Fluid Power Systems & Control - Mechatronics...• Work and Power – Work is done whenever a force...

Introduction to Hydraulics K. Craig 1

Fluid Power Systems & Control


• Pascal’s Law (French scientist Blaise Pascal, 1623-1662)

– Pressure applied on a confined fluid is transmitted undiminished in

all directions, acts with equal force on equal areas, and at right

angles to them.

Introduction to Hydraulics

Eaton Corp.


Eaton Corp.

• Pascal’s Principle applied to the Hydraulic Press and

Hydraulic JackHydraulic Press(Bramah 1795)


• Conservation of Energy applied to the Hydraulic Press

– Conservation of Energy: The net work done on the

system plus the net heat transferred into the system

equals the change in the total energy of the system.

Eaton Corp.


• Hydraulic Power Transmission

– Source of Power is an electric motor or engine.

– Input is the pump (e.g., vane, gear, piston).

– Output is the actuator, either linear or rotary.

Eaton Corp.


Eaton Corp.


• Variable Speed Operation

Maximum SpeedEaton Corp.


Reduced SpeedEaton Corp.

The pressure relief valve protects the system from overload damage. When the load exceeds the valve setting, pump delivery is directed to

the tank. The result is a definite limit to torque or force output.


• Reversible Operation

Eaton Corp.


Eaton Corp.


• Pressure in a Column of Fluid

Eaton Corp.


Eaton Corp.

Eaton Corp.

Oil Level Above Pump

Oil Level Below Pump


• Atmospheric Pressure Charges the Pump

– The inlet of a pump normally is charged with oil by a

difference in pressure between the reservoir and the pump

inlet. It is necessary to have a partial vacuum or reduced

pressure at the pump inlet to create flow.

Eaton Corp.

Pressure differences push oil into a rotary pump, as

successive pumping chambers increase in size

as they pass the inlet.


• Liquids vaporize in a vacuum. This puts gas bubbles in

the oil. The bubbles are carried through the pump and

collapse with considerable force when exposed to load

pressure at the outlet. This will cause damage that will

impair the pump operation and reduce its life. This is

called cavitation.

• In addition, too low an inlet pressure permits air dissolved

in the oil to be released. This oil-air mixture collapses

when exposed to load pressure and causes the same

cavitation damage.

• Driving the pump at too high a speed increases fluid

velocity in the inlet line and, consequently, a lower

pressure condition, further increasing the possibility of

cavitation.

• The recommendation is about 12.2 psia at the pump inlet.


• Positive Displacement Pumps Create Flow

– Pump output is constant (except for changes in efficiency)

regardless of the pressure. Whatever goes into the pump is

forced out.

– The sole purpose of a pump is to create flow; pressure is

caused by resistance to flow.

Eaton Corp.

raise a load


Maintenance of pressure alone is no indicator of a pump’s condition. It is necessary to measure flow at a given pressure to determine whether a

pump is in good or bad condition.

Eaton Corp.


• How Is Pressure Created?

– Pressure is created when the flow of fluid is restricted. The

resistance may come from a load on an actuator or a

restriction (or orifice) in the piping.

Eaton Corp.


Eaton Corp.

Eaton Corp.

With the faucet completely closed, all

10 GPM will go through the relief valve at 1000 psi.

A relief valve should be used with a positive displacement pump.


• Parallel Flow Paths

– Liquids always take the path of least resistance.

– When parallel flow paths offer different resistances, the

pressure will increase only to the amount required to take

the easier path.

Eaton Corp.


Eaton Corp.

When the pump outlet is directed to two actuators, the actuator which needs the lower pressure will be the first to move. Since it is

difficult to balance loads exactly, cylinders which must move together are often connected to each other mechanically.


• Series Flow Path

– Pressures are additive when resistance to flow is connected

in series.

– The pressure at the pump is the sum of the pressures

required to open individual valves.

Eaton Corp.


• Pressure Drop Through an Orifice

– An orifice is a restricted passage in a hydraulic line or

component used to control flow or create a pressure

difference (drop).

– In order for oil to flow through an orifice, there must be a

pressure drop through the orifice. If there is no flow, there is

no difference in pressure across the orifice.

– An increase in pressure drop across an orifice will always be

accompanied by an increase in flow.

Eaton Corp.


Eaton Corp.

Eaton Corp.


• Force equals Pressure times Area

Eaton Corp.

Hydraulic Press


Eaton Corp.

• Speed of an Actuator

– How fast a piston travels or a motor rotates

depends on its size and the rate of oil flow into it.

Eaton Corp.

The force or torque of an actuator is directly

proportional to pressure and independent of flow.

The speed depends on the amount of fluid flow without

regard to pressure.


• Velocity in Pipes

– Fluid velocity is inversely proportional to pipe cross-sectional

area.

– Friction of a liquid flowing through a pipe is proportional to

the velocity if the flow is laminar (smooth). Should the flow

become turbulent, friction varies as the square of the

velocity.

Eaton Corp.

A = area (in2) V = velocity (ft/s) Q = flow rate (GPM) 1 Gal = 231 in3

VA = Q(0.3208)


• Work and Power

– Work is done whenever a force is exerted through a

distance.

– Work = Force x Distance

– Power is the rate of doing work or work/time.

– 1 hp = 33000 ft-lbs/min = 550 ft-lbs/sec = 746 watts (N-m/s)

– Power P = Flow Rate Q x Pressure P

– P (hp) = Q(GPM) x P(psi) x (0.000583)

– The horsepower required to drive a pump will be higher

since the system is not 100% efficient.


• Design of a Simple Hydraulic System

Eaton Corp.


Eaton Corp.


Eaton Corp.

The rate at which the load must travel will determine the pump size. Extending the rod 30

in. will require 300 in3 of fluid. If the total travel should take 3 sec., a flow rate of 100

in3/s is required, or 26 GPM. If the maximum pressure is 1000 psi and the pump efficiency is

83%, then a 18.2 hp pump is required.


Eaton Corp.

• More on Pressure

– Pressure results whenever there is a resistance to

fluid flow or to a force which attempts to make fluid

flow, as from a mechanical pump or gravity.

Pressure “head” comes from the weight of the fluid. The

pressure at any depth is always the same and is due to the

weight of the fluid above it.

A “head” of one foot (0.30 m) of water is equivalent to 0.433 psi

(0.03 bar, 9 kPa). A “head” of oil is equivalent to about 0.4 psi per foot

(0.09 bar, 9 kPa per meter).


• Atmospheric Pressure and the Mercury Barometer

Eaton Corp.

At sea level, atmospheric

pressure due to the weight of air is 14.7 psia (1.01 bar, 101 kPa). At higher altitudes, this

pressure is less; below seal level, it

is more.

Pressure less than atmospheric pressure is called a partial vacuum.


• Pressure and Vacuum Measurement Scales

Eaton Corp.

Introduction to Hydraulics K. Craig 34Eaton Corp.

• How Flow Is Measured

– Flow is the action in the hydraulic system that

gives the actuator its motion. Pressure gives the

actuator its force, but flow is essential to cause

motion. Flow is created by a pump.

– Flow rate is a measure of the volume of fluid

passing a point in a given time. Velocity is

distance per unit time.

Constantflow rate;

Varying velocity

1 GPM = 231 in3/min


• Flow and Pressure Drop

– Whenever a liquid is flowing, there must be a

condition of unbalanced force to cause motion.

– Fluid flowing in a constant-diameter pipe must

overcome friction and, hence, there is a pressure

drop.

Friction in pipes results in a pressure drop.


Eaton Corp.

Eaton Corp.

Fluid seeks a level or levels

depending on the pressure.

Note that in hydraulic circuit design, the

pressure required to move the oil mass and to overcome friction must be added to the pressure needed to

move the load.


• Laminar and Turbulent Flow

Eaton Corp.

Laminar flow is in parallel paths; friction is minimized.


Eaton Corp.

Turbulence results in flow resistance; friction is greatly increased, heat is generated, operating

pressure increases, and power is wasted.


• Bernoulli’s Principle

– Hydraulic fluid in a working system contains

energy in two forms: kinetic energy by virtue of the

fluid’s weight and velocity, and potential energy in

the form of pressure.

– Swiss scientist Daniel Bernoulli demonstrated that

in a system with constant flow rate, energy is

transformed from one to the other each time the

pipe cross section size changes.

– Bernoulli’s Principle says that if the flow rate is

constant, the sums of the kinetic energy and

pressure energy at various points in a system

must be constant. Therefore, if the kinetic energy

decreases (increases), it results in an increase

(decrease) in the pressure energy.


Eaton Corp.

The sum of pressure and kinetic energy is constant with a constant flow rate.


Eaton Corp.

The venturi effect in a gasoline engine carburetor is an application of Bernoulli’s Principle.


Friction and velocity affect pressure.


• Most Common Hydraulic Symbols

Eaton Corp.

working line: carries main stream of flow. pilot line (sensing line): carries fluid used for control

drain line: carries leakage oil to reservoir


Eaton Corp.

Rotating Components


Eaton Corp.

Cylinders


Eaton Corp.

Valves


Entire Hydraulic Circuit with Reservoir Tank

Here the reservoir is vented (vs. pressurized – closed top) with the lines terminating below the fluid level in the tank

(vs. above the fluid level).

Fluid Power Systems & Control - Mechatronics...• Work and Power – Work is done whenever a force...

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