Hydraulics Versus Pneumatics

By Clifford Mayhew

Fluid Power

Lesson Plan

Pneumatic and Hydraulic Control Comparison

Pneumatic Control Is:

Clean

Fast

Intrinsically Safe

Overload Safe

Inexpensive for Individual Components

Hydraulic Control:

Is Infinitely Controllable

Produces Extremely Large Forces

Requires High Pressures

Requires Heavy Duty Components

Pneumatic and Hydraulic Dangers

The dangers of the use of compressed air include:

Air Embolism

Hose/Pipe Whipping

Noise

Crushing/CuttingThe 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

Differences in SymbolsSymbols Reminder

Hydraulic Pump Compressor

Pumps and Motors differ only by filling in the direction arrow or leaving it white.

Supply and Pilot arrows are also filled in or left white.

Pneumatically Actuated and supplied 3/2 Pilot Spring

Hydraulically Actuated and supplied 3/2 Pilot Spring

Differences in SymbolsCylinders and other actuators also differ with respect to supply and direction arrows.

Hydraulic Double Acting Cylinder Pneumatic Double Acting Cylinder

Hydraulic Filter Pneumatic Filter

Many symbols do not change, for example the Filter symbol.

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.

Differences in SymbolsHydraulic valves have a crossover to Tank.

Pneumatic valves tend to have two Exhaust outlets to Atmosphere.

Differences in Medium

Hydraulic Fluid is viscous and can be various types with varying Viscosities including:

Hydraulic fluid can have many additives including:

Hydraulic Oil is Hygroscopic and can be easily Oxidised at high temperatures.

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.

Flame RetardantMineral OilSynthetic OilWater Glycol

Anti OxidantsLubricity ImprovementAnti Foaming AdditiveAnti Wear Additives

Differences in Principles and Properties

Air is Compressible.

Oil is considered Incompressible.

Actuator demand is measured in m3 per hour or operationCompressor output is measured in m3 per hour Free Air Delivery (FAD)

Gas laws such as Boyle’s and Charles’s Laws govern medium behaviour

Both Hydraulics and Pneumatics are described with Pascal’s Law and F=PA

Bernoulli’s and other Fluid Flow Laws govern medium behaviourActuator demand is measured litres per minute for a specific speed

Pump output is measured litres per minute

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.

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.

Differences in Pressure and Force

Pneumatic Pressures and Forces

Hydraulic Pressures and Forces

Force Calculator

Force

Pressure

Area

Produced at 10BarUsed at 0~6 BarForces up to 5000Kg

Produced and used at 200~400BarForces up to Thousands of tonnes

Differences in ConstructionProduction

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.

Differences in ConstructionValves 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.

Differences in ConstructionHoses, Pipes and Connectors

Braid (Reinforcement)

HoseFittings

(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.

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

Application Example Video

Moving and light clamping of components is easily, cleanly and quickly achieved using a pneumatic control system.

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.

Differences in Circuit DesignPneumatic

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

This is a typical pneumatic Schematic. The Sequence Of Operation of the machine is as follows:

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 ValveAccumulator

3/2 DCV

All the components on the main schematic represent components that control Sequencing, Timing and Positioning

Differences in Circuit DesignHydraulic

This schematic is from a system that manufactures Castings. The system is controlled via a computer.

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.

Counterbalance Valves

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.

Simple Application ExampleHydraulic

Pump

Tank

DCV

Actuator

Filter

Simple Application ExamplePneumatic

FRL

3/2

DCV

5/2

DC

V

Act

uat

or

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.

Electro-Pneumatic and Hydraulic Systems

Push Button Switch

Reed Switch

Proximity Sensor

Solenoid

RelaySolenoid

Programmable Logic

Controller (PLC)