Pneumatics V Hydraulics


These systems normally use air at pressures between 550 kpa - 1030 kpa (80 and 120 psi). These pressures are not particularly high and do not give rise to particularly high forces hence components need not be particularly heavy. Aluminium and plastics are widely used for pneumatic cylinders and other system components, consequently some of these types of devices are available at low cost.

Because of the low cost of many types of pneumatic power tools, many factories have compressed air piped round all workshop areas. These tools often use high speed air turbines with very low stall torque.

As gases are easily compressed, this limits the applications of these devices. They can not normally be used for accurate positioning, except against a stop.

If the medium is compressed air (which it normally is) then slight leakage, or exhaust from power tools, will not usually present any nuisance or hazard. However even in a system operating at modest pressures, the compressibility of gases means that very considerably energy may be stored which will present a safety hazard unless managed correctly. For this reason all pressure vessels for storing any type of gas (or liquid that readily vaporises) are required by law to be regularly inspected.

Air motors may be considered as an alternative to DC drives for general industrial applications - link to Dynatork Air Motors Site.


A wide range of liquids are available. Prior to the 1940s all liquids tended to be oils and flammable. Following the total loss of a US aircraft carrier caused by fire following damage to its hydraulics system in the 1940s, low and non flammable I water based fluids have been developed. It should however be noted that a few cars are still destroyed by fire every year following accidents, due to brake fluid leaks onto very hot exhaust manifolds. As even water based liquids contain a few percent chemicals for corrosion inhibition and to improve lubricity, leaks to the environment must be avoided.

Hydraulic systems work at much higher pressures than pneumatic systems, normally 200 bar, 20 MPa, with aircraft applications between 34 MPa and 69 MPa (3000 psi, aircraft: 5000 to 10000 psi). Components must be manufactured to very close tolerances. Although the hydraulic fluid has high bulk modulus, the high pressures mean that compressibility may have to be considered in some situations. Accurate positioning can often be achieved. To reduce cost and complication, while facilitating increased functionality, there is currently quite a bit of effort going into replacing hydraulic actuation with electric actuation, particularly with automotive systems such as power steering and brakes.

The high pressures mean that frequently very high forces are generated in components and very strong construction, which is sometimes heavy, is usually needed. Cast iron is often used for pump / motor bodies and steel (sometimes heat treated) for valve bodies and actuator cylinders. For this reason, and the need for close tolerances, hydraulic devices tend to be expensive.

By minor changes to valving, pumps become motors and vice - versa. Hydraulic motors usually operate at low speeds and are frequently intended to provide specified (high) torques. Stall torque may be the maximum torque available.

Because of high power density, components and the hydraulic fluids may get hot and oil coolers are common. While at low temperatures most hydraulic oils are mainly a skin irritant, at higher temperatures, above about 60 C, they can be absorbed through the skin and present a much greater hazard. High pressure leaks are a severe hazard.

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David J Grieve, updated 10 July 2003, original 22nd April 2002.