1. Hydraulics Versus Pneumatics
Fluid Power
Hydraulics Versus Pneumatics
Lesson Plan
By Clifford Mayhew

2. Pneumatic and Hydraulic Control Comparison
Pneumatic Control Is:
Clean
Fast
Intrinsically Safe
Overload Safe
Inexpensive for Individual Components
Is Infinitely Controllable
Produces Extremely Large Forces
Requires High Pressures
Requires Heavy Duty Components

3. Pneumatic and Hydraulic Dangers
The dangers of the use of compressed air include:
Air Embolism
Hose/Pipe Whipping
Noise
Crushing/Cutting
The dangers of working with high pressure oil can be infinitely more drastic:
High Pressure Oil Injection
Oil Burns
Crushing/Cutting
Carcinogens
This injury is a result of placing the hand in front of a jet of leaking hydraulic fluid at around 180 Bar

4. Differences in Symbols
Symbols Reminder
Hydraulic Pump
Compressor
Pumps and Motors differ only by filling in the direction arrow or leaving it white.
Hydraulically Actuated and supplied 3/2 Pilot Spring
Supply and Pilot arrows are also filled in or left white.
Pneumatically Actuated and supplied 3/2 Pilot Spring

5. Differences in Symbols
Cylinders and other actuators also differ with respect to supply and direction arrows.
Hydraulic Double Acting Cylinder
Pneumatic Double Acting Cylinder
Many symbols do not change, for example the Filter symbol.
Hydraulic Filter
Pneumatic Filter
Remember however that the physical construction is completely different.
For example, hydraulic filters can be either Suction Strainers (suction side of the pump), Pressure Filter (pressure side of the pump) or Return Filter (in the return to tank line). Each filter requires different properties.

6. Differences in Symbols
Hydraulic valves have a crossover to Tank.
Pneumatic valves tend to have two Exhaust outlets to Atmosphere.

7. Differences in Medium
Hydraulic Oil is Hygroscopic and can be easily Oxidised at high temperatures.
Hydraulic Fluid is viscous and can be various types with varying Viscosities including:
Flame Retardant
Mineral Oil
Synthetic Oil
Water Glycol
Hydraulic fluid can have many additives including:
Anti Oxidants
Lubricity Improvement
Anti Foaming Additive
Anti Wear Additives
Air can hold Moisture which can turn into Condensation at the Dew Point.
The pneumatic components must remove the Condensation from the air and provide Lubrication.
Air is safe under a wide range of operating temperatures.

8. Differences in Principles and Properties
Pneumatic systems rely on a supply of Compressed air flowing through Pipes to Actuators. The Force for work is produced due to the Pressure of the Air acting on the Area of the actuator.
Air is Compressible.
Gas laws such as Boyle’s and Charles’s Laws govern medium behaviour
Actuator demand is measured in m3 per hour or operation
Compressor output is measured in m3 per hour Free Air Delivery (FAD)
Hydraulic systems rely on a supply of incompressible fluid flowing through Hoses to Actuators. The Force for work is produced due to the Pressure of the Oil acting on the Area of the actuator.
Oil is considered Incompressible.
Bernoulli’s and other Fluid Flow Laws govern medium behaviour
Actuator demand is measured litres per minute for a specific speed
Pump output is measured litres per minute
Both Hydraulics and Pneumatics are described with Pascal’s Law and F=PA

9. Differences in Pressure and Force
Pneumatic Pressures and Forces
Produced at 10Bar
Used at 0~6 Bar
Forces up to 5000Kg
Hydraulic Pressures and Forces
Produced and used at 200~400Bar
Forces up to Thousands of tonnes
Force Calculator
Force
Pressure
Area

10. Differences in Construction Production
The hydraulic Power Pack contains the Pump, Tank (Reservoir), Filters and commonly a Relief Valve for protection of the system. The unit is usually local to the machine that is using it. Hydraulic pumps are usually Positive Displacement devices which means they displace all the oils they pump.
The Pneumatic Compressor installation usually includes a Dryer and Receiver. The unit is usually remote from the machine that is using it.

11. Differences in Construction Valves and Actuators
Pneumatic valves and actuators are generally of light construction as they need to deal with pressure up to a maximum of 10 Bar.
The cost of these components is cheap when compared to the much more heavily constructed hydraulic components.
Hydraulic valves and actuators are much more heavily constructed than pneumatic components. This is because the components must deal with pressures up to 400 Bar+. Hydraulic actuators can be very large when compared with common pneumatic actuators. Hydraulic components are much more expensive than standard pneumatic components. A standard hydraulic DCV is in the region of hundreds of Euro, a standard application pneumatic valve would typically cost tens of Euro.

12. Differences in Construction Hoses, Pipes and Connectors
Braid (Reinforcement)
Hose
Fittings
(Connectors)
Hydraulic hoses and connectors are heavily constructed to hold the higher pressures. Rubber hoses are steel Reinforced (Braided) to Strengthen them.
Pipe Y
Tee
Straight Coupling
Fittings
(Connectors)
Pneumatic Pipes and Fittings are of light construction. Pneumatic Pipe is made from nylon and generally connects to the fittings using ‘Push Fit’ connectors.

13. Differences in Application
Hydraulic systems are used where large forces are required such as in earth moving equipment, heavy cutting, Pressing and Clamping
Pneumatic systems are used for relatively light moving, Clamping and Process operations

14. Application Example Video
Lifting a car on a Car Ramp does not require high speed or clean control systems. Large forces are required to lift the heavy car. This application is particularly suited to the use of hydraulics.
Moving and light clamping of components is easily, cleanly and quickly achieved using a pneumatic control system.

15. Differences in Circuit Design Pneumatic
This is a typical pneumatic Schematic. The Sequence Of Operation of the machine is as follows:
The operator places a circuit board into an open drawer on the machine.
The operator then closes the drawer, pushing in the Slide Cylinder.
The drawer immediately locks shut (Shot Bolt) and the Press Cylinder extends to cut the edges off the circuit board.
After a short delay, (Timer) the press cylinder returns.
Once the press cylinder has returned, the shot bolt retracts and the drawer, with the Cut To Size circuit board is opened for the operator.
Timer

16. The highlighted timer is a Sub Circuit of the main schematic
The highlighted timer is a Sub Circuit of the main schematic. The timer contains a Accumulator, Restrictor, Check Valve and 3/2 DCV
Restrictor
Check Valve
Accumulator
3/2 DCV
All the components on the main schematic represent components that control Sequencing, Timing and Positioning

17. Differences in Circuit Design Hydraulic
This schematic is from a system that manufactures Castings. The system is controlled via a computer.

18. Counterbalance Valves
This circuit shows components that are used to control heavy loads in the hydraulic system. The system is called a Counterbalance Circuit because it hydraulically counterbalances the load.
A pneumatic system would not normally need to control loads of this magnitude, therefore this type of circuit would not be seen in a pneumatic schematic.
Counterbalance Valves

19. Simple Application Example Hydraulic
Actuator
DCV
Pump
Filter
Tank

20. Simple Application Example Pneumatic
Actuator
5/2 DCV
3/2 DCV
FRL

21. Electro-Pneumatic and Hydraulic Systems
Control of Electro-Pneumatic and Hydraulic systems using Electrical control systems is similar for both media types.
Both systems would use Solenoid actuated valves, either Directly Actuated or Indirectly Actuated.
An Electronic system would commonly incorporate Push Button Switches (for human input), Reed Switches (to detect cylinder position), Proximity Sensors and Photocells (to detect machine/component position).
An Electronic system would also commonly incorporate Relays and computer controlled systems such as Programmable Logic Controllers.

22. Electro-Pneumatic and Hydraulic Systems
Push Button Switch
Reed Switch
Proximity Sensor
Programmable Logic Controller (PLC)
Solenoid
Relay