How does Hydraulic Press stop so fast after shaterring something? Why isn’t there any momentum?
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There’s often a retaining spring, or the mass of the piston is too great to move quickly through pneumatic pressure alone.
The press is moving incredibly slowly and with great force. So it doesn’t really have much momentum. Also the hydraulic fluid controls the press in both directions. So when the hydraulic pump stops, it is basically locking the piston in place.
Liquids are not compressible to any significant degree. So even if you have a liquid with high pressure before you shatter something when the cylinder moves a small bit the pressure inside drops and there is no force behind it.
Even if the direction of the moment is down you need to create a vacuum in the system for it to move if no more airflow in. that means air pressure keeps it up.
A pneumatic system with air pressure will behave differently because it is compressible so if you use it and you shatter what is in the way the pressure is still high when the cylinder moves a bit. It will likely move as far as it is possible to move before it sops.
The press moves super slowly and isn’t particularly massive, compared to the forces on it, so the momentum present is minimal.
If you have something strong enough, the press setup can bend a little bit, and so the whole thing sort of BANGs when the object breaks as all that tension releases, but that’s about it.
A hydraulic press is just a metal bar being pushed on by liquid coming from a pump. This giant metal bar moves very slowly because the pump only moves a little bit of liquid at a time, but the force available is exceptionally high, which is why it can crush things with ease.
Think of it like a screw. You’re turning a screw, tightening a clamp down on a coffee mug, until it shatters, but the screw stays right where it was.
This is because liquids are **functionally** incompressible. The idea of hydraulic pressure is partially a misnomer. That pressure isn’t stored in the system like a compressed gas or a spring. It’s like that screw. The pressure reading on the gauge tells us its instantaneous working pressure, or the force being delivered to the load, as a function of kPa, Bar, or PSI present in the fluid. You can see it linger if the load is pushing back on the cylinder, sometimes called clamping force, but it will instantly disappear the moment it clears the load.
They do not surge forward because a) the fluid in the rear of the cylinder, which was pushing forward, cannot be compressed and stores no energy. When the load disappears, the system is already at equilibrium. It cannot drop, because the liquid also cannot be put into a vacuum but for a few microseconds and immediately nulls out. And b) there is fluid in the front of the cylinder, used to retract it, which is also preventing the piston from traveling forward (in the case of double acting cylinders, which most YouTube Crushing Stuff presses are). This is much the same as how the threads of a screw prevent movement in both directions, unless the screw is turned.
Fluid power transmission gets very eli10 as you keep looking. I can explain more if you have more questions
Edit: yes, you can compress liquids in the realm of physics, but this explanation was too long already as it was.
The relative incompressablilty of the fluid has a lot to do with it. The hydraulic pump is high pressure not high volume. When you let the valve close by letting the control lever go, the short amount of time doesn’t let a lot of fluid flow. The press alone is not moving a lot of mass so it stops easily.
what momentum? the press was moving slowly to begin with, so after the thing shatters, it just continues at the speed it was already moving, which was slow
what you’re probably thinking about what it would do if the press used gas instead of liquid. liquids don’t compress, and that’s why they’re used for this purpose. this means you can only fill a container until it’s full with a liquid. with a gas, after it’s full, you can continue to put more gas into the container, and the gas just squishes in there.
so if you put a gas in there, once the thing shatters, that gas wants to escape since it’s being held under pressure (picture releasing an inflated balloon that you haven’t tied yet), so the air forces the piston outward at a crazy high speed. with a liquid, since it can’t compress, as soon as the container ruptures, the liquid just pours out normally like out of a broken glass or mug.
this is also why they use liquids to pressure test air tanks. air tanks pressure tested with air = bomb. air tanks pressure tested with water = safe.
this is also why air jacks are considered dangerous. i heard a story of a guy who ignored safety warnings and used an air jack on the bumper of a car. like one of those big external chrome bumpers from an 80’s car. he bent over the front of the car, and the bumper came off (you aren’t supposed to use them for jacking a car up), and the pressure in the jack(s) shot the bumper into the man’s face, severely disfiguring him. it crushed every bone in his face. had it been a hydraulic jack (means they have liquid in them), the car would’ve fallen and the bumper would jumped a few inches at most (from it storing energy from bending ever so slightly).
Usually it’s compressing something slowly there’s no force to react oppositely more than the opposing force if that makes sense? For example if a melon is being squashed the forces go out in other directions even if it’s slowly via the squashed melon. Put a compressed bottle under there it’ll come out wherever the weakest point is which is the bottle head. There’s always forces opposing the compressor but it’s just not strong enough.
Take a glass and put a ping pong ball in it. Then slowly fill up the glass with water.
The ball will rise up, pushed by the water. When you stop filling the water, there isn’t any significant momentum to keep pushing the ball.
Now replace the ball with a cylinder that fits snugly in the glass, but not so tight it can’t move given a little push. Then put a small hole in the bottom of the glass connected to a piece of tube. Fill the glass with water through the tube.
Again, the cylinder will rise up and when water stops being filled, there’s still no real momentum to continue pushing the cylinder so it stops.
Assuming you mean *”Why does hydwualic pwess stop as soon as can of beans makes huge explosion”* then there’s an additional aspect which would be present in an automated press (I think hydraulic press channel is controlling the press manually and he stops as soon as something shatters).
An automated press will want to trip its sequence as soon as it detects a sudden drop in press force as this would indicate a fault with the hydraulics. This would either be the thing its pressing has shattered / come off (no more resistance from the object), or the hydraulic seals have blown and fluid isn’t going where it should be. In either case you’d want the machine to stop, raise an error and have the operator investigate before continuing.
Short answer: to protect the machine from damage.
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