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Many designers have long accepted leaks as
inherent to hydraulic systems,
even though advances in technology
should have eliminated hydraulic leakage a long
time ago. Hydraulics suffers
a similar identity crisis
when it comes to
noise. Noise certainly
cannot be eliminated, but
a number of products and
techniques exist to at
least bring noise down to
an acceptable level. The
problem is that noise reduction
is a complex subject,
and investing a great
deal of time, effort, and
money may produce only
modest improvements.
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Sources of noise
A hydraulic system’s
greatest contributor to
noise is the power unit,
Example #1. Noise not only
emanates directly from
the electric motor and
pump, but also is caused by pressure
fluctuations in the hydraulic fluid and
by components — either resulting from
these pressure fluctuations or from
physical vibration. Transmitting vibration
of the pump-motor assembly to the
reservoir can transform this physical
vibration into sound — in the same way
a loudspeaker transforms electromagnetic
vibrations into sound.
Electric-motor noise comes from
bearings, the rotor and stator assembly
(the characteristic hum), and, especially,
the fan. A standard electric motor
contains a fan with blades designed
to provide cooling whether the motor
shaft rotates clockwise or
counter-clockwise. A fan
designed for rotation in
only one direction will
generate less noise, so the
expense of this option
may be warranted if the
application demands
quiet operation.
Pump noise stems from
rolling and sliding of bearings
and pumping elements
(vanes, pistons, rotors,
gears, etc.), plus
pressure fluctuations that
result from the cyclical nature
of the pumping process.
Metal housings,
whether part of the hydraulic
pump or an electric
motor, do little to prevent
noise from being transmitted
to the surrounding environment.
Moreover, because
the pump generally
is coupled to an electric
motor (and the coupling itself
is a source of noise),
noise control often involves
treating the pumpmotor
combination as a
unit. This design technique has produced
power units where the pump-motor combination
is submersed in oil or where the
entirepower unit is submerged in the reservoir.
This techniaue uses liquid to
dampen sound waves by acting as a buffer
between the pump-motor housing
and the surrounding atmosphere.
Valve noise has occurred in cabs of
construction and other mobile equipment
for years. Often, a high-frequency,
random noise occurs when
fluid, traveling at high velocity through
the valve, undergoes a rapid and severe
drop in pressure. This causes air dissolved
in the fluid to form bubbles
which, when they collapse, generate
noise. Other types of noise — such as
chattering, squealing, or buzzing — is
generated when poppet-type valves do
not seat properly.
Fortunately, most of these problems
can be eliminated through better system
design or by incorporating cushioning
features into valves. A current
trend replaces direct-operated valves
with joystick-controlled remote electrohydraulic
valves. This process of
removing the hydraulics from the
equipment cab offers other advantages
beyond providing a quieter workplace
environment.
Fluid conductors (tubing, hose,
fittings), often are overlooked asnoise sources. However, pressure
pulsations in plumbing can distribute
noise over a large area. Pressure pulsations
can shake hose and tubing,
causing rattling and eventual leakage.
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Although reducing fluid-borne noise
can be complicated, many manufacturers
suggest rules of thumb to help reduce
noise. For example, terminating a
long run of metal tubing with a section
of hose at each end helps isolate noise
sources, Example #2. One might be
tempted to simplify the design by instead
specifying a single section of
hose. Hose, however, is very sensitive
to pressure pulsations, so in long sections
it can be a greater source of noise
than metal tubing or pipe.
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Ex. #2- Many simple techniques can be employed to quiet a noisy hydraulic systems. Among these are incorporating hose into long tubing assemblies, tiop, and using a 60mesh screen positioned 30o from horizontal.
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Securing tubing to framework with
resilient clamps eliminates rattling
and banging noise. However, care
should be taken not to confine tubing
too tightly, because lines may need to
undergo thermal expansion. On the
other hand, allowing a tube to fit too
loosely could cause wear as the tube
constantly rubs against a metal clamp
surface. Likewise, resilient grommets
should be used when a hose or tube
passes through a hole in framework,
covers, etc.
Actuators, especially hydraulic
motors, also generate noise. Hydraulic
motors sometimes are considered to
generate noise equivalent to that of
pumps. However, hydraulic motors
often operate at relatively slow
speeds, so motors generally operate
much quieter than pumps do.
Prevention and cure
The power unit generally holds potential
for the greatest reduction in
noise for a given amount of time, effort,
and expense exerted. As mentioned,
an optional cooling fan may
reduce noise from the motor. Also,
using a motor that operates at 1200
instead of the usual 1800 rpm may reduce
noise. However, expect a 1200-
rpm motor to be larger, heavier, and
more expensive.
Pump noise may be reduced by
running a large pump at a lower than
normal speed (which can also increase
pump life) or specifying four
or five small pumps for a power unit
instead of the usual one or two large
pumps. Size and the type of pump
(piston, vane, gear, etc.), number of
pumping cycles per rotation, system
pressure, and, especially, pump speed
all influence noise. Check with the
manufacturer for assistance in determining
what parameters will best suit
your application.
In addition to specifying quiet
pumps and motors, you can also reduce
noise by
using vibration-damping mounts to
mount the pump and the motor to a
subframe
mounting the subframe to the
power unit frame using vibration-damping
mounts
installing a flexible coupling between
the motor and pump (and
aligning it properly before startup)
using hose sections between tubing
and components that are mounted to
framework, and
as a last resort, treating noise as a
symptom rather than at its cause may
be the only recourse for some applications.
Installing sound-damping
materials around the motor-pump or
power unit not only adds expense and
complexity to the system, but complicates
maintenance and may hinder air
circulation for cooling. Acoustic filters,
which use internal reflections
and resonant frequiencies to cancel
out noise, may also be effective.
However, they must be tailored to the
application and tend to be expensive.
Not allowing air to dissolve in hydraulic
fluid goes a long way toward
preventing cavitation, both in the
pump and in downstream components.
Cavitation usually causes
noise when air bullbe suddenly collapse
as fluid becomes pressurized in
the pump. Air can be removed most
effectively when fluid is in the reservoir.
Given enough time, air will separate
from the fluid, so the path from
the return line to the pump inlet
should be as long and with as little
turbulence as possible. In addition,
incorporating a fine-mesh screen promotes
removal of air. Furthermore,
tests have shown that positioning a
60-mesh screen 30o from horizontal
may remove as much as 90% of entrained
air, Example #2.
Another method of quieting
the power unit is to reduce pressure pulsations. Accumulators often are
specified for this purpose, but their effectiveness is limited because
they dampen pressure pulsations within a range of frequencies for a given
size and precharge pressure. Moreover, accumulator calculations are
complicated, and several accumulators may be required to dampen the full
range of pulsation frequencies experienced by a system.
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 Ex. #3- Oscilliscope readings, shown above, illustrate how suppressor dramatically reduces pressure pulsations, and, therefore, noise. |
 Surge suppressor, shown installed on pump outlet in photo above, takes up much less space than accumulator. |
An alternative is to mount an inline
surge suppressor to dampen pulsations
over a wide range of frequencies,
Example #3.One such a suppressor
consists of a housing containing an
annular area that holds a pressurized
charge of nitrogen, a cylindrical
membrane, and a perforated tube,
Example #4. Under normal operation,
fluid simply passes through the suppressor
by entering one end of the
tube and exiting the other. However,
if pressure increases — from pump
pulsation, for example — the fluid
passes radially outward through the
tube perforations, overcomes the nitrogen
charge pressure, and expands
the diaphragm outward. Allowing
pressure fluctuations to act against
the pressurized nitrogen cushions the
vibration, so output pressure is much
smoother — and, therefore, pump operation
is quieter. Moreover, sizing is
simple, because the suppressor is selected
according to the size of the
pump discharge line.
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 Fig. 4. Surge suppressor uses pressurized nitrogen gas, elastic membrane, and metal tube containing hundreds of holes to dampen pressure pulsations. In cross-sectional drawing above, a sudden increase in pressure causes hydraulic fluid to flow radially outward, through holes in the perforated tabe, and against the membrane, which is loaded against a charge of nitrogen gas. At low fluid pressure, the membrane contracts around the perforated tube. The small size of the holes prevents the membrane from extruding through them. |
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Finding out more
However you decide to make the
hydraulic systems you design run
quieter, component manufacturers
prove an invaluable resource. Not
only can they provide specifications
on components, but they may also
have useful literature containing
more information on noise control of
hydraulic systems. Engineering service
laboratories who specialize in
design and testing of hydraulic systems
may also provide solutions.
Whether affiliated with major component
manufacturers or engineering
laboratories, application engineers
possess a wealth of knowledge that
may include solutions to noise problems
very similar to those experienced
by your applications.
But resources don’t end
there. Dozens of books, technical reports, and papers exist to help you
learn more about controlling noise in hydraulic systems. Calling on these
resources may not make you an expert on the subject, but you’ll certainly
be more able to decide which solutions are most practical for your
applications.
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