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Otto Bock MOVOHOOK 2GRIP 10A80 P [pimped]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Otto Bock MOVOHOOK 2GRIP 10A80 P [pimped]; published April 8, 2009, 17:32; URL: https://www.swisswuff.ch/tech/?p=166.

BibTeX: @MISC{schweitzer_wolf_1571792730, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Otto Bock MOVOHOOK 2GRIP 10A80 P [pimped]}}, month = {April},year = {2009}, url = {https://www.swisswuff.ch/tech/?p=166}}


5 Comments

Due to that mostly very good performance of the Otto Bock MovoHook 2Grip both initially and in due course of performance testing, I figured that the only significant problem so far - loose joint - warranted serious dedication to get it fixed.

The Otto Bock MovoHook 2Grip 10A80 hook turns out to be very nice with just one exception:

  • Moderately civil look
  • Moderately light weight design
  • Perfect choice of two non-linear spring settings
  • Perfect cable suspension with ball bearing
  • Useful hook shape and size
  • Wiggly joint
  • Otherwise very sturdy and reliable make

Otto Bock had offered me to swap defective material but not to improve the design significantly after I had submitted both problem and solution (below) to them. I decided that their hooks were too valuable for me to just have them forget to get that joint built the right way.

Maybe a prosthetic manufacturer can risk to forget that I am not that patient type of client that puts up with a wiggly prosthesis (which really is very unnerving), perceives themselves as taxi driver for the cause of cheap engineering (which adds up in terms of valuable time during work hours) and that will try to get insurance to pay for repeatedly wrecked material (as Otto Bock made it clear that they suggested I get ergotherapy and their products were not to be over-used without a metric specification of what they understand as 'use').

So after I had identified the problem - wiggly joint- the first hook (I have a total of two of them - one for dirty work, one for clean work) was pimped. Now the second one has undergone the same procedure.

What did we do?

Problem: wear down of joint after usage of about 6 months

Otto Bock boldly advertises this: "Cable-Controlled Upper Extremity Prostheses - These body-powered prostheses are a time-tested for durable functionality" as well as "With our (...) world-class body-powered products, Otto Bock has set the standard for all your Upper Extremity prosthetic needs". That is laudable but depending on the results,  testing may not be all that's needed, but acting on test results would be required.

The new Movohook did not completely withstand the normal usage I was putting it through. It was suggested by Otto Bock that I may have been *over*using the prosthesis - but as no manual or instruction sheet restricted me to use the hook for daily activities, as I have enough muscle power to operate the open/close function without even thinking about it, I do not see myself at fault at all. I operate the hook smoothly and very frequently and under work and normal house/hobby conditions.

Besides, where's y'alls normative constraint? A metal hook that costs 1200 CHF should - by definition - be built to a strength that allows one to tow a truck, to push locomotives, no further questions asked. Cheap hooks that wiggle may cost no more than 180 CHF - by definition. That is what I expect before looking at the product.

So after about 6 months, the hook tips wiggle around 3-5 mm and that is a clear offense, a slap into the face to anyone who is interested in technical items. Also this is a clear discrepancy to Otto Bock's statement "der MovoHook 2Grip ist durch neuartige Gleitlager wartungsfrei" [Otto Bock document 647G204 500 01.08 MD] (English: the MovoHook 2Grip hook does not have to be serviced due to a new gliding joint). The German advertising of Otto Bock asserts this: "Bei zugbetätigten Prothesen kann anstelle einer Systemhand auch ein Hook als Greifgerät eingesetzt werden. Hooks eignen sich besonders für Arbeiten, bei denen es auf höchste Griffpräzision ankommt, wie zum Beispiel beim Hantieren mit kleineren Gegenständen wie z.B. Schrauben oder Nägeln. Aber auch bei harter manueller Arbeit bzw. bei Tätigkeiten in einem schwierigen Umfeld (z.B. Lösungsmittel, Feuchtigkeit) ist ein Hook ein zuverlässiges und weitgehend unempfindliches Greifwerkzeug." (Google translation to English: "In the case of traction-actuated prostheses, a hook can also be used as a gripping device instead of a system hand. Hooks are particularly suitable for work in which high precision is required, such as when handling small objects such as, for example, Screws or nails. However, even with hard manual work or in a difficult environment (e.g., solvent, moisture), a hook is a reliable and largely insensitive gripping tool." -- My experience is that this hook is not what I call reliable, it is particularly not suited for what I call high precision, and it is not insensitive to hard work.

Dissassembled MovoHook

On the way to figuring out how to fix this, this is what we got after disassembling the MovoHook: the fixed claw features joint surfaces whose planes appear to be slightly diverging (and thus not exactly parallel). The moving claw's joint seems to be covered with a little whitish plastic disk. I am not sure what part of this joint classifies as 'neuartige Gleitlager'.

The fixed part of the claw features a joint that will keep the bolt in place (two holes, that is) and that seems to be somewhat stabilized by the plastic disk. The rest of the joint surface appears to be metal without particular treatment that appeared to be visibly covered with dark smudgy dust. The metal showed small spots of brown discoloration that appeared to be slightly corroded. All in all I did not get the impression of a joint that was constructed to last a very long time in a very stable state but maybe in some regions of this world, plastic discs rubbing against untreated metal surfaces are state of the art joints

Solution: rebush joint

We considered various options for fixing this. A ball bearing was discarded due to an expected fast wear-down. This means that the joint would benefit most from being rebushed (ausbuchsen) using a brass bush pressed to the joint surfaces (Messingbüchse).

As a result of the brass inlay, the joint surfaces are now strictly parallel (which they did not appear to be beforehand), and all surfaces are covered with brass.

Rebush result:

It is definitely correct to state that this forgoes any warranty claims through Otto Bock. Given Otto Bock's representatives' perception that they produce high precision technology, and given my perception of recording and documenting Otto Bock product performance and problems, I would believe that I am far better at judging what type of quality does or does not work under daily strain. I also believe that self delusion should be allowed to define a clear limit.

And so I very seriously considered the question of what worth Otto Bock's warranty could possibly ever be - given the circumstances: that they call a rather cheapish looking plastic disc rubbing against untreated oblique metal surfaces acting as a 3-5 mm wiggle joint a "new gliding joint that is service free". When I read that first, I let that sink in for a while.

So, we did not just play with the hook - no, this is a top class pimped item now.

In all modesty, this product is a lot better now than it ever was. I will get my future hooks rebushed right after the purchase. Replacing the bush (Büchse) costs a couple of bucks, it's not necessary to go back to getting a new hook from anyone just for that reason.

Remark regarding quality

Why would a company do that?

A company will reduce manufacturing cost that if they expect mass sales - it's not worth saving a 3 CHF item or even a 50 CHF item on a 1200 CHF product if you only sell four of them. They will only be able to justify cheap production - that invariably dissatisfies customers - only once the 50 CHF per piece start to amount to an annual saving of at least around 50'000 CHF - so they could plan to sell at least 1'000 hooks per year. At least. Any smaller figures do not justify the risk of getting exposed with faulty products over the very small amount of money saved and even then, a client would choose a 1250 CHF hook with a full strength design over a 1200 CHF hook with design faults any time.

But the upper extremity prosthetic market is excruciatingly small to begin with, the market for expensive highly functional parts (the MovoHook could target that type of market if it was built to last 80 years without servicing) even smaller, and the market for extremely expensive high-tech prototypes (some bionic myoelectric prostheses) definitely seems to be tiny.

Which is why I do not understand the rationale behind really cheap manufacturing of bolts, wrists or hook joints. I really don't get it.

Master, the product has been built now. Shall I immediately start fixing it?

Operational and design based analysis by myself and Stephan Müller, technical analysis and work done by MDP Meili, Kaltenbach, Switzerland - all issues discussed together. If you have similar questions, get in touch with Roman Meili.

Inability to grasp thin thread

The Movohook 2Grip eventually - maybe after a year of use - developed a problem whereas it would not grasp very thin items any more. Single sheet paper and threads would entirely slip through. Even a well functioning hook may experience this with a thread. Now: my disability (amputation) causes a lack of grip, and the hook is sold to remedy this grip problem. As we can see, this not remedying my disability. This reverberates further than one feel comfortable about. But let's not obsess about a thread: hooks are for men, and men don't sew, right guys? Where is good old chauvinism when we need it to save the day?

The actual issues was worse. This would affect the "strong" grip lever setting whereas the "low strength" setting was not affected.

The hook claws close when spring lever in "low" setting as the following image shows.

But with the spring setting in "high", the hook stays open by enough to not grasp a sheet (or a few sheets) of paper. The tolerances for the lever - to switch grip strength - and the spring length appeared to be too narrow. The problem itself is technically very interesting: the harder lever had banged a tiny groove into the softer hook body, allowing the lever to sit a tad bit higher. With that, the spring was entirely contracted to the point where its pulling mechanism became a push mechanism.

Close inspection showed that the apparently soft lever metal had suffered a curved dent from repeatedly banging against the hook body.  A remedy here consists in not having an edge there, but allowing the lever to lie against a rounded fitting shape of the hook body. I mailed Otto Bock this problem and I hope they will fix their design.

The spring is totally contracted here.

The issues of the hook were communicated to Otto Bock. I told them the reason I had made sure they see the hook is so they can remedy the design issues this devices has. I told them the real quick and dirty fix was to shorten the spring so it would not push but pull given Otto Bock's low tolerances for this dent to cause issues.

That was when I halted all plans to equip myself with Otto Bock devices. I rolled these back to wait and see. The issues of the Movohook 2Grip were (a) bolt adapter not fitting to wrist, (b) hook joint cheaply made with failure for wiggle almost built in, and (c) the lever / spring tolerances wrongly designed for long term stability. I never heard from them again, so I guess they do not care about customer requests for improved design.

There are other devices that work a lot better

  • Hosmer hooks and TRS Prehensors all have ball joints that do not wiggle under very heavy constant use and over many years.
  • The Toughware PRX V2P has switchable grip power and is very robust.
  • Using any TRS Prehensor or Toughware Equilux (new as of 2017) allow for voluntary closing control, which allows a far greater and better grip force control than the VO hooks.
  • Massively improving the cable shredding issue and wearing a wide area contact shoulder anchor (instead of a large German manufacturer's parts) allows to run a hook device at constant maximal grip strength setting, doing away with the need to lower grip strength of the hook at any time really.
  • Wearing PUPPCHEN wrist and connectors / adapters effectively removed the Otto Bock bolt issue entirely.

 

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