Critical Speeds and Whirling: Master the Dynamics of Rotating Machinery

Question 1

Which factor primarily influences the critical speed of a rotating system?

Accepted Answer

Stiffness of the support structure

Answer

Rotational speed (rpm)

Answer

Mass of the rotor

Answer

Damping present in the system

Question 2

Which element does NOT contribute to the critical speed of a rotating shaft?

Accepted Answer

Clearance in the bearing

Answer

Shaft rigidity

Answer

Shaft diameter

Answer

Density of the shaft material

Question 3

What is the purpose of a Campbell diagram?

Accepted Answer

Predicting critical speeds of a rotating shaft

Answer

Determining natural frequencies of a structure

Answer

Calculating damping ratios

Answer

Evaluating stability of rotating systems

Question 4

Which bearing type is most suitable for preventing whirling?

Accepted Answer

Hydrodynamic bearing

Answer

Plain bearing

Answer

Roller bearing

Answer

Ball bearing

Question 5

Describe the shape of a whirling mode:

Accepted Answer

Elliptical

Answer

Circular

Answer

Sinusoidal

Answer

Parabolic

Question 6

Which of the following is NOT a primary factor affecting the critical speed of a rotor?

Accepted Answer

Fluid film bearing properties

Answer

Rotor mass

Answer

Rotor stiffness

Answer

Operating speed

Question 7

What is 'whirling' in the context of rotating machinery?

Accepted Answer

Unstable lateral vibration of a rotating shaft

Answer

Torsional vibration of a rotating shaft

Answer

Axial vibration of a rotating shaft

Answer

Radial vibration of a rotating shaft

Question 8

What is the purpose of a Campbell diagram?

Accepted Answer

To graphically represent critical speeds as a function of operating speed

Answer

To calculate the natural frequencies of a rotor

Answer

To determine the damping ratios of a system

Answer

To predict the fatigue life of a component

Question 9

Which technique is commonly employed to increase the critical speed of a rotor?

Accepted Answer

Increasing rotor stiffness

Answer

Decreasing rotor mass

Answer

Balancing the rotor

Answer

Reducing damping

Question 10

What is the primary cause of whirling in steam turbines?

Accepted Answer

Unbalanced steam forces

Answer

Shaft misalignment

Answer

Bearing wear

Answer

High operating speed

Question 11

Which of the following is NOT a preventive measure for whirling?

Accepted Answer

Excessive damping

Answer

Proper rotor design

Answer

Adequate bearing support

Answer

Regular maintenance and monitoring

Question 12

Which of the following is NOT a cause of whirling in rotating machinery?

Accepted Answer

Proper lubrication

Answer

Unbalanced masses

Answer

Misalignment of bearings

Answer

Eccentricity of rotating components

Question 13

What is the primary purpose of a balancing machine?

Accepted Answer

To identify and correct unbalanced masses in rotating components

Answer

To measure the critical speeds of rotating shafts

Answer

To align bearings in rotating machinery

Answer

To determine the stiffness of rotating components

Question 14

Which type of whirling occurs when the frequency of the exciting force is equal to the natural frequency of the system?

Accepted Answer

Synchronous whirling

Answer

Half-speed whirling

Answer

Quarter-speed whirling

Answer

Supercritical whirling

Question 15

What is the effect of damping on the amplitude of whirling?

Accepted Answer

Damping typically decreases the amplitude of whirling

Answer

Damping typically increases the amplitude of whirling

Answer

Damping has no effect on the amplitude of whirling

Answer

The effect of damping on the amplitude of whirling depends on the excitation frequency

Question 16

Analyze the role of damping in mitigating vibrations near critical speeds in rotating machinery. Describe common damping mechanisms employed in such systems.

Accepted Answer

Damping generally reduces vibration severity near critical speeds, enhancing system stability.

Accepted Answer

Common damping mechanisms in rotating machinery include viscous damping, friction damping, and structural damping.

Answer

Damping plays a negligible role in determining critical speeds in rotating systems.

Question 17

How does the log decrement help determine the stability of a rotating system?

Accepted Answer

It reveals the damping ratio, which is key to stability.

Answer

It accurately predicts the system's critical speed.

Answer

It's mainly used for non-rotating systems.

Answer

It's unnecessary for well-balanced systems.

Question 18

Which condition can cause whirling instability in a rotating shaft with a disk attached at its midpoint, assuming negligible damping?

Accepted Answer

Disk mass exceeds shaft mass

Answer

Shaft is perfectly balanced

Answer

Shaft rotates below its first critical speed

Answer

Shaft has very high stiffness

Question 19

The critical speed of a shaft is defined as the speed at which:

Accepted Answer

The natural frequency of the shaft coincides with the operating speed

Answer

The shaft fails due to fatigue

Answer

The shaft experiences maximum deflection

Question 20

Which of the following analytical techniques is used to determine the critical speeds of a shaft?

Accepted Answer

Eigenvalue analysis

Answer

Static deflection analysis

Answer

Dimensional analysis

Answer

Finite element analysis

Question 21

What is the purpose of a balance weight in a rotating system?

Accepted Answer

To counteract the effects of inherent mass imbalance, reducing vibration

Answer

To increase the stiffness of the shaft

Answer

To absorb shock and impact loads

Answer

To reduce the operating speed

Question 22

Which of the following is a consequence of whirling in rotating machinery?

Accepted Answer

Excessive vibration, noise, and reduction in component lifespan

Answer

Increased efficiency and performance

Answer

Reduced operating costs

Answer

Improved stability and safety

Question 23

The Campbell diagram is used to:

Accepted Answer

Graphically illustrate the natural frequencies of a shaft as a function of operating speed

Answer

Determine the mode shapes of a vibrating shaft

Answer

Calculate the critical speeds of a shaft using a graphical method

Question 24

How can damping affect the critical speed of a shaft?

Accepted Answer

Damping can increase the stability margin by reducing the amplitude of vibration near critical speeds

Answer

Damping can decrease the critical speed and increase the operating speed

Answer

Damping has no effect on the critical speed of a shaft

Question 25

What is the significance of the first critical speed?

Accepted Answer

It is the lowest speed at which whirling can occur and is often the most critical to avoid

Answer

It has no practical significance in machine design

Answer

It is always higher than the operating speed of the machine

Answer

It is the only critical speed that can cause catastrophic failure

Question 26

Which of the following is NOT a factor that can contribute to whirling in rotating machinery?

Accepted Answer

Balanced masses

Answer

Fluid flow

Answer

Shaft stiffness

Answer

Misalignment

Question 27

What is the primary purpose of a Laval nozzle in a steam turbine?

Accepted Answer

To increase steam velocity and reduce moisture content

Answer

To increase steam pressure

Answer

To reduce steam temperature

Question 28

What are the typical consequences of operating a machine above its critical speed?

Accepted Answer

Excessive vibration, bearing failure, and fatigue damage

Answer

Increased efficiency

Answer

Reduced noise levels

Question 29

A shaft operating at its critical speed will experience:

Accepted Answer

Resonance

Answer

Torsion

Answer

Bending

Answer

Damping

Question 30

Which of the following is a commonly used method to identify critical speeds experimentally?

Accepted Answer

Vibration analysis

Answer

Acoustic analysis

Answer

Visual inspection

Answer

Thermal analysis

Question 31

A rotating shaft with a mass unbalance will experience:

Accepted Answer

Eccentric loading

Answer

Torsional loading

Answer

Shear loading

Answer

Bending loading

Question 32

The purpose of a Campbell diagram is to:

Accepted Answer

Plot critical speeds of a rotating shaft

Answer

Predict resonance frequencies

Answer

Calculate damping coefficients

Answer

Determine shaft stiffness

Question 33

Which type of whirling occurs when the operating speed exceeds the critical speed?

Accepted Answer

Forward whirling

Answer

Backward whirling

Answer

Transverse whirling

Question 34

The ratio of the operating speed to the critical speed is referred to as:

Accepted Answer

Speed ratio

Answer

Stiffness ratio

Answer

Damping ratio

Answer

Unbalance ratio

Question 35

A rotating shaft is experiencing significant vibrations at a specific speed. What is this speed most commonly referred to as?

Accepted Answer

Critical speed

Answer

Natural frequency

Answer

Operating speed

Answer

Resonant frequency

Question 36

What is the primary cause of whirling in rotating machinery?

Accepted Answer

Asymmetrical mass distribution or external force excitation

Answer

Friction between moving parts

Answer

High operating temperature

Answer

Excessive lubrication

Question 37

A rotating shaft with a larger mass will generally have a ____ critical speed compared to a lighter shaft with the same geometry.

Accepted Answer

Lower

Answer

Higher

Answer

Similar

Answer

Unpredictable

Question 38

Which of the following methods can be employed to reduce or eliminate critical speed problems in rotating machinery?

Accepted Answer

Stiffening the shaft, adding damping materials, balancing the rotating components, or operating at a different speed

Answer

Increasing the operating speed

Answer

Decreasing the stiffness of the shaft

Answer

Reducing the mass of the rotating components

Question 39

The critical speed of a shaft can be calculated using the following formula: ωn = √(k/m). What do 'k' and 'm' represent in this equation?

Accepted Answer

k = stiffness, m = mass

Answer

k = stiffness, m = damping coefficient

Answer

k = mass, m = stiffness

Answer

k = damping coefficient, m = mass

Question 40

A rotating machine operating at a constant speed suddenly experiences excessive vibration. What could be the most likely explanation?

Accepted Answer

The operating speed has coincided with a critical speed

Answer

The motor is overloaded

Answer

The lubrication is inadequate

Answer

The bearings are worn out

Question 41

What is the primary function of a vibration damper in rotating machinery?

Accepted Answer

To absorb and dissipate vibrational energy, reducing the amplitude of vibrations

Answer

To improve lubrication

Answer

To increase the critical speed

Answer

To reduce the mass of the rotating components

Question 42

A rotating shaft with a higher stiffness will generally have a ____ critical speed compared to a less stiff shaft with the same mass.

Accepted Answer

Higher

Answer

Lower

Answer

Unpredictable

Answer

Similar

Question 43

Which of the following factors significantly influences the critical speed of a rotating shaft?

Accepted Answer

Shaft length, shaft diameter, and mass of rotating components

Answer

Type of bearings used

Answer

Operating temperature

Answer

Material type of the shaft

Question 44

A rotating machine is designed to operate at a speed below its calculated critical speed. What is a potential benefit of this design choice?

Accepted Answer

Reducing the risk of severe vibrations and avoiding resonance

Answer

Increasing operating efficiency

Answer

Reducing wear on bearings

Answer

Minimizing noise levels

Question 45

A shaft of length 'L' is supported by two bearings and is subjected to a whirling force 'F' at its mid-span. If the critical speed of the shaft is 'ω_c', then the natural frequency of the shaft is given by:

Accepted Answer

ω_c / √2

Answer

ω_c / 2

Answer

ω_c

Answer

ω_c √2

Question 46

Which of the following is NOT a factor affecting the critical speed of a shaft?

Accepted Answer

Material density

Answer

Length of the shaft

Answer

Shaft diameter

Answer

Bearing stiffness

Question 47

A rotor is supported on two bearings with a bearing stiffness of 'k'. If the rotor mass is 'm', then the whirling frequency is given by:

Accepted Answer

√(k/m)

Answer

m/k

Answer

√(m/k)

Answer

k/m

Question 48

The phenomenon of whirling can occur when:

Accepted Answer

The operating speed is close to the critical speed.

Answer

The shaft is perfectly balanced.

Answer

The bearings are overlubricated.

Question 49

Which of the following methods can be used to reduce the critical speed of a shaft?

Accepted Answer

Adding a flywheel to the shaft.

Answer

Reducing the shaft length.

Answer

Increasing the shaft stiffness.

Answer

Decreasing the bearing stiffness.

Question 50

A shaft with a critical speed of 'ω_c' is operating at a speed of 'ω'. If the damping ratio of the system is 'ζ', then the magnification factor is given by:

Accepted Answer

1 / √(1 - (ω/ω_c)^2 + (2ζω/ω_c)^2)

Answer

1 / (1 - (ω/ω_c)^2 + (2ζω/ω_c)^2)

Answer

√(1 - (ω/ω_c)^2 + (2ζω/ω_c)^2)

Answer

(ω/ω_c)^2 / √(1 - (ω/ω_c)^2 + (2ζω/ω_c)^2)

Question 51

The Campbell diagram is a graphical representation of:

Accepted Answer

Whirling frequencies versus operating speeds.

Answer

Damping ratios versus operating speeds.

Answer

Critical speeds versus bearing stiffness.

Answer

Magnification factors versus operating speeds.

Question 52

Which of the following is a common source of excitation for whirling in rotating machinery?

Accepted Answer

Unbalance

Answer

Bearing wear

Answer

Electrical noise

Answer

Misalignment

Question 53

A rotor is supported on two bearings with a bearing stiffness of 'k' and a damping coefficient of 'c'. If the rotor mass is 'm', then the logarithmic decrement is given by:

Accepted Answer

2πζ√(k/m)

Answer

ζ√(k/m)

Answer

2πζ(k/m)

Answer

ζ(k/m)

Question 54

Which of the following techniques can be used to prevent whirling in rotating machinery?

Accepted Answer

Balancing the rotor

Answer

Adding a flexible coupling

Answer

Reducing the bearing stiffness

Answer

Increasing the operating speed