Hydraulic Systems Flashcards
Basic components
1 - Energy converters (pumps, motors, cylinders)
2 - Energy controllers (directional pressure/ flow control valves)
3 - Accessories (resovoirs, filters, accumulators, sensors)
Hydraulic system advantages
- Easy speed + position control
- Facilitates stepless control
- Can easily change direction of movement
- Capable of accumulating energy
- Smoothly provides safety mechanism
- Allows for combination with electric controls
stepless control = continous power adjustment, so no predefined levels such as high, medium, low. Instead continuous range.
Oil Hydraulic system advantages
Compared to Pneumatics & Water hydraulic systems, oil has:
- Superior lubricity
- Better rust resistance
- Easy maintenance
- Compact system
- Can operate under high pressure
- Superior control & response speed
Hydraulic approximations
Unless stated otherwise, we make the following assumptions.
- Hydraulic fluid considered incompressible
- No loss of fluid pressure through:
1. Directional control valves
2. Pipes, filters and coolers
3. Check valves (1 direction) - Gauge pressure in return line is 0
- Dynamics of hydraulic liquid are neglected
Hydraulic system standards
Standards important as components must be interchangeable and able to perform to known standards.
And so symbols can be interpreted so circuit diagrames can be understood by everyone.
British standards, ISO (international standards organization) & CETOP are all organizations committed to producing standards
Pressure valves
Pressure valves influence pressure in hyrdraulic system or part of hydraulic system.
Represented by squares in which arrows represent flow direction
There are set and adjustable pressure valves. (Adjustable have an arrow through spring)
Pressure relief valve
Releases excess pressure in the system to protect other components.
Works by opening when pressure exceeds a set limit, allowing fluid to bypass the circuit and return to the reservoir.
Pressure regulator valve
Maintains a consistent pressure level in hydraulic system. Works by adjusting flow of fluid to maintain a set pressure downstream. Ensures stable pressure for specific components/ operations.
Pressure sequencing valve
Controls the sequence of hydraulic actions by allowing flow only when a predetermined pressure is reached.
Works by opening when inlet pressure reaches a set value, allowing flow to pass through.
Used to ensure specific operations occur in the correct order, (In machinery with multiple hydraulic cylinders).
Pressure counterbalance valve
Controls movement of load, esp where gravity may cause uncontrolled motion.
works by providing back pressure to maintain control over a load’s descent/motion + prevent it from free-falling or running away.
Used in systems with heavy loads or vertical movements to ensure safe and controlled operation.
Hydraulic cylinders
Hydraulic cylinders convert hydraulic energy to mechanical.
Generates force when extension and retraction.
DCV alternately directs fluid to diff sides
Single acting cylinder
Actuation: Operates in one direction, typically extending, and relies on other means (springs/gravity) for retraction.
Provides force and movement in one direction only. Used in applications where force is needed in only one direction, typically for lifting/pushing tasks.
Double acting cylinder
Actuation: Operates in both extending and retracting directions.
Provides force and movement in both directions. Used in applications requiring bidirectional movement and precise control.
Telescopic cylinders
Actuation: Consists of nested stages that extend/ retract to offer varying stroke lengths.
Provides extended stroke lengths while maintaining a compact design. Ideal for applications where space is limited but a long stroke length is required. Used often in construction equipment and hydraulic machinery.
Hydraulics equation
Q = flow
.
Y = distance piston moves
P = pressure
Q1 = A1 x Y
Q2 = A2 x Y
.
Y = (Q1/A1 = Q2/A2)
Hydraulic restrictor
hydraulic restrictor/ flow control valve limits flow rate of hydraulic fluid through a specific path in a hydraulic system.
How hydraulic resistor works
Flow Regulation - fluid flows through the restrictor + size of the orifice/position of valve determines rate at which the fluid can pass.
Pressure Drop - Fluid flows through restricted path = resistance which causes pressure drop across restrictor.
Controlled Speed - By adjusting restrictor’s settings, flow rate can be controlled = speed of actuators/ hydraulic motors controlled.
Hydraulics restrictive equation
Q = Kr (sqrt(Pa - Pb))
Q= flow rate
Kr = restrictor coefficient
Pa - Pb = pressure drop
In restrictors flow rate is proportional to sqrt of pressure drop
Hydraulic power
Hydraulic power unit provides necessary power by converting mech power from drive motor
Components in supply:
- Hydraulic pump
- Oil filtration unit
- Filters
- Heaters
- coolers
Reservoir also plays a part in conditioning fluid
- filtering/gas separation by built in baffle plates
- cools through surface.
Hydraulic pumps
Pumps convert mech eng (rotation of motors/engines) to fluid energy.
3 types (usually)
- Vane
- Piston
- Gear
Piston Pump mechanism
Piston pumps works by using reciprocating pistons to pressurize hydraulic fluid. It uses the following mechanisms.
1 - piston movement/induction stroke. Where 1 of the pistons move back/forth away from the inlet valve = creates a vaccum so hydraulic fluid drawn into cylinder.
2 - compression stroke.Piston moves back infront of inlet valve and compresses fluid + increases pressure.
3 - discharge stroke - Piston moves to outlet port = pushes pressurized fluid out of pump, into hydraulic system.
4 - repeat
Piston pump advantages
- high efficiency (85-95%)
- ease of operation at high pressure
- displacement easily changed by angle of swash plate
- ease of conversion to variable displacement type
Piston pump disadvantages
- low suction ability
- pumps usually large + costly
- highly susceptible to foreign substances in oil
- valve plate is easily damaged + when damaged efficiency drops
Vane pump mechanism
ozperates by using vanes mounted on a rotor to pressurize fluid. Used the following mechanisms
1 - rotor rotation : Inside pump, rotor with slots rotates in eccentrically places cam ring.
2 - vane movement : Spring-loaded vanes in rotor slots + they rotates, centrifugal force causes the vanes to extend/retract.
3 - intake stroke: During intake stroke, expanding space between vanes = vacuum, which draws hydraulic fluid into the pump.
4 - compression stroke: As the rotor rotates, vanes retract due to spring tension, trapping fluid between vanes and the cam ring. This compression increases the fluid pressure.
5- discharge stroke: Fluid then pushed out of outlet port as vanes pass the outlet, maintaining a continuous flow.
Vane pump advantages
- minimized discharge pressure pulsation
- compact + lightweight for high output
- less efficiency degradation due to wear
- reliable/ease of maintenance
- generally quieter
- less susceptible to fluid contamination
Vane pump disadvantage
Efficiency lower (75-90%)
Suction ability is medium
Variable displacement can be achieved by changing eccentricity of cam ring.
Gear pump mechanism
operates by using intermeshing gears to pressurize fluid. Uses following mechanisms.
1 - gear rotation: inside pump, are two gears (typically spur), rotate in opposite directions in tight-fitting housing.
2 - fluid intake: As gears rotate, expanding spaces created between teeth + housing, creating vacuum that draws fluid into inlet port.
3 - fluid compression: Gears continue to rotate, expanding spaces between the gear teeth gradually decrease in size, = compresses fluid + increase in pressure.
4 - fluid discharge: Pressurized fluid forced out of pump through outlet port as gears mesh + push fluid towards the outlet.
Gear pump advantages
- small, simple + inexpensive
- low noise level
- small discharge pulsation
- hardly susceptible to foreign substances in oil
- good suction ability
Gear pump disadvantages
Low efficiency and drops as gears wear out (75-90%)
Hydraulic pump - flow rate
In an ideal pump volumetric flow rate =
Qp = Dp x ωp
Qp = volumetric flow rate
Dp = pump displacement
ωp = angular velocity of drive shaft
Dp in variable displacement pump can be varied
Hydraulic pump - drive torque
In an ideal pump
Tp = Δp x Dp
Tp = drive torque
Δp = pressure difference
Dp = pump displacement
DERIVATION
Input power = Tp x ωp
Output power = Δp x Qp
In ideal pump
Input power = output
So
Tp x ωp = Δp x Dp
Hydraulic pumps - mechanical efficiency
Due to friction, torque required is actually higher.
Mp added (mechanical efficiency)
Mp = Ideal torque/Actual torque
Tactual = (Dp x Δp)/mp
Mp usually 85-95%
Hydraulic pumps - Volumetric efficiency
Due to leakage, volume flow rate is less than ideal.
Vp added (Volumetric efficiency)
Vp = actual flow rate/ideal flow rate
Q actual = Vp x Dp x ωp
Vp usually 80-95%
Hydraulic pumps - overall efficiency
np = output power/input power
Overall efficiency = np
np = Vp x Mp
DERIVATION
Δp x Q actual/ T actual x ωp
np = Δp x Vp x Dp X ωp / ((DpxΔp)/mp) x ωp
= np = Vp x Mp
Hydraulic motors basics
Hydraulic motors have a similar construction to hydraulic pumps.
Convert hydraulic power to a mechanical rotating force.
Hydraulic motor types
Different types available
- fixed displacement
- variable displacement
- bi direction
They are available in gear, vane and piston types as the pumps are
Hydraulics motors - equations
Vm = ideal flow rate/actual flow
-»
Qm = Dm x ωm/ νm
Hydraulic motors - equations II
Output torque depends on difference between motor inlet/outlet pressure
Mm = Actual torque/ideal torque
-»
Tm = Mm x Dm x ΔP
Overall efficiency = ηm
ηm = νm x Mm
Hydrostatic transmission definition
Hydrostatic transmission accepts rotary power from a prime mover (usually internal combustion engine) and transmits it to a load.
Difference between this and hydraulic motor is a component in hydraulic systems whilst hydrostatic transmission is a complete system.
In simple terms, hydraulic transmission is a system that uses hydraulic supposed to transfer power 
Hydrostatic transmission - pros
- Speed, torque, power and direction can be regulated, alternative to gears belts, a chains.
- transmits high power in a compact size
- Exhibits low inertia
- Doesn’t creep at zero speed
- High efficiency of a wide range of torque- speed ratios
- Can locate input/output shafts wherever required
- Maintains controlled speed, regardless of load
- Can transmit power from a single prime mover to multiple locations
- Faster response than mechanical/electromechanical transmissions
- Can remain stalled and undamaged
Hydrostatic transmission types & configurations
The configuration of a hydrostatic transmission determines its performance characteristics.
- Fixed displacement pump & motor
- Variable displacement pump & fixed displacement motor
- Fixed displacement pump & variable displacement motor
- Variable displacement pump & motor
Hydrostatic transmission - Fixed displacement pump & motor
- Simplest and most inexpensive type.
- Applications are limited due to efficiency, as pump displacement is fixed.
- Pump must be sized to drive motor at fixed speed under a full load
- When full speed isn’t required, fluid from pump outlet passes over relief valve = energy wasted in heat form.
Hydrostatic transmission - variable displacement pump, & fixed displacement motor
- Torque output constant at any speed because only depends on fluid pressure and motor displacement ( motor is fixed )
- Increasing/decreasing pump displacement = increases/ decreases motor speed whilst torque remains fairly constant
- Therefore power increases with pump displacement
As P = torque x angular displacement
Hydrostatic transmission - fixed displacement pump & variable displacement motor
- delivers constant power
- if flow to the motor is constant + motor displacement is varied to maintain the product of speed and torque constant. Then power delivered is constant.
- Decreasing motor displacement increases motor speed, but decreases torque a combination that maintains a constant power.
Hydrostatic transmission - variable displacement pump & motor
- theoretically this provides infinite ratios of torque & speed to power
- With motor at maximum displacement, varying pump output directly varies speed + power output whilst torque remains constant
- decreasing motor displacement at full pump displacement, increases motor speed to its maximum; torque varies, inversely with speed and power remains constant
Hydrostatic transmission - flow expressions
νp x Dp x ωp = Dm x ωm/ νm
ωm/ωp = νp x νm x Dp/Dm
In an ideal transmission, rotational speed can be changed by changing the ratio of pump to motor displacement .
If Dm reduced = output speed reduced
Hydrostatic transmission - pressure expressions
mp x Tp/ Dp = Tm/ Mm x Dm
Tm/Tp = mp x Mm x Dm/Dp
In an ideal transmission, torque can be changed by changing the ratio of pump to motor displacement .
If Dp reduced = output torque reduced
Pressure control valves - definition
Pressure control valve control pressure in a hydraulic circuit.
Pressure control valves - types
Relief valves - limit max pressure
Sequence valves - control sequence of one or + actuators
Counterbalance valves - prevents overload by balancing load
Unloading relief valves - release excess pressure
Pressure reducing valves - maintain low set pressure by diverting excess pressure to resovoir when set limit reached
Break valves - adjust pressure to engage/release brakes, allowing precise control.
Balancing valves - distribute fluid evenly among circuits in system to maintain equal pressure and prevent overloading.
Pressure switches - detect changes in pressure and activate/deactivate circuits accordingly.
Pressure control valves - equations
Q = Cd x Ao x sqrt (2 x Δp/ρ )
Cd = discharge coefficient
Ao = minimum cross sectional area
Directional control valve - definition
Directional control valves establish a path through which fluid can travel in a hydraulic circuit
Directional control valves - uses
- To distribute hydraulic energy in a fluid power system
- To control the start, stop, change, acceleration in direction of flow of pressurised fluid
- To extend,retract, position hydraulic cylinder and other components in motion
Directional control valves - classifications
- Continuously operating - these valves have two end positions, and can have any number of intermediate switching positions with varying throttle effect. E.g Proportional and several valves.
- Digitally operating - these have fixed switching positions. These valves are central to hydraulic pneumatics. They are known simply as DCVs
Directional control valves - basics
- used to allow/block fluid using finite positions
- valves contain ports that are external openings for fluids to enter/leave via connecting pipelines
- DCVs can be classified by
1. Type of construction: poppet valves, rotary valves, spool valves
2. No. Of working ports: 2 way, 3 way, 4 way
3. No. If switching positions: 2 positions, 3 positions
4. Actuating ,mechanism: Manual, mechanical, solenoid, hydraulic/pneumatic, indirect.
DCV symbols
Positions - shown by a square for each switching position
Ports/Ways - shown by small dashes on outside of box
Functions - shown by arrows that represent direction of flow
Actuations - controls active position of dcv, shown by a symbol on left or right of the dcv
DCV port symbols
P - pressure supply
A, B - actuators or working lines
R, S, T - exhaust or return
Z, Y - control (pilot) lines
DCV - Poppet valves
Poppet valves consists of housing bore in which one or more seating elements (moveable) e.g ball or cones.
DCV - poppet valves pros
- no Leakage as provides absolute sealing.
– long useful life, as no leakage.
– may be used with the highest pressures, as no
hydraulic sticking (pressure dependent deformation)
or leakages in the valve.
DCV - poppet valve cons
– large pressure losses due to short strokes
– pressure collapse during switching phase due to
negative overlap (connection of pump, actuator and
tank at the same time).
DCV - spool valves
Spool valves consist of a spool, a cylindrical component w large diameter machined to slide in a very close fitting bore of a valve body.
Amount of liquid, primarily, depending on the gap between the school and the housing . (Usually less than 20μm)
DCV - spool valve flow equation
Theoretical approximation
Kν = Cd x π x d x sqrt(2/ρ)
Q = Kν x X x sqrt(Δp)
Kν = valve constant
X/d = spool diameter
Cd = discharge coefficient
DCV - spool valve 2/2-way
This is the simplest dcv spool valve (aka a check valve)
- fluid only flows in one direction
- could be normally opened/closed (NO/NC)
- actuated by pilot (hydraulic actuation), manual, mechanical, or solenoid
In diagram if arrow In left box is connected to outside ports = opened
If ports and arrows not connected/in seperate boxes = closed
DCV - spool valves 3/2 - way
3/2 way is used to alternatively pressurize and exhaust (relieve) one working port.
- these valves used to operate single acting cylinders
- can be NO or NC
- available w various actuation methods
- most commonly only have two positions but in some cases a neutral position may be needed
When A (actuator port) connected to P (pressure supply) in right hand box = closed
When A (actuator port) connected to T (return/exhaust) in left hand box = open
DCV - spool valve 4/2 - way
4/2 way valves used to operate double acting cylinders.
Aka impulse valves as only have two switching positions (no mid position)
Used to reciprocate ( move back/forth) or hold at one end.Used in machines where fast reciprocation cycles needed.
Valve spool moves short distance to operate from positions, used for punching, stamping or other machines needing fast action
DCV - spool valve 4/2 - way II
4 ports, 2 box sections. Can move to two positions.
In one position spool connect port P t A while B connected to T allowing fluid to flow from supply to one of the working ports whilst exhausting the other.
In the other position, the connections are reversed with port P connected to B and A to T. Which changes the direction of fluid flow. (Reverses it)
DCV - spool valve 4/3 - way
primary function of four-way valve is to alternately pressurize an exhaust two working ports A/B
Generally used to operate cylinders/fluid motors in both directions
Used when need to stop or hold actuator at some intermediate position within stroke range
Or where multiple circuit/functions must be done from one hydraulic power source
DCV - spool valve 4/3 - way configurations
4/3 way valves have diff configurations
- open centre
- closed centre
- floating centre
- regenerative center
- tandem centre
DCV - spool valve 4/3 way - neutral position
- Neutral Position (Center Position):
• valve blocks flow between all ports.
• no connection between any of ports, so fluid can’t flow through valve.
• typically used to stop flow or hold load in static position.
DCV - spool valve 4/3-way closed centre
DCV - spool valve 4/3 - way tandem centre
DCV - spool valve 4/3 - way open centre
Hydraulic accumulator
Stores potential energy of an incompressible fluid held under pressure by an external source against some dynamic force
can be used to maintain pressure, absorb shocks, or supplement pump flow in a hydraulic system.
Hydraulic accumulator types
- Weight loaded (gravity)
- Spring loaded
- Gas loaded
Hydraulic accumulator - weight loaded
- consists of vertical heavy wall steel cylinder with a piston and packaging to pressure leakage
- this type, creates constant fluid pressure throughout volume output regardless of rate/quantity of output
- con = large sized & heavy so unsuitable for mobile usage
Hydraulic accumulator - spring loaded
- pressure depends on size/preloading of spring
- pressure exerted on fluid ≠ constant
- typically delivers relatively small oil at low pressure. Tend to be heavy/large for high pressure/ large vol systems
- shouldn’t be used for applications needing high cycle rates bc spring will fatigue & lose elasticity = inoperative accumulator
Hydraulic accumulator - gas loaded
Various options for interface.
- seperator: physical barrier between gas & oil
- no seperator: able to handle large vol of oil. Con: absorption of gas in oil due to lack of seperator. Or leakage.
Piston - can handle very high/low temp system fluids, expensive to manufacture
Diaphragm- small weigh/vol ratio = suitable exclusively for mobile applications.
Hydraulic accumulator - auxiliary source
Main purpose = store oil being delivered by pump during portion of work cycle —> able to use smaller pump
Accumulator can then release stored oil on demand to complete cycle (serves as 2ndary power source)
Hydraulic accumulator - emergency source
In some systems, safety dictates cylinder be retracted even if normal oil pressure supply lost (due to pump/electric failure)
= function called back drivable
Such function requires use of an accumulator as emergency power
The main difference is that the auxiliary source supplements the primary power source, while the emergency source serves as a backup in case of primary power failure.
Hydraulic accumulator - shock absorber
Accumulator can be used to eliminate/reduce high pressure pulsation/hydraulic shock.
Accumulator should be installed as close to shock source as possible
Oil from pump flow to the accumulator first & when that is filled oil moves to cylinder
Hydraulic filters
Hydraulic fluids need filtration to remove contaminants/reduce them to acceptable level.
As hydraulic systems may malfunction due to clogging/internal wear
When hydraulic fluids contaminated, systems damaged and fail to provide optimal performance
Selecting a hydraulic filter
Absolute/nominal filters - Nominal: designed to remove specified size range of contaminants. Don’t guarantee removal of all particles within that range.
Absolute: designed to remove all particles above a specified size, providing a higher level of filtration efficiency compared to nominal filters.
Average pore size - average size of openings in filter through which fluid passes. typical units = micrometers (µm) or microns.[
βμm value = how many time more particles above specific size in μm are located in filter intake than filter return.
Filtration grade - 20-40μm to 1-2μm
Selecting a hydraulic filter lI
Selecting/positioning of filter = largely based on sensitivity of hydraulic component to dirt.
Return filter - (10-25μm) built straight onto oil resovoir/in return line. Cheaper than high pressure filtering
Suction filter - (60-100μm) mainly used where required cleanliness of fluid ≠ guaranteed
Pressure filter - (3-5μm) in pressure line ahead of sensitive devices.
Offline/bypass filter - lower filtering capacity/filter on part of hydraulic fluid.
Filtration can be done on surface/deep bed based on application/composition of filter/ location
Hydraulic heat exchanger
For smooth system operation, necessary to exchange heat w working fluid & keep it at certain temp.
Hydraulic heat exchanger - cooler
For hot regions - cooler
- energy transferred by prime mover transformed into thermal energy
- higher temp deteriorates working fluid = shorter fluid life
Hydraulic exchanger - heater
Heater - for cooler reigon
- viscosity of fluid increases
- suction resistance increases = more pressure losses
Hydraulic reservoirs
Requirements:
- set up with method to shut out foreign substance e.g using baffle plates, filters
- components can be easily detached for maintenance
- needs oils level gauge for safety
- return/suction line installed below oil level.
- capacity needs to be 3-5 x larger than pump output capacity ( 2 x in closed circuit)
- surface area —> heat generated radiated from reservoir surface (determined by presence of heat exchanger)
Hydraulic fluid definition & characteristics
Hydraulic fluid: used to transmit power in hydraulic systems
Characteristics
- stable quality/performance despite temp fluctuations (e.g viscosity)
- shouldn’t cause erosion of rubber/coating inside pipes
- shouldn’t cause erosion of metal
- good oxidation stability
- non compressible
- invariable shear ability
- high ability to eliminate foaming
- maximum resistance to fire
- boiling/freezing point
Hydraulic fluid - functions
Hydraulic fluids must be able to fulfill a variety of tasks
- pressure transfer
- lubrication of moving parts
- cooling
- cushioning of oscillations caused by pressure jerks
- corrosion protection
- scruff removal
- signal transmission
Hydraulic fluid types
Petroleum based
1. General
2. Turbine oil w additives
3. Special fluid
- antiware, fluid w high viscosity, fluid for low temp, fluid for high temp.
Synthetic fluid (fire resistance)
1. Phosphate ester
2. Fatty acid ester
3. Chlorinated hydrocarbon based
Water based (fire resistance)
1. Water glycol
2. Water in oil emulsion
3. Oil in water emulsion
