Hydraulics 2 Flashcards
1
Q
Cavitation vs. Aeration
A
-cavitation is a vapour bubble and aeration is just air
2
Q
Causes of cavitation
A
- really anything that causes low pressure
- clogged inlet filters
- pump is mounted too high above the reservoir
- fluid is
3
Q
Cavitation
A
- is the violent collapse of entrained vapour bubbles
- gas boils out of fluid when pressure is low enough
- high pressure causes the gas bubbles to implode
4
Q
Effects of cavitation
A
-
5
Q
Effects of Areation
A
-
6
Q
Aeration
A
- occurs when air becomes mixed with fluid
- air leaks into the system
- air bubbles implode under pressure
7
Q
Piping
A
- the smaller the pipe the faster the flow
- remember if you half the size of a pipe the area becomes 1/4 of the original area.
- recommended flow inside a circuit/piping is 7/20 feet per second
8
Q
Causes of Aeration
A
- not as violent as cavitation
- leak in the inlet
- fluid level too low in tank
9
Q
Boiling water
A
- raising the temperature
- lowering he pressure
10
Q
Bernoulli’s Principle
A
- only applies when something is moving. Pascals is used when something is not moving.
- for a constant flow rate, as velocity of a fluid increases, the pressure decreases. Conversely, if the pressure increases, the velocity of fluid decreases.
11
Q
Why does Bernoulli’s works
A
-the law of conversation of energy.
-if velocity increases, speed energy increases
-to balance energies, energy must be lost.
-
12
Q
Energy
A
-capacity to do work
13
Q
Work
A
- a function of applying a force over a distance
- applying a force with no movement, no work is being done
14
Q
Power
A
- the rate at which work is done, it is a ratio of work done over an amount of time
- do work faster=more power
- do work slower=less power
15
Q
Horsepower
A
- hp=gpm X psi X .0007
- each horsepower that is not converting into mechanical energy is converted to heat.
16
Q
Efficiency
A
- tells how well something is working
- efficiently = what you put in divided by what you get out.
17
Q
Power efficiency
A
- how much power is obtained from the actuator compared to the power you put into the system through prime mover.
- expressed in a percentage and is arrived at by dividing the output horsepower by the input horsepower multiplied by 100
18
Q
Volumetric Efficiency
A
- how much fluid an actuator discharges in comparison to the amount of fluid intake.
- this will change with the pressure and speed the pump is operating at.
19
Q
Open circuit
A
- most widely used in industry
- return line does not connect directly to the pump inlet
- uses a reservoir
20
Q
Reservoir
A
- hold 3x rated pump outlet
- cool the oil
- de area tea the oil
21
Q
Closed Loops
A
- widely used in mobile equipment
- return line connects directly to pump
- small reservoir (3/4 the capacity of charge pump
- compact unit
22
Q
Charge pump
A
-returns fluid from the case drain back to the main pump inlet
23
Q
Hydrostatic drive
A
-use a charge pump attached to the main pump
24
Q
Closed loop vs. Open loop
A
-no DCV’
-closed loop has no cavitation
-closed has cooling issues
-
25
Case drain
- leaking oil that gets put back into system
- provides flow to reservoir for fluid that flows by piston seals.
- without pressure case drain pressure would continue to build and blow out case or shaft seals
- if case were pressurized, pressure behind piston ball would counteract extension/retraction.
26
Basic Circuit
Electrical energy to mechanical energy to hydrodynamic energy (energy transfer) to mechanical energy
27
Hydrodynamic energy
-the study of fluids in motion
28
Fluid
- transmits power from input to output
| - oil better lubricator, better seal, better for heat control, compressibility versatility
29
Filters
-inlet filters protects pump, coarse (removes large particles), min inlet restrictions
30
Max pressure relief valve
- provides overload protection
- diverts excess flow to reservoir
- sets max system pressure
30
Valves control
- direction to the fluid
- flow
- speed
31
Directional control valves
- directs or stops flow to different parts of the circuit/actuator
- can be operated manually, hydraulically, electrically or pneumatically
- sliding spool, rotary spool and poppet
32
Flow control valve
-controls the speed of actuators by controlling flow to and from them
33
Cylinder
- linear actuator
| - actuator is the output of the system
34
Pressure reducing circuit
- maintain a reduced pressure in a branch of the circuit
- read pressure downstream
- once pressure reaches set pressure the valve will close
- check valve remains closed on extension
35
Sequencing circuits
-force two actuators to work in sequence, but setting the pressure of the second one you want to move higher than the first
36
Meter in flow control
- controls flow into a cylinder (extension/retraction)
- works for opposing loads only (like going down a hill in a car and letting off the gas doesn't slow you down)
- to control retraction with metering in
- look at arrow on check valve to see if it's meter in/meter out
37
Meter out control
- controls flow out of cylinder (extraction/retraction)
- cylinder can only move as fast as fluid leaves Rod end
- when in doubt meter out
- like when you're going down a hill in a car and you need to apply the brakes to slow down and not just letting your foot off the gas
- look at arrow on check valve to see if it's meter in/meter out
38
Bleed off control
- ensures no flow goes over the relief valve when flow to the cylinder is reduced
- opposing loads only
39
Pressure intensification
-pressure is higher in one branch of the circuit than what the relief valve is set for
40
Metering control
- when metering flow in or out, the excess flow produced is wasted over the max pressure relief valve
- flow over relief=running at max pressure = waste of energy
41
Counterbalance circuit
- a pressure control valve that is utilized to control the movement of a vertical load.
- prevents the load from moving faster than the hydraulic oil
- normally closed which restricts the flow leaving an actuator as it lowers the vertical load
- internally or externally sensed
42
Internally Piloted
- internally set so the static pressure from the load alone is sufficient to cause the valve to open.
- valve senses pressure internally from the inlet line
- set at 10-30% higher
- disadvantages load change requires valve adjustment and back pressure results in reduced max downward force
43
Externally piloted
- takes the pilot pressure from the cap end of the cylinder
- removes necessity of high pressure in rod end counteracting downward force
- increasing load will not result in loss of control
- disadvantage higher cap end pressure
44
Deceleration circuits
-used to advance the cylinder at full speed until it is close to the work piece, then the sensor on the valve slows down the cylinder by diverting flow through flow control valve
45
Hydraulic motor control
- produce rotary motion and torque
| - most motors are bi-rotational
46
Cross port relief valve
-cushion valves for start and shock
47
"Hunting" in a valve
-a valve trying to figure out if it's supposed to be open or closed and it jerks open and closed.
48
Regenerative circuit
- using a 2-1 cylinder, a regenerative circuits can provide equal speed and force on extension and retraction
- doubles speed, 1/2 force and no retraction.
49
Pilots
-pressurized fluid through small lines and passages
50
Direct acting relief valve
- used for small flows or quick acting for accurate control
| - when pressure in system overcomes spring tension, flow is diverted to reservoir
51
Pilot operated relief valve
-used for energy efficiency, which they provide by minimizing the pressure
52
Cracking pressure
- pressure required to open the valve and allow the first drop of fluid past the seat
- minimal pressure of setting
- minimal flow to reservoir
- effective area decreases as poppet opens - f (to open)= PxA smaller A requires more P
53
Full flow pressure
- pressure required to hold the valve open during full flow
- much greater pressure than cracking pressure
- all flow to reservoir
- as flow increases a higher pressure is needed to overcome the pressure drop -flow increases, pressure decreases
54
Pressure override
-difference between the full flow pressure and the cracking pressure
-requires system to produce an extra 500psi when full flow is diverted to tank
-
55
Main stage Balanced piston
-in a pilot operated relief valve
-always 20 psi pressure override
-
56
Venting a relief valve
-externally draining the relief valve from a remote location (unloading system)
-
57
Unloading valves
- used to depressurized part of, or the entire system to allow pump to operate under minimal pressure (unloading pump)
- used in accumulator circuits
- operates like normal relief but with a check valve
58
Pilot operated unloading valve
-two pump system allows larger pump to unload and smaller to maintain system pressure
59
Pressure reducing valve
- operate a branch of a circuit at a pressure below the setting of the maximum pressure relief valve
- normally open
- piloted from outlet
- external drain
- reverse flow bypass
60
Sequence valves
- controls order of operation
- normally closed
- piloted from inlet port
- external drain because there could be pressure build up downstream
61
Counterbalance valve
- control vertical cylinders so they don't fall freely due to gravity
- normally closed
- piloted from inlet
- reverse flow bypass
- varying loads require an external pilot so you don't have to adjust it all the time
62
Brake valves
- used on discharge from hydraulic motors
| - they prevent overrunning and pressure buildup on deceleration.
63
Directional control valve
- control the direction of flow in a circuit
- use controlling elements such as a rotary spool, sliding spool, or poppet
- can be
64
Decent actuation
-a detent will hold the spool in a position until it is shifted
65
Check valves
- allow flow in only one direction, always opposite the direction of intended flow
- inline check valves will cause a pressure drop because the ball valve is blocking flow, except for right angle.
66
Right angle check valve
-can handle flow up to 3x's as high as those handled by in line check valves because the poppet moves out of the flow path when it opens.
67
Pilot to open check valve
- requires pilot pressure to open in one direction (normally closed)
- areas(ratio) on check valves need to be greater than area of the cylinder
- used to hold cylinder in place
68
Pilot to close check valve
-requires pilot pressure to close valve (normally open)
69
Flow control valve
-controls the speed of an actuator
70
External drains
-if there is pressure down stream
71
Tandem center
-pump to tank and A&B are blocked
72
Closed center
-
73
Cylinder leaks when DCV is closed
-there is internal leakage somewhere
74
Pressure compensated flow control
- flow remains constant as long as pressure drop across orifice remains constant.
- if load were to change, pressure difference at orifice will change
- this will change the speed of actuator
75
By pass compensator
- bleeds off excess flow to tank
- system pressure never more than 20psi over load pressure
- system does not operate at relief setting
- has a built in relief valve
- disadvantage is it only works with meter in
76
Restrictor pressure compensator
-uses a piston to restrict flow to main spool
77
Stacking valve
- Designed to be bolted together in a stack instead of mounted separately
- main point is they have o-rings
- compact, clean, easy to replace. Reduce piping
- 1 stack per actuator so the stack provides all the instructions for that actuator
- relief valve is always on the bottom
78
Cartridge valve
- contact design of valve installed on a manifold block to minimize leak points
- poppet used instead of a spool
- screw in (low flow, low pressure)or slip in (high flow, high pressure)
