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When balancing an overhung rotor, one of the two following procedures should be taken:
1 BALANCING OVERHUNG ROTORS BY CLASSIC SINGLE-PLANE STATICCOUPLE METHOD:
Figure 42 helps explain methods of balancing overhung rotors. Classically, Bearing A is most sensitive to static unbalance whereas the bearing farthest from the fan wheel to be balanced (Bearing B) is most sensitive to couple unbalance. Since Plane 1 is closest to the rotor center of gravity (C.G.), static corrections should be made in this plane while measuring the response on Bearing A. On the other hand, measurements should be made on Bearing B when making couple corrections in Plane 2. However, placing a trial weight in Plane 2 will destroy the static balance achieved at Bearing A. Therefore, in order to maintain he static balance at Bearing A, a trial weight placement which will generate a couple must be used. Thus, a trial weight of identical size should be placed in Plane 1 at an angle 180° opposite the trial weight in Plane 2。
Therefore, either the data collector can be used using single-plane balance software or the single-plane graphic technique previously explained can be successfully employed on many overhung rotors, particularly if the ratio of rotor length-to-diameter (L/D) is less than approximately .50 (where L is length of the rotating component on which correction weights will be placed and D is the diameter of this component.
Following below will be a description of this classic single-plane balancing technique for overhung rotors.
a. SET UP DATA COLLECTOR OR SPECTRUM ANALYZER INSTRUMENT
The data collector, photo-tach and transducer should be set up as previously described under Sections K & L and Figure 40 showing the two-plane balancing procedure. Alternatively, the analyst may wish to employ either a swept-filter analyzer that drives a strobe light, or a spectrum analyzer which will fire a photo-tach for phase measurements.
b. TAKE INITIAL MEASUREMENTS
Take initial measurements of 1X RPM amplitude, frequency and phase before adding any trial weights. Measurements should be taken on both the outboard and inboard bearings in both vertical and horizontal directions. The radial direction measurement having the highest amplitude will normally be employed for initial balancing (however, after correcting unbalance in the radial direction, measurements will have to taken in the other directions to ensure amplitudes in all directions likewise acceptable).
c. DETERMINE IF THE DOMINANT UNBALANCE PROBLEM IS STATIC OR COUPLE UNBALANCE
Looking at the amplitude and phase measurements taken on both bearings in the radial and horizontal directions, determine if the problem is dominated by either static or couple unbalance. If phase differences between the outboard and inboard bearings are between 90° and 180° in both the vertical and horizontal directions, the dominant problem will be couple unbalance. On the other hand, if these differences are both anywhere from 0° to approximately 40°, a static unbalance is dominant. Of course, phase differences ranging from approximately 40° to 140° are truly dynamic balance once again with a combination of static and couple. If the problem appears to be mostly couple unbalance, use couple unbalance procedures outlined below. However, if the problem appears to be predominantly static or dynamic unbalance, employ static balance procedures. For now, we will assume that the problem is mostly static.
d. MAKE A SINGLE-PLANE STATIC BALANCE
Referring to Figure 22, use single-plane techniques taking measurements on bearing A and placing trial and correction weights in Plane1.
e. DETERMINE IF RESULTANT VIBRATION AMPLITUDES MEET REQUIRED CRITERIA
After completing the single-plane static balance using Plane 1, repeat vibration measurements on both the outboard and inboard bearings in each direction (including axial) and ensure that amplitudes now meet allowable criteria.
f. IF CONSIDERABLE COUPLE UNBALANCE NOW REMAINS, CONTINUE WITH SINGLE-PLANE BALANCE FROM BEARING B
Overhung rotors often have large cross-effects which means that single-plane balancing from Plane 1 will often cause high vibration on bearing B. Therefore, the analysts will perform another single-plane balance, this time making their measurements from bearing B farthest from the component to be balanced. When they arrive at the single-plane correction weight solution, they should place this weight in Plane 2; and then place an identical size correction weight in Plane 1 180° away from the weight location in Plane 2.
g. DETERMINE IF AMPLITUDES NOW MEET ALL CRITERIA
After completing the single-plane couple correction, the analyst must again make measurements in horizontal, vertical and axial directions on each bearing and determine that all amplitudes now meet allowable criteria. Often, further balancing must be done at this point beginning with another single-plane balance using Bearing A and Plane 1 which might possibly be followed by another couple balance correction.
h. IF ALLOWABLE CRITERIA CANNOT BE MET IN ALL THREE DIRECTIONS OF EACH BEARING, PROCEED TO TWO-PLANE BALANCE PROCEDURE OUTLINED BELOW
Sometimes, this single-plane approach will not successfully reduce amplitudes below allowable criteria in all three direction on each bearing, particularly if the L/D ratio is greater than .5 or if the component to be balanced is located far away from the closest bearing. If this happens, two-plane techniques outlined below will have to be taken.
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发表于 2011-7-7 08:28
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