Carnes arm - hand function

After I discussed the history of the Carnes arm, here is a description of one of the more intricate mechanisms found in later Carnes hands [4]. I find it absolutely fascinating to also consider how history seems to repeat itself.

[thanks to Mark Lesek for amending parts of my content after giving the 1942 patents a thorough analysis]

Was this a original Carnes mechanism? - Carnes sold a patent license to the German authorities in 1916. The Germans published Carnes-related technical details in 1920 that had not been patented by Carnes by then [4]. So the question arises whether the below detailed mechanism was an original Carnes design or whether it was built by German engineers.

  • It cannot have been the Germans. The German constructors that built the Carnes arm and that modified it added only minor design changes to mechanisms themselves such as adding or omitting single springs or cogwheels. They added modifications to cable and socket design to allow users to connect to Sauerbruch cineplasty. They also modified the material and shape used for the outer shell of the prosthesis and they adapted the Carnes arm to a number of different levels of amputation. But we are looking at an original Carnes mechanism here [see below!] that was unknown in the US patents at the time. Furthermore, the German government already oversaw a rather large number of well functioning but simpler hand mechanisms that had been developed up to that time, many of which are available for study [4]. Yet, none matched the Carnes mechanism's apparent sophistication. So the Germans would not have paid Carnes so much money for the patent if that patent would not have seemed to be worth it or if they would have had a comparable thing around the corner.
  • Carnes must have carefully designed and timed his patents to some type of strategy. The Carnes patent descriptions available up to around 1910-1920 detailed a versions that are different to the mechanisms in the construction plans as sold to the Germans, and Carnes must have made the real plans available to the Germans only after successful negotiatons. An integral part of successful negotiaton certainly were the instrumental shells [Fig. 191 #8 and #9, see below] that covered up the triangular parts of a crank shaft [Fig. 191a, below] - so in other words, even if you could study the mechanism in front of your eyes, the operative parts were magically hidden from inspection and taking them apart would render them useless. This is nothing but ingenious in relation to selling clockwork type mechanisms. As 1920 came, the Germans already had four years of extensive experience with the patents they had bought from Carnes and were obviously sobered by the experience. Despite absence of US Patent material, the Germans published detailed descriptions and figures [4] of the whole stuff they had bought seeing as if from any practical point of view the Carnes arm had become obsolete by then. It was not until twenty years later on August 1st 1940 (when Germany was aggressively extending and other countries struggling to defend themselves) that Carnes submitted yet another design to the US Patents where it was only accepted and published considerably later (US Patent 2287781, published on June 30th  1942, but see the article about the history of the Carnes arm); this patent apparently contained spring design elements introduced by the first hands of Daniel B. Becker.

Maybe William Carnes was aware of the fact that patents would expire after 20 years time and so he thought it would be better to preserve this mechanism for exploitation by his family until well into the sixties. Maybe he waited until World War II was about to look like a success to Germany where he had sold the patent licenses so successfully in 1916 and maybe he just sold them a limited time access to these designs - but it is hard to say what was the rationale behind submitting a design that was already published in German books by 1920 to the US patent office in 1942 - and not only did the German authors include greater detail but far greater clarity.

The mechanism detailed below may have been new to prosthetics - but not new. If we look at the history of previous inventions regarding upper extremity prosthetics, William Carnes must have been most definitely talented in using these ideas to put together his arm. But ground-breaking new principles that left far behind what previous thinkers had established seemed to be more the domain of other people, such as Vanghetti who in all likelihood was the one who paved the way for Sauerbruch. So we could assume that the mechanism used inside the 1-cable operated Carnes arm (below) could have been copied from another patent - and that then would easily explain why its submission to the US patent office was delayed for so many years - after all, Carnes must have built this design around 1915, and only filed patents over twenty years later. Peculiar if you consider that everything else he built seemed to have been patented at once.

We could also speculate that in order to hit it big with his German clients, Carnes may have built an intricate and impressive mechanism for this purpose, that he had a certain Mr. Smith showcase and present the unit, that he then made the deal with the German authorities but made sure this design remained undisclosed on American soil where he may have continued to produce far simpler, more robust and possibly considerably better mechanisms. Maybe he was embarrassed to learn that the mechanism found a large rejection rate and that Sauerbruch, despite early agony and pain to get his ambitious project started, turned out to be the better prosthetic provider in terms of units sold, patients operated and happy just until about 1952.

It remains undisputed that this piece of engineering is worth a very close study and consideration. Also, Carnes obviously built and patented a number of different arms - so any given arm may or may not match a patented design, and maybe some of the patents were never built, or at least not in large numbers.

Was the know-how of olden times forgotten and is it sensational to have it back? - The amount of engineering going into these clockwork type hands is absolutely amazing. So it surprises that we do not find these any longer in today's prosthetics. However there are a number of reasons that I could listed based on my overall impression of the literature at hand and own experience of using terminal devices:

  • First of all, the amount of force required to operate the Carnes arm using cable pull must have been high. So high, in fact, that a user that was trained and skilled for several years would still break out in sweat after 15 minutes of operation.
  • Secondly, the weight of such an arm must have been relatively high. Personally I prefer lighter weights such as aluminum hooks over anything else even though weight differences for me are almost minimal. The Carnes arm must have been heavy though.
  • Thirdly, the effective tasks solved with that arm as opposed to using a very simple arm cannot have been worth the effort; by and large this becomes obvious already when I compare my extremely comfortable silicon liner / pin lock / socket / hook setup with a hard socket / myoelectric arm: while the myoelectric arm appears to be technically advanced and modern, all other aspects of it (price, weight, comfort, functional range, duration of function, reliability of function) make that product tank miserably right there.

So, long term usage / popularity did not reflect its apparent technical supremacy. According to an independent report, Carnes arm usage had dropped to negilible percentages only after just about four or five years. That does not mean no one used the Carnes arm any more (as always, some die hard fans always remain) but by and large, the Carnes arm had stopped to be what people wanted.

This is also reflected by the fact that a state-of-the art text in Germany's 1920 that densely and to the point details all available solutions contains close to 200 pages of text, and the complex mechanism of the Carnes arm takes up just about 9 pages of these; simpler and more comfortable, attractive, lightweight or functional arms seemed to be far more popular, not to forget the Sauerbruch hand, Fischer hand or Huefner hand. This points to a hype-versus-usefulness discrepancy worth studying up close.

The efficiency of a mechanism that contains so many little parts must be doubted: it appears right from the outset that frequent repairs must have been necessary because when at first an amputee would do the Carnes arm yank (to switch open/close) too softly, it would have not worked; if that person would then do the yank harder (to make sure it really switches) things would then easily become difficult to manage from a service / support / maintenance point of view. Metal parts then would have to be extremely expensive with hardened parts and minimal slippage that at the time seems hard to attain (because for some modern day prosthetics manufacturers, such precision seems still out of reach). The more parts interact, the larger the total metrical error will add up, the more banging will start to occur inside the mechanism, the larger the amount of deformation relative to part sizes and the larger the tolerance fields, so an earlier failure is more likely to occur. So unless real modernization - minimalizing internal friction and slippage, hardening parts, reducing weight and improving stability, better precision for manufacturing - happens, I cannot see such an arm surviving a so-called heavy usage client's first three weeks.

After all, what I call 'average usage' is termed 'ultra heavy usage' by prosthetics manufacturers which tells us just more about their understanding of, well, life, rather than what I do with these parts. Back in the days after World War I, the German authorities mostly equipped young soldiers coming back from the war - in other words, men ready to recover, train, work and play hard. Wrecking a prosthetic arm that is built for sofa sitting usage is absolutely not a problem - I am pretty fit myself, and I can rip out a fully mounted steel cable with a single yank if I don't pay attention, I jammed the new Otto Bock Movowrist within the first seconds of testing, I permanently jammed the Otto Bock two-way hand within the first minutes of usage and so on and I completely dismantled an Otto Bock perlon string setup inadvertently within minutes of first usage.

So a minimum amount of engineered stability is important, even more if hard yanks are part of the operation such as the open/close switch for Carnes arms. And going for repeated repairs sucks ever so much. It really does. It is not a remote machine that breaks - it is a 'half mechanical body part / half prosthetic ego' that is hurt and disappointed, sad and angry when a prosthetic arm breaks, not at all comparable to a book shelf leaning over. It is a big deal to see your prosthetic arm fail, not a comfortable feeling, particularly if failure occurs suddenly.

Conversely, simple and sturdy mechanisms really rock and roll - our first quick release wrist prototype was ever so simple to mill and even simpler to put to use immediately after it fell out of the machine, and it remained wiggle free for at least six months of 'ultra mega heavy usage'. Then it got replaced by a more advanced version that we are currently developing.

There are two hand mechanisms that Carnes constructed:

  • The Carnes arm for below elbow amputees is operated by two cables. One pulls it open, the other one closes it.
  • The Carnes arm for above elbow amputees contains a hand with a switch: the switch changes between opening and closing the hand. Due to the rather complex switch, that hand is rather heavy.

Carnes hand for two cables


This mechanism is intended for below elbow amputees. There are two cables (Fig. 188, 1 and 2; Fig. 189, D and E). Personally, even though I am a below elbow amputee and theoretically could wear a two-cable-harness I do find a 1-cable solution more versatile and comfortable.

For sake of my own preference I will therefore only go in depth describing the one-cable solution (below) - the hand that is operated by one cable and that contains that (rather heavy) switch mechanism. If the two-cable harness is something you are into, you will find details in the relevant printed matter (books, articles, patents).

Carnes hand for one cable pull-mechanism

[click on images for full size]


This hand contains a worm drive (Schneckenwelle) with two worm axles - one to open, one to close the hand. The tricky part about the Carnes 1-way hand is the mechanism to switch between these using just one cable.

The hand [Fig. 190 and 190a] contains a centrally located mechanism that is taken out for illustrative purposes and shown in detail [Fig. 191]; one particular part is a camshaft (Nockenwelle) that is highlighted in the overview [Fig. 190 and 10a: #15] as well as in the mechanism's detail [Fig. 191, #15] and shown in its entirety separately [Fig. 191a].

There is a driveshaft [Fig. 191, #1] that holds / provides the axle for both worm drives [Fig. 191, #2]. Two chains [Fig. 191, #3 and #4] connect to the switch/gear drive [Fig. 191, #5]. The mechanism provides that the connection of these two chains to the gear drive [#5] is mutually exclusive in that either chain [#3] or chain [#4] connects to it.

A belt [Fig. 191, #6] is fixed to the gear drive [Fig. 191, #5] and that belt is connected to the cable that can be pulled by means of the shoulder harness. When the cable is not pulled (but relaxed), the belt [Fig. 191, #6] will wrap once around the gear drive [Fig. 191, #5]. When the cable tension is relieved, the belt [Fig. 191, #6] is pulled back onto the gear drive [Fig. 191, #5] by a spring [Fig. 190, #7].

Both chains [Fig. 191, #3 and #4] are attached to shells [Fig. 191, #8 and #9]. These shells are shaped like tubes but they have an intricate inner mechanism that locks one shell to turn with the main gear drive [Fig. 191, #5] in one direction, the other shell in the other direction.

The locking mechanism that are contained within these shells (cross sections see Fig. 191 [#12, #13]) are brought into their working positions by a part of the gear drive that is contained inside it - a camshaft (Nockenwelle) [Fig. 191a]. In order for the camshaft not not slip but be able to forcefully turn the shells' locking mechanisms, the camshaft contains two areas of triangular cross section (as can be seen in Fig. 191a).

I will now describe the locking mechanisms contained inside the shells [Fig. 191, #8 and #9]. These triangular faces sitting on the camshaft [Fig. 191a] constitute the little areas against which springs [Fig. 191, #10 and #11] press. These springs press against locking levers [Fig. 191, #12 and #13].

The position of the locking levers against the triangular parts of the camshaft appears to be critical: when one locking lever [#12] sits on a triangular facet, the other lever [#13] sits on a triangular edge and vice versa. Due to a minute off-set / off-center / excentric mounting, locking lever [#12] peeks out of the surrounding shell [#9] (see bottom diagram of Fig. 191 with outline of lock lever indicated on outer perimeter of shell [#9]), whereas locking lever [#13] is completely contained inside the other shell [#8].

When the cable [Fig. 191, #6] is pulled, the gear drive [#5] is turned and with it, the nose of lever [#12] that then carries with it the surrounding shell [#9]. This causes one chain [#3] to operate one of the worm drives [#1] and due to that, chain [#3] is stretched / unwound from driveshaft [#1] and piled up on the gear drive [#5]/shell [#9]. At the very same time, chain [#4] that is attached between driveshaft [#1] and gear drive [#5] in reverse direction to chain [#4] is pulled the other way and wound from its wind-up on shell [#8] onto the drive shaft [#1]. This may become clearer by studying the chains' directions in the top view of the mechanism [Fig. 190].

To allow switching between both lock levers [#8 and #9], the camshaft [Fig. 191 #15, Fig. 191a] is placed excentrically inside the gear drive [Fig. 191, #5] and contains a switch wheel [#15]. By letting go of the cable pull, the spring [#7 in Fig. 190/190a] pulls the switch wheel [#15, Fig. 191] against a little latch [#16, Fig. 191]. This will turn the switch wheel [#15, Fig. 191] and the attached camshaft [Fig. 191] by 60 degrees so that the orientation of triangular face / edge towards both lock mechanisms [#12 / #13] is reversed. So the camshaft's triangular parts' orientation is instrumental to making this switch possible.

References

[1] (21. 6. 1921) Tätigkeitsbericht der Prüfstelle für Ersatzglieder Charlottenburg, Archives of Orthopaedic and Trauma Surgery 19(2): 551- 578

[2] Karpa M (2004) Die Geschichte der Armprothese unter besonderer Berücksichtigung der Leistung von
Ferdinand Sauerbruch (1875-1951). Dissertation, Hohe Medizinische Fakultät, Ruhr-Universität, Bonn, Deutschland.

[3] Monika Burri - Aurel Stodolas Entwurf für eine Handprothese [ETHZ]

[4] Gocht H, Radike R, Schede F (1920) Kuenstliche Glieder. Stuttgart: Ferdinand Enke (with 689 figures and 2 tables).

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Carnes arm - hand function; published 30/01/2010, 22:46; URL: https://www.swisswuff.ch/tech/?p=268.

BibTeX: @MISC{schweitzer_wolf_1603886315, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Carnes arm - hand function}}, month = {January},year = {2010}, url = {https://www.swisswuff.ch/tech/?p=268}}