Hydraulic brakes Hydraulic presses Hydraulics at work Pneumatics Pneumatics at work

Hydraulics is the science of forces created by moving liquids or liquids at rest. Technically hydrostatics is the term used for the science of liquids at rest, and hydrodynamics for liquids in motion. The prefix "hydra" implies that the liquid used is water, but oils also play an important role in hydraulics. Hydraulic systems can involve enormous forces, caused by the fact that liquids cannot be compressed, and have many applications.

Hydraulics is a most efficient way of making the most of a limited amount of force. When a force is exerted on a solid object, the object will transmit that force in the same direction, for example a billiard cue or a cricket bat hitting a ball. With liquids the force is exerted equally in all directions. This means that if a mass of 1kg was placed on a piston with an area of 1cm² the force applied would be transmitted through the liquid in the pipe and the same force per cm² will be exerted at the other end of the pipe no matter what the pipe's diameter.

Hydraulic brakes
At one time car brakes worked by a system of levers applying force to the brake linings against the brake drum. Now brake fluid is used to transmit that force much more effectively. Pressure on the brake pedal applies a force to the master cylinder that then transmits the force to the braking mechanism. Large trucks also use hydraulic pressure, but here the hydraulics are used to hold the brakes OFF, and the force is released as the brake pedal is depressed. Then the greater force to apply the brakes is able to take over. The "sigh" as a truck or bus releases the brakes is the hydraulic pressure being reapplied.

Hydraulic presses
To raise a car while being serviced requires a great force. This is applied by hydraulic pressure where force from a pump is applied to a small-diameter piston, the force is transmitted by oil in the sealed system, and that force is multiplied by raising a much greater diameter piston at the car-raising end. The force magnification is proportional to the relative surface areas of each piston. However, the distance moved by the larger diameter piston is much less than the distance moved by the smaller piston.

A long easy push on the small cylinder will lift a larger mass, a smaller distance on the larger cylinder.

Hydraulic press

Hydraulics at work

Pneumatics
This is the branch of physics dealing with gases at rest or in motion. Compressed air is the most common gas used for pneumatic applications. Because energy is required to compress gasses, energy can be stored in compressed gasses. This stored energy can be released to do work, by decompressing the gas. A good example of stored energy being released is the sound produced when a compressed gas horn is activated. Any system using energy stored as a compressed gas is called a pneumatic system.

One of the simplest pneumatic systems is a bicycle pump. The pump has a simple valve allowing air to pass into the barrel as the handle is pulled out. Then, when the handle is pushed in again the valve closes and air is compressed. You can feel this force if you hold your finger over the end as you push the handle. The compressed air is then used to pump up a bicycle tyre to high pressure so that the ride will be smoother and friction reduced.

Pneumatics at work
Pneumatic pressure is used in many everyday work situations. Examples are pneumatic drills, high speed dentist's drills, tightening and loosening wheel nuts on a car or truck, and sirens where air passes at high pressure over a spinning disc with holes in it. Robots in automation systems often operate by pneumatic pressure. Pneumatics are also used in underground mining tools where electric motors might create sparks and potential explosions.

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