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Comparison between Hydraulic and Pneumatic Systems
In this post, the Comparison between Hydraulic and Pneumatic Systems is given in the tabular form. But before the table learns some basic definitions, advantages, and disadvantages of Hydraulic and Pneumatic Systems respectivly.
Hydraulic System
Hydraulics is the study of incompressible liquids and hydraulic devices are the devices that use oil as their working medium. Since the density of the oil does not vary with pressure, it is used as a working medium in hydraulic systems. Liquid-level systems are a class of hydraulic systems. The factors such as positiveness, accuracy, flexibility, high horsepower-to-weight ratio, fast starting and stopping, precision, simplicity of operation, and smooth reversal of operation make the hydraulic system find wide applications. The hydraulic system that is non-linear in nature is stable and satisfactory under all the operating conditions.
The operating pressure of hydraulic systems is between 1 and 35 MPa. It varies based on the application. The weight and size of the hydraulic system vary inversely with respect to the supply pressure for the same power requirement. A very high driving force for an application can be obtained by using high-pressure hydraulic systems. Due to the advantages of electronic control and hydraulic power, the electronic system in combination with the hydraulic system is widely used.
Advantages of Hydraulic System
The advantages of hydraulic system are:
- The fluid (oil) used in the hydraulic system acts as a lubricant.
- Fluid in the system is used to carry the heat generated conveniently to the heat exchanger.
- Hydraulic devices have a higher speed of response.
- Hydraulic actuators can be operated at different conditions without damage (continuous, intermittent, reversing, and stalled condition).
- Flexibility exists in the design, due to the availability of linear and rotary actuators.
- When loads are applied, the speed drop is small due to the presence of low leakage in the actuators.
Disadvantages of Hydraulic System
The disadvantages of hydraulic system are:
- The cost of a hydraulic system is higher when compared to an electrical system.
- Occurrence of fire and explosion due to the usage of non-fire-resistant liquids.
- Difficult to maintain the system due to the presence of leakage.
- Failure occurs in the system due to the presence of contaminated oil.
- Availability of hydraulic power is less when compared to electrical power.
- Difficult to design a sophisticated hydraulic system due to the involvement of non-linear and complex characteristics.
- Has poor damping characteristics.
Applications of Hydraulic System
Hydraulic system is widely used in the control applications such as power steering and brakes in automobiles, steering mechanisms of large ships, control of large machine tools, etc.
Devices Used in Hydraulic System
Devices such as dashpots, pumps and motors, valves, linear actuator, servo systems, feedback systems and controllers are used in hydraulic systems.
Pneumatic Systems
In pneumatic systems, the compressible fluid that is used as a working medium is air or gas. The pneumatic system is most frequently used in industrial control systems due to the fact that it provides a greater force than the force obtained from electrical devices. In addition, it provides more safety against fire accidents. The dynamic equations for modeling of pneumatic systems are obtained using conservation of mass. The devices in the pneumatic systems involved the flow of air or gas through connected pipelines and pressure vessels. The variables involved in the pneumatic systems are mass flow rate qm and pressure P. The electrical analogous variables to the mass flow rate and pressure are current and voltage respectively.
Gas Flow Resistance and Pneumatic Capacitance
The basic elements of the pneumatic systems are resistance and capacitance. The resistance to gas flow is due to the restrictions present in the pipes and valves. Hence, the gas flow resistance R is defined as the ratio of the rate of change in gas pressure to the change in the gas flow rate.
R=\frac{Change in gas pressure difference in N/m^{2}}{Change in gas flow rate in kg/sec}The pneumatic capacitance C of a pressure vessel depends on the type of expansion involved. The pneumatic capacitance of a pressure vessel is defined as the ratio of change in gas stored to the change in gas pressure.
C=\frac{Change in gas stored in kg}{Change in gas pressure in N/m^{2}}Advantages of Pneumatic System
The advantages of pneumatic system are:
- The working medium used in the pneumatic systems is non-inflammable and offers safety from fire hazards.
- The viscosity of gas or air is negligible.
- Return pipelines are not necessary since the air or gas can be exhausted once the working cycle of the device comes to an end.
Disadvantages of Pneumatic System
The disadvantages of pneumatic system are:
- Response of the pneumatic system is slower since the working medium is compressible.
- The power output of the pneumatic system is less.
- The accuracy of the actuator in the system is less.
Applications of Pneumatic System
Pneumatic system is used in guided missiles, air craft systems, automation of production machinery and automatic controllers in many fields.
Devices Used in Pneumatic System
Devices such as bellows, flapper valves, relays, actuators, position control systems, and controllers are used in pneumatic systems.
Comparison between Hydraulic and Pneumatic Systems
The comparison of hydraulic and pneumatic systems based on the common characteristics between the systems is given in the table below.
Table ∣ Hydraulic system vs Pneumatic system
| Characteristics | Hydraulic system | Pneumatic system |
| Compressibility | Working fluid (oil) cannot be compressed | Working fluid (air or gas) can be compressed |
| Lubricating property | Oil is a good lubricant | Air or gas lacks this property |
| Operating pressure | More | Less |
| Accuracy of actuators | Good at all velocities | Poor at low velocities |
| Leakage in the system | Internal leakage is permissible, but external leakage should be avoided | External leakage is permissible, but internal leakage should be avoided |
| Requirement of return lines | Necessary | Not necessary |
| Operating temperature | 200C-700C | 50C-600C |
| Sensitivity to temperature | Sensitive | Insenstive |
