Turbine Fan Trim Balance

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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 gs
• 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 analyzers 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 300 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 BalanceWhere 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 the 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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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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ReviewVibration

• 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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ReviewWhy 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 youve already bought additional insulation
  for your company jet to reduce noise, you didnt
  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
• sales_at_acessystems.com