-2021?li=shops&optionId=204957859

How to install

You will need a screw, nail, bolt, or wire to fix this lever extension into place through the holes there. I ultimately used a 40 mm M5 screw and a rubber ring containing a safety nut.

Etc

[Further prototype experiences will go here]

 

Disclaimer: all tryout, order, and use risks your own - I do not guarantee anything here - however, it is an offer that is free from my side, no money generated towards me with this.

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Extending prosthetic gripper lever length for better grip strength control - TRS JAWS XFS (strong spring) [prototype]; published 15/03/2021, 16:53; URL: https://www.swisswuff.ch/tech/?p=11686.

BibTeX: @MISC{schweitzer_wolf_1620622544, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Extending prosthetic gripper lever length for better grip strength control - TRS JAWS XFS (strong spring) [prototype]}}, month = {March},year = {2021}, url = {https://www.swisswuff.ch/tech/?p=11686}}