Imbalance In Rotating Machinery Engineering Essay

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Imbalance in rotating machinery is very common source of vibration; in addition vibration generally causes decrease in service life of rotating machinery. For this reason balancing of rotors is an important factor to improve design quality of rotating machineries. Secondly; balancing of rotors can be done by using balancing machine. Moreover this lab report will present numerical investigation of both static and dynamic balancing of rigid rotors; subsequently it includes a table of results obtained of both static and dynamic balancing of rigid rotors, as well as a clear drawing of test rotor. Finally this lab report indicates procedure of static and dynamic balancing on rigid rotors.

Introduction:

First of all; vibration in any rotating machinery is the result of mechanical inaccuracies including mass unbalance, and other causes. Secondly; mass unbalance is the most important factor in the rotating machinery especially if the rotating speed is very high in this case the vibration level will increase too. For this reason rotor balancing is very important in modern design. Subsequently the generated force is depending upon the weight of the unbalanced mass and the magnitude of rotating speed. Finally balancing is process of trying to improve mass allocation of rotor in order to make rotor rotates smoothly.

Theory:

Unbalance in rotor is consequence of uneven allocation of mass, and that causes rigid rotor to vibrate. Moreover vibration is generated by the interaction of unbalanced mass with the radial acceleration due to rotation and together generates centrifugal force and then the rotor will transmit the vibration to the bearings.

Balance is a process of trying to improve the distribution of rotor mass; in order to make the rotor rotates without centrifugal force. Moreover this can be done in two ways. The first one by adding small masses to the rotor, and the second way by drilling to remove fixed quantities of material.

Static unbalance is known as eccentricity of the gravity centre of a rotor, because of certain mass located at certain radius from rotation centre. In order to solve the problem an equal mass is placed at 180° to unbalanced mass at the same radius as shown in figure (1). Finally static balancing can be done if and only if the diameter of the rotor is more than 7 to 10 times its width, and usually treated as single plane rotors.

Dynamic unbalance is a combination of static and couple unbalance, moreover it is necessary to make vibration measurements while the machine is running, and then add the mass to it. Finally Fig (1)

there are two classifications of rotors rigid or flexible. A rigid rotor is one whose service speed is less than 50% of its first critical speed. Above this speed, the rotor is said to be flexible.

Test rigid description:

Rotor rigidity:

There are several classifications of rotors, depending on flexibility, operating speed, and others (ISO 5243).

Class 1 is rigid rotors; this covers 90% of application

Class 2 is rotors that are not rigid or that have special characteristics of mass distribution but that can be balanced using a modified balancing technique (choice of correction planes is the key here).

Class 3 and 4 are flexible rotors

Note some motors need to be balanced at specific speeds, at two speeds or even when hot. Thermal effect can cause distortion that in turn causes unbalance, which can cause more distortion.

The mass eccentricity 'e' is the measurement of the unbalance in terms of the displacement of the mass axis and the shaft axis. Units are linear; inches or mm. The eccentricity multiplied by the rotor mass gives the rotor unbalance. The units are the combination of mass and eccentricity; ounce-Inches / Gram- Millimetre

Shaft axis and Mass axis

Fig (2)

P1 a b c P2

Method of balancing:

Rotor can be balanced by aligning equal quantity of mass at 180° to unbalanced mass.

For example; the rotor is symmetrical except for the unbalanced m at radius r:

U = m*r = M*e U = M*e so e = U/M = m*r/M

Experimental procedure:

Firstly; experimental procedure differs from single plane rotor to double plane rotor.

Single plane rotor procedure:

1. Run the machine and record the initial rotor vibration and phase angle.

2. Stop the machine and firmly attach a small Trial Weight to the rotor. Record the Trial Weigh mass and position. (Ensure that the Trial Weight is firmly attached to the rotor. It is a serious safety risk if the Trial Weight is not firmly attached and flies off while running the machine).

3. Run the machine and record the new rotor vibration and phase angle.

4. Stop the machine and remove the Trial Weight.

5. Input the data obtained from the 3 steps above in a single plane balancing program to calculate the required balancing weight and position.

6. Firmly attach a balancing weight of the required mass to the calculated position on the rotor.

7. Run the machine and re-measure vibration. If required, perform a trim balance.

Double plane rotor procedure:

1. Measure and record the initial rotor vibration and phase angle on both bearings.

2. Stop the machine and attach a small Trial Weight to plane 1 of the rotor. Record the trial, weigh mass and position. (Ensure that the Trial Weight is firmly attached to the rotor. It is a serious safety risk if the Trial Weight is not firmly attached and flies off while running the machine).

3. Run the machine and record the new rotor vibration and phase angle on both bearings.

4. Stop the machine and remove the Trial Weight from plane 1. Attach a small Trial Weight to plane 2 of the rotor. Record the trial, weigh mass and position. (Ensure that the Trial Weight is firmly attached to the rotor. It is a serious safety risk if the Trial Weight is not firmly attached and flies off while running the machine).

5. Run the machine and record the new rotor vibration and phase angle on both bearings.

6. Stop the machine and remove the Trial Weight from plane 2.

7. Input the data obtained from the 3 steps above in a two plane balancing program to calculate the required balancing weights and positions.

8. Firmly attach balancing weights of the required mass to the calculated positions on planes 1 and 2 of the rotor. (Ensure that the Balancing weights are firmly attached to the rotor. It is a serious safety risk if a Balancing Weight is not firmly attached and flies off while running the machine).

9. Run the machine and re-measure vibration. If required, perform a trim balance.

Results obtained:

Record table (1): Calculated tolerance

Eccentricity (e)= 8 µm

Radius

Rotor mass

G

Service speed

Test speed

49.7mm

0.693 kg

2.5

3000 rev/min

1467 rev/min

No. of runs

Unbalance

Angle

Balancing mass

Balancing angle

1

0.49 g

241°

0.4 g

240°

Remaining unbalance: 0.069 g / 53°

Record table (2): Calculated tolerance:

Eccentricity (e) = 8 µm

Rotor mass

a

24.63mm

b

25 mm

c

15.15mm

Service speed

3000 rev/min

P1

P2

G

2.5

Test speed

1264 rev/min

0.9285 g

R1

37.5 mm

R2

37.5mm

P1

P2

No. of runs

Unbalance

Unbalance

Balancing mass

Balancing mass

Mag

Phase

Mag

Phase

Mag

Phase

Mag

Phase

1

0.82

326°

0.7

115°

0.8

326°

0.7

115°

2

0.394

271°

0.450

79°

0.4

271°

0.5

79°

3

0.396

224°

0.174

63°

0.4

224°

0.2

63°

4

0.398

263°

0.150

53°

0.4

263°

0.2

53°

Remaining unbalance

0.038

267°

0.093

294°

Discussion:

Initially; static method of balancing should be used with some types of rotors, especially narrow plane rotor to reduce time of balancing and improve the results. Moreover couple method of balancing can be used with rotor which has wide plane distance. Finally it's important to apply ISO standards for both static/ couple method balancing.

There are many advantages associated with well-balanced rotor. Balancing of rotors minimizes the noise and vibration produced during rotation, as well as it increases bearing and machine life which lead to have high operation efficiency of machine.

Inaccuracy of balancing machine is very rare but inaccuracies may come from the operator or equipment is used in balancing i.e.; the scale.

From my point of view balancing doesn't have disadvantages, because it is important for any rotating part.

Conclusion:

Rotating parts are widely used industry and every part rotates needs to be balanced to ensure smooth running. Subsequently balancing is essential to manufacture of rotors, because when the speed of any rotating part increases the effect of unbalance increase as well.

Appendices:

The following definitions are from ISO 1925, Balancing Vocabulary. This Standard contains a more complete listing of definitions and a copy is recommended for reference.

Static Unbalance The unbalance is distributed equally from the CG and on the same side of the rotor. The PIA is displaced parallel to the shaft axis.

Dynamic Unbalance The unbalance is distributed in a manner that the PIA is not parallel to and does not intersect the shaft axis. Dynamic Unbalance is a combination of static and couple unbalance.

Narrow Plane Rotor A rotor where the distance between the correction planes is less than 1/3 the distance between the bearing journals.

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