The following blog relates to those who field balance using a photo or laser tach and reflective tape.
By far the most common pitfall to field balancing is a problematic tach signal. When one balances a rotor using one’s field balancing unit (VIBXPERT® II, VIBXPERT, or VIBSCANNER®) the equipment is recording the energy displayed at the frequency of the signal from the tachometer. To help visualize the importance of a clear tachometer signal that is exactly 1 pulse per revolution, look at figure 1.
What amplitude will your equipment record if the tach pulses:
1. 1,195 times per minute?
2. 2,002 times per minute?
3. 2,006 times per minute?
4. 2,011 times per minute?
5. 2,013 times per minute?
We often start a balance job by haphazardly placing our tach and tape. Because both the tach and tape are well-engineered, we may go on without a problem. But just a little attention to some of the common tach signal problems is usually all it takes to avoid having to restart a botched attempt at field balancing. What should be avoided when setting up a tachometer?
1. Don’t place your tach too close to the rotor. Most tachometers are used in the fieldwork sending some type of light out and bouncing it back, so they have a sending function and a receiving function. The wavelength of the light is such that not just any light will be accepted by the receiver, but only that wavelength of light sent out by the sending unit. So the receiver counts a pulse every time that wavelength of light appears (or disappears, depending on whether you are triggering by leading or trailing edge). The receiver is no smarter than that, we must supply the rest of the intelligence. When we put the receiver too close to the rotor, even a poor reflector may be able to bounce back enough of the light signal to create a pulse. The balancing technician should determine the distance from the rotor to set up their tach with the understanding that they want a good signal bounced back from their chosen reflector AND ONLY THEIR CHOSEN REFLECTOR! Most often, a 6-inch space is sufficient.
2. Don’t place your tach pointing perpendicular to the rotor. Earlier we stated that “both the tach and tape are well-engineered”. One thing most of us field balancers take for granted is the reflective tape. This tape is actually a well-engineered tool. Reflective tape is faceted in such a way that light can strike it at an acute angle, and still be reflected right back along the axis from which it came. This allows the tach to be staged at such an angle that light will strike the rotor, even a rotor that is itself a good reflector, and be reflected off and away from the receiver UNTIL the tape comes into the line of the light, and then with its special faceting, it will bounce the light back to the receiver. This gives one clean pulse every time the tape comes around, and only when the tape comes around.
3. Don’t use old reflective tape that may not be in proper working condition. Make sure the tape is clean and in good shape. Reflective tape works very well when it is clean and in like-new condition, but can get dirty or even deteriorate if conditions are right. Replacing a small piece of tape is most often very quick, easy, and cheap compared to extra balancing runs or possibly even worse.
4. Don’t use a tach with dirty lenses. Make sure the tach lenses are clean and in good shape. When your lens is dirty, it forces you to do things (in order to get a strong enough signal to go through the dirty lens) that aren’t conducive to a clean, clear, once per revolution pulse; like move the tach too close to the rotor, or place it at a 90° angle to the rotor.
Doing everything we have suggested here could take all of 5 minutes (if you work slowly) at the beginning of a field balance job, but it could save a lot!
Balancing, Condition Monitoring, Vibration Analysis by Mike Fitch CRL