Hydraulics 101

You pass a construction site and see large machinery busy on the job. Engines drone along. Suddenly, there’s a load roar. Machines lift tons of earth and steel. How can these machines do this so easily? Because hydraulic power does the lifting.

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

Hydraulics work on the same principle as the mechanical lever, a simple machine that functions like a seesaw. If one end of the lever is pushed down, the other end moves up. If the lever is well designed, then a machine can lift a very heavy object without much effort. This is especially true when the lever’s fulcrum is close to its load.  

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

How does a hydraulic cylinder work? 

In a hydraulic device, a hose or pipe connects a cylinder with a small diameter to a cylinder with a large diameter. Within the pipe is the non-compressible fluid. By applying a force to the small cylinder, a much greater force is applied to the large cylinder. The force, in this case, is multiplied by an amount equal to the difference in the area of the cylinder.

When a cylinder of a diameter A1 applies a force of F1, it travels a distance of d1. The cylinder connected to it with a diameter of A2 will exert a force of F2 and travel a distance of d2.

Because the force is greater, the distance that the large cylinder moves will be smaller. Remember that Work is equal to the force applied multiplied by the distance travelled (W=fd).

The setup of a hydraulic system can allow machines to do some amazing things. You can connect two cylinders with pipes or a flexible hose across a long distance. This lets you put the first cylinder in one room, and the second cylinder in another part of a building, easily transmitting the force around corners or through walls. 

Hydraulics are often used for moving parts of mechanical systems that need to lift or push heavy objects. The landing gear in an aircraft use several hydraulic cylinders to move the wheels into place and to cushion the aircraft's landing.

What is a hydraulic power unit? 

In simple hydraulic systems, the small cylinder has to move a much larger distance than the large cylinder. That means the small cylinder is limited in the distance it can push or pull. To make a hydraulic system even more powerful, the small cylinder can be replaced with a pump. This creates something that works like the small cylinder, but with an infinite “travel distance.” The pump continues to draw fluid out of a reservoir and pushes the big cylinder until the reservoir is empty. These hydraulic power units (HPUs) are installed on construction equipment and heavy lifting rigs. Most large airplanes have HPUs to move the flaps on wings and rudders. A single HPU can feed dozens of cylinders, limited only by the pump power and reservoir capacity.

What are hydraulic fluids?

The choice of the fluid inside the hydraulic system is important as it flows under tremendous pressure, easily thousands of psi (pounds per square inch).

Did you know? 

The SI-based unit of pressure is the Pascal (Pa). 1 psi = 6894.76 Pa.

Hydraulic oils are specially formulated so that they function consistently across wide pressure and temperature ranges. Also, gases do not dissolve into them. This is important because dissolved gases form bubbles under certain conditions, which can create problems.

Did you know? 

Hydraulic and pneumatic systems are similar. Hydraulic systems use fluids under pressure, while pneumatic systems use gases. Each has its advantages and disadvantages.

Hidden Hydraulics 

Hydraulic components are often hidden on machinery. Cars use hydraulics to stop four tonnes of steel with a light press of your foot (with the help of the brake booster). Manufacturing and shipping businesses use robots that can lift and move packaging and other parts using hydraulic-powered arms.

Hydraulics are all around you! The next time you notice a lot of work done with ease, see if you can spot the hydraulic systems.

Did you know? 

An airplane engine may require as much power as two or three cars to start up its hydraulic system because of the high pressure needed.