Introduction to Hydraulic Units 1

Hydraulic System

Fig 1. Schematic diagram of simple hydraulic system

The hydraulic systems comprise of a number of parts for its proper functioning. The component of the hydraulic system are storage tank, filter, hydraulic pump, pressure regulator, hydraulic cylinder, control valve, piston, and leak proof fluid flows pipelines. The schematic diagram of a simple hydraulic system is shown in figure 1. It comprises of:

• Storage tank
• A movable piston connected to the output shaft in an enclosed cylinder
• Electric pump
• Filter
• Pressure regulator
• Control valve
• Leak proof closed loop piping.

The output shaft transfers the motion/force while all other parts help to control the system. The fluid tank is a storage for the liquid used as a transmission media. The liquid used as transmission media is generally high-density incompressible oil. To remove dust or any other unwanted particles from the oil, it is filtered before pumped by the hydraulic pump. The capacity of the pump is governed by the hydraulic system design. These pumps normally deliver constant volume in each revolution of the pump shaft. Hence, the fluid pressure can increase indefinitely until the system fails. To avoid such circumstances the pressure regulator is used which redirect the excess fluid back to the storage tank. The direction of movement of the piston is controlled by changing liquid flow from port A and port B. The movement of the cylinder is controlled by using control valve which directs the fluid flow. The fluid pressure line if connected to the port B the piston rises and if it is connected to port A the piston lowers down. The valve is used to stop the fluid flow in any of the port. The leak proof piping is also very important due to safety, environmental hazards and economical aspects. Other accessories such as travel limit control, electric motor starter, flow control system and overload protection may also be used in the hydraulic systems.

Pumps

Hydraulic pumps function according to the displacement principle. In hydraulic pump, there is the existence of mechanically sealed chambers in the pump. Through these chambers, fluid moves from the inlet (suction port) of the pump to the outlet (pressure port). The sealed chambers make sure that there is no direct connection between the two ports of the pump. As a result, these pumps are best suited to operate at high system pressures and are ideal for hydraulics.

Different Types of Hydraulic pump

Hydraulic pumps are designed and manufactured on the basis of different functional and hydraulic system requirements, such as required range of pressure, operating medium, type of drive, etc. There is a large range of design principles and configurations behind hydraulic pumps. Therefore, not every pump can fully satisfy all sets of requirements to an optimum degree. There are three types of hydraulic pumps:

1. Hydraulic Gear Pumps

• Internal gear pump
• External gear pump
• Gear ring pump
• Screw spindle pump

1. Hydraulic Vane Pumps

• Single chamber vane pump
• Double chamber vane pump

1. Hydraulic Piston Pumps

• Radial piston pump
• Axial piston pump

Note: Gear pumps operate with fixed displacement volume, whereas vane and piston pumps may operate with fixed as well as variable displacement volumes.

Application of Hydraulic pump

Hydraulic pumps are the device that is used to transfer power through the hydraulic liquid. These pumps have various applications in automobiles, automatic transmissions, controllers, material handling systems, compressors and household items.

• A backhoe uses an engine driven hydraulic pump to drive the articulating parts of the mechanical hoe.
• The hydraulic pumps are commonly used in the automotive vehicles, especially in power steering systems.
• The hand operated hydraulic pump is used in a hydraulic jack where many strokes of the pump apply hydraulic pressure to lift the ram.
• The lift system of the tractor is operated by the hydraulic pumps. These are used in automatic transmissions and material handling systems in industries.
• Many precise controllers are developed by using hydraulic pumps. The commonly used compressor is operated by reciprocating pumps.
• The hydraulic pumps are also used in routine household systems like power lift and air-conditions. Therefore, it can be said that the hydraulic pumps have significant applications in industries as well as one's routine life.

Control Valves

In a hydraulic system, the hydraulic energy from a pump is converted into motion and force by the means of an actuator. The control of these motion and force is one of the most important functions in a hydraulic system and is carried out by the control valves. The appropriate selection of control valves makes sure of the desired output and safe functioning of the system. So, to control the hydraulic outputs, different types of control valves are required.

There are basically three types of valves employed in hydraulic systems:

1. Directional control valves

• Check valve
• Directional spool valve
• Directional poppet valve

2. Pressure control valves

• Control task: Variable throttle valve

1. Pressure relief valves
2. Pressure reducing valves
3. Pressure drop valves
4. Pressure difference valves
5. Pressure valves with additional electric switch-off

• Switching task: Constant throttle valve

1. Pressure shut-off valves
2. Pressure sequence valves

3. Flow control valves

• Throttle valve: flow Δp dependent
• Control valve: flow Δp independent

Classification based on actuation mechanism

1. Manual actuation

In this type of control valve, the spool is operated manually. Manual actuators are push button hand lever, and pedals etc.

2. Mechanical actuation

The DCV spool can be operated by using mechanical elements such as roller and cam and rack and pinion, roller and plunger etc. In these arrangements, the end of the spool is of roller or a pinion gear type. The plunger or rack gear or cam is attached to the actuator. So, the mechanical elements gain some motion relative to the actuator (cylinder piston) which can be used for the actuation.

3. Hydraulic actuation

This type actuation is usually known as pilot-actuated valve and a schematic is shown in Figure. In this type of actuation, the hydraulic pressure is directly applied on the spool. The pilot port is located on one end of the valve. Fluid entering from pilot port operates against the piston and forces the spool to move forward. The needle valve is used to control the speed of the actuation.

4. Solenoid actuation

The solenoid actuation is also known as electrical actuation. The schematic of solenoid actuation is shown in the figure below. The energized solenoid coil creates a magnetic force which pulls the armature into the coil. This movement of armature controls the spool position. The main advantage of solenoid actuation is its less switching time.

Fig 2. Solenoid actuated Valve

5. Pneumatic actuation

DCV can also be operated by applying compressed air to a piston at either end of the valve spool. The construction of the system is similar to the hydraulic actuation as shown in Figure. The only difference would be the actuation medium. The actuation medium is the compressed air in pneumatic actuation system.

References

1. Technical book, “Construction Machinery Training”, Instate, Imlambad
2. Harris, F. and McCaffer, “Management of Construction Equipment”, Macmillan Education Ltd. London, UK.
3. Erich J. Schulz, “Diesel Equipment I and II”, Mcgraw-Hill book co.
4. Frank Harries, Ronald McCaffer, “Construction of Plant Excavating and Material Handling”, Granda Publishing.
5. SAE Handbook Volume 4
6. “Caterpillar performance Handbook”, Edition 33, Caterpillar Inc, Peoria, Illinois, USA.