Rotor balancing

Velocity is chosen cause it's usually has the flatest curve.

Acceleration is chosen where low frequency noise is problem. Displacement is chosen to Aviod high frequency noise.

How to balance

Adding compensating masses to the rotor at prescribed locations. Removing fixed quantities of material for example by drilling.

Perform field balancing by making alterations to the mass distribution of the rotor by:

Adding trial masses. Measure the resulting phase and magnitude of bearing vibration.

(3)Determining the balance quality. Classifying rotor to be balanced according to the level of acceptable vibrations.

As for different types and sizes of machines, the level of vibration regarded as excessive various considerably.

(7) Measurement check.

Before using the efects of trail mass to calculate the correction mass,check that the results are suitable

It's possible to have two equal masses placed symmetrically about the center of gravity, but positioned at 180 ° from each other,the rotor is in static in balance. There is no eccentricity of the center of gravity

But when the rotor turns the two masses cause a shift in the inertia axis, so that it's no longer aligned with the rotation axis, leading to strong vibration in the bearing, the unbalance can only corrected by taking vibration measurements with the rotor turning and adding correction masses in two planes.

Rigid rotor balancing

By making correction in any two arbitrarily selected planes.

Unbalance

Common source of vibration in machine with rotating parts.

There are four possibilities can rise.

Delta fay is the difference between the phase measured before and after trail mass was mounted. Delta V is a difference between the vibration level measured before and after the trial mass is mounted.

Couple unbalance example

Disk that has swash run out with no static unbalance.

The maximum residual unbalance after correction should be determined.

Dividing rotor into equal sigments Ex:every 45°. Move trail mass to each postion and measure the vibration. Plot results. Mres=|Vresr|/|Vt|*Mt. And fay is the postion of residual mass. Vertical line from the highest point.

Unbalance is an important factor in modern machine design where :

High speed and reptability are significant considerations.

Balance quality for rigid rotor

If it isn't perfectly balanced rotor, the forces at each end will be equal, but the allowable residual specific unbalance will be different for each bearing.

Balancing

Improve the mass distribution of rotor. Rotor rotates in its bearings without uncompressed centrifugal forces.

(1) Delta fay less than 25 ° Delta V less than 25 %

Increase trail mass

Dynamic unbalance correction

Is corrected in two planes.

Couple unbalance correction

Is corrected in two planes. Rotor diameter is less than 7 to 10 of its width.

Static unbalance correction

Is corrected only in one axial plane. Rotor diameter more than 7 to 10 of its width.

Flexible rotor balancing

Is more complicated because of the elastic deflection of the rotor.

(1) Preforming a frequency analysis.

Is the unbalance the cause of the excess vibration or not.

Balancing of rotor increases the useful life of machinery as:

It prevents excessive loading of bearings. Aviod fatigue failure.

Correction mass and correction radius

It's impossible sometimes to mount the correction mass at the same radius of trial mass. eM=mr M=rotor mass. m=unbalance mass. r=correction radius. e=specific unbalance. m1r1=m2r2=eM.

Flexible rotors

Its service speed is above 50% of its first critical speed.

Rigid rotor

Its service speed is less than 50% of its critical speed.

Maximum residual mass

MMR=su*MR/Rc Su:specific unbalance required in gmm/kg. MR: rotor mass kg Rc:correction radius mm A suitable trail mass is 5 to 10 mes the value of the maximum residual mass.

Couple unbalance

Mass axis intersects the running axis.

Static unbalance

Mass axis is displaced only parallel to the shaft axis

Dynamic unbalance

Mass axis is not coincidental with the rotational axis. A combination of static and couple unbalance

Basic theory

Mass components rotates. The force also rotates tries to move the rotor along the line of action of the force. Vibration will be transmitted to the rotors bearing. Any point on the bearing will experience this force once per revolution.

(5) Single plane static balancing

Mount an accelerometer and tachometer probe and connect them to the instruments. Run the machine at its normal operating speed. Measure and record the vibration level and phase angle.

Two plane dynamic balancing.

Mount an accelerometer and tachometer probe and connect them to the instruments. Run the machine at its normal operating speed. Measure and record the vibration level and the phase angle for each plane in turn.

(2) Delta fay less than 25 ° Delta V more than 25%

Move trail mass

General balancing procedure :

Preforming a frequency analysis. Selecting the best measurement. Determing balance quality. Selection of trail masses. Single plane static placing. Double plane dynamic balancing. Measurement check.

(3) Delta fay more than 25 ° Delta V less than 25 %

Proceed

(4) Delta fay more than 25 ° Delta V more than 25 °

Proceed

Types of rotors:

Rigid rotor. Flexible rotor.

If Length to diameter ratio of the rotor more than 0.5 balance correction is

Single plane : 0-500rpm. Two plane : above 500 rpm.

If Length to diameter ratio of the rotor less than 0.5 balance correction is

Single plane: 0:1000 rpm. Two plane: above 1000 rpm.

Mount the correction mass at the postion indicated by the correction angle, a positive correction angle indicates that the angle should be measured in the direction of rotation, for a negative correction angle, measure agnist the direction of rotation.

Start up the machine again and measure the residual unbalance. If care has been taken with the balancing procedure and proper balancing equipment, the level of residual vibration should be small.

Calculate the values of the correction mass and angle required. Mount the correction mass at the postion indicated by the correction angle.

Startup the machine again and measure the residual unbalance. If care has been taken with the balancing procedure and proper balancing equipment, the level of residual vibration should be small.

Stop the machine and Mount a trial mass of suitable size in plane No. 1, marking its position.

Startup the machine and record the new vibration level and phase angle for each plane in turn. Stop the machine and remove the trial mass.

Types of unbalance :

Static unbalance. Couple unbalance. Dynamic unbalance.

Mount a trail mass of suitable size in plane 2,the trail mass used in plane 1 can be used again, and mark its postion. Start the machine again and measure the vibration level and phase angle once more, for each plane in turn.

Stop the machine and remove the trial mass.

Stop the machine and Mount a trial mass of suitable size arbitrarily in the correction circle, the plane where is the correction is to be made,mark the position of the trail mass. Start up the machine and record the new vibration level and the phase angle.

Stop the machine and remove the trial mass. Calculate the value of the correction mass and angle required.

(2) Selecting the best measurement. .

The parameter with the flatest curve, The most horizontally for vibration measurement requires the smallest dynamic range in the measuring instruments, so the signal to noise ratio is higher.

principle of balancing

The size and position of the correction mass must be determined.

(4) selection of trail masses

The specific unbalance is used to calculate the size of trail masses, which are used during balancing to make temporary alterations to the mass distribution of the rotor to determine the relationship between the specific unbalance and the bearing vibrations.

The position of center of gravity divides the rotor in the ratio 1/3 : 2/3.

The sum of the moment about the center of gravity must be zero. There fore the residual specific unbalance at bearing A is 2/3 of the total residual specific unbalance, while at bearing B its 1/3 total.

The relationship between the three parameters as a function of frequency

The three: displacement, velocity and acceleration curves has different slopes,but peaks in the spectrum ocurr at the same frequencies. The same information about vibration levels obtained but the way the presentation is presented differs.

The effects of this trial corrections enable the amount and position of the required correction mass to be determined.

The values are usually calculated with the aid of a bocket calculator.

Balancing instrumentation. Single plane balancing. Two plane balancing.

When choosing an instrument for balancing, it is important to look at the other things it can do. Like wise,when selecting equipment for general vibration measurement or machine condition monitoring,its important to consider whether it can be adapted easily for balancing.

Vibration due to rotor unbalance causes :

uneven distribution of mass . Interaction between unbalanced mass with axial acceleration generate centrifugal force.