# CRITICAL SPEED QUESTION ANSWERS CLASS NOTES

**CRITICAL SPEED QUESTION **

**ANSWERS CLASS NOTES**

Skip to content
# CRITICAL SPEED QUESTION ANSWERS CLASS NOTES

** **

### Critical speed is the rotational speed

### which is equal to the numerical value

### of the natural frequency of vibration.

### Then rotational speed and natural

### frequency are in resonance. At this speed,

### a rotating shaft becomes dynamically

### unstable with large lateral amplitudes of

### vibration due to resonance. Critical

### speed is applicable to rotating

### machinery such as a shaft, gear,

### propeller or a lead screw. It is also

### called as whirling speed or whipping

### speed or potentially destructive

### rotational speed.

### CRITICAL SPEED

#### All shafts act as beams (simply supported or cantilevers) when stationary. All beams deflect with or without external loads. Thus rotating shafts will also deflect during rotation. The deflection causes unbalancing of the mass of the shaft & mass of the pulleys mounted. The unbalanced mass of the rotating object causes vibration. These vibrations increase with the increase of shaft speed of rotation. The rotational speed becomes numerically equal to the natural frequency of vibration. That speed is the critical speed.

### Factors on which critical speed depends

#### (i) Type of support (simply supported or cantilever type)

(ii) Length of the shaft

(iii) Diameter of the shaft

(iv) Stiffness of the shaft

(v) Mass of shaft

(vi) Other masses mounted on the shaft

(vii) Magnitude of deflection

(viii) Magnitude of unbalance of the masses with respect to the axis of rotation

(ix) Type and the amount of damping available in the system

#### It is absolutely necessary to calculate the critical speed of a rotating shaft to avoid excessive noise and vibration.

### Calculation of the critical speed (Nc)

#### calculating the critical speed requires natural frequency. There are two methods to calculate the natural frequency of vibration i.e. Rayleigh- Ritz method (slightly overestimates) and Dun Kerley’s method (slightly underestimates). For any orientation of the shaft, use Rayleigh–Ritz equation.

#### Natural frequency, f = critical speed (Nc)

Natural frequency f = Nc = (30/ π) (g / δst)^{0.5}

g = acceleration due to gravity (9.81 m/s^{2})

δst = Shaft vertical static deflection when placed horizontally

Nc is in RPM

#### NOTE: As rotational speed increase, δst also increase and hence natural frequency also change. However some value of rotational speed becomes equal in magnitude to the natural frequency of vibration. This rotational speed is the Critical Speed. At this speed, shaft may come out of the bearing. Avoid critical speed It at all costs.

From actual practice and experience, there is a suggestion. That the maximum operating speed should not exceed 75% of the critical speed.

## Similar Posts

### DEGREES OF FREEDOM CLASS NOTES FOR MECHANICAL ENGINEERING

### VIBRATIONS MULTIPLE CHOICE QUESTIONS (MCQ) WITH ANSWERS

### GEARS CLASS NOTES INTERVIEW SHORT QUESTION ANSWERS

### THERMODYNAMIC QUESTION ANSWERS CLASS NOTES

### BELTS MULTIPLE CHOICE QUESTIONS (MCQ) WITH ANSWERS

### BELT DRIVE CLASS NOTES FOR MECHANICAL ENGINEERING

Byadmin

**CRITICAL SPEED QUESTION **

**ANSWERS CLASS NOTES**

(ii) Length of the shaft

(iii) Diameter of the shaft

(iv) Stiffness of the shaft

(v) Mass of shaft

(vi) Other masses mounted on the shaft

(vii) Magnitude of deflection

(viii) Magnitude of unbalance of the masses with respect to the axis of rotation

(ix) Type and the amount of damping available in the system

Natural frequency f = Nc = (30/ π) (g / δst)

g = acceleration due to gravity (9.81 m/s

δst = Shaft vertical static deflection when placed horizontally

Nc is in RPM

From actual practice and experience, there is a suggestion. That the maximum operating speed should not exceed 75% of the critical speed.

Byadmin

DEGREES OF FREEDOM CLASS NOTES FOR MECHANICAL ENGINEERING Body motion is controlled by number of degrees of freedom it possesses. Degrees of freedom is the minimum number of independent parameters to describe a motion without violating any constraint imposed on it. Degrees of freedom does analysis of the dynamic systems in practical life. Fig. Six Degrees…

Byadmin

VIBRATIONS MULTIPLE CHOICE QUESTIONS (MCQ) WITH ANSWERS MCQ increases depth of understanding. It gives a high degree of clarity. Vibration is a periodic motion. It is like reversal loading. It is highly dangerous. Vibrations are free & forced vibrations with /without damping. Vibrations causes unpleasant sound. It weakens the foundation of a structure. …

Byadmin

GEARS CLASS NOTES INTERVIEW SHORT QUESTION ANSWERS Question answers makes a topic highly clear and understood. This helps in the design & manufacture of gears. Gears are used in power transmission. Discuss the advantages and disadvantages of an involutes profile. A. Advantages of involutes profile Manufacturing is accurate and easy. Pressure…

Byadmin

THERMODYNAMIC QUESTION ANSWERS CLASS NOTES What is the Difference between a control volume and control surface? ANS: Fig. Control Volume & Control Surface Control volume is a 3 dimensional space in a thermodynamic system (say a compressor or pump). There will be mass transfer, energy transfer and momentum transfer across the control volume. Select control…

Byadmin

BELTS MULTIPLE CHOICE QUESTIONS (MCQ) WITH ANSWERS MCQ increases understanding and level of clarity. Fundamentals are easily applied in real life applications. Power transmission is by friction. Belts, ropes, chains and gear do power transmission. There is a driver and a driven. Driver is the pulley or gear mounted on the motor or engine shaft….

Byadmin

BELT DRIVE CLASS NOTES FOR MECHANICAL ENGINEERING Belts, chains and gears transmit power from one shaft to the another shaft. Use belts and ropes where distance between the two shafts is large. The chains are used for intermediate distances. The gears are used for shorter distance between the shafts. Gear drive is a positive…