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Machinery Balancing Blog

Balancing Armatures to the EASA Standard

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IN ANTICIPATION OF THE UPCOMING EASA CONVENTION IN TORONTO, IT'S APPROPRIATE TO PUBLISH AN ARTICLE ON BALANCING TO THE EASA STANDARD. VISIT US AT BOOTH 305.


BALANCING ARMATURES TO THE EASA STANDARD

Section 2.7 of the EASA Standards covers the balancing of armatures.The following discussion will provide a quick way to calculate and apply the Standard. If your balancing machine does not read out in units of weight (ounces, grams) look at paper titled CONVERTING AMPLITUDE READINGS(MILS, IN/SEC, G’S) TO UNBALANCE TOLERANCES (OZ-IN, GRAM-IN, GR-MM) to assist in using the Standard.


The EASA Standards for the Repair of Electrical Apparatus includes a section (Section 2.7) on balancing.This reads as follows;

“Dynamic Balancing should be to the level specified by the customer.In the

absence of a requested level, dynamic balancing to balance quality grade G2.5

(ISO 1940/1) should enable the machine to meet final vibration limits as defined

in 6.4.6.”

Reference:EASA Standards dated February, 1995, Section 2, Page 2.

The following procedure presents an easy way to calculate the tolerance required for an armature based upon the referenced specification (Quality grade G2.5 (ISO 1940/1).

DISCUSSION

To understand and apply the referenced ISO Standard to rotor balancing requires a knowledge of three facts; the speed of the rotor, the dimensional shape of the rotor and the weight of the rotor.Applying the Standard to motor armatures is simplified since the speed is always known and the dimensional shape is generally symmetrical.This allows for a simplified approach for easy calculation of the tolerance based upon weight.

For reference only, the general formulas for calculating unbalance tolerances to the ISO Standard are as follows;

(a) Acceptable Level of Unbalance in ounce inches = 6.015 x G x W/N

(b) Acceptable Level of Unbalance in gram inches= 170.5 x G x W/N

(c) Acceptable Level of Unbalance in gram mm=9549 x G x W/N

were G is the balance quality grade from the ISO Standard

W is the weight of the rotor in pounds (a & b) or Kgs (c)

N is the operating speed of the rotor

and the acceptable level of unbalance calculated is total for the rotor.

Since the EASA Standard states the Quality Grade required (2.5) and most motor armatures are symmetrical (allows the total unbalance to be divided by 2) and RPM is always known, we can easily calculate the tolerance based on weight.

As an example, to calculate ounce inches of unbalance for a 3600 RPM symmetrical armature;

Acceptable level on balance per plane in ounce inches  = 6.015 x G x W/N

2

= 6.015 x 2.5 x W/3600

2

= .004177 x W / 2

= .002088 x W

For any 3600 RPM motor armature, the balance tolerance per plane can be determined from this formula by simply knowing the weight.

The following chart will show the simplified formulas for 3600, 1800, 1200, 900 and 600 RPM as well as several examples.

FORMULA 3600 RPM 1800 RPM 1200 RPM 900 RPM 600 RPM

Allowable Unbalance per plane (oz - in) =

.002089 x W

.004177 x W

.00627 x W

.00835 x W

.01253 x W

Allowable Unbalance

per plane (gm - in) =

. 0592 x W

.1184 x W

.1776 x W

.2368 x W

.3552 x W

W = Weight of the Rotor in pounds

EXAMPLES

Unbalance Tolerance per plane in both ounce inches and gram inches


Rotor Weight
in Pounds
3600RPM1800RPM1200RPM900RPM600RPM
oz-in/
plane
gm-in/
plane
oz-in/
plane
gm-in/
plane
oz-in/
plane
gm-in/
plane
oz-in/
plane
gm-in/
plane
oz-in/
plane
gm-in/
plane
2500.52214.81.04429.61.56844.42.08859.23.13388.80
5001.04529.62.08959.23.13588.84.175118.46.265177.6
10002.08959.24.177118.46.270177.68.350236.812.53355.2
20004.177118.48.354236.812.54355.216.70473.625.06710.4
25005.223148.010.44296.015.68444.020.8859231.33888.0
40008.356236.816.71473.625.08710.433.40947.250.121421
500010.45296.020.89592.031.35888.041.75118462.651776
750015.67444.031.33888.047.03133262.63177693.982664
1000020.89592.041.77118462.70177683.502368125.33552
1500031.34888.062.66177694.052664125.33552188.05328

PROCEDURE

STEP 1.

Select the proper formula above based on the RPM to balance a rotor to G2.5 from the ISO Standard and comply with the EASA Standard.

STEP 2.

Weigh the rotor and calculate the proper tolerance per plane from the formula.

STEP 3.

When the proper tolerance has been calculated, divide the tolerance by the radius at which weight will be added.

EXAMPLE:

The unbalance tolerance for a 1000 pound, 3600 RPMrotor is 59.2 gram inches per plane.

The radius to the balance ring is 10 inches.

Weight = unbalance tolerance / radius = 59.2 / 10= 5.92 grams per plane

STEP 4.

Balance the rotor until the unbalance tolerance has been met.

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.

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