BALANCING OF ROTATING MASSES

          We have already discussed that whenever a certain extra amount of mass is attached to a rotating shaft, it produces (or exerts) some centrifugal force. Due to this force the shaft will bend and produces vibrations in it. In order to prevent the effect of centrifugal force, another mass is attached to the opposite side of the shaft, at such a position so as to balance the result of the centrifugal force of the primary mass. This can be done in such a way that the centrifugal force of both the masses are made to be equal and opposite. The method of providing the second mass so as to counteract the effect of the centrifugal force of the first mass, is termed as Balancing Of Rotating Masses.

The following cases are more important from the subject point of view:

1. Balancing of a single rotating mass by a single mass rotating within the same plane.

2. Balancing of a single rotating mass by two masses rotating in different planes.

3. Balancing of various masses rotating in the same plane.

4. Balancing of various masses rotating in different planes.

We will discuss these cases, in detail, in the following posts.

Balancing Of Rotating Masses -- Introduction

Introduction:

          Now-a-days, high speed of engines and other machines is quite common. It is, therefore very important that all the rotating and reciprocating parts should be completely balanced as far as possible. If these rotating or reciprocating parts are not properly balanced, the dynamic forces develop in those bodies. These developed forces not only increases the loads on bearings and stresses in the various members, but also produce vibrations. In the upcoming classes we discuss the balancing of unbalanced forces caused by rotating masses, in order to minimise loads on bearings, vibration of rotating parts.

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.