In article .com,
"Jake" wrote:
I'm posting this to this group because it seems that this is where most
of the hydraulics related questions go.
[...]
You can make your cylinders basically any size you like--the force
generated equals the oil pressure times the area of the piston, so you
can scale 'em down a bunch if you're willing to deal with higher
pressures. And with a "hand" you probably won't be needing a whole
buttload of force on the intricate bits like fingers--you can easily
build a system using different pressures in different sections.
A piston less than 1/4" across has an area of 1/2 a square inch, so
20psi oil pressure gets you ten pounds of force, which ought to be
plenty for a finger. Step it up to 50psi for hand-smashing fun and
you're still dealing with negligible pressures and won't need
hydraulic-use rated hardware. Safe to work with, too.
Generally, as components shrink they can handle higher pressures more
easily, too, since there will be less surface area internally. (for
example, a hypo needle has a burst pressure in the neighborhood of
10,000psi) If you don't need a whole lot of flow use small lines.
Cheaper fittings to boot. Size it so fluid won't have to move faster
than 20 feet per second.
If you use hydraulics your biggest headache in terms of construction
would be the lines and hoses--that can be mitigated somewhat if you
place all of your valves as close as possible to the cylinders, so you
might prefer the tendons option. Also, if you go with a tendon setup
you have more room to install positional sensors either out at the
joints or along the tendons themselves. And with a tendon setup the
cylinders won't have to move around, so you could plumb them all with
rigid lines, which are better/cheaper than hoses to work with.
I assume you've read about open center and closed center hydraulic
systems. Open center basically has a low-pressure standby mode where
oil just gets pumped straight through the valves and back to the tank.
It's easy to do if all of your valves will be located on the same
manifold, but gets quite a bit more complex if you don't. Closed center
would be the way to go in that case, but you'll need either a variable
displacement pump or some sort of unloader valve arrangement. With a
variable displacement pump you'll wind up with a rather high pressure
standby that could make the hand difficult to control. If the valves
will all be computer-controlled and not in a common manifold, closed
center with an unloader activated by the computer would probably be
best. Make sure to always have a pressure relief valve in case
something goes wrong.
Maintenance depends on how cleanly you assemble the system and how
hot it'll run. You'll probably be running pretty cool, so it's not too
likely that you'll wear out or burn the fluid any time soon. Make sure
your reservoir has a low spot in it and the pump's intake is higher than
it. That way any water that might get in will stay out of the
hydraulics.
You'll get mixed opinions on where to place the filter--some say on
the return to the tank, some say before the pump, some say after the
pump if working pressures allow it. On the return to the tank is a
logical choice if you know you can put the system together cleanly.
Pump intake is a somewhat dangerous place because if your filter gets
clogged too badly it could either cavitate the pump, or worse, get
sucked into the pump.
You could also use a bypass filter setup--a secondary high flow, low
pressure pump that does nothing but take fluid from the reservoir, pump
it through a filter, and back. Tie the main pump's intake to the
filter's output and you'll get the best of all systems and a
low-pressure filter.
If you decide to build parts yourself you can make hydraulic
cylinders pretty efficiently with DOM (drawn over mandrel) tubing. You
only need to clean up and thread the ends and install your plug and
gland nut. Of course, you'll need to make plugs, pistons, and gland
nuts. (:
As far as hydraulic positioning systems, I know some hydraulic
cylinders that need accurate positional feedback have a ballscrew that
is attached to a rotary converter in the base of the cylinder. The nut
is on the end of the rod and the rod is hollow to make room for the
screw. But I think something like that would be extremely expensive and
not available in smaller sizes. Also, some cylinders actually have
threaded rods that are then built back up. The rod is smooth when all
is said and done, but you can place a magnetic pickup by the rod and
count as the threads pass by below the surface. Again, expensive, but I
think more readily available in small sizes.
You'd probably be better off using a sensor outside the cylinder or
even at the joint.
--
B.B. --I am not a goat! thegoat4 at airmail dot net
http://web2.airmail.net/thegoat4/