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CHECKING OR PROVING THE

BALANCE LEVELS IN ROTORS

There are many times when it is necessary to check to see if a specified balance tolerance has been met, to verify that a balancing machine is performing properly, or to convert measurement readings such as Mils to balance terms of ounce inches or gram inches

It is often necessary to state the unbalance level achieved in a balancing machine in terms that are different than those that were used to balance the rotor in the first place.For example, the customer may ask for the final balance level in ounce inches when the rotor was balanced in terms of Mils in the balancing machine. Another case that happens often is the customer states the balance level required and then asks that you prove that the level was achieved.

It is a simple matter to convert mils to ounce inches or gram inches or to show that the balance level expected has been achieved.The following Step by Step procedure shows you how, and is applicable regardless of the type of equipment being used or the readout terms being used.

STEP 1

Balance the rotor in the balancing machine in the normal manner to a level which is deemed acceptable.

STEP 2

Select a test weight to be added to one plane of the rotor.This weight should be large enough to produce a readout that is a least five (5) times the final reading deemed acceptable in Step 1.Also, the following test should be run in both planes, one plane at a time.

Record the amount of weight = ________ (grams/ounces).

Record the radius from the center of the shaft at which the weight

is added.Radius = ___________ (inches/mm).

Multiply test weight times the radius =___________ (oz-in, gram-in, gram-mm)

Add the test weight to the rotor and call the first location 0 degrees for record keeping purposes.

STEP 3

Spin the rotor and record the amplitude in the table on the following page.A table has been provided for the two most common tests (8 holes or 45 degrees and 12 holes or 30 degrees).The amplitude can be in any terms ( mils, in/sec, ounces, grams, volts, etc.). Stop the rotor.

The table provides for space for recording the test for both the right and the left plane if required (A single plane rotor would require only one set of readings).

STEP 4

Remove the test weight from the original 0 degrees position and move to the next location (30 or 45 degrees), making sure the radius remains the same and spin the rotor.Record the amplitude in the table provided.

STEP 5

Continue to move the weight around the rotor and recording the amplitude until

all test data is completed.

            ROTOR WITH 8 HOLES FOR WEIGHTS                          ROTOR WITH 12 HOLES FOR WEIGHTS

STEP 6

Review the Table and determine the highest(Hi) and lowest (Lo) readings for both planes.Using the formula;

Residual Unbalance (Ur) = Test Weight (in oz-in, gr-in, etc.) x (Hi -Lo)/(Hi+Lo).

Example:Test Weight =(6.4 ounces) x(Radius = 5.75 inches)=36.8 ounce inches

Highest reading in the left plane = 11 mils

Lowest reading in the left plane = 9 mils

Residual Unbalance(Ur) = 36.8 ounce inches x (11-9)/ (11 +9)

= 36.8 x 2/20

=3.68 ounce inches for the left plane.

Calculate Ur for the Right Plane also.Record:

Left Plane ____________Right Plane ____________

STEP 7

Compare Ur to the specified tolerance.If Ur for each plane is less than the specified tolerance, then the rotor is at an acceptable level of unbalance.

Note:This test is similar to the ISO Standard test for proving the accuracy of balancing

machines, however, in that case it would be necessary to plot the readings to

graphically display a sine wave.

A simpler, less accurate method of converting mils to ounce inches or gram inches is provided in last weeks article CONVERTING AMPLITUDE READINGS (MILS, IN/SEC, G’S) TO UNBALANCE TOLERANCES (OZ-IN, GRAM-IN, GR-MM)

Although C D International, Inc believes reasonable efforts have been made to ensure the accuracy of the information contained in this document, it may include inaccuracies or typographical errors and may be changed or updated without notice. It is intended for discussion and educational purposes only and is provided "as is" without warranty of any kind and reliance on any information presented is at your own risk.