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Terms in this set (69)
store hydraulic fluid under pressure, this is a form of energy storage
store energy by compressing a gas
is filled with hydraulic fluid until the gas volume is half of its precharge volume
When the pressure system is less than the accumulator pressure
fluid flows out of the accumulator to system.
as fluid flows out the gas decompresses and losses pressure.
the pressure constantly decreases inside the accumulator
when the accumulator pressure is less than system pressure
the fluid flows from the system into the accumulator
Nitrogen typically used to fill gas-charged accumulators (why?)
Nitrogen is inert and does not support combustion.
At high precharge pressures (1000-1500 psi) air is extremely dangerous
can be very dangerous if not done carefully.
the precharge pressure should be checked frequently in the initial stages of operation
Bladder type accumulators
it's precharged with nitrogen, energy stored in form of a compressed gas.
gas and hydraulic fluid separated by a synthetic rubber bladder .
Fluid pumped in compresses gas and increases pressure inside accumulator
protects bladder from damage as it contacts fluid port
similar to bladder type, a synthetic rubber separates fluid and gas.
advantages of bladder and diaphragm type over piston type
no moving parts (no lubrication needed)
can be used with fluids that have poor lubricating qualities
less sensitive to contamination
piston separates gas charge from the hydraulic fluid.
pressure is a function of gas compression and varies with the volume of oil in the accumulator
no separator between gas and hydraulic fluid the gas is then applied directly to the hydraulic fluid.
but the gas can become absorbed into hydraulic fluid and cause problems downstream
store energy in the form of a compressed spring.
spring applies force on the piston that exerts pressure on the hydraulic fluid
fluid pumped in raises the weight.
advantage: constant pressure throughout operation
disadvantage: very large weight needed ( dangerous)
sizing an accumulator
because pressure drops as fluid flows out of the accumulator, it will not provide flow at a constant pressure
so you must determine minimum and maximum operating pressures
pump fills accumulator when
the accumulator pressure reaches unloading valve setting ( pump unloaded back to tank)
the check valve
isolates the accumulator from pump ( accumulator stays at high pressure while pump is unloaded)
when the present pressure is reached
pressure switch shuts down the prime mover.
the relieve valve is still needed as a safety backup
when cylinder reaches end of stroke then the accumulator gets charged.
when the accumulator is fully charged the pressure switch shuts down the prime mover
accumulator supplies enough flow to compensate for leakage and keep pressure on the press
accumulator can also be used to
provide emergency pressure and flow is electrical power lost
and it can absorb shock
discharge valves are needed for accumulator circuits to
relieve accumulator pressure to the tank safely
"boosters" are also used to generate pressure greater than those achievable with standard hydraulic pumps alone
pressure is increased and flow is decreased by the same factor`
sizing pressure intensifiers
need to calculate the pressure and flow at the outlet of the intensifier given the pressure and flow at the inlet (from pump)
pressure intensifer application
is to use compressed air on the low pressure side (inlet) and hydraulic fluid on the high pressure side
this is called hydropneumatic or air-over-oil intensifies
ex: clamps and stamping presses
convenient and relatively inexpensive
store hydraulic fluid
provide heat exchange
allow contaminants to settle
allow air to escape
*heat generated during operation due to pressure drops across components
excess heat is radiated through reservoir walls and carried away by airflow around the reservoir, cooling the fluid
heat exchange through fluid flow around the reservoir surfaces ( convection)
air exists in 2 forms in a hydraulic system
1. dissolved air 2.entrained air 3. free air
air in solution with the oil, has no adverse affects on the system
air in the form of small bubbles--can be dissolved air that has come out of solution or from an external leak
air pocket in the system
includes a strainer to filter out contaminants
main return flow path, typically has a filter
only for external drain lines from components, does not have a filter as it must have as little resistance to flow as possible
prevents fluid from going directly from return line to pump inlet.
it forces fluid to dwell, allows time for heat to be dissipated, promotes heat exchange, allows entrained air to escape, allows contaminants to settle
access for cleaning
refill reservoir with oil, allows air to escape, and prevents pressure or vacuum building inside reservoir
rule of thumb
select a reservoir that is 3x the system flow rate
having a reservoir above pump
provides a positive pressure at the pump inlet, reducing the possibility of cavitation
heat removal is required in systems that cannot dissipate heat quickly
heat exchange's keep operating temp around 140
air cooled-heat exchanger
similar to automobile radiator
fan blows air over bundle of tubes that hold hydraulic fluid, bundle works better more surface area for convection
surface area exposed between hot fluid and cooling fluid
is the most important factor for heat exchangers.
water cooled heat exchanger
cool tap water sent through tubes via water inlet. Hot fluid enters the shell via the hydraulic fluid inlet and flows around the tubes, cooling the fluid
**fluids flow in opposite directions known as counterflow. counterflow maximizes heat exchange
sometimes hydraulic fluid needs to be heated up
can preheat fluid to avoid damage caused by cold start, can use a shell and tube design, or use an electric element heater
constant heating or cooling. they are typically coupled with a thermostat and electronic controls
Because of high pressures involved
seals and bearings are a very critical element
provide a seal between surfaces that are not moving relative to one another
seal between moving surfaces
dynamic seal that prevents contaminats from getting in from the environment
bearings allow a moderate amount of
side loading to be tolerated
seal with a circular cross section, amount of deformation is called a squeeze, which varies from 10-25%
o-rings used for static sealing
used for dynamic applications, seals pressure from only one direction, must install the seal so the U faces the pressure
if used as a piston seal then you must install two of them back-to-back
are extremely effective as pressure is applied it presses the ring lips more firmly against the surfaces
is when the deal begins to squeeze into the gap between surfaces
frequently a problem in high pressure applications
cant use natural rubber as its not compatible with petroleum based hydraulic oil
materials that are hard at room temp but can be molded easily when heated
softer than plastics but harder and stronger than elastomers
oil cleanlines is important for proper operation and longevity. Pumps and Motors are VERY sensitive to contamination.
external-dirt and grime from outside system
internal-component/seal wear and tear
other types of contamination
coarse filter made of a wire mesh
can be cleaned and reused, removed particles greater than 150 micrometer
filters out particles down to 5micrometer and smaller
cellulose paper filter
filter with bypass check valve
filter is bypassed if it becomes clogged
return line filter
fine filter, inexpensive since it can be low pressure
pump outlet filter
only necessary in contamination sensitive systems to catch particles generated by pump wear
measure the performance of a filter to ensure the required level of filtration is achieved.
beta ratio means the filter is catching more particles
pressure read before and after orifice
flow rate determined from the pressure drop
rotameter ( flowmeter)
float allowed to move freely inside a vertically mounted, tapered tube
flowing fluid enters bottom of the tube and lifts the float.
as float moves up the tube diameter increases, which decreases the flow force
the faster the flow rate the greater the flow force and the float rises higher
measures the velocity of the fluid stream
fluid velocity used to find the flow rate
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