User:Thanupillai

INTRODUCTION

When man invented the wheel, he very quickly learnt that if it wasn’t completely round and if it didn’t rotate evenly about it’s central axis, then he had a problem! What the problem he had? The wheel would vibrate causing damage to itself and it’s support mechanism and in severe cases, is unusable. A method had to be found to minimize the problem. The mass had to be evenly distributed about the rotating centerline so that the resultant vibration was at a minimum.

UNBALANCE

The condition which exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal forces is called unbalance or the uneven distribution of mass about a rotor’s rotating centerline.

sub Heading Rotating centerline

The rotating centerline being defined as the axis about which the rotor would rotate if not constrained by its bearings. (Also called the Principle Inertia Axis or PIA).

sub Heading Geometric centerline

The geometric centerline being the physical centerline of the rotor. When the two centerlines are coincident, then the rotor will be in a state of balance. When they are apart, the rotor will be unbalanced. Different types of unbalance can be defined by the relationship between the two centerlines. These include:

sub sub Heading of Static Unbalance

where the PIA is displaced parallel to the geometric centerline.

sub sub Heading of Couple Unbalance

where the PIA intersects the geometric centerline at the center of gravity. (CG)

sub sub Heading of Dynamci Balance

where the PIA and the geometric centerline do not coincide or touch. The most common of these is dynamic unbalance. Causes of Unbalance: In the design of rotating parts of a machine every care is taken to eliminate any out of balance or couple, but there will be always some residual unbalance left in the finished part because of 1. slight variation in the density of the material or 2. inaccuracies in the casting or 3. inaccuracies in machining of the parts.

Sub Heading Why balancing is so important?

1. A level of unbalance that is acceptable at a low speed is completely unacceptable at a higher speed. 2. As machines get bigger and go faster, the effect of the unbalance is much more severe. 3. The force caused by unbalance increases by the square of the speed. 4. If the speed is doubled, the force quadruples; if the speed is tripled the force increases by a factor of nine! Identifying and correcting the mass distribution and thus minimizing the force and resultant vibration is very very important

Balancing

Balancing is the technique of correcting or eliminating unwanted inertia forces or moments in rotating or reciprocating masses and is achieved by changing the location of the mass centers. The objectives of balancing an engine are to ensure: 1. That the centre of gravity of the system remains stationery during a complete revolution of the crank shaft and 2. That the couples involved in acceleration of the different moving parts balance each other.

Types of balancing

a) Static Balancing:

  i) Static balancing is a balance of forces due to action of gravity.
  ii) A body is said to be in static balance when its centre of gravity is in the axis of rotation.

b) Dynamic balancing:

  i) Dynamic balance is a balance due to the action of inertia forces.
  ii) A body is said to be in dynamic balance when the resultant moments or couples, which involved in the acceleration of different moving parts is equal to zero.
  iii) The conditions of dynamic balance are met, the conditions of static balance are also met.

    In rotor or reciprocating machines many a times unbalance of forces is produced due to inertia forces associated with the moving masses. If these parts are not properly balanced,the dynamic forces are set up and forces not only increase loads on bearings and stresses in the various components, but also unpleasant and dangerous vibrations.Balancing is a process of designing or modifying machinery so that the unbalance is reduced to an acceptable level and if possible eliminated entirely.

BALANCING OF ROTATING MASSES

       When a mass moves along a circular path, it experiences a centripetal acceleration and a

force is required to produce it. An equal and opposite force called centrifugal force acts radially outwards and is a disturbing force on the axis of rotation. The magnitude of this remains constant but the direction changes with the rotation of the mass. In a revolving rotor, the centrifugal force remains balanced as long as the centre of the mass of rotor lies on the axis of rotation of the shaft. When this does not happen, there is an eccentricity and an unbalance force is produced. This type of unbalance is common in steam turbine rotors, engine crankshafts, rotors of compressors, centrifugal pumps etc.

      The unbalance forces exerted on machine members are time varying, impart vibratory

motion and noise, there are human discomfort, performance of the machine deteriorate and detrimental effect on the structural integrity of the machine foundation.

      Balancing involves redistributing the mass which may be carried out by addition or

removal of mass from various machine members Balancing of rotating masses can be of 1. Balancing of a single rotating mass by a single mass rotating in the same plane. 2. Balancing of a single rotating mass by two masses rotating in different planes. 3. Balancing of several masses rotating in the same plane 4. Balancing of several masses rotating in different planes