[kmcclary]

Originally Posted by IndyStef

BIG difference!
Air is compressible, hydraulic fluids are not (usually oil, rarely water)! That means that pneumatic systems can store energy relatively easily in a tank, but are not very accurately positionable under changing loads. Hydraulic systems are a lot heavier, and all energy has to be provided by the pump at the time when its needed (unless a pneumatic tank stores energy - in which case you could use pneumatics right away).

Yes, the most cited air/hydro differences are: compressibility (vs non), and a MUCH higher average working pressure in hydraulic systems.

But it is not true that a hydraulic system HAS to be limited by the pump's output. There are devices commonly known as "hydraulic accumulators" or "hydraulic surge suppressors". (You can web search on them.) These devices DO accumulate energy for hydraulic systems in the same way that the air tanks accumulate energy for pneumatic systems.

The main parts of a hydraulic accumulator are an expandable chamber for the fluid, working against a "spring force" of some kind. This is often implemented as a very high pressure vessel with a flexible diaphragm of some kind bisecting it and air on the other side to act as the spring, or as a hydraulic piston with a VERY heavy duty spring, air, or displaced vertical weight giving the back force.

For example: Smart multi-car hydraulic "elevator array" systems in buildings can use the unused elevator cars as "accumulators". They SWAP energy for "nearly free rides" with valves by sending one or more empty cars one way as the full one goes the other, or transferring the surplus "down" energy into a BIG standard accumulator to be reclaimed later for "up" use. Down can ALWAYS be had at any time by simply opening a valve. A Smart Array system (hydraulic OR electric based) can save the building owners a LOT in energy costs over the long haul.

Anyway... Standard compact hydraulic accumulators are NORMALLY used for one of these reasons (there may be others, but these are the most common):
A) To store energy for high frequency actuation you otherwise couldn't directly GET out of a pump;
B) Act as a "shock absorber" to help reduce "water hammer" effects, or
C) Allow a MUCH smaller "average capacity" pump to be used instead of a large "peak" one, for serious cost savings.

Just like with air, once an accumulator is pumped up, you can go crazy for a while with the actuators. However, pressures in a HYDRAULIC accumulator vessel can often exceed several THOUSAND PSI. The volume depends on your application.

The total energy stored is related to the total displacement volume times the maximum rated working pressure. IOW, a HECK of a lot of stored hydraulic energy can be had in just a TINY accumulator package, as much as a HUNDRED times higher than is available in OUR air based systems of the same size. "A couple thousand PSI" systems are fairly common. Anything over "a few thousand PSI" hydraulic systems DO exist, but are VERY dangerous, and take special plumbing parts and rules to avoid ruptures.

For safety, systems with hydraulic accumulators MUST include a way to CAREFULLY purge the energy stored in them. The easiest schematically is to add a simple bleeder/purge valve between the high pressure side and the oil reservoir.

BTW, hydraulic systems can be much more complex, too. Some components (like decent servo valves) may require microscopic oil filtering to eliminate even tiny contaminants for proper operation. I've once been in situation where even cracking a system for a SIMPLE change may lead to a DAY of lost time refiltering the entire oil load. But that's an extreme case...

I worked with accumulated systems when I helped a well known auto company develop their hydraulic active suspension system. Trust me, you quickly develop a HEALTHY respect for the kind of stored energy that can make a multi-ton car make significant corrective motions several hundred times a second, using only a hydraulic pump similar in size to our FIRST air pump, and a hydraulic accumulator not much bigger than a large grapefruit! Your health and safety hinges on proper procedures and rituals, and you think TWICE before taking ANY action in the lab.

During R&D tests, we even once had a full sized car jump over a foot straight up into the air on powerup due to a software error. (Whoops...) You might say, "there were puddles everywhere"... Trust me though, not ONE of them was the car's hydraulic fluid!

...And THAT is why we don't include even NON-accumulated hydraulics in FIRST contests, and seriously limit working pressures! If you think an air piston can be dangerous, imagine a TINY hydraulic system which can EASILY produce THOUSANDS of pounds of crushing force in a gripper using with an itty bitty cylinder. In addition, hydraulic oil spray from a leaky high pressure connection intersecting a person can "pressure inject" oil directly under or even THROUGH your skin. NOT fun, and it may require some messy surgery to attempt to clean it out. (I've seen it happen... Luckily, it was an arm and not a FACE...)

But ACCUMULATED hydraulics are WORSE. They're that dangerous WELL AFTER the power is cut off. One mistake with a well sealed but unpurged multi-K PSI accumulator based system (even DAYS later) could easily be crippling, or fatal. On the suspension project, we had some SERIOUS safety classes before we could even get NEAR the project hardware.

Now, I don't mean to scare anyone off from the field. Accumulated Hydraulic systems are VERY USEFUL, and some things simply can't be easily done WITHOUT them. But they're VERY serious systems, not something to "play with" as a first time user. Air is MUCH better for learning on, and teaches you the same things with fewer safety issues. NEXT comes Basic Hydraulics, and THEN Accumulated and "High Pressure" Hydraulics.

I'm SURE we'll be sticking with low pressure, "limited storage" air for THIS contest. I just want people to know that the field of "Accumulated Hydraulics" EXISTS, and can be quite exciting!

(I hope this has been informative...)

- Keith