Hydraulic Systems Flashcards

Question

Vane pump advantages

Answer 1

- 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

Answer 2

Efficiency lower (75-90%) Suction ability is medium Variable displacement can be achieved by changing eccentricity of cam ring.

Answer 3

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.

Answer 4

- small, simple + inexpensive - low noise level - small discharge pulsation - hardly susceptible to foreign substances in oil - good suction ability

Answer 5

Low efficiency and drops as gears wear out (75-90%)

Answer 6

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

Answer 7

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

Answer 8

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%

Answer 9

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%

Answer 10

Overall efficiency = np np = Vp x Mp DERIVATION np = output power/input power = Δp x Q actual/ T actual x ωp np = Δp x Vp x Dp X ωp / ((DpxΔp)/mp) x ωp = np = Vp x Mp

Answer 11

Hydraulic motors have a similar construction to hydraulic pumps. Convert hydraulic power to a mechanical rotating force.

Answer 12

Different types available - fixed displacement - variable displacement - bi direction They are available in gear, vane and piston types as the pumps are

Answer 13

Vm = ideal flow rate/actual flow ->> Qm = Dm x ωm/ νm

Answer 14

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

Answer 15

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

Answer 16

- 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

Answer 17

The configuration of a hydrostatic transmission determines its performance characteristics. 1. Fixed displacement pump & motor 2. Variable displacement pump & fixed displacement motor 3. Fixed displacement pump & variable displacement motor 4. Variable displacement pump & motor

Answer 18

- 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.

Answer 19

- 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

Answer 20

- 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.

Answer 21

- 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

Answer 22

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

Answer 23

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

Answer 24

Pressure control valve control pressure in a hydraulic circuit.

Answer 25

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.

Answer 26

Q = Cd x Ao x sqrt (2 x Δp/ρ ) Cd = discharge coefficient Ao = minimum cross sectional area

Answer 27

Directional control valves establish a path through which fluid can travel in a hydraulic circuit

Answer 28

- 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

Answer 29

1. 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. 2. Digitally operating - these have fixed switching positions. These valves are central to hydraulic pneumatics. They are known simply as DCVs

Answer 30

- 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.

Answer 31

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

Answer 32

P - pressure supply A, B - actuators or working lines R, S, T - exhaust or return Z, Y - control (pilot) lines

Answer 33

Poppet valves consists of housing bore in which one or more seating elements (moveable) e.g ball or cones.

Answer 34

- 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.

Answer 35

– 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).

Answer 36

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)

Answer 37

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

Answer 38

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

Answer 39

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

Answer 40

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

Answer 41

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)

Answer 42

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

Answer 43

4/3 way valves have diff configurations - open centre - closed centre - floating centre - regenerative center - tandem centre

Answer 44

1. 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.

Answer 45

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.

Answer 46

1. Weight loaded (gravity) 2. Spring loaded 3. Gas loaded

Answer 47

- 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

Answer 48

- 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

Answer 49

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.

Answer 50

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)

Answer 51

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.

Answer 52

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

Answer 53

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

Answer 54

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

Answer 55

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

Answer 56

For smooth system operation, necessary to exchange heat w working fluid & keep it at certain temp.

Answer 57

For hot regions - cooler - energy transferred by prime mover transformed into thermal energy - higher temp deteriorates working fluid = shorter fluid life

Answer 58

Heater - for cooler reigon - viscosity of fluid increases - suction resistance increases = more pressure losses

Answer 59

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)

Answer 60

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

Answer 61

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

Answer 62

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

Hydraulic Systems Flashcards

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