index balancing

Introduction to Machine and rotor Balancing with free Software

Introduction to Balancing

Since unbalance can not be measured directly, the force resulting from unbalance or thevibration it causes are measured instead. Vibration and force are used on balancing machines.The use of vibration is for in-place balancing. It seems to work well when all the conditions aresatisfied. Rather than mention vibration and force repeatedly just vibration is to used in furtherdiscussions.

Balancing is easy if provided the vibration measured is due to solely to unbalance. If there areother vibration causes present at the same time and at the same frequency things get difficult.This can happen a lot, unfortunately. Balancing on a stand is normally easier than in-placebecause the stand is designed to support and spin the rotor with minimum effect from outsidesources but in-place balancing is distinct.

Since in-place balancing has many advantages a lot of attention is paid to this topic; however,there are bad things than can occur. If eccentricity, misalignment, looseness and structuralnon-linearities produce vibrations that are confused with unbalance vibration then balancingcauses may be difficult to achieve. Special attention should be paid to this subject.

The act of balancing is consisted in various steps; calibrating the system, measuring the initialunbalance, making corrections, and verifying the results.

Unbalance Sources

Unbalance is generally invisible. Unbalance is the result of various minor imperfections thatcauses an unequal weight distribution on the rotating centerline. Below are some of the morecommon sources of unbalance: Dirt deposit buildup Eccentric bore Tapped fluids and moisture Unequal parts assembly radius Keys & Keyways

1 / 22


Eccentric machining Welding distortion Wear and corrosion Blowholes and inclusions

The buildup of these imperfections causes unbalance. The most precisely made part canhave an unacceptable unbalance.

A very common source of unbalance results from the buildup of clearances and toleranceswhen parts are assembled into a final rotating unit. The resulting unbalance is illustrated in theexample of an eccentric rotor installation illustrated on the next page.

Balance Machine

A balancing machine is a measuring tool used for balancing rotating machine parts examplesincluye rotors for electric motors, fans, turbines, disc brakes, disc drives, propellers andpumps. The machine machine generaly consists of two rigid pedestals, with suspension andbearings on top supporting a mounting platform. The item under balance is bolted to theplatform and rotated. As the item is rotated vibration in the suspension is detected withsensors and data is utized to determine the amount of unbalance in the part. Along with phase information, the balance machine can determine how much and where to add weights tobalance the part.

2 / 22


How It Works With an item (rotating part) resting on the bearings, a vibration sensor is attached to thesuspension. In some machines, a velocity sensor is used. Accelerometers, which measureacceleration of the vibration, can also be used.

A photocell also called a phaser, or key phaser, proximity sensor, or encoder is used todetermine the rotational speed and relative phase of the rotating part. This phase informationis used, filter the vibration information to determine the amount of movement, or force, in onerotation of the part. Also, the time difference between the phase and the vibration peak givesthe angle at which the unbalance exists. Amount of unbalance and angle of unbalance give anunbalance vector.

Calibration, performed by adding a weight at a given angle. In a soft bearing machine, trialweights must be added in correction planes for each part. This is because the location of thecorrection planes along the rotational axis is unknown, and therefore it is unknown how mucha given amount of weight will affect the balance. By using trial weights, you are adding aknown weight at a known angle and getting the unbalance vector caused by it. This vector is then compared to the original unbalance vector to find the resultant vector, which gives theweight and angles needed to bring the part into balance. In a hard-bearing machine, thelocation of the correction plane must be given in advance so that the machine always knowshow much a given amount of weight will affect the balance.

Types Of Balance Machines Static balancing machines differ from hard- and soft-bearing machines in that the part is notrotated to take a measurement. Rather than resting on its bearings, the part rests vertically onits geometric center. Once at rest, any movement by the part away from its geometric centeris detected by two perpendicular sensors beneath the table and returned as unbalance. Staticbalancers are often used to balance parts with a diameter much larger than their length, suchas fans. The advantages of using a static balancer are speed and price. However a staticbalancer can only correct in one plane, so its accuracy is limited. A blade balancing machine attempts to balance a part in assembly, so minimal correction isrequired later on. Blade balancers are used on parts such as fans, propellers, and turbines.On a blade balancer, each blade to be assembled is weighed and its weight entered into abalancing software package. The software then sorts the blades and attempts to find theblade arrangement with the least amount of unbalance. Portable balancing machines are used to balance parts that cannot be taken apart and put ona balancing machine, usually parts that are currently in operation such as turbines, pumps,and motors. Portable balancers come with displacement sensors, such as accelerometers,

3 / 22


and a photocell, which are then mounted to the pedestals or enclosure of the running part.Based on the vibrations detected, they calculate the parts unbalance. Many times thesedevices contain a spectrum analyzer so the part's condition can be monitored without the useof a photocell and non-rotational vibration can be analyzed.

Phase

The rotating force has a circular frequency of ? = 2pf, and each revolution is equal to 360degrees or 2p. A quarter of a revolution is 90 degrees and so on. Point A is 90 degrees fromB. Point C is 180 degrees from A. Since point A was selected as the starting point, all otherpoints have a phase angle related to A at 0 degrees.

There is a 90 degree phase difference between the dashed line waveform and the solid linewaveform. Since the dotted waveform starts later in time, its phase is 90 degree lagging.

Occasionally, phase is measured using a stroboscopic light that is synchronized to flash onceevery revolution of a shaft. A keyway or chalk mark will appear to stop at a fixed angle.

Phase can also be measured using a reference pulse from a reflective tape strip. A circuitmeasures the time to reach the waveform’s peak displacement as the tape passes under aphotocell. The angle will be displayed on a digital meter.

Protractors and Angles

The baseline is the straight line through the index from 0 degrees CW to 0 degrees CCW. Theindex is the intersection of the vertical line at 90 degrees and the baseline.

To measure an angle with a protractor the following steps need to be used and see theillustration below. 1. First place the protractor’s index at the intersection of the lines that from the angle to be

4 / 22


measured. 2. At the same time it is necessary to align the protractor’s baseline over line A. 3. Then find the angle at which line B crosses the protractor’s angle scale. 4. Finally, read the angle paying attention to the direction in which the angle increases.

Vectors and Scales

Vectors are utilized to represent values that have both direction and amount, such asunbalance, force, velocity, vibration and acceleration.

Using an appropriate scale, amount is shown as the vector’s length.

Direction is the vector’s angle that is always in degrees, from 0 to 360 degrees.

Vector length is measured in millimeters or inches that is not a unit of measure for values like,unbalance (oz-in), force (pounds), or vibration (g, mils, or in/sec.) so some scale of length perunit of vector value is used.

For example, the vector, starts at its origin point in the direction of the right side of the page. Itis divided into seven equal parts and each part is ¼ of an inch long. A vector value of one milis assigned to each ¼ inch and the scale is .25 inch per mil.

The direction scale is set with 0 degrees vertically on the page with the angles increasingclockwise. Therefore, the direction of the sample vector on the right is 90 degree. This vectoris 7 mils at 90 degrees.

A second vector of 5 mils at 135 degrees is exposed at the right. The same .25 inch per milscale is used.

5 / 22


The same vectors can represent forces in pounds. For example, each ¼ inch can represent10 pounds. The scale would become .025 inch per pound or .25/10. The two vectors would be70 pounds at 90 degrees and 50 pounds at 135 degrees.

The scale must be subdivided into smaller increments if the vector amount is a number like5.85. If possible, the subdivisions would be in increments of .1 or .2 like shown below.

When you are working with more than one vector the following rules apply: All of the vectors must represent the same value, for example vibration in mils. Each vectors need to be drawn to the same scale. 1 mil = 1 inch, for example. The angles must start with 0 degrees at the same place, vertical, and increase in the samedirection, clockwise.

Rigid Versus Flexible Rotors Defined

Introduction

Rotors are classified according there balancing requirements. A rotor is rigid when itsunbalance may be corrected in any two arbitrarily selected correction planes and after thecorrection its residual unbalance does not change significantly at any speed up to themaximum service speed and when running under conditions that closely approximate the finalsupporting system.

A rotor is flexible when it does not satisfy the definition above due to the elastic deformation.

Rotors that operate approximately below 70 percent of their first rotor bending mode are rigidand rotors operating above 70 percent of that critical are flexible. Observe the illustrationbelow.

So to determine a rotor’s balancing requirement it is necessary to determine if the rotor

6 / 22


operates or not below 70% of its critical speed.

Flexible Rotor Classifications

The American national standard S2.42.1982, ¨Procedure for Balancing Flexible Rotors¨ dividethe rotors into 5 main classes. Each class needs a different balancing technique. The fivemain classifications are outlined as a general reminder. A careful study of the standard isrecommended for a full understanding. Below some useful terms are defined.

Class 1– Rigid Rotors Rotors with unbalance that can be corrected in two arbitrarily selected correction planes andafter that unbalance does not significantly change at any speed up to maximum servicespeed.

Class 2- Quasi-rigid Rotors Rotors that can’t be considered rigid but may be balanced utilizing modified rotor balancingtechniques. These rotors are classified as to whether unbalance distribution is either known orunknown with effective balancing techniques for each. ¨Quasi¨ means ¨almost but not quite¨having dynamic behavior of a rigid rotor.

Class 3- Flexible Rotors Rotors that can0t be balanced utilizing modified rigid rotor balancing techniques but need theuse of high speed balancing methods like larger four and two pole generator rotors.

Class 4- Rotors with Flexible Components Rotors that can qualify as class 1, 2, or 3 but have component that are flexible or withflexibility attached. A motor with a centrifugal switch for example.

Class 5- Single speed Flexible Rotors Flexible rotor balanced for only one speed of operation.

7 / 22


Single Plane Balancing

Discussion Single-plane balancing is the procedure in which the mass distribution of a rigid rotor isadjusted to ensure that the residual static unbalance is in specified limits or when the balancecorrection are restricted to only one correction plane.

Rotors with only static unbalance will be narrow ones that are mounted perpendicular to thejournal centerline in practice but in many cases couple unbalance is not within specified limitsand single-plane balancing will not be effective. Single-plane balance is usually chosen sinceit seems easier and less time consuming. Single-plane balancing can happen on parallel knifeedges in a balancing machine or in-place.

Setup - In-place Balancing First install a vibration pickup at the support bearing with the highest vibration possible. For details see sensor placement for balancing. Then establish a phase reference system For details see phase reference systems, And a scale is necessary for accurate weights. The balance beam or an electronic scale issuitable. The small electronic scales are the best for field use. For details on trial and balance weight materials see correction weights.

Data Measurements and Procedure

Measure Original Readings The machine should run at normal operating RPM or selected balancing speed. The machine and vibrations should be allowed to stabilize before taking data. Vibration amplitude and phase at each bearing should be measured and recorded. Single-plane balancing record form can be used.

Measure Trial Weight’s Effect The machine is to be stopped and added a safe trial weight to the correction plane. Trial weight amount and angular position should be recorded. The machine is to be returned to speed; the same speed for all readings has to be used.

8 / 22


Vibration amplitude and phase should be measured and recorded.

Calculate Balance Corrections The machine is to be stopped and trial weights removed after marking position. Data should be entered (bold face) into a calculator or computer or use vector solution. Correction weights and their angular position for each plane should be determined. Add the balance correction to the rotor carefully

Some balancing instrument will automatically record readings as they are measured and willprovide a solution without entering any data as described.

Measure Balance Correction Weight Effects Machine should be returned to speed. Measure and record vibration phase and vibration. Compare results with vibration or balance tolerances. Assure correction weights are safe and permanent if acceptable. Machine should be returned to operation. Proceed with trim corrections if unacceptable.

Trim Corrections Initial correction weights rarely satisfactory balance a rotor. Additional weights that are calledtrim corrections can be needed to meet vibration and balance tolerances. Heavy sport anglecalibration and unbalance sensitivity constants are used to convert subsequent vibrationreadings into trim corrections.

Trim Procedure Plot or enter latest phase reading or vibration amplitude. Required trim correction should be calculated. Trim correction should be added to the correction plane. As needed, repeat trim corrections until rotor is balanced. Verify that vibration at all bearings is satisfactory and if not, two-plane balancing may benecessary.

Knife Edge Balancing

9 / 22


The static unbalance in a rotor placed on parallel knife edges has downward torque or forcethat overcomes friction forces to turn the rotor. The heavy spot soon settles to the bottom,allowed to turn freely. Place correction weights at the top to balance. The rotor should berepositioned 90 degrees and allow the heavy side to settle to the bottom once again. Moreweight should be added to the top. This should be repeated until the rotor no longer turn. Thecorrection weights have to be applied symmetrically on the plane of the center of gravity.

This procedure may also be done on rolling element bearings on a balancing stand. This is agood way to remove gross static unbalance before spin up in the stand.

Single-Plane Vectors

The trial weight and the trial weight’s effect should be recorded after following the single planebalancing procedure the data for the original readings. A few sample readings are recordedbelow.

Make a copy of the single-plane balancing record and build the vector diagram following thisexample.

Procedure SYM Data Measure Original Readings O 5 mils @ 30° Add a Safe Trial Weight TW 3 ozs @ 0° Measure Trial Weight’s Effect O + T 6 mils @ 150° Calculate Balance Weight’s Effect Bal Wt 1.58 oz @ 33° CCW Measure Balance Weight’s Effect O + T1 1 mil @ 270° Make Trim Corrections Tr + Wt 1.42 oz @ 9° CW Final Vibration Reading O + T2 0.18 mil @ 20°

Vector Equations: Bal. Wt. = TW x O/T Trim Wt. = Bal. Wt. x O/T

Choose a scale for all vectors first. In the example, 1/8” = 1 mil. On the small diagrams theconcentric circles are spaced at 1/8” intervals. Draw the “O” vector using the scale of 1 mil equals 1/8” first. Draw “O” 5 mils (5/8”) at 30o

10 / 22


and label the vector “O”. Then draw the “O + T” vector by using the same scale. Draw “O + T” 6 mils (6/8”) at 150o andlabel the vector “O + T”. Build the “T” vector by connecting the end of “O” to the end of “O + T” and label this vector“T”. The length of the “T” vector should be measured using the same scale of 1 mil equals 1/8”and record the length in mils, in this example T= 9.5 mils. Then measure the angle between “O” and “T” which in this example the angle is 33o. Thesolution is: Bal. Wt. = TW x O/T = 3 oz. x 5/9.5 = 1.58 oz. Angle = 33o CCW

Remove the trial weight and place the 1.58 oz. balance weight at 33o counter clockwise fromits initial position at 0o to balance. Assuming the radius for the trial weight and the balanceweight are equal.

Record the effects of the balance correction weight just added running the machine. For thenext example, the new vibration is 1 mil at 270o. If the vibration is unacceptable furtheradjustment to the balance weight is necessary. The details continue on the next page.

Adjustments to the balance weight will be required if the initial balance weight does notbalance the part well enough.

Draw the “O + T1” to the same scale as the “O” vector was drawn. The diagram scale hasbeen doubled to ¼” = 1 mil.

Build the new “T1” vector by connecting the end of the “O” the end of the “O + T1”.

Using the same scale measure the length of the “T1”. The length should be recorded in milswhich in this example T 1 = 5.56 mils.

11 / 22


Then, using the same scale, measure the angle between “O” and “T1”. The angle is thisexample is 9° CW.

The solution is: Trim Wt. = Bal. Wt. x O/T1 = 1.58 oz. x 5/5.56 = 1.42 oz. Angle = 9° CW

Remove the balance weight and adjust it to 1.42 oz. to make the trim adjustment. The 1.42oz. trim weight should be placed at 9o clockwise from its previous position. Assuming thebalance weight radius is not changed.

Record the results by running the machine and additional trim correction are done the sameway.

Two Plane Balancing Procedure - Rotor between Bearings

Discussion Rotors that are mounted between bearing are balanced by attaching balance weight in twocorrection planes that are selected where the weights can be safely placed. They are normallyadded at the ends of the rotor and near the support bearings. Corrections may be added inany two correction planes for rigid rotors. Multiple correction planes may be required forflexible rotors. For details, see flexible rotor balancing.

Setup Vibration pickups should be installed at each support bearing. For details, see sensorplacement for balancing. A phase reference system should be established. For details, see phase reference systems. A scale is necessary for accurate weights. Balance beam scales utilized for shop balancingare accurate but they are not rugged enough for use in the fields. Small and accurateelectronic scales are more suitable. For derails on trial and balance weight materials, see correction weights.

12 / 22


Data Measurements and Procedure Measure Original Readings Run the machine at selected balancing speed or operating RPM Allow the vibrations and the machine to stabilize before taking data. Vibration amplitude and phase should be measured and recorded before taking data. Observe two-plane balancing record form.

Measure 1st Trial Weight’s Effect The machine should be stopped. A safe trial weight should be added to a correction plane,left or right. Record the trial weight’s angular position and amount. The machine should be returned to speed and the same speed for all readings has to beused. Vibration amplitude and phase should be measured and recorded at each bearing.

Measure 2nd Trial Weight’s Effect The machine should be stopped and remove the trial weight from the previous step. A safe trial weight should be added to the other correction plane. Record the trial weight’s angular position and amount. The machine should be returned to speed. Vibration amplitude and phase should be measured and recorded at each bearing.

To Calculate Balance Corrections First enter the data from previous steps (in bold face) into a calculator or computer. Then determine the correction weights and their angular position each plane. Finally stop the machine and carefully add the balance corrections to the planes. There are balancing instruments that automatically record readings when they are measuredand they provide a solution without entering data.

To Measure Balance Weight Effects Machine should be returned to speed. Vibration amplitude and phase should be measured and recorded at each bearing. Compare the results with vibration or balance tolerances. Assure weights are safe and permanent if acceptable. Machine is to be returned to operation. Proceed with trim corrections if unacceptable.

13 / 22


Trim Corrections The initial correction weights rarely balance a rotor satisfactory. Trim corrections are theadditional weights needed to meet balance and vibration tolerances. Heavy spot anglecalibration and unbalance sensitivity constants used to solve for the initial correction are alsoused on subsequent vibration readings for trim correction.

Trim Procedure The latest vibration amplitude and phase readings should be entered. Calculate the trim corrections. To each correction plane, trim corrections should be added. Initial balance correction weight are not to be disturbed. Observe the illustration below. Until rotor is balanced, repeat trim corrections as needed.

Two Plane Balancing Record Form

Using the Form

The two plane balancing record form organizes balancing data for entry into balancingprograms and works as a permanent data record. This form is designed for TI-74S BasicaleProgrammable Calculators but it may be utilized with others.

In the form there are three data columns. The two correction planes columns are labeled,¨Left, Near, and A¨ and ¨Right, Far, and B¨ so they will match switch positions on differentinstruments and to match different programs. Correction plane names may be anything just asthe positions are consistent during the procedure.

The balancing data column items will identify the measurements and that match the names inthe TI-74S program. The original reading is RUN 1. The reading with the trial weight in theNEAR correction plane is RUN 2. The reading with the trial weight in the FAR correction planeis RUN 3.

14 / 22


The numbered squared in the upper left corner of the data blocks will identify the data for useelsewhere on the form. For calculations, blocks 1 through 10 are entered into the TI-74S. The11 and 12 blocks are the balance correction weights to be added.

Also provided are the data blocks for alternative static couple weight corrections.

The trim correction data blocks for two planes and alternative static couple corrections arealso provided. For the two trim runs, enough spaces are provided. A second from should beused for any additional trim runs.

Residual Unbalance

Residual Unbalance and Unbalance Sensitivity are calculated using data from numberedblocks. Residual unbalance is obtained by multiplying the final vibration reading that is takenafter adding the last trim corrections by the unbalance sensitivity.

Use the combine weights program to combine initial weights and trim weights into a singleweight for precise unbalance sensitive. Although, it is not necessary to actually combineweights on the rotor just combining them for residual unbalance calculations.

Calibration Recovery

Rotor calibration constants are obtained from data in blocks 1 through 10 and utilized forcalculating balance corrections and to rebalance identical or the same rotors. The constantsare not stored after the program is exited and they must be recalculated for future balancing.Re-enter data from blocks 1 through 10 to recalculate constants and then select TRIM fromthe menu and enter the new original readings and proceed with balancing as before. For theconstants to be valid, the following have to be the same as the original balancing: balancing RPM, phase mark, sensor position, weight angle reference.

15 / 22


No Phase Balancing

Discussion

Cooling tower fans may present a unique balancing problem. The environment (vapor andfog) and slow speed makes it very difficult to use a strob light for phase measurements. It canalso be easier to balance without phase then to try to install a reflective tape and photocell asa phase reference pulse. The next procedure is presented for use in these circumstances.When it is certain vibration is from unbalance, no phase balancing works best on single planebalancing problems.

Procedure

Balancing with no phase needs: Safe and identical trial weights added in sequence at three different angles. The trial weight positions need to be separated by at least 90°. Trial weight has to be added at the same radius. Trial weight position should be selected 1 and called 0°. Trial weight position 2 can be at for example 120°. Trial weight position 3 can be at for example 240°.

A graphical solution is utilized to solve for the balance weight angle and amount. 4 circles are drawn. Each and every circle represents all angles for each trial weight position. “T” vector is defined by the intersection of the three “trial weight circles”. Balance weight is the trial weight times the ratio of “O” divided by “T”. The balance weight angle is the angle from 0o to the “T” vector.

Vibration readings are taken: 1 x RPM vibration with no trial weight 0 = __________ . 1 x RPM vibration with the trial weight at position 1 0 + T1 = __________ . 1 x RPM vibration with the trial weight at position 2 0 + T2 = __________ . 1 x RPM vibration with the trial weight at position 3 0 + T3 = __________ .

16 / 22


A scale factor, inch/mil, turns vibration into inches for use as the radius for 4 circles. Observethe sample below and the other to a larger scale on the next page.

No Phase Balancing - Example

Procedure Item SYM Data 1 x RPM Original Reading RUN 0 (amplitude) O 24.5 mils Safe Trial Weight at 0° Trial Weight (amount and angle) TW 8 oz. at 0° Measure Trial Weight’s Effect RUN 1 (amplitude) 0 + T1 29.0 mils Safe Trial Weight at 120° Trial Weight (amount and angle) TW 8 oz. at 120° Measure Trial Weight’s Effect RUN 2 (amplitude) 0 + T2 22.5 mils Safe Trial Weight at 240° Trial Weight (amount and angle) TW 8 oz. at 240° Measure Trial Weight’s Effect RUN 3 (amplitude) 0 + T3 50.0 mils Construct Vectors and Measure T Vector T Vector (amount and angle) T 26.2 mils at 68° Correction Weight TW x O/T Corr. Wt. 8 oz x 24.5/26.2 = 8 oz x 0.94 = 7.48 oz at 68° Final Vibration Reading

Machine Name Cooling Tower Fan 12 120 RPM Date April 25 and 26, 2000

No Phase Balancing Record

Procedure Item SYM Data 1 x RPM Original Reading RUN 0 (amplitude) O Safe Trial Weight at 0° Trial Weight (amount and angle) TW Measure Trial Weight’s Effect RUN 1 (amplitude) 0 + T1 Safe Trial Weight at 120° Trial Weight (amount and angle) TW

17 / 22


Measure Trial Weight’s Effect RUN 2 (amplitude) 0 + T2 Safe Trial Weight at 240° Trial Weight (amount and angle) TW Measure Trial Weight’s Effect RUN 3 (amplitude) 0 + T3 Construct Vectors and Measure T Vector T Vector (amount and angle) T Correction Weight TW x O/T Corr. Wt. Final Vibration Reading

Machine _________________Name _____________________Date________

Trial Weights

Trial weights and permanent correction weights are the two kinds of weights used inbalancing. Trial weights are temporary and are added to a rotor to calibrate the machinesresponse to unbalance. A trial weight must be safe above all.

A safe trial must be large enough to make a significant change to the rotors 1x RPM vibrationbut not so large to risk a damage to the machine or the people nearby.

The trial weight should: Decrease or increase 1x RPM vibration by at least 30% or Change phase angle by at least 30 degrees or A combination of the two

The three vector diagrams show the effects of the trial weight on the O vector to create the0+T vector. The calibration of machine response to unbalance will not be accurate andbalancing will not go well if the T resultant vector is too small.

Safe Trial Weight Size

Nearly all balancing software includes formulas for calculating safe trial weights. The rotor’s

18 / 22


weight is required that is often unknown. The formula’s results are usually too conservativeand the weight too small. The author and experienced balancers rarely rely on these instead,but in its place, inspect the rotor to observe what size the machine’s manufacturer and othershave utilized on prior balancing efforts. Rotor inspection also exposes the type of weightsand the way they were attached. These are important things the balancer will also need.

Trial weight size depends on RPM, rotor weight and the radius at which the weight isattached. Unbalance and its correction is weight time distance in gr-in or oz-in. So it isimportant to determine a safe trial weight in terms of gram inches or ounce inches and thendivide by the radius at which it will be attached. This amount is the weigh for attachment onthe rotor.

Ideas for Trial Weight Materials

Balancing wax is a sticky and light material that may be pushed under a ledge or against avertical surface with caution. If the rotor speed is under 1200 RPM and surface is clean thebalancing wax and duct seal may be utilized for temporary weights. Washers, nuts, small boltsand other metal bits may be imbedded in the wax or duct seal for more weight. This trialweights come loose and become dangerous missiles as evidenced by the pieces of wax thatis stuck to the ceiling of many balancing shops.

The rotor should be checked to be balanced for places to add washers, nuts and bolts.Sometimes an existing bolt may be removed and a longer one can be added. A weight maybe made that spans adjacent bolts. Weights may be made and can serve as both permanentand trial weights. For some ideas see balance weights you can buy.

For some rotors that do not have convenient ways to add trial weights consider taping orstrapping them to the rotor’s circumference. This method is usually used on the smoothsurface of paper machine rolls. Observe the illustration below.

A 1” x 3” x 8” bar stock will weigh approximately 108 oz. If the radius of the roll surface is 10”the trial weight is 1080 oz-in and at 300 RPM will have a 172 centrifugal force. At 600 RPMthe same weight sill have a 689 centrifugal force. So the strapping has to be chosen with care. Wooden wedges under the strap may be utilized to adjust strap tension to keep the weight

19 / 22


in place. The angular position can be changed easily.

Tacking welding pieces of steel sheet or bars to rotors where welding is allowed can also beeffective. The rod should be included as part of the weight. The tack welded weight may beknocked off for moving and size adjustments.

Removing weight by milling, grinding, shaping or drilling for trial weights is not practical. Thesemethods are often reserved for permanent corrections after the heavy spot position has beenestablished. Grinding and drilling are the most effective of these methods.

The methods to make trial weights are too numerous to mention all. The toughest challengefor the balancer is figuring a way to make weight corrections. The methods mentioned hereought to offer some ideas.

Common Material Density Aluminum = 1.584 oz/cu in = .099 lb/cu in Brass = 4.912 oz/cu in = .307 lb/cu in Cast Iron = 4.160 oz/cu in = .260 lb/cu in Copper = 5.152 oz/cu in = .322 lb/cu in Lead = 6.560 oz/cu in = .410 lb/cu in Steel = 4.528 oz/cu in = .283 lb/cu in Stainless = 4.640 oz/cu in = .290 lb/cu in

Metal density varies depending on fabrication and alloy. These values can be used forestimating material amounts. Balancing weights should be weighed on a scale.

Balance Weights You Can Buy

Balancing Wax

Balancing wax is used as temporary trial and balance weights and it is pliable and sticks tomost dry and clean surfaces yet easy to remove. Balancing wax is also light and good from

20 / 22


small trim corrections. Washers, nuts and similar bits of metal may be mixed with the wax foradded weight. The wax should be placed under a rim or shoulder so the centrifugal force willnot cause it to come loose and turn into a dangerous missile. Wax should not be used forpermanent corrections. It is for temporary use when locating the unbalance heavy spot forcorrections that are made by removing material.

For sources of balancing wax, check with your suppliers.

Balancing Weights

C-Clamp weights are used for permanent and trial corrections. These weights are given insets of sixteen, two of each size such as: 1.75, 2.75, 3.75, 4.50, 5.50, 6.25, 7.25 and 8.00ounces. A set screw is utilized to hold them in place. Weight ought to be attached to theleading edge of fan blades or rotor so the centrifugal force will hold them in place. Theyshould be tack welded for safety as a permanent weight.

Balancing weights are obtainable from several suppliers.

Balance Clips

Curved weights are used as permanent or trial weights on squirrel cage fan blades. Theseclips are available in 12.5, 25, 50, 75, 100, 150 and 200 grain weights (15.3 grains = 1 gram).Balance clips come in sets of 700 clips with 100 in each size.

These clips sets are sold by several suppliers.

Large balancing clips that are up to 1 oz are available from local trane company parts center.

21 / 22


EPO Dynaweight Balancing Compound

It is a two-part epoxy putty that is used for permanent and trial weights and it remainsworkable 3 hours at room temperature after it is mixed. The uncured mixture may by utilizedfor temporary weights, increased, moved and adjusted as need to obtain final balance. Afterthe adjustment the mixture is cured for permanent corrections. The mixture cures in 15minutes at 250 degrees with a heat gun.

22 / 22