Hydraulic Systems

Muhajir Ab. RahimSchool of Mechatronic Engineering

The Basic Hydraulic System

Consist of these components

1. A fluid, usually oil

2. A tank, or reservoir, to hold a supply of fluid

3. Fluid conditioning devices to keep the fluid clean and cool

4. A prime mover, usually an electric motor or engine, to drive the pump

5. A pump, to cause the fluid to flow

6. Conductors, usually pipe or tubing, to carry the fluid

7. Valving, to control fluid flow, direction and pressure

8. One or more actuators, usually cylinders or hydraulic motors, to do the work

Hydraulic Fluid

• Water?

Evaporates, freezes, leaks, rust parts, poor lubricant• Petroleum oil

Viscosity

• Viscosity is a measure of oil’s resistance to flow. • It is determined under laboratory conditions by

measuring the time required for a specific amount of oil at a specific temperature to flow through a small opening.

• The higher the viscosity, the longer it takes.• High viscosity= thick, Low viscosity= thin• Viscosity index is a measure of how much the viscosity

changes when the oil is heated.• High viscosity index, if the viscosity change very little.

(good quality- lubricant capability remains constant)

Pour, Flash and Fire Points

• Pour point is the lowest temperature at which a given oil will flow enough to be useful

• Flash point is the temperature at which oil will flash, or light briefly if touched with a flame

• Fire point is the temperature at which a given oil, if touched with a flame, will catch fire and stay lit for at least five seconds.

• The auto-ignition temperature is the level of heat at which a given oil will catch fire by itself.

Functions of Hydraulic Fluids

1. Transmit force to the actuators doing work of the system

2. Conduct heat away from moving parts

3. Lubricate moving parts to reduce heating, lessen wear, and increase efficiency

Desired characteristics of hydraulic fluids

1. Appropriate viscosity

2. Lubricating ability

3. Freedom from damaging chemicals

4. Resistance to foaming

5. Fire resistance

6. Oxidation resistance

*Additives are used to achieve desired characteristics in hydraulic fluid

Contaminants

Contaminants in hydraulic fluid are a major source of

problems in system operation

The three principle sources of contaminants are;

1. Newly installed components

2. Air entering the tank

3. System operation, generating chemicals, gum, sludge, varnish and moisture

Hydraulic Reservoir (Tank)

The functions of a tank are

1. Store fluid

2. Protect fluid

3. Provide means for checking fluid level

4. Provide means for adding fluid

5. Cooling

6. Remove contaminants

• Strainer is used to block large contaminant particles from entering the system

• Baffle is a wall in the tank, to disrupt the flow between return line and pump inlet line (for more even fluid temperature and more thorough settling of contaminants to the tank bottom)

• Magnetic plug is used to attract metal particles. (should be removed and cleaned periodically)

Tank Schematic Diagram

Tank with pump inlet

line

Tank with immersed strainer

Tank with strainer in

pump inlet line

Tank with strainer in pump inlet line,

filter in return

Pump

• The purpose of pump is to cause fluid flow

• The motor or engine that drives a pump is called a prime mover

• The most common type of pump is the external gear pump

• Positive-displacement pumps mean that fluid must be moved when the pump is running

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System schematic showing location of the pump and motor

TANK

STRAINER

PUMP

MOTOR

Pump Schematic Diagram

Pressure Relief Valves

A pressure relief valve has two functions:1. Set a maximum

operating pressure level for the system

2. Protect system components such as the pump, valves, and lines from overpressure.

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System schematic showing location of the pressure relief valve

TANK

STRAINER

PUMP

MOTOR

PRESSURE RELIEF VALVE

Pressure Reducing Valve

• The purpose of pressure reducing valve is to regulate or limit pressure for an individual circuit branch or actuator

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System schematic showing location of the pressure reducing valve

TANK

STRAINER

PUMP

MOTOR

PRESSURE RELIEF VALVE

PRESSURE REDUCING VALVE

Directional Control Valve

• The purpose of directional control valve is to deliver fluid to an actuator at the proper port at the appropriate time.

• A way is a flow path through a valve.• A port is an opening in the valve body.• A position is a specific setting of a spool which

determines flow paths.• Directional control valves are described by specifying the

number of ways, the number of positions, the means by which the valve is operated, and for 3-ways valves, whether open or closed center.

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System schematic showing location of the directional control valve and actuator

TANK

STRAINER

PUMP

MOTOR

PRESSURE RELIEF VALVE

PRESSURE REDUCING VALVE

DIRECTIONAL CONTROL

VALVE

ACTUATOR

Actuator

• Device which converts fluid energy into mechanical force and motion.

• Cylinder is used to produce motion in a straight line, which we call linear motion.

• Rotary actuator (hydraulic motor) is used to convert hydraulic energy into rotating motion about an axis.

Quiz

A hydraulic system is said to have a mechanical advantage of 40. If the input piston, with a 12 inch radius, has a force of 65 pounds pushing downward a distance of 20 inches, find

i. the volume of fluid that has been displacedii. the upward force on the output pistoniii. the radius of the output pistoniv. the distance the output piston moves