HYDRAULIC AND PNEUMATIC MACHINES

Introduction

In many industries, there are three main methods for power transmission from one point to another. They are electrical transmission, which is through wire and transformers; mechanical transmission, which is through shafts, gears, belts, chains etc.; and the last is the fluid power transmission, which is through liquids or gas in a confined space. The fluid power transmission method comprises the hydraulic system and the pneumatic system.

Fluid power is the technology that deals with the generation and transmission of forces and movement of mechanical elements or systems with the use of pressurised fluids in a confined system. Both liquids and gases are considered fluids. Fluid power systems include a hydraulic system (hydro means water in Greek) and a pneumatic system (pneuma means breath).

Oil hydraulic system employs pressurised liquid petroleum oil and synthetic oil.

Pneumatic system employs compressed air which is released to the atmosphere after performing the work.

 

Hydraulic Devices

A hydraulic device is a device that uses liquid power flow as the powering medium.It works by pushing the hydraulic liquid through the numerous hydraulic motors and hydraulic cylinders which apply pressure onto the various parts of the device.

Examples of hydraulic devices are:

1. Hydraulic cylinders are actuation devices that utilise hydraulic fluid to produce linear motion and force.
2. Hydraulic pumps deliver high-pressure fluid flow to the pump outlet.
3. Hydraulic valve contains and transfers the flow and pressure of hydraulic fluid in hydraulic power systems. They range from simple shut-off valves to the precision control valves.
4. Hydraulic motors are powered by pressurised hydraulic fluid and transfer rotational kinetic energy to mechanical devices.
5. Hydraulic regulators maintain the output pressure at a set value.
6. Hydraulic reservoirs are storage tanks that hold liquid or gases used in fluid power applications. They are usually rectangular, cylindrical, T-shaped and L-shaped. They are made of steel, stainless steel, aluminium or plastic.
7. Hydraulic accumulators use incompressible fluid and a compressed gas, spring or raised mass to store energy, absorb shock, eliminate noise and counter-balance loads.
8. Hydraulic rotary actuators use a pressurised, incompressible fluid to rotate mechanical components.
9. Hydraulic clutches are assemblies consisting of elements for the connection and disconnection of shafts (clutch) in equipment drives.

 

General examples of hydraulic devices are brakes, cardiovascular systems, jacks, shock absorbers in vehicles, cylinders which raise and lower the blade of bulldozers, elevators in shorter buildings, car lifts at most garages, parts that make a lowrider go up and down, a dentist or barber chair, wheel drivers on construction equipment and dump mechanisms on dump trucks.

 

Pneumatic Devices

Examples of pneumatic devices are:

1) Air brakes on buses and trucks are a compressed air brake system. This is a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake pad needed to stop the vehicle.

Air brakes on trains are actuated by compressed air. Modern trains rely upon a fail-safe air brake system.

 

2) An air compressor or gas compressor is a mechanical device that increases the pressure of air or gas by reducing its volume. An air compressor is a specific type of gas compressor which is similar to pumps. It increases the pressure on a fluid and can transport the fluid through a pipe.

 

3) A Barostat system is a device used to maintain constant pressure in a closed chamber.

 

4) Cable jetting is used to install cables with optical fibers in underground polyethylene ducts.

 

5) Dental drill is used during dental procedures, usually to remove decay tooth structures.

 

6) HVAC control system: The HVAC Healing Ventilation and Air Conditioning) control system applies regulation to a heating and/or an air conditioning system.

 

7) Exercise machines are used for physical exercise.

 

8) A pipe organ is an organ that produces sound by driving wind through pipes.

 

9) A player piano is a self-playing piano containing a pneumatic mechanism that operates the piano action via pre-programmed perforated music paper.

 

10) A pneumatic actuator mainly consists of a piston, a cylinder and valves or ports, which is of different types including tie rods, cylinders, rotary actuators, gripper and vacuum generators.

 

11) A pneumatic bladder is used to seal drains and ducts to contain chemical spills or gases. They are used for the containment of chemical spills, oil spills or fire water, usually in the form of booms.

 

12) A jack hammer or drill is a hammer that uses compressed air as the power source.

 

13) A pneumatic tyre is a ring-shaped covering that fits around a wheel’s rim to protect it and enable better vehicle performance.

 

14) A pressure regulator is a valve that automatically cuts off the flow of a liquid or gas at certain pressure.

 

15) A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed.

 

16) A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum.

 

17) A pneumatic lego is a kind of lego brick that uses air pressure to perform various actions using pneumatic concepts.

 

Components of Hydraulic and Pneumatic Machines

The basic components of a fluid power system are essentially the same, regardless of whether the system uses a hydraulic or a pneumatic medium.

There are five basic components used in a fluid power system. They are:

1. Reservoir or receiver.
2. Pump or compressor.
3. Line (pipe, tubing or flexible hose).
4. Control valve.
5. Actuating device.

 

Components of Hydraulic Machines

Fig. 15.8 shows a simple scheme of a hydraulic machine with its basic components.

 

Pump set

The hydraulic pump supply fluid to other components in the system. The electric motor is the power source to drive the pump.

 

Control valves

Directional control valves route the fluid to the desired actuator. It consists of a spool inside a cast or steel housing. It is used in several places in hydraulic machines.

 

Actuator

This includes the hydraulic cylinder which is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke.

 

Reservoir

The hydraulic fluid reservoir holds excess hydraulic fluid and accommodates volume changes from cylinder extension and contraction, temperature driven expansion and leaks. It is also designed to aid in the separation of air from the fluid and to work as a heat accumulator to cover losses in the system when peak power is used.

 

Accumulator

It is a common part of hydraulic machinery; its function is to store energy by using pressurised gas.

 

Hydraulic fluid

It is the life of a hydraulic circuit; it is also known as tractor fluid. It is usually petroleum oil with various additives; some require fire resistant fluids, depending on their applications.

For the hydraulic fluid to do work, it must flow to the actuator and/or motors, then return to the reservoir. It is thereafter filtered and re-pumped.

 

Filters

These are important part of a hydraulic system. Metal particles are continually produced by mechanical components and need to be removed along with other contaminants. Thus, filters aid the removal of these particles and contaminants from the hydraulic system.

 

Components of Pneumatic Machines

A pneumatic machine carries power by employing general air or compressed gas as a fluid for transmitting energy from an energy-generating source to an energy-using point to accomplish useful work. Fig. 15.9 shows a simple circuit of a pneumatic machine with its basic components.

 

Function of the Components

The functions of a pneumatic machine’s components are as follows:

1. The storage reservoir is used to store a given volume of compressed air.
2. The compressor is used to compress the fresh air drawn from the atmosphere.
3. The pneumatic actuator converts the fluid power into mechanical power to perform useful work.
4. The piping system carries the pressurised air from one location to another.
5. External power supply (motor) is used to drive the compressor.
6. The valves are used to control the direction of flow rate and pressure of the compressed air.

 

Operations and Uses of Hydraulic and Pneumatic Machines

The general operation of the fluid power system is to transmit and control power through pressurised fluids.

 

Operation of a Hydraulic Device

The piping in Fig. 15.9 is of closed-loop type with fluid transferred from the storage tank to one side of the piston and returned from the other side of the piston to the tank.

1. When the position of the valve is changed to the upper part, the pipe pressure line is converted to point A and thus the load is raised.
2. When the position of the valve is changed to the lower part, the pipe pressure line is converted to point B and thus the load is lowered.
3. When the valve is at the centre position it locks the fluid into the actuator or cylinder (thereby holding it in position), causing all the pumped out fluid to return to the tank via the pressure roller.

 

Operation of a Pneumatic Device

Air is drawn from the atmosphere through an air filter and raised to the required pressure by an air compressor. As the pressure rises, the temperature also rises, then an air cooler cools the air with some preliminary treatment to remove the moisture. The treated pressurised air then needs to be stored to maintain the pressure in the storage reservoir.

A pressure switch is fitted to start and stop the electric motor when pressure fails and reaches the required level respectively. Three positions change over the valve, delivering air to the cylinder which operates in a similar way to a hydraulic circuit.

In a general term, once actuated, compressed air enters into the tube at one end of the piston and hence, imparts force on the piston. Consequently, the piston becomes displaced (moved) by compressed air expanding in an attempt to reach atmospheric pressure.

 

Uses of Hydraulic and Pneumatic Devices

Hydraulic systems usually use some form of liquid to do work. Examples are car jacks, car lifts, hi-polymer water saws, hydraulic pump system used to close large switches (circuit breaker) in a high voltage switchyard (substation), water supply in the house, hydraulic power cleaner which is used to force out a narrow outlet at high pressure to clean cars, dirty paths, etc.

Pneumatic systems use air pressure to perform similar functions as outlined above for hydraulic systems. Examples are air compressors used to remove or install nuts on car wheels, pressure power cleaner for blowing away dirts from unreachable places, etc.