Advantages/disadvantages of Pneumatic & Hydraulic Actuators

PneumatiActuators

A set of devices into with one or more pneumo engines, which are determined to start mechanisms or some other objects by means of pressed working gas is called pneumatic actuator,or pneumo actuator.The devices intended for transformation of potential and kinetic energy of the stream of compressed gas in mechanical energy of the output link that can be, for example, a rod of the piston, a shaft of the turbine or the case of the jet device is called pneumatic engines of the automated actuator.All pneumatic actuators can be subdivided into the following types:
diaphragm pneumatic actuators;
pneumatic power cylinders;
gas-engine pneumatic actuators;
The principle of transformation of potential or kinetic energy of the gas stream into mechanical energy of the engine output link of the engine provide the base for division into types

Advantages of PneumatiActuators:

simplicity of realization relatively to small back and forth motions;
sophisticated transfer mechanisms are not required;
low cost;
high speed of moving;
ease at reversion movements;
tolerance to overloads, up to a full stop;
high reliability of work;
explosion and fire safety;
ecological purity;
ability to accumulation and transportation.

Disadvantages of PneumatiActuators:

compressibility of the air ;
impossibility to receive uniform and constant speed of the working bodies movement ;
difficulties in performance at slow speed;
limited conditions – use of compressed air is beneficial up to the definite values of pressure;
compressed air requires good preparation

Hydraulic Actuators

Hydraulic Actuators, as used in industrial process control, employ hydraulic pressure to drive an output member. These are used where high speed and large forces are required. The fluid used in hydraulic actuator is highly incompressible so that pressure applied can be transmitted instantaneously to the member attached to it.
Pascal’s Law
Pressure applied to a confined fluid at any point is transmitted undiminished and equally throughout the fluid in all directions and acts upon every part of the confining vessel at right angles to its interior surfaces.
Since pressure P applied on an area A gives rise to a force F, given as,
F = P×A
Thus, if a force is applied over a small area to cause a pressure P in a confined fluid, the force generated on a larger area can be made many times larger than the applied force that crated the pressure. This principle is used in various hydraulic devices to such hydraulic press to generate
very high forces

Advantages of Hydraulic Actuators

Variable hydraulic actuators are widely used as drives of machine tools, rolling mills, pressing and the foundry equipment, road and building machines, transport and agricultural machines,etc. A number of advantages in comparison with mechanical and electric transfers explains such their wide application
infinitely variable control of gear-ratio in a wide range and an opportunity to create the big reduction ratio;
small specific weight, i.e. the weight of a hydro actuator is in ratio to transmitted capacity(0,2…0,3 kg / kWt);
opportunity of simple and reliable protection of the engine from overloads;
small sluggishness of the rotating parts, providing fast change of operating modes (start-up, dispersal, a reverser, a stop);
simplicity of transformation of rotary movement into reciprocating one;
opportunity of positioning a hydraulic engine on removal(distance) from an energy source and freedom in making configuration.

Disadvantages of hydraulic actuators:

efficiency of a volumetric hydraulic actuator is a little bit lower, than efficiency of mechanical and electric transfers, and during regulation it is reduced;
conditions of operation of a hydraulic actuator (temperature) influence its characteristics;
efficiency of a hydraulic actuator is a little reduced in the process of exhaustion of itsresource owing to the increase in backlashes and the increase of outflow of liquid (falling of volumetric efficiency);
sensitivity to pollution of working liquid and necessity of high culture service.
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