Otto bock wrist unit, terminal device compatible bolts

May 2008

Initially (May 2008) my test socket was equipped with a MovoWristFlex wrist unit. Since that wrist jammed within the first minutes of testing it still on site at the orthopedic technician's workshop, they immediately replaced it with a standard Otto Bock wrist unit without ratchet.

After using initial equipment for a while, it emerged that some Otto Bock parts did not withstand the normal usage I was putting it through. Some wrist and bolt parts appeared to require a detailed look.

As I will require some non-standard add-ons (hobbies: not covered by insurance, such as crafting, gardening, ..) we were also looking into a standard interface specification for both wrist units and bolts.

In order to start that project, I asked Otto Bock's prosthetics department to give away part specifications including detailed bolt diameters and tolerances as the current situation (wrist slipping or hard to lock) made us wonder what was going on.

Yet, Otto Bock representatives do not give away design specs - including diameter tolerances. That is funny, because not only are widely varying bolt diameters not a particular high security information - the reverse is true: they should be public as they are typically paid for by government agencies or public insurances.

Now, while withholding hook and bolt tech specs, an Otto Bock representative also suggested that I may not adhere to a "normal" amount of usage.

If they share no technical tolerances and specifications, how can they expect anybody to distinguish "normal" from "excessive" usage.

This type of entertainment could be straight out of Dilbert.

Terminal device bolts

As it turned out, our Otto Bock factory issue bolts exhibit a rather wide diameter range - I own four bolts, two of which sit fully fixed against rotation when the wrist is locked, but one rotates with moderate force and the fourth one rotates just by cable pulling:

We thus decided to measure the bolts ourselves. So, these Otto Bock bolts feature diameters range around values:

Bolt 1 (previously, this was the hook shown in the video above, now revised with new Otto Bock bolt): 1,599 cm (one diameter; other not measured); this bolt is at the upper and of the size of our current selection and I can almost not completely lock the wrist - conversely, after wearing this thick bolt for a while, the other two bolts (below) start to exhibit a greater rotation slip than before.

Bolt 2, on prosthetic hand: 1,588 cm (one diameter) - 1,590 cm (other diameter) (with locked wrist, this still can be rotated).

Bolt 3, hook: 1,585 cm (one diameter) - 1,588 cm (other diameter) (with locked wrist, this still can be rotated). Needless to say that this is not useful as "work hook".

The following image gives an idea about the bolts and terminal devices at hand:

Our own first series of bolts that was done experimentally yielded these results: 1,583 cm (free rotational slip), 1,573 cm (wiggles), 1,581 cm (wiggles).

This warrants further questions:

  • How are Otto Bock's metric tolerances controlled? According to some information I obtained, some of their machines are precise down to at least 0.02 mm if not down to the micrometer range.
  • Does Otto Bock implement a quality control model that assumes that significant testing is done by the client? Given their product prices (one bolt is sold to the end user for 80 CHF) this is worth asking.


While we now identified the problem of widely varying bolts, we identified another problem: my current wrist exhibited a slow increase in bolt adapter diameter over a period of 6 months. We dismantled, took apart and examined the wrist closely (great stuff for the 'steam punk aficionado' in you):

A view from the back shows an intricate spring mechanism designed to hold the circular bolt groove in place:

This mechanism now can be opened carefully.

We see a circular spring with two bent ends. One of the ends leans against a conical screw obviously designed to be adjusted while the spring bends out of shape:

About that spring; if you closely examine the angles of the ends that are bent from the circular shape you will find that the end to the right (which leans against that conical screw in its intended position inside the wrist) is slightly deformed compared to the end to the left (picture below). That may be the result of using Otto Bock's bolts of the upper end of the diameter range. In other words, wide bolt tolerances are obviously able to interfere with the delicate mechanical stability of this piece of iron.

This picture shows how the bent end of the spring leans against a conical screw:

December 2008

We now defined the following problem list:

  • Terminal device bolts by Otto Bock vary in diameter within a range that spans "free wheeling" (rather small) and "can almost not close wrist lock" (rather big). Even two out of three bolts that were available at the prosthetic technician's office apparently did not lock in my wrist.
  • Wrist unit bolt adapter does not maintain same diameter over several months if used in conjunction with Otto Bock's varying diameter bolts: particularly large diameter bolts seem to have the capacity to distend the metal parts of the bolt adapter.
  • The terminal device cannot be rotated without fiddling with the lock that can be jammed if a bolt has a large(r) diameter. That makes it impossible to wear a silicon glove that covers the wrist.
  • The bolt/wrist connection may not just slip (rotation) but also wiggle.

Otto Bock Switzerland was notified of these observations as of September 2009, but did not provide anything I would perceive as constructive answer that would solve any of the issues.

In an answer from Otto Bock Germany, they recommended I get ergotherapy, they reinforced their perception of producing medical quality precision stuff, and they told me they would use their legal services against me in case they felt that'd be necessary.

At the moment I read that they responded to differing bolt diameters 'with ergotherapy' I first had to laugh so hard I almost swallowed my liver.

Then I reconsidered that suggestion and found they must have meant for me to get my own wrist unit built:

As part of 'ergotherapy', three methods are significant:

Competence oriented method

  • selected craft / tool usage [1]
  • exercises from life and hobbies [2]
  • acquisition of lost abilities [3]
  • training abilities

Expression oriented method

  • using creative means to express, to shape, to exhibit and to communicate [4]
  • use and emphasize emotions [5]
  • use music, use materials [6]

Interaction oriented method

  • group dynamics, group dynamical process, work and deal with issues within a group, experience group togetherness [7]
  • group members can have different roles [8]

As you see from the following annotations, I am closely following ergotherapy aspects:

[1] using CAD software, milling machines, screw drivers, and many more

[2] discussing, devising, pondering, and drafting are exercises that are rooted deeply in life

[3] building a wiggle free wrist is a step towards regaining abilities, definitely

[4] what did you think this website was, an exercise in hiding?

[5] while Otto Bock recommends ergotherapy I am not sure how they deal with negative emotional reactions targeted against some apparent product flaw, but I am very happy to read that my own emotions and their expression generally are considered part of healthy ergotherapy (which was what Otto Bock recommended)

[6] music, aluminum, Ramax, brass

[7] yes, team work is a challenge and fun

[8] we have members that design *and* craft, and i help with design *and* test; so we do implement different roles.

It is my personal conviction that if an industry is able to build motors that run 300'000 kilometers and drill hammers that allow for lock and unlock of steadfast drill mounts using little more than two fingers, there necessarily must be a way to build a prosthetic wrist, hook and hand that delivers solid positioning and motion for at least 80 consecutive years of so-called 'heavy duty' usage. And it is my personal conviction that it is up to me as customer of such an industry to see to it, if necessary personally, that state-of-the art technology will happen as long as no one else is up for the job.

The customer service representative was faced with the information that their resulting bolt problems were consistent with extremely cheap off-shore (such as, far east) manufacturing and absent quality control while still charging a premium (80 CHF) for a single bolt; in fact, my prosthetist had alleged to me later, that someone inside Otto Bock had confirmed that notion when discussing these bolts. Be whatever, I invited the Otto Bock representative to apologize to me. That was on September 22nd 2009.

After all, they caused me

  • a number of trips to the prosthetist
  • a number of trips to the factory where we built the real wrist unit
  • a lot of time investigating and researching

Respect is the ultimate currency - and they sure are not aware of that.

I am very happy about the occasional support of Otto Bock, and about the support of good friends. Particularly, I thank Stephan Mueller, Roman Meili, Remo Quinter and Peter Schneider.

January 2010 - new wrist

We drafted our first own wrist unit (probably as part of the 'ergotherapy' that Otto Bock suggested). Continue here.

Cite this article:
Wolf Schweitzer: - Otto bock wrist unit, terminal device compatible bolts; published 25/11/2008, 13:07; URL:

BibTeX: @MISC{schweitzer_wolf_1638538075, author = {Wolf Schweitzer}, title = {{ - Otto bock wrist unit, terminal device compatible bolts}}, month = {November}, year = {2008}, url = {} }

9 thoughts on “Otto bock wrist unit, terminal device compatible bolts

  1. I very liked this post. Can I copy it to my site?
    Thank in advance.

    Sincerely, Your Reader.

  2. You cannot copy this to your site. As it is subject to updates, it does not make sense to copy it.

  3. If he plans to translate to Russian, that might make sense...

  4. True.

    But the e-mail address is invalid, his/her site seems to be copied together from other sources with mostly' marketing based stuff.

    Maybe better to consider a "this page in Russian" button on our English pages as this is already a concession (not a national language here, local colleagues complained already) ... and besides, this is *really* subject to constant updates.

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