Eli5 How do jacks work?


How does a little 50 lb tool lift a 900 lb object with just a few light pumps?

In: Physics

A hydraulic hand jack is basically a lever with savings. There are little one-way valves set up such that every big crank you give the handle results in a relatively small exchange of fluid in the hydraulic piston which stays in place when you lift the handle again. In this way, rather than having to exert 900 lbs of force over 12 inches of space, you only have to exert 45 pounds of force over 240 inches of space.

Your weak arm is using a long lever to pump tiny amounts of fluid per pump into a cylinder with considerablt larger diameter than the pumping piston. Leverage, via hydraulics.

Youtubing a video of “how a hydraulic jack works” is probably going to be clearer than reddit can explain in text. However, I shall try.

First, when you push down on that lever, it amplifies how much force is applied to the lever’s piston at the base of the lever arm, which only moves a tiny amount to compensate. So if you apply 10 lbs of pressure on the handle, and move 20 inches, the piston at the base of that lever will only move 2 inches, but apply 100lbs of force. (numbers are approximate, its the relationship that counts.) Now here comes the part you can’t readily see. There’s hydraulic fluid underneath that small piston attached at the base of the lever. When that piston moves, it shoves that hydraulic fluid into the large cylinder that has a large cylinder to match. It’s shoved in there with just as much pressure. So you’re pushing on the small piston with say 100 lbs of force. There’s a piston in the large cylinder that’s connected to the actual jacking shaft, and it’s about 10x as large as the small piston. So with just as much pressure (force over area) with 10x the area, you’ll amplify the force of the small piston by 10, but again loose travel to compensate. 10lbs and 20 inches on the handle, 100lbs and 2 inches at the small piston, means you get about 1000 lbs of force and 0.2 inches out of the jack as a whole. 100x as much as you put in, but you move the lever 100 times further than the jack actually moves.

Fluids do not compress under pressure. So if you fill a hose with a fluid, and the hose and fittings can hold the force, you can push fluid through the system to move something on the other side. This makes hydraulics essentially flexible levers without the drawback of bend in solid levers. So your Jack is operated by a lever that exchanges distance for force (your lever travels farther in exchange for generating more force; torque) and transfers that force through the fluid that can’t be compressed and raises the cylinder. When the fluid pushes through, it opens a one way valve, and when that pressure is released, the valve closes, allowing you to charge for another cycle (the upstroke).

When you turn the little pin to lower the jack, you are manually opening the one way valve.

So, in effect, you are traveling your arm farther than the cylinder is raising and the difference is what generates the excess force. Similar to when you are trying to lift something heavy with a seesaw, if you just make the side you’re manipulating longer, you can lift more weight.