28-05-2013, 03:24 PM
BALANCING OF INLINE AND RADIAL ENGINES
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INTRODUCTION
The internal combustion engine of multi-cylinder type are in common use. Each cylinder carries piston which reciprocates and is responsible for creating disturbing force on foundation. Each cylinder will have primary and secondary disturbing forces which have to be balanced. The cylinders are arranged in a line with 2, 3 or more. Two cylinders or banks of four or six cylinders may be arranged with centre lines at angle resulting in a V-formation. The cylinders may be arranged on a circle with central lines along radii meeting at centre on a single crank pin giving configuration of a radial engine. In W-engine three cylinders or three rows of cylinder are arranged with their centre lines at some acute angle.
In each case the cylinders do not fire at the same time but two or more may fire at a time. Their firing order will not, however, affect unbalanced disturbing force or balancing process which are only affected by weight or mass of reciprocating and rotating parts.
PRIMARY BALANCE OF MULTI-CYLINDER IN-LANE ENGINE
In multi-cylinder in-line arrangement all the cylinder centre lines are parallel and in the same plane which is on the same side of the crank shaft centre line. This appears to be the most common configuration in use.
For the reciprocating parts weighing R and crank rotating at ω rad/s, the condition for balanced state will be defined by two statements.
DIRECT AND REVERSE CRANK METHOD
By this method, the reciprocating unbalancing can be converted into an equivalent rotating unbalance by using the direct and reverse crank for primary and secondary. This method is very useful for V-engines and radial engines.
COMPLETE BALANCE OF A SINGLE CYLINDER ENGINE
We can draw conclusion that complete balance, comprising balance of primary and secondary forces and primary and secondary moments, is normally not possible. In a single cylinder engine, however, we can create design that may satisfy the condition of complete balance. Our description below will show that such a proposal, though may be feasible, will increase the cost and make a single cylinder engine much bulkier than with partial balance.
BALANCING MACHINES
You have been exposed to the calculation of disturbing forces that arise due to unbalanced system of forces in machines and machine parts. The possible suggestions for design have also been offered for complete or partial balancing.
The designer is likely to take all measures in making rotating parts of machines perfectly balanced so that out-of-balance force or moment are eliminated, yet either due to slight variation in material density or inaccuracies in casting and machining residual errors may exist. Such residual out-of-balance force and couple have to be determined for deciding and executing practical measures for achieving 100% balanced system. For this balancing machines are needed. The need for determining actual state of machine part (whether static or dynamic) cannot be overlooked even if such imbalances are small. We must remember that the centrifugal forces and moments due to these forces are proportional to square of angular speed and hence small increase of angular speed may result in considerable increase in unbalanced forces and moments even if the inaccuracy in machine part is very small.
In such cases where a rotating part is of large diameter and comparatively of narrow width the static balancing may be sufficient. Dynamic couples in such cases will be so small that their effects may not cause much disturbance. However, for parts which are long axially, dynamic balancing may be necessary. The requirement of balancing has translated into existence of several balancing machines of static and dynamic types to help determine the balanced state of rotating parts. There are machines which can measure both static and dynamic unbalance.
Static Balancing Machines
A very simple balancing machine to find out-of-balance disc and ring is shown in Figure 21.15. It essentially consists of a heavy base extended in a column to sufficient height. It carries a scale on which a needle indicator moves. The scale may read unbalance in units of Nm. A beam or an arm is supported on a knife edge near the middle of the height. When left to itself the beam will be horizontal. At one end of the machine a mandrel can be carried in a matching groove. On the other end similar mandrel will carry a hanger to support dead weight.
Rotating part, whose balance is to be checked is placed on mandrel which is placed in groove as shown. Dead weights are placed in hanger to bring the indicator near zero on the scale to indicate that the dead weight can balance the weight of part. The part is then rotated or displaced on mandrel either by hand or by a motor to see if the beam remains horizontals in all positions of the part to be balanced. If there is additional mass in the part as shown by small circle at a distance from r from the centre then the beam will tilt towards the dead weight as shown in Figure 21.15. If the additional mass moves to far off position then the beam will tilt in the direction of the part. The additional mass being anywhere between extreme horizontal to extreme vertical positions will cause the beam to occupy some intermediate position.
Dynamic Balancing Machine
The cradle in machine of Figure 21.16 may be made to rock about knife edge if it is placed in the centre as shown in Figure 21.17. The axis of knife edge, QQ is perpendicular to axis is rotation, MB. Presence of unbalanced force or an unbalanced moment will cause the cradle to rock about the knife edge axis, QQ. This machine (Figure 21.3), thus is a dynamic balancing machine. Before any part is tested in this machine it should be statically balanced so that only effect of unbalanced moment is observed.
Measurement of Unbalanced Force and Moment
In dynamic balancing machine as separate device is used for measurement of force and couple which are not balanced. The basic principle is to apply known force and couple on the cradle in a direction to oppose the effect of disturbance. This is achieved by rotating masses which are statically balanced on vertical shaft but being in different planes (parallel planes) exert a moment in vertical plane which is transferred to cradle. This imposed moment can be measured and varied by changing the distance between planes of rotating masses and also be changing their relative position in the parallel planes. Thus, the device requires rotation of shaft, change of distance between the planes of rotation of masses and the angular positions of the masses in parallel planes. Yet another requirement will be to effect the changes when the part to be balanced (P in Figures 21.17 and 21.18) is rotating on the cradle.
SUMMARY
The methods of calculating the balanced or unbalanced situations in in-line and V-engines have been brought out. The IC engines are often used in multi-cylinder configuration. Many configurations are balanced by properly placing the cranks round the crank shaft. However, no single method can be used to determine balancing. The methods deal individual cylinders and then unbalance is found in terms of forces and couples. The method of reverse crank has been discussed to find residual unbalance. Providing balance for secondary forces is much cumbersome as it needs rotating masses which would revolve at twice the speed of crank of the engine. Such masses will be required to rotate at very small distance from the axis.
Machines that can measure unbalance force in a rotating part have been designed and made. They are used practically. Narrow parts that do not create significant moment or some parts are balanced by static balancing machines which consist of balanced beam. One end of the beam carries the part to be balanced and other end carries the dead weight. The swing of balanced beam measured on the machine indicates the out-of-balance force which can be removed by adding or removing material from the part to be balanced.