How Hydraulic Jack Lift Heavy Weight With Small Input Force ?


Hydraulic jack always be an option if we work with heavy equipment. For example when we want to change car’s wheel. To do that, we need to lift the vehicle. And hydraulic jack is a good choice to do that job.

But how the hydraulic jack can lift the heavy weight with small input force ?

in this article, I will show you inside the hydraulic jack when it works.

hydraulic jack animation

Hydraulic jack work using pascal’s law. With this principle, the hydraulic jack has an ability to lift a weight that has two or three time heavier than the input weight.

But here, I want to talk a lot about physics. I just want to show you how powerful this pascal principle, of course in simple explanation.

So, it can be said that the area very influence the weight can be hold. If you ever see a raft. When you stand in the raft with small area, you can sink. But if we make the raft larger, it can accommodate two or three people without sink.

It is a proof that the area very affected with the weight can be accommodate. In small area, we just need small force to make it sink. The larger the area, the bigger the force we need to make it sink.

Now we back to pascal’s law.

You can see there is a container with different diameter. Input diameter has smaller diameter than the output. Then, we put a piston on each cylinder. If we putting the load on the piston, then piston will move down and it contain the pressure.

Just like what we discuss before, due to small area on input piston it make the value of pressure bigger. The pressure will be continued to the output piston with the same value. But because the area of output piston is larger, the weight or force contained from the piston is larger.

To make it simple, if the output piston two times larger so the output force two times bigger. If it three time larger, so the output force three times bigger.

So the conclusion, hydraulic jack capable to lift high load due to there is a significant differ between input and output piston.

Following the principle, when we press the input piston the output piston will move up. And when we release the input piston, the output piston should be move down. But in fact, the output piston keep on position when we release the input piston.

Even when we press input piston for the second time, the output piston will move up again and again.

It happened because hydraulic have a pair of one valve. This mechanism only allow the fluid to pass in a direction depending the pressure value.

Just like this ilustration. When we give a pressure on the right side, the valve will open. And the fluid flow through the valve. But when we change the pressure on left side, the valve will close. And it will cut the flow, so the fluid is not flow back to the right side.

The hydraulic jack have two one way valve. This is located between the cylinder, and the one located on the reservoir path. Both of them have opposite direction. So, when we press the input piston, the first valve open, and another one will close. It make the fluid flow to the output cylinder.

It makes the piston rise.

When we release the input piston, there are increase of volume in the input cylinder. It will change the valve position. In this position, reservoir valve open and another one closed. So this volume enlargement, will suck the fluid from the reservoir tank.

To move the output piston down, we use a screw to bypass the fluid directly to the reservoir.