Transcript No Slide Title

A $20,000.00 installation of
additional cabin sound proofing is
less effective than a $500.00 fan
trim balance for reducing noise
caused by an out of balance fan.
The balance job is also 100% more
effective in reducing vibration!
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Turbine Fan
Trim Balance
.
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Goals for this Seminar
Each attendee should
Understand Basic Vibration
Understand the Fundamentals of
Fan Trim Balancing
Understand Why Balancing is Beneficial
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Overview
What is vibration?
What are the effects of vibration?
How is vibration measured?
What are the options in reducing vibration?
How is vibration analyzed when balancing?
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What is Vibration?
For the purpose of fan trim balance in a
Turbofan Engine, vibration can be described as
the unwanted, unproductive,
cyclic oscillation of the
fan assembly about its
rotational axis.
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REMEMBER...
All noise and/or vibration is not generated by an
imbalance in the fan.
To verify the vibration source, a vibration
survey should be conducted
and manufacturers’
limitations adhered to.
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Examples of
Vibration-Related Complaints
Passenger complaints of noise in the cabin
Higher than normal EVM (Engine Vibration
Monitoring) system indications
Physical movement of airframe (buzz in the
seat, yoke, rudder pedals)
Malfunctioning or failed avionics
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What Are the Effects of Vibration?
Vibration excites natural frequencies causing
significant vibration at the components.
The expended energy from vibration causes
wear of components, reduced performance,
passenger discomfort and reduced fuel
economy.
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How Is Vibration Measured?
Sensor Types
Sensor Engineering Units (EUs)
Characteristics of Different Sensor Types
Sensor Specifications
Sensor Mounting
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How Is Vibration Measured?
Sensor Types
Displacement - Measures physical
change of position.
Velocity - Measures the rate
of change of displacement with time.
Acceleration - Measures the
rate of change of velocity with time.
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How Is vibration measured?
Engineering Units
Mils (0.001 inches) - displacement
IPS (Inches Per Second) - velocity
gs (equivalent gravities) - acceleration
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How Is Vibration Measured?
Modifiers
Peak
Peak to Peak
Average
RMS
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How Is vibration measured?
Sensor Characteristics
Displacement Sensor
Measure change in position
Typically reported in mils Peak to Peak
More sensitive to low frequencies
Directly related to movements due to imbalance
Seldom used in balance. (However, displacement units
(Mils) are sometimes used)
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How Is Vibration Measured?
Sensor Characteristics
Velocimeters/Velometers
 Measure velocity
 Typically reported in IPS Peak
 More sensitive to medium frequencies
 Directly related to energy from imbalance
 Often used in balance
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How Is Vibration Measured?
Sensor Characteristics
Accelerometers
Measure acceleration
Typically reported in g’s
More sensitive to higher frequencies
Directly related to force caused by imbalance
Used in balancing (after conversion to velocity or
displacement)
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How Is vibration measured?
Sensor Characteristics
Specifications
Sensitivity (millivolts per Engineering Unit)
Temperature range
Physical size
Physical weight
Mounted resonance frequencies
Use manufacturer recommendations for engine
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How Is Vibration Measured?
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How Is Vibration Measured?
Sensor Characteristics
Sensor Mounting
Use engine manufacturer recommended mount
Mount sensor per manufacturer instructions
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Options for Reducing Vibration
Remove and replace the faulty component.
Repair the faulty component.
Use active cancellation systems.
Dynamically balance the fan.
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How Is Vibration Analyzed
A vibration sensor measures vibration in a given
direction.
The once per revolution tach signal provides a
timing reference from which the phase angle is
calculated.
The relationship between the two is used to identify
the magnitude of the vibration and the phase angle
or radial location of the heavy spot on the fan.
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How Is Vibration Analyzed
 The vibration sensor generates an
electrical signal as the heavy spot on
the fan passes its location and sends
the signal to the analyzer. The analyzer
measures its voltage, the time of its
occurrence and records these in
memory.
 The reflective tape triggers a response
in the LASETACH® as it passes the
optical laser beam. The LASETACH
then sends an electrical signal to the
analyzer. The time of arrival is
recorded in the analyzer’s memory.
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How Is Vibration Analyzed
 The vibration sensor and reflective
tape are installed on the engine. The
LASETACH is mounted up to 30 feet in
front of the inlet and aimed at the
spinner where the tape is to trigger the
tach event. The mass (heavy spot on
the fan) is located by relative
occurrence of the tach event
(reflective tape passing in front of
laser beam) and mass passage at the
radial sensor location. In this slide,
the tach event is about to occur and
the vibration event is near zero
between the positive and negative
vibration events.
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How Is Vibration Analyzed
 As rotation of the fan continues, the
mass is now located opposite the
vibration sensor. This is the point of
the maximum negative going peak in
the sine wave. From this point, the
mass will move back toward the zero
crossing (between negative and
positive) then upward toward the
sensor. This compresses the
piezoelectric element inside which
generates the voltage output. That
voltage is sent along the connecting
cable to the analyzer where it is
measured and converted to
engineering units (Mils, gs, IPS).
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How Is Vibration Analyzed
 The mass is about to enter a point in
the rotation where the positive upward
movement begins in the vibration
event. The strength of the electrical
signal increases from this point and
peaks as the mass reaches the sensor
location.
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How Is Vibration Analyzed
 In this slide the mass, or heavy spot
on the fan, is approaching the position
of the vibration sensor. Notice that the
reflective tape (now at the 3:00
position) has traveled 270 degrees
since the tach event. The maximum
amplitude of the vibration, as
measured by the vibration sensor
occurs here.
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Fundamentals of Balancing
Overview
What information is required
How the data is collected
How the balance solution is computed
How the balance weight is installed
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Fundamentals of Balancing
What information is required?
Engine speed(s) for balancing
The vibration amplitude for each speed
The angular reference (phase) of the vibration for
each speed
Influence on the fan of weight addition for each
speed
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Fundamentals of Balancing
How to select balance speeds.
Use Manufacturers Recommended Speeds
Use Speed of the complaint
Use Vibration Survey to Select Speed(s)
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Fundamentals of Balance
What is an Influence?
A ratio of how much weight is required to counter a
measured out of balance condition
Typical units are (grams/IPS) with a phase lag
It is used to compute a balance solution
It can be estimated for the first run
It should be refined on subsequent runs from
actual vibration measurements
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Fundamentals of Balance
How is the required data collected?
Install vibration sensor
Install speed sensor
Run aircraft
Collect average magnitude and phase data for each
speed and sensor of interest
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Fundamentals of Balance
How is balance solution calculated?
The balancer calculates a solution based on the vibration
magnitude and phase data collected for each speed
It presents the solution to the user, such as “Place 2
Grams of weight at 90 degrees” or “Install a -2 weight in
hole number 2”
Install/record the weight added and its location
Run the engine again to verify predicted results
Refine solution if necessary
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Fundamentals of Balance
How is the solution refined?
Actual vibration changes and weight additions are used to
compute a more accurate estimate of the influence at each
speed
This recomputed “accurate” influence is used to estimate a
new balance weight
Balancer should record actual weights added
Influence should be further refined on each additional run
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Fundamentals of Balance
Notes on calculated solutions
All turbine fans are different!
Sophisticated balance equipment adjusts for these
differences after the first weight placement
Vibration will not always go down with the first weight
placement
If the vibration is not reduced by the second adjustment,
check mechanical condition and/or balance process
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Fundamentals of Balance
How are the balance weights installed?
On first run, place closest weight in closest hole
On later runs, split weights between holes for more accurate
solution
If removing the spinner, index it so it can be reinstalled in
the same location and orientation
Beware of different length bolts used in weight placement or
spinner attachment
Use the engine manufacturers instructions
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Fundamentals of Balance
Where to place the weights
NOTE: If your balancing equipment gives you
specific bolt or hole locations and class weight
information for addition of trim weights, these steps
are not necessary.
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Fundamentals of Balance
Where to place the weights
1. Index the fan back to the
point where the tach event
occurs. (0 or 360 degrees)
2. Rotate the fan assembly, in
the direction of rotation, the
number of degrees specified in
the balance solution. The
location for the weight is now
adjacent to the vibration
sensor.
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Fundamentals of Balance
How is weight split between holes?
Compute weights between holes so that the total
effective weight is the same as the desired weight
If class weights (a fixed set of available weights are
used on the engine), use an optimizing algorithm to
select the best weight combination to approximate
the desired weight.
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Fundamentals of Balance
Summary of Steps:
Install vibration sensor
Install the once per revolution tachometer
Perform a vibration survey and verify balance necessary
Remove any existing trim balance weights
Collect magnitude and phase data for each speed of
interest
Compute weight necessary for balance
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Fundamentals of Balance
Summary of Steps: (continued)
Convert the solution into a class weight for the
required weight and to a bolt or hole number for
angle.
Present the solution to user as “Place -2 weight at
hole #3" for instance
Ask user for the actual weights installed
Run the engine again and collect vibration data.
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the
Fundamentals of Balance
Summary of Steps: (continued)
Verify predicted results.
If acceptable, balance job is complete
If not, compute refined estimate of the influence
and continue the process at computing weight
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Why Is Balancing Beneficial?
Reduces vibration and noise induced stress on crew
and passengers.
Airframe, avionics, and engine systems sustain less
damage. Useful life is extended.
Operational efficiency is increased because the energy
previously used to generate noise and vibration is now
used to generate thrust.
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Review
Vibration
What is vibration
Examples of vibration and related complaints
Effects of vibration
How vibration is measured
How vibration is analyzed for balancing
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Review
Why is Balancing Beneficial
Reduces vibration and noise induced stress on
crew and passengers.
Airframe, avionics, and engine systems sustain
less damage. Useful life is extended.
Operational efficiency is increased because the
energy previously used to generate noise and
vibration is now used to generate thrust.
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Conclusion
If you’ve already bought additional insulation
for your company jet to reduce noise, you didn’t
get what you paid for. For much less money
and time a Fan Trim Balance is the best choice
for a quiet smooth engine.
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Contact
www.acessystems.com
1-865-671-2003
[email protected]
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