Regardless of which of these systems you use, the speed and phase angle source provides several pieces of the balancing puzzle. The speed portion is obvious. It provides an indication of the rotation frequency (RPM) of the fan so that you may adjust controls to match the frequency of interest, or required balancing speed. The balancing equipment also uses this input to synchronize the out of balance mass passage with the actual speed of the fan.
An "overall" or "composite" vibration reading is the sum total of all vibrations being generated. The oscillation event can only occur once per each revolution of the fan. When the vibration sensor measures that oscillation event and compares it to the input of the tachometer, all non-synchronous vibration events can be electronically filtered out leaving only the vibration attributed to the condition of the fan. And finally, the phase angle information provided to the analyzer is used to calculate the radial location of the dominating mass (the heavy spot on the fan causing the oscillation) relative to a known fixed point, usually the vibration sensor.
The balance procedure
As stated before, equipment varies in many aspects. Refer to the maintenance manual for recommendations on balancing equipment. Be sure you have a sufficient supply of the class weight set for the engine. Most newer engine models incorporate a dual-purpose bolt that attaches both the spinner and the balance weights to the engine. There are design variations in engines and engine sub-models that require the spinner to be removed to add or remove trim balance weights.
Weights may also be attached with rivets or may necessitate changing a null weighted screw with a specifically weighted one. Later models such as the TFE731-60 require only the bolt to be removed when adding weights. Some engine manufacturers number the weight locations relative to an indexed point. In the case of the TFE731, the holes are not numbered.
The technician conducting the balance job picks any one of the holes and designates it as No. 1. The placement of the reflective tape along an imaginary line between the leading edge of the hole and the tip of the spinner insures that the optical tachometer will trigger as the hole, now designated as No. 1, passes the light beam.
If holes on the engine you are balancing are numbered by the manufacturer, review the maintenance manual for specific guidelines on tape placement. Since the laser can be positioned or aimed to trigger at any desired point in the engine rotation, we will define its point of trigger at the six o'clock position as viewed from the front of the engine looking into the intake.
When the position for the tape is defined, clean the area with a quality degreaser, then dry the area thoroughly before applying the tape. Make sure there are no bubbles or loose edges in the tape. These act as airfoils at high speed and will cause the tape to come loose which results in the loss of tach signal.
When the tape is attached, rotate the fan until the tape is at the six o'clock position. Mount the laser tachometer on top of the wing. Power the laser from the analyzer and aim the laser beam to strike the reflective tape midway in its length. Tighten the mount and recheck the alignment. Adjust as necessary.
For this example, we will place the vibration sensor at the twelve o'clock position. Again, be sure to use the recommended sensor mount. Cables from the tachometer and the vibration sensor are routed along the fuselage to the cockpit and secured about every two to three feet with duct tape. The cables are connected to the analyzer then the analyzer is placed in the cockpit where it will be operated by the technician running the engines.
Fan Trim Balance
At what speed do you bal-ance?
If you are attempting to stop a vibration or noise at a reported speed (RPM), balance at the speed where the problem is reported. Fan-generated vibration and noise usually occurs at a specific power setting, but may continue to varying degrees throughout a power range.
For routine balancing, engine vendors will usually specify a speed, RPM, or power setting at which the balance should be performed. This isn't always the best path to a solution for the problem. Vibration changes in amplitude as speed changes. It may even reoccur at multiples of the problem speed.