Bowden cable mount for prosthetic arm [patent, explained]
The motivation for this patent was to avoid frequent cable tears, cable rips or cable damage when using metal cables such as the ones used for gear or brake control on bicycles.
A normal cable mount for a prosthetic arm..:
- (a) contains a cable sheath, that is
- (b) mounted on the prosthetic socket
- (c) with one (or two) side mounts.
- (d) The cable sheath curves, or is able to curve, by being flexible.
- (e) It extends towards the upper arm, shoulder and back.
- (f) The control cable - usually made of steel, but also made of kevlar, nylon or other material - runs through that sheath.
- (g) The sheath mount then is abraded, internally, always along the same (inner) curve of the sheath.
- (h) With steel cables, the abrasion build-up then causes an increase of friction that will start to damage a steel cable.
- (i) With increasing force for the voluntary opening (VO) hook, more rubbers or spring tension is place on the hook, and increasing forces also accelerate the abrasion rate.
- (j) This in turn leads to very early cable damage.
So in other words, a conventional prosthetic arm setup for body powered arms contains a type of sheath and cable mount that has a very self limited survival rate. I would usually wear up a steel cable within four to ten days.
I always wondered why that is, given that bicycle brake cables last a hell of a lot longer, with more forces on the brakes than I usually have rubbers on my hook.
So I found that the necessary design feature of any bicycle mounted Bowden cable was the following:
- (1) every bike mounted cable sheath is mounted on two sheath end mounts, let us call them A and B
- (2) the direct (metric) distance between A and B is always shorter than the length of the cable sheath, that connects A and B
- (3) due to that design property, pulling on the cable that runs through the cable sheath will force the cable sheath to approximate the direct line between A and B
- (4) but since the cable sheaths of bicycle control / gear / brake cables have embedded wires, the sheaths may bend but not shorten
- (5) the resulting forces will create pull between A and B, but since a bicycle frame usually has a rigid build and geometry, the bike will likely not significantly deform under the presence of a strong brake cable pull ; )
- (6) with that, the exact 3D course of the cable sheath is undetermined, but constrained
- (7) and that causes the sheath / cable interaction forces to be dynamic and complex, which is totally different from the resulting experience stated above under points (g) to (j).
The design of a prosthetic arm mounted cable sheath then embodies actual / true Bowden principles if it conforms to the following requirements (and these are explained in my patent):
- there is a belt or flexible structure like that, that does not extend in length when pulled but that can bend; usually, a fabric like a seat belt can provide this property;
- it contains two sewn on mount points for a bicycle cable sheath, let us call these A and B;
- and a cable sheath, that is some 3-4 cm longer than the direct distance between A and B when the belt is stretched, is then placed into these mounts; we tried 2 cm and I guess it was not enough, but really "longer" and not "a lot longer" is what you try to aim at;
- then you take your steel cable through that setup and Bob's your uncle
I wear that for 9-12 months service free, under massive loads for hook rubbers / pull force. Cable sheaths usually directly off a bike store, I went through a few Shimanos and a few others over the last years.
Obviously this setup also fails - just a lot less often.
- Current failure points are in the manufacturing details of A and B: the better the sheath mount and the cleaner the sheath cut, the longer the life span of the cable.
- What fails first now, is the cable sheath end, where the plastic, after about a year of wear and tear, gets hard and brittle.
Images: currently all parts for the sheath mount are hand made.