EXERCISES - Balancing Of Rotating masses

EXERCISES

1. Define Balancing? Write a short notes on Static Balancing? and Dynamic Balancing? State necessary conditions to achieve them?

2. Why is balancing of rotating parts (or masses) necessary for highspeed engines?

3. Explain how a single revolving mass is balanced by two masses revolving in different planes? 

4. Explain the method of balancing a single rotating mass by another single rotating mass in same plane?

5. Explain how the different masses rotating in different planes are balanced?

6. Explain the method of balancing of different masses revolving in same plane?

Balancing of Rotating Masses -- OBJECTIVE TYPE QUESTIONS

Balancing Of Rotating masses

Objective type questions

1. The balancing of Rotating and Reciprocating parts of the engine is necessary when it runs at

(a) Slow speed                                         (b) Medium speed 

(c) High speed                                         (d) No speed

2. Which of statements is true for static balancing of shaft,

(a) The net dynamic forces acting on the shaft is zero           
(b)  The net couple due to dynamic forces acting on the shaft is zero

(c) Both (a) and (b)                          (d) None of the above statements

3. For Dynamic balancing of shaft,

(a) The net dynamic forces acting on the shaft is zero           
(b)  The net couple due to dynamic forces acting on the shaft is zero

(c) Both (a) and (b)                          (d) None of the above statements

4. Which of the following statements is correct about the balancing of a mechanicalsystem?

(a) If it is under Static balance, then there will be dynamic balance too           
(b)  If it is under Dynamic balance, then there will be Static balance too

(c) Both Dynamic and Static balance have to be achieved separately
(d) None of the above mentioned statements

5. A distributing mass m₁ attached to a rotating shaft may be balanced by a single mass m₂ attached in the same plane of ratation as that of m₁ such that 

(a) m₁.r₂ = m₂.r₁           
(b)  m₁.r₁ = m₂.r₂

(c) m₁. m₂ = r₁.r₂

6. Which of the following statements are associated inorder to have a complete balancing of the several revolving masses in different planes 

(a) The resultant couple must be zero           
(b) The resultant force must be zero

(c) Both resultant force and couple must be zero
(d) None of the above

7. Which of the following statements are associated with complete dynamic balancing of rotating systems? 

(1) The resultant couple due to all inertia forces is zero           
(2) The support reactions due to forces are zero but not due to couples

(3) The system is automatically statically balanced
(4) Centre of masses of the system lies on the axis of rotation

(a) 1, 2 and 3 only           (b) 2, 3 and 4 only

(c) 1, 3 and 4 only           (d) 1, 2, 3 and 4 

Balancing of Reciprocating Masses -- Introduction

Introduction

          We have discussed in previous chapters, the various forces acting on the reciprocating parts of an engine. The result of all the forces performing on the body of the engine due to inertia forces only is known as unbalanced force or shaking force.  Thus if the resultant of all the forces due to inertia effects is zero, then there'll be no unbalanced force, however even then an unbalanced couple or shaking couple will be present. 

          Consider a horizantal reciprocating engine mechanism as shown in below Fig.,

Let   F_R  = Force required to accelerate the reciprocating parts,

         F_I   = Inertia force due to reciprocating parts,

         F_N  = Force on the sides of the cylinder walls or normal force acting on the cross-head guides, and

         F_B  = Force acting on the crankshaft bearing or main bearing.

          Since F_R and F_I are equal in magnitude however act in opposite direction, therefore they balance one another. The horizantal component of F_B (i.e F_BH) acting along the line of reciprocation is also equal and opposite to F_I. This force F_{BH =} F_U is an unbalanced force or shaking force and required to be properly balanced.

         The force on the sides of the cylinder walls (F_N) and the vertical component of  F_B(i.e F_BV) are equal and opposite and thus form a shaking couple of magnitude  F_N X x _or F_BV X x. 

          From the above statements we tend to see that the imapct (or) effect of the reciprocating parts is to produce a shaking force and a shaking couple. Since the shaking force and a shaking couple differ in magnitude and direction during the engine cycle, therefore they cause terribly objectionable vibrations.

          Thus the purpose (or  aim) of balancing the reciprocating masses is to eliminate the shaking force and a shaking couple. In most of the mechanisms, we can produce the shaking force and a shaking couple by adding appropriate balancing mass, But it is usually not practical to eliminate them completely. In otherwords, the reciprocating masses are only partially balanced. 

Note : The masses rotating witht he crankshaft are normally balanced and they do not transmit any unbalanced or shaking force on the body of the engine.