How a hydraulic press multiplies force

HostI have always found it wild how a person can step on a brake pedal and stop a huge truck moving at sixty miles an hour. It's not like the person is that strong on their own, but somehow that small push turns into a massive grip on the heavy wheels. How does a bit of oil in a tube actually turn a soft tap into that much power?

GuestIt all comes down to a very simple rule about how liquids behave when you trap them in a space. If you have a jar full of air and you push down on a lid that fits tightly, the air just bunches up. It gets squished. You can feel that springy fight as the air gets packed closer together. But if you fill that same jar with water or oil and try to push down, it feels like hitting a brick wall. The liquid doesn't want to get smaller. It refuses to be crowded. Because it can't be squished, any push you give it at one end has to go somewhere else immediately. It passes that push along to every single spot inside the container at the exact same strength.

HostSo it's like a solid rod of metal, but it can go around corners?

GuestThat's a great way to think about it. It acts like a hard link that can bend and snake through pipes. If you have a long garden hose full of water and you poke the water at one end, the water at the other end has to poke out at the same time. But the real magic happens when you change the size of the holes at the ends of those pipes. Imagine a thin pipe connected to a very wide pipe, both filled with oil. On the thin side, you have a small plug, and on the wide side, you have a large plug. When you push down on the small plug, the liquid carries that push over to the large plug. But because the large plug has a much bigger face, that push is now spread out over a larger area.

HostWait, I don't see how spreading it out makes it stronger. If I take a spoonful of peanut butter and spread it over a whole loaf of bread, it's thinner, not more powerful. Why does spreading the push make the machine stronger?

GuestIt's because of how the liquid talks to the surface of the plug. Let us say the big plug is ten times the size of the little one. Every tiny square inch on the face of that big plug is getting the full strength of the push you started at the small end. The liquid doesn't care that the plug is bigger; it just knows it has to push against every bit of wall it touches with the same weight. So, if you push with ten pounds of weight on the small side, every section of the big side that's the same size as your small plug also gets ten pounds of push. Since there are ten of those sections on the big plug, you now have a hundred pounds of push moving up. You have traded the size of the tool for the strength of the move.

HostThat sounds like we're getting something for nothing. If I can just keep making the second plug bigger and bigger to get more strength, why don't we use this to move mountains with a pinky finger?

GuestWell, here is the catch. You're right that you get more strength, but you lose out on distance. This is the part that people often miss. If you push the small plug down by ten inches, the big plug might only move up by one inch. You have to move a lot of liquid through that thin pipe just to fill up a tiny bit of the space under that big, wide plug. It's a trade. You can have a light push that moves a long way, or a heavy push that moves a tiny bit. You're not making new energy. You're just stretching it out. To crush a car in a scrapyard, the pump has to move back and forth hundreds of times, pushing lots of oil, just to make that giant heavy plate move down a few feet.

HostSo it's like a trade-off between how far you move and how hard you can push. But why do we use oil? Could we do this with a gas if we just pumped enough of it in there?

GuestYou could try, but it would be a mess. Gas is way too bouncy. If you used air in a car brake, when you stepped on the pedal, the air would just squish and get hot. You would spend all your energy packing the air molecules closer together instead of moving the brake pads. By the time the air was squished enough to actually push back, you would've already hit the car in front of you. Liquid is the key because it's stubborn. It stays the same size no matter how hard you shove it.

HostDoes the liquid ever get tired or wear out? I mean, if it's being shoved with thousands of pounds of weight all day, something has to give.

GuestThe liquid itself is fine, but the work does turn into heat. When you force oil through those tight valves and pipes at high speed, it rubs against the walls and itself. That friction warms it up. If a big machine runs for a long time, the oil can get hot enough to burn you. Also, the seals that hold the oil in can only take so much. If you try to lift something that's way too heavy, the push will eventually find a weak spot. A rubber ring will snap or a pipe will burst, and the oil will spray out like a fountain.

HostIt's funny to think that the whole system relies on the fact that water and oil are just too stubborn to get out of the way.

GuestThat's the heart of it. Even the strongest steel frame will snap before the oil gives up and squishes, so the only real limit is how much heat the system can take before the seals melt.

HostThose crushing videos feel a lot different now that I know it's all about that long, slow push of oil doing the heavy work behind the scenes.