Fan Trim Balance

Fan Trim Balance By Jerry Justice June 1999 The first indication of an out-of-balance fan in a business jet is, more often than not, a complaint from the guy sitting in the plush, white leather passenger seat. Although an engine meets or...

Check the calibration or data sheet supplied with the sensor and make note of the sensitivity. Sensitivity is the amount of voltage output from the sensor relative to an engineering unit of measured vibration. For instance, a velocity sensor might have a sensitivity of 20 mV/IPS (20 millivolts per Inch Per Second). This means that the sensor will produce 20 millivolts for every Inch Per Second of velocity transferred to the sensor. Your equipment should have the most common sensors used with the engine preloaded in the balance program. If not, you may be required to enter the sensitivity manually. With any sensor, you must also be acutely aware that the method by which they are attached to the engine (the sensor mount) can make or break the effectiveness of an entire system.

If the mount resonates at any of the operating speeds being checked, it induces a false vibration signature into the sensor and, of course, back to the analyzer. The material, size, weight, and construction of the mount all play a role in its resonance response. Here again, if the engine or aircraft manufacturer recommends a particular mount for the sensor, use it. Don't look for the easy way out. If the sensor mount is expensive, complicated, and hard to install, it's the price you pay for accurate data.

Speed (Phase Reference) Equipment
When it comes to collecting phase data there are four basic methods being used today:

A strobe light gives a visual relationship of where the out-of-balance mass is located on propellers and some turbofan engines. You'll need two technicians to do the balance with the strobe light — one to run the engines from the cockpit, and the other to stand in front of the running engine to operate the strobe. Collecting accurate phase data by this method is a hit and miss prospect. Remember that it's a visual reference only and a difference of ten degrees is hard to estimate at a distance of thirty or forty feet. The strobe cannot be used with equipment that calculates the balance solution automatically because the strobe is not capable of supplying a phase reference to the analyzer. This means you, the operator, must calculate the phase angle for adding weight.

A more advanced method of collecting speed and phase data is the photoelectric cell. It uses a strip of reflective tape placed on the spinner as a trigger. It is usually mounted somewhere in the intake of the engine by opening the cowling, removing an inlet sensor, and placing the photoelectric cell in the hole vacated by the sensor. It must be firmly positioned to project its beam on the reflective tape as it passes the photocell's position. The reflected light then triggers a signal as it is received back at the photocell. The photocell sends the once-per-revolution signal to the balancing equipment, which is then used to automatically calculate the phase angle for adding weight. The time required in opening and closing the cowling, removing the inlet sensor, and mounting the photocell can add an hour or more to an otherwise short job. Another limitation of the photocell is its range. Optimum range for the photocell is 12 to 18 inches from the reflective target.

A large step up from the photocell is the laser tachometer. It works on the same principal as the photocell with a strip of reflective tape but proximity to the tape is extended out to 30 feet. The laser is attached to a locking swivel head, which is then mounted on top of the wing or on the side of the fuselage with duct, or speed tape. The laser beam is aimed at the reflective tape attached to the spinner and the swivel mount locked in that position. This eliminates the need to open cowling or remove sensors. As with the photocell, the Lasertach sends the once-per-revolution signal back to the analyzer. The analyzer uses the input to calculate a phase angle and RPM. The photo on page 48 shows the reflective tape positioned for both a laser tachometer (tape near the tip of the spinner) and the photo electric cell (located nearest the fan blades). The eight bolts shown in the spinner assembly hold the spinner in place. When trim balance weights are required, the designated bolt is removed and a specific class weight similar to a washer is placed on the bolt. The bolt is then reinstalled.

Newer engine designs are using variations in a single tooth design on a phonic wheel as a once-per-revolution signal generator. The single tooth may be missing, shorter, longer, or offset when compared to the other teeth on the wheel.


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