Midterm Review Flashcards
Power systems that use liquid as transmitting media
Hydraulic systems
Greek roots:
Hydro
Aulis
Water
Pipe
Fluid power systems do work by transmitting ______ via ______.
Force via fluid
A fluid power system that uses air or other gases
Pneumatic systems
Any influence capable of producing a change in the motion of a body
Force
Any force that can stop or retard the movement of a body
Resistance
Ex: friction and inertia during accelerations
______ is always present between contacting surfaces of two objects when they are moving across one another
Friction (as resistance)
The reluctance of a body to change in its motion.
Directly related to the quantity of matter in a body.
Inertia (as resistance)
More mass -> heavier -> harder to move
______ is a force which can cause a body to move
Energy
Energy can neither be created nor destroyed, only transformed or transferred
Law of conservation of energy
Energy in the system that is not used to perform work becomes ______ energy
Heat
The pump produces ______ energy
Hydraulic
______ energy is produced when hydraulic fluid moves a piston
Kinetic – energy in motion
______ Energy is produced when piston has raised an object
Potential
______ Energy is produced by friction in the pump, motor, piston and hydraulic fluid
Heat
______ takes place when a body or object is moved through a distance by force
Work
Formula to calculate work
Force (lbs) x distance (ft) = work (ft-lbs)
Formula to calculate hydraulic force
Pressure (psi) X piston area (sq in) = force (lb)
______ is the amount of work done in a given time
Power
1 hp =
33,000 ft-lb/min
550 ft-lb/sec
Which of the following is not a normal function of a hydraulic fluid reservoir?
a) eliminate harmonic imbalance
b) remove heat from the fluid
c) help settle out foreign materials
d) store hydraulic fluid
A. Eliminating heat is NOT a normal function of a reservoir.
Directional control valves are sized by:
a) nominal size of the ports
b) physical size of the valve
c) size of operating handle
d) all of the above
A. Directional control valves are sized by the nominal size of the ports.
What are four primary purposes of hydraulic fluid?
- transmit power
- lubricate moving parts
- cool & dissipate heat
- seal clearances between parts
What does the relief valve do?
a) prevents over loading of the prime mover
b) prevents pumps and other components from being damaged
c) controls fluid temp in the system
d) both a & b
D. Both A & B.
How is horsepower in a hydraulic system calculated?
Power = GPM x PSI x 0.000583
What component is represented by a circle with a black arrow pointing OUT?
Pump
Which of the following factors have the least effect on flow rate?
a) pressure
b) fluid cleanliness
c) fluid temperature
d) orifice size
B. Fluid cleanliness will have the LEAST effect on flow rate.
What is the purpose of the cylinder cushions?
a) to protect the cylinder from falls
b) to control fluid flow
c) to prevent piston from hammering the end caps
d) all of the above
C. Cushions prevent the piston from hammering the end caps
The types of energy that might be used in a hydraulic system include:
a) chemical energy
b) static atmospheric energy
c) kinetic energy
d) all of the above
C. Kinetic energy.
In most cases, a flow compensator is built into the external gear pump housing.
True or false.
False. Gear pumps don’t have compensators!
What is the difference between the schematic symbols for a pressure relief valve and a sequence valve?
The pressure relief valve is in the closed position.
The sequence valve is in the open position.
What are 4 primary advantages of a hydraulic system?
- infinitely variable speed
- reversible while in motion
- overload protection while maintaining output force
- high power in a relatively small, lightweight package
How much can a cylinder with a 7” diameter piston and a force of 2000 psi lift?
a) 10 US tons
b) 38.47 US tons
c) 22.22 US tons
d) none of the above
B. 38.47 US tons
Area = 7 x 7 x 0.785 = 38.465 sq. in.
Force = 2000 psi x 38.465 = 76,930 lbs
76,930 / 2000 (lbs in a ton) = 38.465 tons
Weight is ______.
Always a downward force
A hydraulic cylinder changes hydraulic power into:
a) smooth, fast power
b) back to electrical energy
c) straight in-line motion
d) back to fossil fuel
C. Linear motion
The rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure (which is a frequent cause of structural damage) is called:
a) kinetic energy
b) cavitation
c) aeration
d) boiling point
B. cavitation
A vane type pump may have variable displacement if:
a) it has discharge port modulation
b) it is of the hydraulically balanced type
c) its inlet port has fuzzy little feelers to catch the large particles
d) it is of the hydraulically unbalanced type
D. a vane pump may have variable displacement if it is of the hydraulically UNBALANCED type.
How fast will a cylinder extend if:
GPM = 15
Piston diameter = 8.5”
Area = 8.5 x 8.5 x 0.785 = 56.72 Speed = (GPM x 231) / EPA Speed = (15 GPM x 231) / 56.72 Speed = 61 in/min
To improve cycle life, check valves generally have hard steel seats and poppets.
True or false.
True.
Vane pumps cover the low to medium high volume ranges with operating pressures up to 5000 psi.
True or false.
False. 5000 psi is too high. Vane pumps have much lower max pressures, ~3000 psi
What is the input component of a hydraulic system called?
a) actuator
b) check valve
c) pump
d) pressure relief valve
C. pump
What are the TWO common units of measuring viscosity?
Saybolt Universal Seconds (SUS)
Centistokes (eSt)
How many basic classifications of fire resistant hydraulic fluids are there?
Four.
In what two forms do we find energy in the hydraulic fluid?
kinetic and potential
An atmosphere is equal to ______.
14.7 PSI
What are two state valves?
Consists of a pilot-operated main stage and a pilot stage that is usually either electrically or pneumatically-controlled
Restriction check valves have:
a) limited poppet movement
b) a pilot that limits flow
c) an orifice in the poppet
d) none of the above
C. restriction check valves have an ORIFICE in the poppet
Viscosity index indicates what?
Resistance to flow over a temp range
______ differential is needed for fluid to flow
Pressure
A liquid (hydraulic fluid) cannot be ______.
Compressed
A pump delivers 8 GPM to a cylinder that has a 10” diameter piston. What is the speed of the cylinder?
Area = 10 x 10 x 0.785 = 78.5 Speed = (GPM x 231) / EPA Speed = (8 GPM x 231) / 78.5 Speed = 23.54 in/min
The Law of Conservation states:
Energy can neither be created nor destroyed. It can only be transformed.
A hydraulic pump delivering 12 GPM @ 3000psi is developing how many horsepower?
hp = (psi x GPM) / 1714 hp = (3000 x 12) / 1714 hp = 21
What are the two basic types of pilot-operated check valves?
Pilot to open
Pilot to close
Pressure is defined as a force exerted on:
a unit of area.
P = F x A
What is the pressure at the bottom of a 20ft column of oil?
8 psi.
4 psi for every 10ft (or .4psi for every foot)
What determines the speed of an actuator?
a) pressure of the hydraulic fluid
b) flow (or GPM) of the fluid
c) the work the actuator is doing
d) the viscosity of the oil
B. the flow or GPM determines the speed of an actuator
With which component does the design of a hydraulic circuit begin?
a) pump
b) directional control valve
c) actuator
d) pressure relief valve
C. the actuator
Five possible effects to a system with a fluid with too high a viscosity
high resistance to flow increased power consumption higher operating temperatures increased pressure drop difficulty separating air from fluid in res.
Flow is measure in:
GPM
Express 30 psig in psia
30psig + 14.7 = 44.7 psia
Gauge pressures do not account for atmospheric pressure, so to get psia atmospheric pressure must be added
______ is the results of water and oil mixing
Emulsification
A strainer is a coarse filter, usually metal, with pores larger than:
50 microns
The rod wiper, sometimes called the scraper, is made of synthetic and polymer materials. The importance of this device cannot be overlooked as it keeps foreign material from entering the system.
True or false.
True.
If a force of 11,000 lbs is applied over an area of 23 sq. in., what is the pressure?
Pressure = Force / Area Pressure = 11,000 / 23 Pressure = 478.26 psi
What pressure is usually available to push liquid into the pump inlet?
Atmospheric
All piston pumps operate on the principle that a piston reciprocating in a bore will draw fluid in as it is drawn from the cylinder and expel the fluid as it moves back into the cylinder.
True or false.
True.
Which of the following is NOT a typical type of internal valve element commonly found in directional control valves?
a) poppet
b) disc
c) rotary
d) sliding spool
b. disc
Name two things that could create a tendency for a liquid to flow
A pump or the weight of the liquid
Viscosity describes what characteristic of a fluid?
its resistance to flow
Heat in a hydraulic system is produced by:
friction in the pump motor, pump, piston, and fluid
A pilot valve usually requires ____ psi system pressure to operate.
75 to 100 psi
How is pressure created?
a) pressure is caused by the pump turning at too high of a RPM
b) pressure is the result of flow being restricted or resisted
c) pressure is the result of a closed pump inlet valve
d) all of the above
b. pressure is the result of flow being restricted or resisted
Efficiency of a system is its ability to change energy from one type or source into another with ______ ____.
minimal loss
ISO means ______.
international standards organization
How is viscosity affected by heat?
Fluid gets less viscous
Which type of fire-resistant hydraulic fluid is best for high temp operation?
synthetics
How many PSI in one atmosphere?
14.7 PSI
What is Bernoulli’s principle?
If the flow is constant, the sums of kinetic energy and the pressure energy must be constant at various points in the system
How is viscosity affected by the cold?
Fluid gets more viscous
A bar is equal to how many atmospheres?
1.01 says the midterm review
actually one atmosphere is equal to 1.01 bar
How are rust and corrosion prevented?
incorporated additives in the fluid
Pascal’s Law states:
when pressure is exerted on a confined liquid, the pressure is transmitted equally in all directions through the liquid
A cylinder that uses a spring or the load (or gravity) to retract is called a:
single acting cylinder
Chemical properties of a hydraulic fluid determine:
how long the fluid will function before it must be replaced
A double acting cylinder moves under power in which direction(s)?
in both directions, extending and retracting
How much is atmospheric pressure in psig?
Zero.
What is the output component of a hydraulic system called?
a) pump
b) actuator
c) directional control valve
d) prime mover
B. actuator
Which type of hydraulic fluid has the best natural lubricity?
Petroleum
______ air is any quantity of air that is in excess of the saturation level at a given pressure
Entrained
A cylinder’s capacity to lift a load depends on:
Piston size and the fluid pressure applied to it. Force = area x pressure
hydraulic reservoirs are fitted with a baffle for what reason?
slow migration of fluid returning from system, to help aid in cooling
How can the GPM of a gear pump be varied?
by varying the pump RPM
gear pumps are fixed displacement
Express 5 GPM in cubic inches per minute.
1 gallon = 231 cu. in.
5 x 231 = 1155 cu. in.
Particles grinding between moving parts is what type of wear?
a) abrasion
b) erosion
c) explosion
d) none of the above
A. abrasion
What is demulsibility?
The ability of a fluid to separate out of water
what is the function of the pump?
to create flow
What determines the GPM of the pump needed in a hydraulic circuit?
the speed or rate at which the work must be done
What is the prime mover?
power source that drive the pump
What is located on the side of the reservoir to indicate tank level?
Sight glass.
Viscosity is a _______! High viscosity lubricates better but reduces flow, increases power consumption and temp.
Compromise
Pros and cons of water based fluids.
Pros: highly fire resistant, excellent cooling, cheap.
Cons: low max. operating temp, poor corrosion protection, hard to mix additives
Pros and cons of synthetic fluids:
Pros: highly fire resistant, excellent cooling, operate well at high temps, suitable for high pressure.
Cons: very expensive
Sizing a reservoir
Pump GPM x 3
Solid contamination interferes with:
heat transfer process
lubrication ability
blocks or plugs orifices
what kind of valve regulates reduced pressure in certain portions of a circuit?
pressure reducing valve
what is “cracking pressure”?
the point at which the valve opens
what is the defining characteristic of a counterbalance valve?
its used to maintain control over a vertical cylinder so that it will not free fall because of gravity
Gear pumps are referred to as ______ because high pressure at the pump outlet imposes an ______ load on the gears and bearings
unbalanced
Where might you find a filter or strainer? Three places.
Inlet strainer (on the inlet line between the tank and pump)
Pressure line filter (after pump and relief valve, picks up finer particles)
Return line filter (also good for fine particles, a must on high performance components)
a device whose primary function is the retention of solubles
filter
these valves stop and control the direction of fluid. what are the three types?
directional control valves. three types are poppet, rotary and sliding spool
a one-way directional valves is:
a check valve
pilot operated check valves have two positions:
pilot to open
pilot to close
what kind of valve is most commonly found on an anchor winch?
rotary valves
Pumps convert _____ energy into ______ energy
mechanical into hydraulic
Pump delivery is proportional to:
drive shaft speed (RPM)
Volume of liquid transferred from inlet to outlet in one revolution
displacement
volumetric efficiency accounts for:
leakages
How are pumps rated?
max operating pressure capability, and theoretical output flow at a given RPM
Pumps with fixed displacement cannot change their displacement without replacing certain components.
True or false.
True
What is a vane pump’s minimum starting speed required to throw the vanes out against the ring?
600 RPM
What holds a vane pump’s vanes out against the ring?
Centrifugal force
A(n) ______ vane pump with a round cam ring design has a somewhat limited pressure capability because of it’s side-loading
unbalanced
How does a balanced design differ from an unbalanced design? Vane pumps.
a balanced configuration has a elliptical shaped cam ring, with inlet and outlet ports on each side, 180* apart, preventing side loading
Square vane-type pumps are hydraulically ______ and have a ______ displacement
balanaced
fixed
which type of pump is least tolerant to contamination?
piston pumps
Pressure compensators are found on what type of pump?
Piston pumps
three factors affecting flow rate
pressure (pressure drop)
fluid temp (viscosity)
orifice size
three types of flow control circuits
meter-in
meter-out
bleed-off
two types of flow control valves
pressure compensated
non pressure compensated
what is the function of a pressure compensated flow control valve?
it automatically compensates for pressure changes and maintains its setting, even as the work load changes
A restrictor type check valve uses ______ to restrict flow
an orifice
a condition of flow in a passage that is typified by slow movement of fluid in a relatively straight path along the centerline of the passage
laminar flow
a term loosely applied to almost any moving cylindrically shaped part of a hydraulic component which moves to direct flow through the component
spool
the opposition to current flow offered by the components, conductors and load
resistance
energy that a substance or body has by virtue of its mass and velocity
kinetic energy
the volume of fluid discharged by a pump in a given time, usually expressed in GPM
pump delivery
a condition of equal opposed hydraulic forces acting on a part in a hydraulic component
hydraulic balance
a secondary passage for fluid flow
bypass
a valve that allows fluid to flow in one direction, yet stops flow in the opposite direction
check valve
the volume mass or weight of a fluid passing through any conductor per unit of time
flow rate
a device for converting hydraulic energy into mechanical energy
actuator (ex: cylinder or ram)
a narrowing of the passage size
orifice
14.5 psig
1 bar
a valve shoe’s primary function is to direct or prevent flow through selected channels in this component
directional control valve
a mechanical divide that is used to contain a seal, o-ring or gasket in a specified space to result in a leak-proof connection between two or more mechanical components
clamp
work per unit of time measured in horsepower or watts
power
a seal, made from robber or other synthetic material
gasket
the order of a series of operation or movements
sequence
the measure of pressure at the base or other reference point of a column of fluid, normally measured in feet of water
head
the measure of the capability of a pump to raise fluid from a lower to higher lever at its inlet port without damage to the pump
lift
a separator found in the reservoir
baffle
mechanical device which contains a fine mesh filter normally attached to the top of the reservoir
air breather
a cylinder that has an extend port only
ram (single acting)
air trapped in the hydraulic fluid
aeration
the transfer of power from one state into another. the movement of weight over a specified distance
work
a pressure storage reservoir in which fluid is held under pressure and released when it’s required to perform a specific task in the hydraulic system
accumulator
a rotating component of a pump which transfers energy from the motor that drives the pump to the fluid being pumped by accelerating the fluid outwards from the center of rotation
impeller
a type of flared fitting
JIC fitting
