Chapter 16 - Hydraulic Systems Flashcards
(159 cards)
1
Q
Define hydraulic
A
- water pipe
2
Q
How is a specific amount of force expressed?
A
- newtons
- pounds
- pounds-force`
3
Q
Define the following term in relation to force and pressure:
- pressure
A
- the force per unit area
- pressure = force / area
4
Q
Define the following term in relation to force and pressure:
- pascal
A
- one newton per square metre
5
Q
Define the following term in relation to force and pressure:
- atmospheric pressure
A
- pressure of the atmosphere at sea level is 14.7 psi
- 14.7 psi = 101.35 kPa = 1.01 bar (usually rounded off to 1 bar)`
6
Q
Define the following term in relation to work, power and energy:
- a joule
A
- 1 joule = 1 N x 1 m
7
Q
Define the following term in relation to work, power and energy:
- 1 ft-lbf
A
- 1 ft-lb = 1 lbf x 1 ft
8
Q
Define the following term in relation to work, power and energy:
- 1 watt
A
- 1 newton lifted 1 metre in 1 second
9
Q
Define the following term in relation to work, power and energy:
- 1 horsepower
A
- 746 watts
10
Q
State the law of conservation of energy
A
- Energy cannot be destroyed, but it may be converted from one form to another
11
Q
What are the properties of liquids?
A
- no definite form, but takes the shape of its container
- virtually incompressible
- when force is applied to a fully contained liquid, the liquid shows the same resistance to compression as a solid
- extremely flexible, yet as unyielding as steel
12
Q
What does Pascal’s law state?
A
- Pressure applied to a confined fluid at rest, force is transmitted equally in every direction and always at right angles to the containing surface
13
Q
How is mechanical advantage gained with hydraulics?
A
- by using Pascal’s Law, liquid can be used to gain a mechanical advantage, by acting as a force multiplier between two different surface areas
- force is gained at the expense of distance moved
14
Q
How is fluid flow measured?
A
- measured by the volume of fluid passing a given point in a unit of time
15
Q
How is velocity measured?
A
- the distance travelled by the fluid in a unit of time
16
Q
Describe laminar and turbulent flow
A
- laminar flow - the fluid at the outer walls of conductor moves more slowly than the fluid in the centre of the conductor
- turbulent flow - the fluid particles move in a random pattern rather than parallel to the direction of flow
- can be caused by high velocity, obstructions or projections in the fluid stream, sharp bends and/or roughness in the conductor, a large number of bends in the system, or a combination of these
17
Q
List the main causes of excessive friction in hydraulic lines
A
- excessive length of lines
- excessive velocity (because the lines are too small)
- excessive number of bends or fittings, or unsuitable bends or fittings
- sustained flow at high pressure
18
Q
What does Bernoulli’s Principle state?
A
- if the flow rate is constant, the sum of the kinetic energy and the potential energy at various points in the system is constant.
- therefore, whenever the velocity (kinetic energy) of a fluid increases, the pressure (potential energy) decreases
19
Q
What is the purpose and what are the types of hydraulic actuators?
A
- hydraulic actuators convert hydraulic energy into mechanical energy
- cylinders - used to create linear motion
- motors - used to create rotary motion
20
Q
Describe a single-acting cylinder
A
- applies force, or is pressurized, in one direction only
- the return action is accomplished by an external force such as gravity, a spring, or a small-diameter auxiliary piston
21
Q
Describe rod and ram cylinders
A
- rod - in a cylinder, the cross-sectional area of a piston rod is LESS than half of the piston face area
- ram (plunger) - in a cylinder, the cross-sectional area of a piston ram is MORE than half the piston face area
22
Q
Describe a spring return cylinder
A
- cylinder rod returned by a spring
23
Q
Describe a diaphragm spring-return cylinder
A
- diaphragm cylinders, such as pancake types, are used to provide short strokes with large forces
- large forces are available due to its large piston area.
- rolling diaphragms are used for longer strokes
- very little friction to overcome
- zero leakage
24
Q
Describe a double-acting cylinder
A
- hydraulic fluid delivered under pressure to both sides of the piston, producing a force in either direction
- the force exerted by the piston is larger on the cap end than the rod end
- if the same fluid supply exists on each side, the speed of the piston travel is also different, being faster on the rod end retraction
- piston speed differences are overcome by using valves
25
Describe a double acting cylinder with a double-ended piston rod
- in a double-rod cylinder, where the rods are the same diameter, the forces on the piston can be the same. Because equal amounts of fluid are displaced, the force and the rate of travel is the same
26
Describe cylinders with cushions
- cylinders (often double-acting) are designed with a cushion device at one or both ends of the stroke
- this slows down the piston as it approaches the end of its travel
- the cushioning is created by slowing the flow of oil being discharged, the cushion of oil tends to absorb any shock.
- as the piston enters the cushioning area, the normal discharge is blocked off by the cushion sleeve
- the remaining oil is forced through one of the following:
- - the clearance between the sleeve and cushion area (fixed)
- - a small orifice controlled by an adjustable needle valve (variable)
27
Describe tandem cylinders
- have two pistons attached to one rod
- this design allows for higher forces at the rod end without an increase in fluid pressure or cylinder diameter
- this is achieved because the pistons total surface area is almost doubled
- tandem cylinders require more linear mounting space
28
Describe telescopic cylinders
- have a series of tubular rod segments called sleeves which fit inside each other
- this permits a working stroke much longer than its retracted length
- available as single- or double-acting cylinders
29
Describe cylinder mounting practices
- cylinders can be oriented in any position and will work successfully, provided they are mounted on a strong and rigid base and aligned with the part they are to move
- to examine specific mounting styles, refer to manufacturers reference material
30
How are motors similar to and how do they differ from pumps?
- like pumps, motors may be of gear, screw, vane, or piston design
- they may have fixed or variable capacity
- they may be designed to run in one direction only (uni-directional), both directions (bi-directional), or oscillating
- a motor can act as a pump, but a pump cannot act as a motor
31
What is the advantage of variable capacity motors versus fixed capacity motors for controlling motor speed?
- if any variation must be controlled independently of the motor, then the motor is considered to have a fixed capacity
- Fixed capacity motors run at a constant rate according to the predetermined flow rate
- When the variation is done in the motor, then the motor is considered to have variable capacity. Regardless of the flow rate, the speed of variable capacity motors can be altered
32
Describe the operating principles of gear motors
- the fluid flow from the system enters the inlet port and travels in either direction around the casing, forcing the gears to turn in the casing
- both gears are driven by the fluid, but only one is connected to the output shaft
- gear motors are fixed capacity hydraulic motors
33
Describe the operating principles of screw motors
- uses the force of the fluid against the face of the screw threads to generate motion
- there are two or more screws in the housing but only one is attached to the drive, the others are idlers which act as a seal between the helical chambers, this prevents reverse fluid flow in the housing
- screw motors have a fixed capacity and operate quietly and free of vibration
34
Describe the operating principles of vane motors
- the fluid flow from the system enters the inlet port(s) and exerts force against the vanes and rotor
- the maximum amount of force is exerted against the vane with the largest area exposed to the fluid, turning the rotor in that direction
- no centrifugal force exists until the rotor is put into motion, therefor springs, or some other mechanical means, are needed to hold the vanes against the casing
- Vane motors may be balanced or unbalanced and have fixed or variable capacity
35
Describe the operating principles of radial piston motors
- pressurized fluid enters the cylinder block in the centre forcing the pistons outwards against the reaction ring
- the rotor and cylinder block centre-lines are offset causing the pistons to move to the farthest point
- the offset between the rotor and the cylinder block may be fixed or variable, altering its capacity
- fluid flow is as follows:
- - during the half of the cycle that the pistons extend, the pressurized fluid enters the motor
- - during the other half they contract and the fluid is exhausted into the reservoir line
- the cylinder block is connected to the output or drive shaft
- the casing is held stationary
36
Describe the operating principles of axial piston motors
- the pressurized fluid enters the valve plate, forcing the pistons towards the swash plate
- the angle of the swash plate causes the housing to rotate as the piston is forced to the furthest most point of travel
- this angle determines the capacity of the motor
- - during one half of the cycle, pressurized fluid enters the motor as the pistons travel to their full extent
- - during the other half of a cycle, fluid is exhausted in the reservoir line
- axial piston pumps are available with a fixed or variable housing angle
37
Describe the operating principles of oscillating motors (rotary actuators)
- designed to give shaft rotation of less than 360 degrees
- actuators commonly consist of a single, moveable vane mounted in the rotor
- - fluid enters one port, forcing against the vane and rotating the rotor in one direction
- - when fluid enters the other port, the rotation is reversed
- oscillating motors can develop high torque at low speeds
- their speed and torque can be adjusted in the same way as in standard motors or cylinders
*Excessive travel of the driven component should be controlled by external mechanical stops, not by the vane hitting the housing
38
Define the following in relation to motor calculations
| - displacement
- the amount of liquid handled in one rotation of the motor
| - it is usually expressing in cubic inches per revolution (in3/rev)
39
Define the following in relation to motor calculations
| - pressure
- pressure requirements for motors vary with the size of the motor's displacement
- the larger the displacement of the motor the less pressure required to produce a given torque, and vice versa
40
Define the following in relation to motor calculations
| - torque
- torque output is a function of the system pressure and the motor displacement
- expressed in inch-pounds (in.lb)
= in.lb = psi x in3/rev / 2(Pi)
- torque rating is used to find the size of the motor required for the job
- expressed in inch-pounds per 100 psi (in.lb/100 psi)
41
Define the following in relation to motor calculations
| - mechanical efficiency
- each motor has some slippage, which is fluid that moves through the motor without doing any work. This robs the motor of some torque
- the mechanical efficiency of any machine is usually expressed as a percentage:
-- mechanical efficiency = actual torque delivered / theoretical torque X 100%
42
Define the following in relation to motor calculations
| - motor speed
- Speed (S) is a rotary speed, usually expressed in RPM
- determined by the flow rate or volume per unit time delivered (V) divided by the motors displacement or area (A)
-- S = V/A
V = flow rate = GPM
A = area (displacement) = in3/rev
-- Rotary speed = (flow rate x 231 / Area) X RPM
43
How is a hydraulic motor's torque increased?
- Increase pressure setting
| - Increase displacement
44
How is a hydraulic motor's speed decreased?
- Decrease flow rate
| - Increase displacement
45
What would be the result if you replaced a hydraulic motor with a motor of a smaller displacement?
- Increase in system pressure
- Increase in speed
- Decrease in torque
46
What is the purpose of a DCV?
- used to control the direction of hydraulic fluid flow
- may have single or multiple positions
- - single - maintains a uni-directional flow pattern
- - double - start, stop, or change direction of flow to or from hydraulic actuators
47
How are the number of positions and flow patterns represented by the DCV symbol?
- single -- free check valve
- multiple - two or more squares
- each square represents a position and the flow pattern inside the body
48
How are the ports labeled on the DCV symbol?
- the lower two ports in the symbol are labelled pressure (P) and tank (T)
- the top port(S) are lettered A, B.
49
How is a transitory condition indicated on a DCV symbol?
- an envelope with dashed ends indicate a transitory (in transit) but significant condition between two distinct positions
- this means that the valve passes through this position, but does not stop in it
50
How are flow paths indicated on DCV symbols?
- an arrow indicated the flow path of the fluid through the valve
- the T indicated a blocked flow path or port
- a dot at the intersection of crossing lines indicate flow paths which are connected
- flow paths which cross without being connected do not show a dot
51
How are DCV's identified numerically?
- the First figure indicates the number of ways the fluid can flow. This is usually also the number of ports (excluding pilot ports)
- the Second figure indicates the number of distinct positions
- Example - 3 / 2 designates a 3-way valve (3 ports) with 2 positions
52
List how DCV's are classified other than numerically?
- spool type -- sliding or rotary
- nominal size
- - usually nominal pipe connection size
- - the recommended maximum volume through the valve
- maximum allowable pressure
- port connection
- - threaded; national pipe taper (NPT) or national pipe straight (NPS)
- - flanged or flat face
53
List two types of spools for DCVs
- sliding
| - rotary
54
Compare throttling to non-throttling DCVs
- throttling DCV
- - allows the spool to pass through from one envelope to another at any given rate. Because of this, they are also called - Infinite Positioning
- - they are indicated by parallel lines above and below the envelopes
- example - tracer valve, activated by a plunger against a spring
- non-throttling DCV
- - "snap" into only one of the envelopes at a time. These types include, two-, three-, and four-way valves, and closed-, open-, tandem-, float-, and regenerative centre envelopes
- - normally open valves allow fluid to pass through until the valve is activated
- - normally closed valves block the flow of fluid until the valve is activated
55
Describe the function of the following valve in regards to their centre envelope flow pattern
- closed centre
- all ports are blocked off in neutral
- the actuator is "locked" so that it cannot move out of its position
- over time some movement occurs due to minor internal leaks
- the fluid from the pump must go through the relief valve at maximum pressure, generating heat
- this requires maximum power and wastes energy through heat
56
Describe the function of the following valve in regards to their centre envelope flow pattern
- open centre
- all ports are connected in neutral
- the actuator moves in the direction of any external forces
- the flow from the pump is back to the tank with minimum power demand
- there is no pressure and only a low heat rise
57
Describe the function of the following valve in regards to their centre envelope flow pattern
- tandem centre
- the actuator ports are blocked off, and the flow from the pump is back to the tank
- this centre provides a hydraulic lock to hold the actuator in position
- fluid is allowed to flow from the pump back to the tank
- this requires minimum power and generates a small amount of heat
58
Describe the function of the following valve in regards to their centre envelope flow pattern
- float centre
- ports A and B are connected to the tank, and the pump is blocked off
- this centre allows the actuator to coast to a stop or be moved manually without disconnecting the circuit
- as the actuators movement slows down, oil flows from one side of the actuator, through the centre, and then back to the other side
- the fluid from the pump must go through a relief valve at maximum pressure, generating heat and requiring maximum power
59
Describe the function of the following valve in regards to their centre envelope flow pattern
- regenerative centre
- ports A and B are connected to the pump
- this centre maintains constant pressure to both ports of the actuator
- the fluid from the pump must go through the relief valve at maximum pressure, generating heat and requiring maximum power
60
Describe the purpose, application and types of check valves
- allow the free flow of fluid in one direction only
- available in in-line and right angle mounting
- free check valves use gravity to hold the valve closed and must be mounted in the correct orientation
- spring-loaded check valves use a light spring force of approximately 5 psi to hold the valve seated, regardless of its mounting position
61
What is the purpose for pressure control valves?
- control the hydraulic pressure in all or part of the circuit
- may be wither normally closed or normally open valves
62
Which pressure control valves are normally closed and which are normally open?
- normally closed -- open when pressure reaches a set limit
- - example - relief of sequence valves
- normally open - close when pressure reaches a set limit
- - example - pressure reducing valves
63
How many squares (envelopes) are in a pressure relief valve symbol
- single square
64
Describe the purpose and/or operating principles of
| - a direct-acting relief valve
- direct-acting relief valves are frequently used as safety valves to prevent damage from high surge pressure
- the port on the spring side of the valve seat returns fluid to the reservoir, the other side is attached to the pressure line. it works as follows:
1. while the system pressure is less than the spring force, the valves remains closed.
2. when the pressure exceeds the spring force, the valve opens as the fluid is allowed to flow back to the reservoir
3. as the pressure decreases below spring force, the valve closes again stopping the flow
65
Describe the purpose and/or operating principles of
| - a pilot-operated relief valve
- if a large amount of fluid is to be relieved under a small pressure differential, then a pilot-operated pressure relief valve is used
- the main relief valve has an orifice in it. this allows the system pressure to act equally against both sides of the main valve and the pilot valve. the pilot valve operates as follows:
1. due to equal pressure on both sides of the main valve, only a light spring force is needed to keep the valve closed
2. because the pilot valve has a light spring force, its cracking pressure is low. when system pressure rises, it overcomes the pilot valve spring force
3. the pilot valve opens up, allowing fluid to flow out of the pilot chamber
4. the fluid flows out of the pilot chamber faster than it can be replaced
5. the pressure on the spring side of the main valve becomes less, allowing it to open
6. the valve remains open until the pressure in the system drops, allowing it to close
7. when this happens, the pilot valve closes to equalize pressure on both sides of the main valve
66
Describe the purpose and/or operating principles of
| - an unloading valve
- an unloading valve returns pump output (at low pressure) to the reservoir after the required system pressure has been reached
- an internal or remote check valve is used to maintain pressure in the system
- unloading valves are normally-closed valves and are usually installed in the pump outlet line with a tee connection
- they use a pilot valve to activate the main valve
- they have a sensing or pilot line downstream of the unloading valve
67
Describe the purpose and/or operating principles of
| - a pressure-reducing valve
- when a secondary circuit operates efficiently at a pressure lower than the relief valve setting, a pressure-reducing valve is used to reduce this pressure
- a pressure reducing valve is a normally open valve and is held open by an adjustable spring. it has a pilot line downstream of the valve. it works as follows:
1. fluid from the main circuit enters at the inlet port
2. it flows past the valve and through the outlet port to the secondary circuit
3. pressure on the secondary or outlet side acts on the bottom of the spool through the pilot line
4. when the pressure on the outlet side and against the spool exceeds the spring thrust, the valve partially closes
5. this increases the valve's resistance to flow and reduces the pressure at the outlet port regardless of pressure fluctuations at the inlet port
- the spring chamber is always drained to the reservoir to precent fluid pressure from building up and holding the valve open. Back flow will completely close off the valve
68
Describe the purpose and/or operating principles of
| - a sequence valve
- in hydraulic circuits with more than one cylinder, it is often necessary to work the cylinders in a specific order. This is done by hand, by electrical control, or by sequence valves
- a sequence valve is a normally closed, two-way valve. It has a pilot line that senses the pressure of the inlet port and a line that drains the spring chamber back to the reservoir (external drain). It works as follows:
1. the valve remains closed until pressure of the primary circuit increases to its set limit. this happens when the priority actuator completes and satisfies its function.
2. at this time the pressure through the pilot line forces the valve open and allows fluid to flow through its secondary circuit
3. when the secondary actuator completes and satisfies its function, the fluid is redirected to the next circuit or back to the reservoir by means of a relief valve
- a sequence valve uses a check valve to allow the reverse flow to bypass the normally closed centre and return freely to the reservoir
69
Describe the purpose and/or operating principles of
| - counterbalance valves
- counterbalance valves are used to maintain a set pressure in part of a circuit. This controlled pressure is required to keep a weight such as the platen on a press from falling or to keep rotating load from running away.
- this valve is attached in the exhaust side of the actuator
- a pilot line senses the pressure of the inlet port and an internal drain
- an internal check valve is used to allow free return flow to the reservoir
70
Describe the purpose and/or operating principles of
| - brake valves
- a brake valve is commonly found with a motor
- this valve is a normally closed, pressure control valve, with both a direct and remote pilot connected to the circuit for simultaneous operation
- the direct pilot line acts on a piston area smaller than the area acted on by the remote pilot line (which requires more pressure to move the spool)
- the brake valve is connected to the exhaust side of the motor. It works as follows:
1. the direct pilot line is connected to the inlet side of the valve, sensing the pressure from the motor
2. the remote pilot line is connected to the inlet side of the motor, sensing the pressure from the pump
3. as the DCV shifts out of its centre position, the pressure from the pump (through the remote pilot line) holds the valve open
4. if any external force on the motor increases its RPM, the pressure from the pump decreases. The valve then closes until sufficient back pressure is built up
5. when the DCV shifts to its centre position, the pressure from the pump stops and creates no pilot pressure in the remote line
6. the direct pilot line is now under pressure due to the inertia of the load on the motor. Due to the higher pressure required to move the spool, the valve closes more rapidly
71
What is the purpose for flow control?
- In many hydraulic systems, the speed of a motor or the rate of travel of the cylinder must be regulated
- this is done by controlling the volume of hydraulic fluid entering or leaving the actuator
- in systems using a fixed capacity pump, the regulation is by -- flow control or flow metering valves
72
Define non-compensating flow-control valves
- flow control valves that do not compensate fro any variation in pressure or flow in the system
- as the pressure increases, the flow through the valve increases
73
Describe fixed and adjustable, non-compensating flow-control valves
- fixed, non-compensating flow control valves -- have a pre-set amount of flow in one direction and a free flow in the other
- example -- poppet check valve with sized orifice
- adjustable, non-compensating flow control valves -- have an adjustable amount of flow in one direction and a free flow in the other direction by means of a separate check valve
74
Define pressure-compensating flow-control valves
- maintains a constant rate of flow through the valve, regardless of down-side pressure
- it uses a spring loaded poppet or pressure-compensating spool with a control orifice and a throttling orifice to monitor the flow rate through the valve
75
Describe fixed, pressure-compensating flow-control valves
- has a pre-set orifice sized for its specific application
- as the pressure increases at the control orifice:
1. the poppet moves against the spring tension, reducing the opening at the throttling orifice and, therefore, the flow
or
2. opening the relief port and allowing the excess fluid to return to the reservoir
76
Describe adjustable, pressure-compensating flow-control valves
- has an orifice which can be adjusted to the required flow rate
- this valve acts in a similar manner as the fixed pressure-compensating flow control valve
77
Describe the operating principles of pressure- and temperature-compensating flow-control valves
- as the temperature of hydraulic fluid changes, so does its viscosity. When the fluid warms up, more goes through an orifice
- control is measured by means of a throttle attached to an aluminum alloy or bi-metal rod (temperature-compensating rod)
- the temperature-compensating rod expands and contracts with the changing temperatures, and moves throttle to decrease and increase the orifice
78
Describe the operating principles of flow dividing valves
- hydraulic systems may have more than one circuit
- flow dividing valves control the amount of fluid to each circuit
- they are usually located between the pump and the DCV's
- the valve can deliver equal flow rates or a preset ratio of flow rates into two separate circuits
79
What are the 3 possible stages to convert an electronic signal to activate a main valve?
- electrical signal converting to mechanical movement
- mechanical movement to a piloting stage
- piloting stage to the main valve
80
Define a solenoid and describe how it is used to open and close valves
- simple form of electromagnet (magnet in which the magnetic lines of force are produced by an electric current)
- a coil of an insulated conductor with an armature (plunger) made of iron inside
- when the coil is electrically energized, a magnetic field is produced which attracts the plunger and draws it up the core of the solenoid
- springs are used to return the plunger to its original position
81
Describe the operating principle of a solenoid-controlled pilot-operated valve
- a combination of a small solenoid- controlled pilot valve and the pilot-operated main valve
- the solenoid-controlled pilot valve (Master Valve) directs flow to either end of the pilot-operated main valve (Slave Valve)
- the pilot valve is normally mounted on top of the larger main valve
82
Describe the operating principle of a proportional solenoid
- allows the operator to vary the position of the plunger by varying the amount of current going to the solenoid. This varies the amount of flow
- these are used in conjunction with throttling DCV's, pressure control valves and flow control valves
83
How do hydraulic pumps convert mechanical energy into hydraulic energy?
1. the mechanical action of the pump first creates a partial vacuum at its inlet side
2. this vacuum allows atmospheric pressure in the reservoir to force the hydraulic fluid through the inlet line to the pump
3. the pump's mechanical action then forces the hydraulic fluid to the pump's outlet and into the system
84
What is the basic type of pump used in hydraulic systems?
- positive displacement pumps
85
Define fixed capacity and variable capacity
- fixed capacity -- these run at a given speed, delivering a constant flow rate. They may be gear, vane, or piston pumps
- variable capacity -- these run at a given speed, delivering a variety of flow rates from maximum to zero in one or both direction(s). They may be vane or piston pumps
86
Describe gear pumps
- a fixed capacity pump which has two or more rotors
| - there are external and internal gear pumps
87
Describe external gear pumps
- has its gears meshing on their periphery (outer edges)
- one gear is driven by the other
- the gears carry the liquid from the suction port to the discharge port, around the inner walls of the casing
88
Describe lobe pumps
- has larger spaced between its teeth than conventional gear pumps do
- rotors must be driven by suitable drive gears mounted outside the casing
- lobe pumps deliver a more pulsating flow than conventional gear pumps
89
Describe screw pumps
- an axial flow pump
- may have one, two or three screws which carry the liquid from the suction port to the discharge port
- needs to be driven by external gears
- moves the liquid linearly through the pump, which eliminates pulsations
- there is no metal to metal contact within the pump, which makes operation very quiet
90
Describe internal gear pumps
- has one external gear rotating within an internal gear
- this arrangement is compact
- crescent seal
- gerotor pump
91
Describe vane pumps
- may be unbalanced or balanced, and have a fixed or variable capacity
- variable vane pumps -- may be able to reverse flow through the system
92
Describe hand pumps
- design is similar to hydraulic cylinders and are either single- or double-acting
- check valves are used to maintain correct flow direction
- a release valve is used to return the liquid to the reservoir
93
Describe axial and radial piston pumps
- may be fixed or variable capacity
| - some also reverse flow through the system
94
Describe how pumps are rated
- according to their capacity and pressure
95
How is pressure created in a hydraulic system?
- created by resistance to flow
| - pressure is caused by the workload on the system from the actuator(s)
96
Compare pumps mounted above the reservoir to pumps mounted below the reservoir
- above reservoir
- - the pump must be able to create enough vacuum or pressure drop to overcome :
1. the weight and friction of the liquid
2. the height from the liquid level to the pump's centre-line
- - inlet line should be as short and large as possible
- below reservoir
- - atmospheric pressure helps to push the liquid into the suction side of the pump
- - this gives the advantage of the pump being charged, (pressure-fed) by the suction head of liquid
97
What is the major factor in a hydraulic component failure
- contamination
98
What is the purpose of a magnetic plug in a reservoir?
- attract steel or iron particles
| - normally mounted in the reservoir
99
Define a strainer and state its purpose and usual position in a hydraulic system
- coarse filters, consisting of either a fine wire mesh screen or a screening element wrapped around a metal frame
- remove larger solids from fluids traveling in a straight path
- they are mounted in the reservoir, on the inlet line of the pump
100
Describe proportional filters
- has only a portion of the oil passing through the filtering element, the reset flows directly to the reservoir
- with continuous recirculation, all the oil eventually passes through the filter
101
Describe full-flow filters
- all the oil passes through the filtering element
- gives more filtering action, but builds up resistance to flow as the filter becomes dirty
- filter housing often has a bypass valve
102
Describe mechanical (metal) filters
- considered coarse filters or strainers, and consist of layers of wire screens or discs of perforated steel
- remove the larger solid particles, but do not remove water or very fine solids
103
Describe ABsorbent filters
- inactive -- contain materials such as paper, wood pulp, fabric waste, or wool
- remove fine particles, as well as water and water-soluble impurities
104
Describe ADsorbent filters
- active -- bone-black, charcoal, fuller's earth, and other active clays
- remove impurities by both mechanical and chemical means
- remove all solid particles and insoluble sludge, plus nearly all water and soluble, oxidized material
105
Define a micron
- one millionth of a metre
| - 0.000039 inch
106
What are nominal and absolute filter ratings?
- nominal -- indicates that the filter will remove most particles of that size or larger (depending on particle orientation)
- absolute -- will stop ALL particles of that size or larger
107
How is the filter condition indicated?
- filter indicators
- - guages with green, yellow and red divisions of the dial
- - "tell-tales" with green, yellow and red bands
108
What are the advantages and disadvantages of the following:
| - inlet-side filters
- advantages
- - filters all hydraulic fluid going to the system
- - protect the pump and the relief, or unloading, valves
- disadvantages
- - requires a filter that is large enough to not produce too large of a pressure drop as it becomes dirty
- - volume of fluid requires a in-line filter that is not a stock item, or is very expensive
109
What are the advantages and disadvantages of the following:
| - pressure-side filters
- advantages
- - protects the valves
disadvantages
- - does not protect the pump or main pressure valves
- - filter element and housing must be able to withstand the maximum pressure allowed in the system
110
What are the advantages and disadvantages of the following:
| - return-side filters
- advantages
- - considered best for a fine filter as the fluid is at its highest temperature and therefore at its lowest viscosity
- - removes all solids resulting from wear on parts in the system
- disadvantages
- - does not protect parts from contaminants such as scale and rust (formed in the tank or ingressed)
111
List the purposes of a reservoir
- provides an adequate supply of hydraulic fluid to the system
- allows air in the hydraulic fluid to escape, dirt and water to settle out, and heat to dissipate
112
How much capacity should a reservoir have?
- equal to two to three times the rated pump delivery for one minute
- large enough to keep the fluid level several centimetres above the intake when the system is using the maximum volume of fluid
- there should also be an air space above the fluid level when the system is at its minimum use
113
Describe the construction of a reservoir
- reservoir tank
- - normally made from steel plate, all joints welded
- - sloping bottom with a drain plug at the lowest point
- - removable end caps for easy cleaning
- baffle plate
- - prevents a direct flow of fluid from the return line to the suction line
- - recommended height is about two-thirds the height of the minimum fluid level
- return and suction lines
- - recommended height from bottom of tank = 1.5-2x pipe diameter
- - suction line located on other side of baffle close to the bottom to prevent vortexing
114
What is the importance of a vent, a temperature gauge, and a sight gauge?
- vent -- allow uncontaminated air movement
- temp guage -- monitor excessive heat build up in system
- sight guage -- monitor oil level
115
What are the purposes of heat exchangers?
- ensure correct operating temperature of the hydraulic fluid
- frequently used to cool the oil
- may also be needed to heat the oil for cold weather start-ups
116
Describe water coolers
- usually consist of a nest of tubes in a shell (cylindrical container)
- oil flow is in one direction and cooling water flow is in the opposite direction
117
Describe air coolers
- consist of a nest of tubes that a vertically mounted and have fins for heat removal
- oil flows through the tubes and a fan drives cooling air over the tubes and fins to remove heat
118
Describe heaters
- heat may be added to the fluid by:
1. using electric immersion heaters with thermostat control
2. passing steam or hot water through a coil or length of pip submerged in the tank
3. starting up the hydraulic system and pumping oil over the relief valve at maximum pressure to create heat from fluid friction
119
What are the purposes of accumulators?
- store fluid under pressure for future use as a source of potential energy
- absorb shock waves or dampen pulsations
- maintain constant pressure in the system
120
When using an accumulator, what should be known about it?
- the amount of oil to be added before the gas charge
| - the recommended pressure of the gas charge (pre-charge is a percentage of the operating system's max. pressure)
121
What must be done before any work is started on an accumulator-loaded system?
- read the instructions carefully
- isolate accumulator with a shut-off valve
- discharge or drain accumulator back to tank
122
Describe a weight-loaded accumulator
- a vertical cylinder fitted with a piston
| - a packing gland or oil retaining device keeps fluid in the cylinder
123
Describe a spring-loaded accumulator
- use compression springs instead of gravity to supply resistance
- springs must be evenly loaded to allow even travel of the piston through the cylinder
- must be installed in a clean environment
- does not produce constant pressure through the entire stroke
- the springs exert minimum pressure when the accumulator is at a low volume
- the springs exert maximum pressure at high volume
124
Describe a gas-charged accumulator
- depend on the compressibility (bulk modulus) of a gas to produce the necessary pressure and delivery
- use the principle of Boyle's Law: "at a constant temp, the volume of gas varies inversely to the absolute pressure"
- - P1V1 = P2V2
- 1/3-1/2 charged to system pressure
- available as non-separated, piston, bladder and diaphragm types
GASES USED
- compressed air (low pressure) -- possible rust contamination
- dry nitrogen (med-high pressure) -- inert gas
- Never use oxygen -- explosive
125
Describe a non-separated accumulator
- have no physical barrier between the gas and liquid
- used mainly on low pressure systems
- low pressure limits the amount of gas that dissolves in the liquid
- vertically mounted cylinders
- liquid line connection at bottom end
- pneumatic connection at top end
- high and low level switches required to prevent air from getting into circuit
126
Describe a piston accumulator
- has a free-floating piston between the liquid and the gas
- the piston has two sets of required packing, these seal the two chambers and centralize the piston to prevent metal-to-metal contact
- a bleed hole is used to eliminate any build-up pressure between the seals
127
Describe a bladder accumulator
- have a natural or synthetic rubber bag mounted inside a chamber
- the bag is moulded around the gas valve and mounted to the top, the liquid connection is mounted through the bottom
- a poppet valve is in place to prevent the bladder from being drawn into the liquid connection and rupturing
128
Describe a diaphragm accumulator
- use a natural or synthetic rubber diaphragm mounted in the centre of the chamber to separate the liquid from the gas
- operation is similar to a bladder accumulator
129
Describe the operating principles of a pressure intensifier
- intensifiers for one type of fluid use hydraulic fluid in both sides, those with two types use compressed air or other gas on one side and hydraulic fluid on the other
- the cylinder has two different size pistons connected by a piston rod, or one piston and a plunger
- the lower pressure act on the large piston area and forces the small piston forward, creating a much higher pressure
130
Describe the operating principles of a Bourdon guage
- consist of a calibrated dial face and a pointer attached through linkage to a flexible eccentric metal tube (Bourdon tube)
- the Bourdon tube is connected to system pressure
- as pressure rises the tube tends to straighten slightly due to the difference in area between its inside and outside surfaces
- this action causes the pointer to move around its dial face and indicate the pressure
131
Describe the operating principles of a Schrader guage
- has a calibrated dial face with a pointer attached through linkage to a plunger and bias spring
- the system pressure is connected to the gauge and acts on the plunger
- as pressure increases the plunger is forced against the bias spring, this moves and rotates the pointer around the dial face, indicating the pressure
132
Describe the operating principles of a flow meter
- consists of a weighted device (ball or cylinder) inside a calibrated, tapered tube
- this type of flow meter must be mounted vertically because the weighted device relies on gravity to operate properly
- another style has a turbine in-line with the flow
- a sensing device converts the pulses of the turbine to flow rate - connected to a electrical readout
133
Describe the operating principles of a pressure activated electric switch
- a mechanical device such as a diaphragm, piston or Bourdon tube is used to sense pressure changes
- this device is attached to a switch which opens or closes an electrical circuit if the pressure goes beyond the predetermined level
- the predetermined level may be factory-set or field-adjusted
134
What units are used to measure pressure?
- PSI
- HG
- Feet of head
135
Why is the measurement of temperature important?
- increased temperature is a sign of inefficiency where HP is being wasted as heat
136
List the qualities that should be considered when selecting a hydraulic fluid
- viscosity
- viscosity index
- pour point
- thermal stability
- resistance to oxidation
- resistance to rusting
- resistance to air foaming
- fire resistance
- lubricating qualities
- long life
- cost
- disposability
137
Describe HFA fluids
- High water content fluids
- soluble oils or synthetic chemical emulsions (oil-in-water)
- generally contain 5-10% oil
- excellent cooling ability
- must operate at low temperatures (max 49*C)
- should not be allowed to freeze because oil and water will separate
*always check seal compatability
138
Describe HFB fluids
- Water in oil fluids
- invert emulsions which contain more oil then water (40% water)
- should be checked regularly to maintain the viscosity and the water-oil ratio
- operation similar to HFA fluids
* always check seal compatability
139
Describe HFC fluids
- Water glycol fluids
- consists of 30-40% water dissolved into glycol
- should be checked regularly to maintain correct water-to-glycol ratio
- HFC fluids can endure temperatures below freezing
- these fluids are heavier than oil and should be used with either a very short suction lift, a special inlet design, or in situations where the fluid level is above the pump inlet
* always check seal compatability
140
Describe HFD fluids
- Synthetic fluids
- special compounds which do not support combustion
- they have no water content and can therefore be used at high temperatures with no evaporation problem
- heaviest of the hydraulic fluids and require special pump inlet designs or special pump mounting positions
*always check seal compatability
141
How is hydraulic fluid kept clean during storage?
- keep the spout or bung clear from moisture and other contaminants
- if the containers are to be stored outside, tilt them to prevent water from collecting around any bungs
142
What determines the rate of flow that can pass through a conductor without excessive friction?
- the inside diameter (ID) of a line determines the rate of flow that can pass without excessive friction, heat, and power loss
- velocity for given flow is less through a large opening than through a small opening
- velocity varies inversely as the square of the inside diameter of the line
- as the inside diameter decreases, turbulence and friction increase, causing increased power loss
143
What determines the bursting pressure of a line?
- the wall thickness and the inside diameter determine the bursting pressure of a line
- the greater the wall thickness for a given inside diameter, the higher the bursting pressure
- conversely, the greater the inside diameter for a given wall thickness, the lower the bursting pressure (F=PxA)
144
What type of pipe is used for hydraulic systems?
- pipe is selected for economy and for its ability to carry large flows in the larger sizes of pipe
- seamless steel (black) pipe; preferably pickled is recommended for hydraulic systems
*never use galvanized pipe
145
List the thread types used on hydraulic piping
- American National Pipe Taper (NPT)
- American National Pipe Straight (NPS)
- Dryseal Pipe Taper (NPTF)
146
How is sealing compound applied to the pipe?
- pipe threads should be treated with a sealing compound on the male end before being threaded into a fitting and tightened
- this prevents galling of threads and prevents leakage from the spiral clearance at the roots of the thread
- any sealing device should start about two threads from the end of the pipe
- tape should be wound in the direction to be tightened
- should be wrapped twice around the pipe
- any burrs or sharp edges should be removed from the ID and OD of the ends of the pipe
147
Why is tubing a popular hydraulic conductor?
- it can be easily bent and flared
| - no change in id from pipe to fittings
148
How is tubing size measured?
- sizes are taken from the outside diameter (OD) and are held to close tolerance
- nominal dimensions are given in fractions of an inch or dash numbers
- dash numbers represent sixteenths of an inch (-8 = 8/16" = 1/2")
149
Describe flared and flareless joints
- flared joints
- - rely on the the compression of the tubing material between the inner and outer walls of the flare
- - tube can be made single or double flared
Flare anges for tubing:
- 45* (90* included angle) - SAE standard
- 37* (74* included angle) - Joint Industrial Council (JIC) standard
- flareless joints
- - use an intermediate product to grip the tubing, as the nut is tightened onto the end fitting, the intermediate product is compressed onto the tubing in order to create a seal
150
What factors should be considered when bending tubes?
- to ensure a smooth bend, use proper bending tools and an appropriate bend radius for the tubing size. Generally, the correct bend radius is 3-4 times the tube OD
- use the bend radius to calculate the required cutoff length of tubing
- bend the tube carefully to avoid distortion
- leave a straight length of at least twice the nut length between a bend and a fitting. This allows the connecting nut to slide away from the fitting when necessary
- form bends properly so that the fittings are in alignment.
- allow for expansion or contraction due to temperature when short lengths of tubing are used
151
Describe the basic procedure for tube installation
- start from a fixed point
- use proper hydraulic fittings
- use as few fittings as possible by making bends in the tubing (every fitting is a source of turbulence as well as a potential leak)
- locate the simplest route with the least number of bends
- make sure all joints or fittings can be easily reached for maintenance
- put the tube line where it will not be a hazard to workers
- put the line where it does not interfere with any other equipment
152
What are the purposes of a hose?
- used to connect parts which move in relation to each other, or which are subject to vibration
- a hose has an accumulator action as it bulges slightly with pressure surges
153
Describe the construction of various hydraulic hoses
- the main parts of a hose are the:
-- inner tube - usually synthetic rubber which can withstand temperatures up to 133*C (275F) for short intervals
-- reinforcement - determines the pressure rating - usually single or more braided wire or nylon
-- cover - usually neoprene, which has high resistance to oil, abrasion, and weathering
154
Describe the construction of hose end fittings
- permanent fittings
- - are crimped or swaged on the hose end and are discarded with the hose
- - a hose crimping machine is required to assemble the fittings
- reusable fittings
- - are screwed or clamped to the hose ends and salvaged when the hose is discarded
- - hose for reusable fittings can be purchased in bulk and each section assembled as needed
155
Describe the basic procedure for hose installation
- allow enough slack to avoid kinking the hose at a ridge connection
- do not use a taut hose: pressure tends to bulge the hose and shorten it
- do not twist the hose: this can be checked by markings on the cover (use fittings to avoid long loops)
- follow specifications for minimum bend radius
- install hose lines so parts can be easily reached for maintenance
- keep hoses from rubbing on fixed objects and keep moving objects from rubbing on them (this can be done by clamping or tying the hoses out of the way, or by using hose guards)
- keep the hose away from high heat sources. If the hose cannot be moved, insulate it
156
What standards do fittings and couplings have to meet?
- American National Standards Institute (ANSI), which is the hydraulic industry standard
- Society of Automotive Engineers (SAE), which is the automotive industry standard
- International Standards Organization (ISO), which is the international standard
157
Describe threaded fittings
- threaded ends for port connections can be:
- - NPT (tapered)
- - NPTF (tapered dryseal)
- - NPS (straight)
- NPS threads are used when rotational orientation is important - a locknut and washer allows the fitting to be positioned in any orientation and locked - an O-ring between the fitting and the housing creates the seal
158
Describe flanged fittings
- for the same nominal size, both flange and split-flange fittings have the same bolt-hole pattern - this makes them interchangeable
- tubing sizes above 7/8", split flange fittings are used, the tubing is permanently attached to the flange - an o-ring creates the seal
- when a flanged fitting is used for pipe, it is not split - the pipe is either welded to the flange or threaded into the flange
159
Describe quick disconnect couplers
- often used where hoses are meant to be removed periodically
- they seal hose ends when they are not coupled and allow hydraulic fluid to pass through them when coupled
