week 5 Flashcards
What are the 2 types of actuators?
- Linear (cylinder)
- Rotary (Motor)
What creates pressure?
Resistance to flow.
What is the “job” of the pump?
The create flow.
What are the two types of pumps?
- Fixed Displacement
- Positive or variable displacement.
What does the pump do?
Converts mechanical force and motion into fluid power.
What is the primary function of a control valve?
To control pressure which may include pressure reducing or flow control.
Advantages of Hydraulics
- Variable Speed
- Reversible
- Overload Protection
- Small Packages
- Can be Stalled
- Absence of Vibration
- Quiet Operation
Disadvantages of Hydraulics?
Oily and messy
Catastrophic failure, bursting of lines and loss of control.
Noisy
Combustability of oils, very hot.
What are the 4 basic principles of hydraulics?
- Liquids have no shapes of their own.
- Liquids are practically incompressible.
- Liquids transmit applied pressure in all directions.
- Liquids provide great increases in work force.
What is the purpose of the fluid?
- Power transmission
- Lubrication
- Seal clearances
- Cooling
- Dissipate Heat
- inhibit rust corrosion
- act as a brake when required
What is the basic function of Pilot lines?
Transmits fluid pressure to control another device.
What is the basic function of Working lines?
To carry the fluid (pressure) doing the work.
State Pascal’s Law.
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.
define pressure
To determine the total force exerted on a surface, it is necessary to know the pressure of force on a unit of area.
What is meant by “conservation of energy”?
What is gained in force must be sacrificed in distance.
What is the output component of a hydraulic system named?
Actuator
What is the input component of a hydraulic system named?
Pump
Name 3 advantages of a Hydraulics system.
- High Power/Large load capacity
- Precise positioning
- Smooth movement
What is the origin of the term “hydraulics”?
From Greek words that mean “water” and “pipe”.
What makes petroleum oil suitable as a hydraulic fluid?
- It is minimally compressible (0.5% @ 1,000psi)
- Its lubrication abilities.
What can you say definitely about the pressures on opposite sides of an orifice when oil is flowing through it?
The pressure is different, i.e. there is a pressure drop.
What pressure is usually available to charge
the pump inlet?
the inlet of a pump normally is charged with oil by a difference in pressure between the reservoir and the pump inlet. most pump manufacturers recommend a vacuum of no more than 5 inches of mercury, 12.2 psia.
Why should the pump inlet vacuum be minimized?
to create flow
What is the function of the pump?
to create flow
Why is loss of pressure usually not a symptom of pump malfunction?
Because pressure can be lost only when there is a leakage path that will divert all the flow from the pump.
How is pressure created?
Resistance to flow.
What determines the speed of an actuator?
Its size and the rate of oil flow into it.
Air entrainment — Air in the fluid will cause
poor (slow) performance
Charles Law
The volume of a gas at a constant pressure is directly proportional to its absolute temperature.
Vapor pressure:
Pressure at which a liquid boils for a specific temp.
The boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid. Therefore, the boiling point of a liquid depends on atmospheric pressure. The boiling point becomes lower as the external pressure is reduced.
Advantages of fluid power
Ease and accuracy of control.
Multiplication of force.
Constant force of torque.
Simplicity, safety, and economy.
What is the prime mover?
Whatever drives the pump
Conservation of Energy
Energy cannot be created nor destroyed
Properties of Hydraulic Fluids
viscosity lub film str. pour point flash point demulsibilty resis to foam chem stability anti wear properties compressibility
if the gpm is constant but the pipe diameter changes what happens to the flow velocity?
as the diameter decreases flow velocity increases and vise versa
if the pipe diameter stays constant but the gpm changes what happens to the flow velocity?
as the gpm decreases the flow velocity decreases and vise versa
Pressure is created either by
resistance to flow, referred to as dynamic pressure, or by the potential energy of an object being affected by gravity, known as static pressure.
Viscosity
Resistance to flow at a given temperature
**Single most important property of a hydraulic fluid
Too high of a viscosity causes:
- High resistance to flow
- Increased energy consumption due to increased friction
- Increased input torque requirement at the pump
- Slow or sluggish operation/actuation
- Insufficient separation of air from the oil in the reservoir
- Pump cavitation
Too low of a viscosity causes:
- Increased internal leakage
- Excess wear
- Decreased pump efficiency (volumetric) due to increased leakage and possible cylinder blow-by (could cause increased cycle times/slower machine operation)
- Internal leaking causing an increase in operating temperature
Absolute Viscosity
Resistance encountered when moving one layer of liquid over another.
Kinematic Viscosity
The amount of time needed for a fixed volume of oil to flow through a capillary tube.
Temperature adversely affects:
- Specific weight
- Specific gravity
- Viscosity
Purpose of valves
- Regulates pressure
- Controls fluid rate
- Controls direction of fluid movement
Why does a piston retract faster than it extends?
Due to less volume to fill up the same amount of horsepower.
Pumps
- Convert mechanical energy into hydraulic energy
- Pumps do not generate or manipulate operating pressure in a hydraulic system - they produce flow
Pumping Theory
- Partial vacuum is created at inlet
- Atm. pressure pushes fluid out of the reservoir and into pump intake
- Mechanically, the pump pushes the fluid out of the discharge
- Pumps create flow and pressure as a result of the resistance to that flow
Pump Classifications
Positive Displacement
Non-positive Displacement
Positive Displacement
Delivers the same amt. of fluid per shaft revolution despite the outlet flow of pressure
Non-positive Displacement
Flow rate varies greatly with changes in resistance flow
if viscosity is too low
- internal and external leakage will increase.
- pump slippage will increase and reduce pump efficiency and increase oil temperture
- the system will operate at lower pressure
- there will be a loss of precise control.
if viscosity is too high
- component and fluid temp will increase.
- operation will be sluggish
- pressure drops throughout the system will increase
names sources of contamination:
- Built in dirt produced during repairs
- Ingress of rust, paint, etc from tank or reservoir
- generated in pump, valves, etc
- ingress from cylinder oil seals
- ingress from new oil and refill of oil.
hydrostatic- positive displacement-
name reciprocating pumps properties
hi press low speed 250/ 500 strokes/min up to 700 bar
name three rotary positive displacement pump
- gear- high speed, high volume, low pressure
- vane- low pressure
- piston- high pressure
name two hydrodynamic non- positive displacement pumps
centrifugal
axial flow
DCV do what
start, stop, and control the direction of flow
there will always be a small internal leak in the direction control valve. This leak is dependent upon:
- the gap between the spool and the body
- the length of the gap
- the viscosity of the oil
- the pressure difference between the connections
a relief valve does what and how
it is used to limit the pressure in the system to a preset maximum pressure. this is done by diverting some or the entire pump output to the tank when the pressure setting is reached
relief valve normally open or closed
closed
pressure reducing valve normally open or closed
open
a pressure reducing valve does what and how
they are used to maintain reduced pressure in certain parts of the system. they are actuated by pressure and tend to close as the pressure reaches the valve settings.
a unloading valve does what and how
the unloading valve is often used in systems with two or more pumps. it can unload one pump when the required system pressure is achieved. normally the valve is closed but it opens when the pilot pressure is equal or higher than the preset opening pressure of the valve.
counterbalance valve normally open or close
both one in each direction
a counterbalance is used to
- prevent creeping of heavy loads
- prevent overload
- prevent overspeeding and runaway loads.
- prevent accidents if pipes or hoses break.
what does a counterbalance valve do
They are designed to create backpressure at the return line of the actuator to prevent losing control over the load.
name some basic cylinder construction
- tie rod
- rod bearing
- rod wiper
- rod seal
- rod end head
- rod end port
- piston rod
- body
- air vent
- cushion collar
- piston seals
- cap end head
- cap end port
- piston
- static seal
- cushion plunger
what is the difference between motors and pumps in a hydraulic system?
the pump moves the fluid
the fluid moves the motor
75 percent of hydraulic failures are due to what?
the hydraulic oil being in poor conditions
name points of inspection for hydraulic systems
- oil level
- suction strainer
- all filters
- mounting bolts and brackets
- operating pressures
- oil leaks
- oil temperatures
- speed of the pump
what are three main causes of hydraulic problems:
contamination
heat
leaks
name some other problems
- solenoids not operating
- electrical source not operating
- relief valve, counterbalance valves, set wrong
- human error
leaks can affect hydraulic system operation in three ways
- actuators move slowly
- maximum available system pressure may be reduced.
- actuators may move when they should not.
What are three (3) ways to push fluid into a pump besides using atmospheric pressure?
- Gravity
- Pressurized Reservoir
- Charge Pump
What is the basic operating principle of any positive displacement pump
Pumping chambers increase in volume at the inlet and then decrease as they push the fluid out the discharge ports.
What, if any, change should you expect in the output of a positive displacement pump as the backpressure increases?
Small Decrease.
What condition occurs when the inlet flow is less than the discharge?
Cavitation.
List three (3) methods of ensuring adequate inlet flow?
- Gravity
- Pressurized Reservoir
- Charge Pump
Which seals are contained in the rod bushing?
Rod wiper seals and pressure seals.
Why is an ORB fitting preferable to a tapered thread fitting?
It causes less stress and distortion.
How can you detect an internal leak in a cylinder?
By hot spots.
What is the operating principle of any hydraulic motor?
Fluid pushes on vanes, gear teeth or pistons to turn the drive the shaft.
Which three (3) types of motors are used for low speed and high torque?
- Radial Piston.
- Cam-Type Radial Piston.
- Roller Vane Gerotor.
What does a decease in mechanical efficiency indicate?
An increase in internal friction.
What does a decrease in volumetric efficiency indicate?
Internal Leakage.
On which two (2) factors does motor speed depend on?
- Displacement
* Flow Rate
the minimum horsepower rating of the prime mover can be calculated by what formula
hp = gpm by psi by .0007
the horsepower rating of the prime mover can be calculated by what formula
hp = gpm by psi by .000583
work equals what
force by distance
increase the pressure setting on the motor
increase torque
increase flow rate of motor
increase speed
increase the displacement of the motor
- decrease system pressure
- decrease speed
- increase torque
What are the two type of check valves?
Pilot to open and pilot to close.
force is commonly measured in
pounds or newtons
pressure is commonly measured in
- psi
- bars
- newtons per square meter = pascal
work is commonly measured in
joules or foot-pounds
1 watt is
1 newton lifted 1 meter in 1 second
one hp is
33000 pounds lifted 1 foot in 1 minute
746 watts is
1 hp
1 watt is
.00134 hp
energy cannot be destroyed, but it may be converted from one form to another.
this is the law of conservation of energy
one imperial gallon equals
277 cu in.
one us gallon equals
231 cu in.
velocity is commonly expressed in
m/s
ft/s
