Hydraulics 101

Andres Onu
4 October 2018

Above: Image © Djahan, iStockPhoto.com

You pass a construction site and see heavy, strong machinery doing huge amounts of work. Engines drone along. Suddenly, there’s a load roar. Machines lift earth and steel. What provides the force to do the work with ease? Hydraulic power, a nearly silent component of the system, does the lifting.

Hydraulics allow you to transmit force from one point to another using an incompressible fluid. In most cases, you also increase the force applied at one end for a given amount of effort.

This works on the same principle as the mechanical lever, a simple machine that works like a seesaw. If one end of a lever pushes down, the other end moves up. If the level is well designed, then a machine can lift a very heavy object without much effort.

Cranes at construction site use hydraulic power. Image© baphotte, iStockPhoto.com

A hydraulic cylinder, however, has major advantages over mechanical levers. Let’s look at how a hydraulic cylinder works.

The hydraulic cylinder

In hydraulics, A small diameter cylinder connects to a large diameter cylinder using a hose or pipe. By applying a force at the small cylinder, the large cylinder applies a much greater force. The force, in this case, is multiplied by an amount equal to the difference in the area of the cylinder.

The cost is that the larger cylinder moves a shorter distance.

Have you or anyone you know ever used a car jack? That’s the tool people use to lift up cars to change flat tires. To use the car jack, you need to move the handle over and over in order to lift the car only a few centimetres. Similarly, a large distance, over and over, in order to have the lift move up only a few centimetres.

These diagrams show how to take advantage of hydraulic designs. First, we first can connect the two cylinders with pipes or a flexible hose across a long distance. This allows you to do work on a machine that moves, like an excavator. You can even put the system in one room, and the second cylinder in another part of a building, easily getting around corners or through walls. It is compact and as powerful as needed for the application.

Another amazing feature of hydraulic systems is used in nearly all cases. The example in the diagram above shows a small cylinder moving a long distance. This means that cylinder is limited in the distance it can push or pull. That cylinder can be replaced, however, with a powerful pump. This creates something that is like the small cylinder, but with an infinite “travel distance”. The pump continues to draw fluid out a reservoir and continue to push the big cylinder for a long as is needed. What would a mechanical lever that can do that even look like?

These hydraulic power units (HPU) are installed on construction equipment, heavy lifting rigs and even most large airplanes to actuate the flaps on wings and rudders. A single HPU can feed dozens of cylinders, limited only by the pump power and reservoir capacity.

You may have noticed that the fluid inside the system is important. It flows under tremendous pressure, easily thousands of PSIG (pounds per square inch, gauge pressure).

You must use specific fluids for this. First, you must use liquids. Unlike gases, liquids cannot be compressed. Gases would soak up too much energy when being compressed and would be difficult to control.

Instead, people use specially formulated oils. These oils behave consistently across wide pressure and temperature ranges and gases do not dissolve into them. Dissolved gases form bubbles under certain conditions, creating the same problems as above. Flammability, corrosion resistance, material compatibility, anti-ware and viscosity, are a few of the features people select with the oil type. Water, for example, would fail most of these requirements for most applications.

Did you know?The pressure inside of a hydraulic system can easily be many thousands of PSIG (pounds per square inch, gauge pressure). However, the flow rate at which fluid flows is what can consume huge amounts of power. An airplane engine may require as much power as two or three cars to start up.

Hydraulic components are often hidden when installed on machinery. Cars use hydraulics to stop four tonnes of steel with a light press of your foot (with the help of brake booster). Short elevators, like the one at the Ottawa International Airport parking garage, are lifted and lowered hydraulically (visible through the glass walls of the elevator shaft).

The pressure is all around you! The next time you notice a lot of work done with ease, notice whether hydraulics may be involved.

Did you know? A pinhole leak in a hydraulic system, from a damaged hose for example, is strong enough to cut all the way through your arm and leave the other side.

Learn More

About Hydraulics

How Hydraulics Work (2018) Explain That Stuff

What is Hydraulics? (2016) The Audiopedia

Schuyler Kahgee (2018) Industrial Millwright Apprentice

About careers in hydraulics

Andres Onu, Project Manager (2017) Let’s Talk Science


How Hydraulic Machines Work (Accessed 2018) How Stuff Works

Introduction to the Hydraulic Power Unit (HPU) (2016) Hydra Products

Hydraulics Cylinder Information (2018) Engineering 360

Engineering Essentials: Hydraulic Fluids (2018) Hydraulics & Pneumatics

Andres Onu

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