BALANCING OF ROTATING MASS

Introduction

If the mass center of a component of mass 'm' is rotating at an angular velocity   at a

distance 'r' from the axis of rotation, then the component is subjected to force of mr2

The 'out of balance' forces increase bearing loads, and introduce stresses in the rotor and

framework of a machine. These so called 'inertial forces' may introduce dangerous

vibrations, structural failure or unacceptable noise, and may limit the operating speed

range of a machine. The magnitude of these forces may be reduced or eliminated in the

design stage by 'balancing' the effects of the various mass elements of the device.

Additionally, extra balance masses may deliberately added to a rotating system in order

to cancel out the residual design imbalance.

This experiment involves the balancing of a number of known out of balance masses on a

shaft.

There are two types of balancing.

Static and Dynamic Balance

So far we have considered the case of a wheel, which approximates to a simple disc, having all its mass in or near one plane. If this is statically balanced in the way described, it will run at any speed without vibration. But in a rotating body having a fairly considerable axial length, such as a cylinder, it is important that any local unbalanced mass should be balanced out by a mass as nearly in the same cross plane as possible.

The static method of balancing, in this case, is not reliable because it gives no indication of the position of the bias in relation to axial length. Thus the cylindrical rotor, an armature shaft for instance, shown in Fig. 2, may be heavy at the point A, as indicated by a static balancing test. If this unbalanced mass is counteracted by a weight applied at the point B, the rotor will appear to be in correct balance; but when running at high speed, the effect of the two unbalanced masses will cause local reactions R-R which tend to rock the shaft along its length, or in other words to set up a "couple." In practice, the effect of this may be worse than that of a single unbalanced force which tends to vibrate the structure bodily, and it is often much more difficult to detect and correct.

The method usually employed for dynamic balancing is to mount the shaft in bearings on a frame which is resiliently mounted, usually by some form of spring suspension, so that it is capable of being displaced in any plane by the effect of unbalanced forces. Means are provided for locking the frame while the shaft is run up to a fair speed by any convenient means, after which it is released and allowed to vibrate or oscillate under the effect of the unbalanced forces. In modern dynamic balancing machines, indicating or recording devices are provided to show the position and extent of the unbalanced masses. While it would not be impossible to construct a simple dynamic balancing rig in the home workshop, most of the problems involved in small machines can be dealt with by careful consideration of design, and accuracy in construction of moving parts. It may be mentioned that even the balancing machine, unless of very complex design, may leave certain important considerations out of account.

For instance, suppose that a rotor having an unbalanced mass at J (Fig. 3A) is balanced by adding two smaller masses at the points K, L. The rotor is then in correct dynamic balance, and in the case of a fairly rigid component, such as an armature, it will be perfectly satisfactory in practice. But suppose the same principle is applied to a non-rigid component, such as a crankshaft; in this case, the cancelling masses, being in different planes, exert bending stresses on the shaft, and the latter may be deflected, thereby altering the moment of the masses and putting the system out of balance (Fig. 3B).

This is only one of the many pitfalls in practical balancing, which cause the designer many headaches, and are rarely capable of being dealt with by theoretical calculation. Another example occurs in the case of a rotating body which for practical reasons cannot be made symmetrical in shape, though the moments of mass are calculated and counterweights added where necessary to cancel out and give correct balance as in Fig. 4. When running at high speed, however, the effect of centrifugal forc,e causes the flywheel to distort, and thereby displace the masses to a varying extent, thereby unbalancing them. In case readers think this is an unlikely eventuality, I may say that I once worked on a certain type of flywheel magneto which gave a great deal of trouble through this cause, though dynamic balancing tests gave no indication of the source of error.

Balance weights, whatever their type or purpose, should always be located as close to the plane of the unbalanced mass as possible. Thus, in the case of the crankshaft shown in Fig. 3B, it would be better to attach the counterweights to the crank webs than at the points indicated. The practice of fitting balance weights to external flywheels, therefore, is one that cannot be commended; in the case of an overhung crankshaft, any bias in the flywheel would set up a violent rocking couple. Flywheels should always be at least in static balance, and if of any great width, dynamic balancing is desirable. An exception is made in the case of internal flywheels, as in motor-cycle engines, which are close to the crank pins, and usually form the crank webs.

DISCUSSION

Applications use rotating balancing to maintain there characters properly

1.       Engine crank shaft

2.       Automobile wheel

3.       Grinding wheels

4.       Steam turbines and runners

5.       Compressors

6.       Washing machines

7.       Air craft propellers

Types of Balancing

There are two forms of balancing: static and dynamic.

Static balancing is done by holding the component at its axis, then compensating (by removal or addition of mass) for the "heavy" side of the component. During Static balancing, the component is not rotating, hence "static".  Static balancing is typically done on "flat" parts, or parts that have a large diameter to axial length ratio (pancake like parts, such as fans, pulleys, wheels). 

Dynamic balancing is done on parts that are long compared to their diameters such as rotor assemblies.  These parts require balancing to be done in two planes since the actual imbalance will intersect the center line/axis.  Unless both ends of the part are balanced, mass imbalance will continue to exist.  Rotor assemblies that MCE manufactures are balanced dynamically (in two planes).

Balancing can be achieved by the addition or removal of mass in certain locations.  MCE only provides balancing by use of mass removal which is achieved by abrasive material removal or by drilling/machining.  Note that when designing your part, take into account that material removal will be required and allow for extra material such as a balancing ring or thicker flanges than required by design to achieve mechanical structural integrity. 

Why we need Balancing of rotating Masses

1. It will reduce the unwanted vibrations

2. it reduce the energy waste so it will increase the efficiency of the machine.

3. Vibrating parts wear quickly . so balancing is so important to have a long life time for the shafts.

4. When shaft is unbalanced it bends when it’s rotating so it s subjected to compression and expansion  alternatively so it fails easily.

5.Bearing wear due to large forces on them