How strong would a steam-powered prosthetic arm be?

Question

Partly due to WB's great response to my steampunk questions, I think the world I'll work on next will have to be steampunk :)

If I had a steampunk cyborg running around, what could he do with his arm? I'll take care of maintenance myself; don't worry about that. I'm more interested in the possibilities of how much he could lift etc, if this arm was running just on steam-power.

Obviously, since this doesn't have to really work (and I doubt it really would), I can certainly stretch things a bit (and probably will), but I'd like to have a ballpark figure for lift power in this arm -- most specifically, if it'd be more or less than human strength; with regards to my story, the question is whether an undercover cyborg can be detected easily from the strength of his arm alone.

How strong would a steam-powered prosthetic arm be?
To make things easier, assume that a knapsack is available to be used for extra steam-power hardware; like water tanks and such, in case it can't be done with everything inside the arm. I'd much prefer not to need to knapsack, but it's not completely out of the question.

Answer 1

The actual question is difficult to answer as building steam powered prosthetics is well beyond current science, and even more so beyond the science of the era that actually used such technology. Oddly enough what you apparently wanted to know, whether there is a noticeable difference, is answerable anyway.

It is not really practical to use steam power directly, so the cyborg would use some mechanism to transmit the power. Pretty much the only one applicable to steampunk and at all plausible for prosthetics would be hydraulics.

And hydraulics would actually have a noticeable difference from human muscle. Once the hydraulics lock into position, moving them noticeably with external force almost requires something to break. So the grip of an hydraulic arm would be almost impossible to break. Some padding, gloves and shock absorbers in the system would probably hide this "hardness" in normal use, but any kind of wrestling or other event where human applied strength to the arm would make this noticeable.

As for strength, hydraulics are limited by the pressure used and the area of the pistons. So it really depends what kinds of pressures these systems could withstand and how much space people building would be willing to spend on it. Personally, I would build a system with variable pressure so that the cyborg could balance energy use and risk of mechanical failure to the strength he expects to need. This would imply normally being even weaker than most humans, but on demand having super-human strength available. Even at low pressures the hydraulics would be pretty hard to beat at wrestling though.

Speed of the hydraulics would be dependent on the ratio between the areas of the hydraulic pistons and the valves controlling them. Having reasonable control over speed of movement would require using analog valves with analog logic to control them. This is actually a reasonable steampunk assumption anyway. Maximum speed would be limited by the area of the hydraulic lines. There would certainly be design compromises made to reduce bulk from the relatively long lines.

The power source would be a compact steam engine creating a pressure differential between hydraulic accumulators. Steam engine would have a limited power output and when the cyborg used power at a higher rate the pressure in the accumulators and the system would drop reducing the force the cyborg can apply. By staying still a moment the cyborg could let the pressure increase and have more force available. I think that this would lead to cyborgs thinking before they act. Wait patiently for opportunity and then take fast and decisive action. The redo from beginning.

Answer 2

Well the first thing would be that the person would stand out because steam power needs a 'large' engine and water to turn into vapor. A steam locomotive spews out huge amounts of steam as it goes down the track. An arm would of course need a fraction of power but having a pressurized container holding water over it's boiling point (and keeping it there) to use as a power source is going to be obvious and dangerous. When the arm moves it is going to need to release pressure and give off little puffs of steam somewhere.

I think steampunk would also accept a battery to power a hydraulic arm.

As Ville was pointing out, hydraulics, even ones small enough to work in a arm can be significantly stronger than an human arm. However lifting ability is limited to how strong the rest of the human is and how well the arm is attached. but being able to crush stones, hands and other things would be incredible.

Answer 3

Very strong. A steam engine of the same dimensions and materials as a internal combustion engine has much, much greater torque. This is due to the fact that pressure is exerted over the whole of the piston stroke. If anyone has ever seen one of these small ride on steam trains pull out of the station they will appreciate this. As pointed out above the arm can only be as strong as the body supporting it, although a hand could be more useful, an unbreakable grip might come in hand clutching a rope, for example.

Most probably there would have to be a backpack, as even very small 'flash steam' boilers are quite bulky for their power output, and they operate at very high temperatures, so not the best thing to carry around stepped to your back. If, however, a catalyst is used to produce steam from hydrogen peroxide (H_2O_2) then the apparatus could be contained within the arm, the tanks and reactor built into the bones, perhaps. As the reaction also releases oxygen brass, copper, and bronze would be used, so it fits the aesthetic. Superheated water also releases quite a lot of energy, releasing many times its own volume of steam, so that is another option that could fit into the arm. The last two options would also be cooler in operation, increasing the ability for the cyborg to go unnoticed.

While pistons and cables wrapped around gears at the joints seems the most logical 'muscle' it would be possible to make some variation of 'pneumatic muscle', a tube that shortens in length as it is inflated by the steam, would be more realistic as far as appearance goes. It would, however, have give that a direct piston or hydraulic design might lack, although they can have springs added to compensate.

So what can he do? Given adequate support where the arm is attached he could support his own weight indefinitely, especially if a locking mechanism is included. Crush people's hands and bones fairly easily I would think, although bone is remarkably strong. Probably do the same to light metal pipes. Also, as the hand is metal, and presumably cannot feel pain, can punch much much harder than a normal human.

Answer 4

For a prosthesis of this type I have always thought pneumatic bladders made the most sense, reinforced one the outside witha webbing such bladders could contain quite large pressures, but would also allow for flexibility more similar to an animal than the hydraulic example above.

In the most lightweight cases, assuming you made a system using rubber hoses reinforced with silk and braided wire, it might not even need an extensive harness to be mounted, while allowing for strength comparable to a human or slightly greater (especially in cases of grip and stability) and could double as a platform for mechanical interface to other devices, vehicles or weapons.

Depending on your power source, actuation method and mounting system this would even be possible to build today.

Answer 5

There is a guy who is building a pneumatic powered exoskeleton. As a steam engine creates something similar to the high pressured air used in pneumatics I'd see the raw power you could generate similar, too.

As described here this guy could use the leg part of his exo to lift the rear of a car and the upper part to lift 200kg.

His design is bulkier than a cyberarm but he did it by himself and he could not hide the stuff in his arms because he still has both of them. Whoever builds/equips this cyborg should have more manpower at his service.

So he should be stronger than the average guy. How obvious it would be is, more or less, up to you.

Answer 6

As strong as you like, but you'll have to take the weight penalty for it. Steam power is hot and heavy (and explosive), there are a few questions you need to answer before you can know how strong the arm will be

How are you going to heat the water to generate the steam?
What pressure are you going to make the steam?
How bulky do you want to make the arm?

Once you have answered those questions then you can work out how strong it is. In terms of control you have a choice, you can either build an arm that can pick up eggs and pluck flowers, or you can build an arm that will rip doors off their hinges but it's going to really hard to have it both ways.

Either way, if you're going to make this realistic by current technology, you might as well give the man a mini JCB. It'll be smaller and lighter.