Theory of Operation
Mode l8500C/8500C+ Operators Manual
2-3
The vibrational forces created by a lateral mass imbalance are transferred with equal intensity to th
opposite end of the spinning shaft and from there to any components the wobbling spindle may
contact. In large, rapidly spinning rotor systems such as helicopters and propeller airplanes, this
phenomenon may be felt by passengers and crew as an uncomfortable, resonating “buzz” caused by
the propagation of vibrations through the air frame and cabin. A second consequence is far more
serious, that is, damage to system hardware. If an excessive rotor imbalance is left untreated,
components that are continually subjected to the associated vibrational energies can, over a period o
time, suffer wear, abrasion, fatigue, or even breakage. Such damage is at best costly in terms of aircraft
maintenance, and at worst dangerous to the ship and its occupants.
How can we locate and neutralize a lateral mass imbalance in a rapidly spinning disc? Since we know
that the weighted edge of the rotating disc must be at the top of its rotary travel when the vibrating
spring reaches the point of maximum expansion (positive amplitude), and at the bottom of its arc when
the spring reaches the point of maximum contraction (negative amplitude), it follows that the location
of the weight can be determined from the spring position alone. That is, if we could stop the spinning
disc at the point of maximum vibrational amplitude, the weighted edge would appear at the top of its
trajectory.
The expansion and contraction of the spring can be measured as positive and negative vibration
amplitude, respectively, and plotted in terms of displacement versus time (se e Figur e2-3 on page2-4).
The position of the disc at any given moment is called its
angular position
(expressed in either phas
angle or clock angle). Angular position is a measure of the relationship between a given point on th
edge and some fixed artificial reference (also called
azimuth
) and is computed in terms of the central
angle subtended by the two points. The angular position of the disc thus tells us exactly where th
destabilizing mass is located. Moreover, the magnitude of the vibrational amplitude, that is, the amount
of expansion and contraction seen in the spring, is directly related to the weight of the mass.
With this information, we can now pinpoint the source of the imbalance and either remove the
appropriate amount of weight or add a counterbalancing weight to the opposite edge of the disc. Either
action can correct the out-of-balance condition.
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