Physics of Hydraulics

If you’ve ever seen massive machinery move, you’ve seen hydraulics power in action. It might look slow and unremarkable in operation, but hydraulic technology allows us to very efficiently move extremely heavy objects using liquid.

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HYDRAULIC SAWNational Museum of Science and Technology Leonardo Da Vinci

The word hydraulics combines hydro, which means water in Greek, and aulos, which means pipe in Greek. 

Hydraulics principles

The first principle of hydraulics technology is that liquid is almost incompressible. Fluids can withstand enormous amounts of pressure without changing their volume much. The second principle is that any liquid under pressure exerts equal pressure on all container parts.

Incompressibility of liquid

If we take 2 equal containers of water and seal them with a movable platform, no matter how much weight we put on the platform, the water retains its volume. A rabbit OR an elephant could be sitting on top!

Compressibility of gas

If we compress gas, like air, the more pressure we exert on it, the smaller its volume. This happens because gas molecules are further apart than liquid molecules. Due to this property, gas does not make a good hydraulic medium.

Equal pressure

Under pressure, liquid exerts exactly the same pressure on all sides of a container. As we push on the plunger, fluid spurts out of holes in multiple directions with the same force. Pressure is exerted perpendicular to the container’s surface.

A simple hydraulics model

We can model a simple hydraulic system with 2 connected syringes. When 1 plunger is pushed down, the liquid forces the other plunger up. Hydraulics basically uses this fluid to transfer energy from one point to another.

Linear Hydraulics

Linear hydraulic systems can apply force in one direction. They are usually implemented as a liquid inside a sealed piston, similar to a medical syringe. When force is applied at one end of the system, the other end moves accordingly.

Equal cylinder, equal force

When the cross-section of the movable piston is the same, force is transferred from one end of the hydraulic system to the other at a 1-to-1 ratio. If both ends receive the same force, the system is in equilibrium and nothing moves.

Equal cylinder, unequal forces

Given the same cylinder cross-section, if only one end has force applied, then the other end of the hydraulic system will rise. This happens because the force on one end is transferred by the fluid to the other movable end.

Unequal cylinder

Doubling the surface area of the cylinder exposed to liquid creates a force multiplier effect. Now, applying force on the left side results in equivalently 2 times the force on the right side. However, the liquid’s volume hasn’t changed.

Unequal cylinder

In this last example, no force is being applied to the right side of the cylinder. This results in the the piston rising. But since the piston has twice the surface area, it rises half as high as before.

Hydraulic System

A complete working hydraulic system uses a hydraulic motor to convert the pressure of the hydraulic fluid into work. The system usually consists of a hydraulic pump, hoses, and an actuator, along with a tank to hold the hydraulic fluid.

Hydraulic pump

The hydraulic pump contains a mechanism that drives the flow of fluid. Many types of pumps exist: gear pumps, rotary vane pumps, and axial piston pumps. Pumps can also be set up to directly control the fluid’s direction and pressure.

Directional control valve

This device alters the direction of the fluid. By using this switch, we can redirect the flow of the liquid in order to move the fluid in the opposite direction.


The actuator is the part that moves and does work. By controlling the pressure and speed of the fluid, we can directly control the movement of the actuator. Here, a linear cylinder actuator is shown.

Hydraulic tank and fluid

The hydraulic tank stores excessive hydraulic fluid. Typically hydraulic fluid is an oil–based liquid. The job of the hydraulic fluid includes the transferring power, as well as lubrication and cooling.

Hydraulic hose

Hydraulic hoses are no ordinary hoses. They are designed and manufactured to withstand an incredible amount of pressure from within. Having a leak in a pressurized hydraulic system would be catastrophic.

Hydraulics Applications

Hydraulic power is used everywhere for its compact size and high power output. At the same time, it provides an operator with responsive and precise control, making it ideal for a wide range of machinery. 

Hydraulic equipment can be found in the manufacturing, robotics, and construction industries, among others.

Car brakes

Car brakes work by clamping a calliper around a rotor disk to create friction. Pressing the calliper together requires a lot of force from the foot pedal. Hydraulics–assisted brakes act as a force multiplier to help make braking effortless for the driver.


There are multiple hydraulics at work here. The outriggers are extended using hydraulics to provide stability to the crane platform. The boom is raised and lowered using a linear hydraulic motor.


An excavator is another piece of construction equipment that has a very visible use of hydraulics. The arms are moved by a linear hydraulic motor that help it dig rocks or break concrete. 

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