Engine Balancing

OverviewEngine balancing is one of the important aspects for smooth operation of diesel engine. Balancing helps in reducing engine vibration to an acceptable limit.Unbalanced engine is liable to excite severe vibration and cause damage to engine components and ship structure.

Learning Objectives

Meaning of BalancingStatic Balancing

Dynamic BalancingPartial Balancing of Engine

Complete Balancing of Engine

IntroductionEngine Balancing

Balancing of Gas Pressure Balancing of Moving Parts

Balancing of Moving PartsForce & moment Balancing Static Balancing

Dynamic Balancing

Static BalanceWhen a shaft carries a number of eccentric masses and if the sum of the moments of all the masses about the axis of rotation is equal to Zero for all angular positions, the shaft assembly is said to be in Static Balance

Static BalanceAny multi cylinder crankshaft will be statically balanced, when the cranks are equally spread through out the cycle.

If it is out of balance statically, it will have its centre of gravity off the polar axis.The crank spread is determined from the standard formula: Crank spread =

number of degrees in cycle number of cylinders

Hence, a 6 Cyl 2-stroke engine has a crank spread of 360 60o

6

4

1

2 6 5

4 1

2 5

3

3

6

The closed polygon indicates static balance

Dynamic BalanceAny multi-cylinder crankshaft that is statically balanced may generate an out-of-balance couple or moment when rotating, causing dynamic imbalance.

To achieve dynamic balance, balance weights may be added to eliminate the out-of-balance couple

Dynamic Balancing

Dynamic balancing is a way of controlling vibrations by arranging the overall summation of out-of balance forces and couples cancels out or reduced to more acceptable value There are two groups of forces and couples - related to: revolving mass reciprocating mass

Rotating mass balance is quite easily obtained by choice of crank sequence and balance weight Reciprocating mass balance is more complicated.

Disturbing Forces in Engine

Inertia force due to reciprocating motion Reciprocating motion of piston creates out-of-balance forces acting along the cylinder

Inertia force due to rotating motion Centrifugal force of the crankpin rotating about the main bearing

centre creates a rotating out of balance force

These forces create objectionable vibration and noise in

engine foundation and to the ship structure

Inertia Force Revolving MassInertia force of rotating mass,

F = Mrot x f = Mrot x 2 rThis force acting on the crankpin will act radially outwards at all times and has two components: Vertical component = Mrot x 2 r cos

Horizontal component = Mrot x 2 r sin These components are first order force and will act at a frequency of engine RPM

Inertia Force Reciprocating MassInertia force of reciprocating mass, F = Mrec x f Piston acceleration, f = 2 r (cos + cos 2) n The inertia force is split up into: Primary disturbing or 1st order force, Fp = Mrec x 2 r cos Secondary disturbing or 2nd order force, Fs = Mrecx 2 r cos 2 n F=F +Fp s

First order & Second order forces are n generated by acceleration of reciprocating masses of the running gears.

Free MomentsFrom Engine

Controlling of First Order MomentFirst order moments, arising out of primary disturbing forces, act in both the vertical and horizontal directions.

First order moment is controlled in the following way:

Standard Counterweights / Balance weights

Option 1st order moment compensator

Counterweight / Balance weight

Its an integral part of crank web or separate piece bolted to the web

Counter-balances out of balance forces / momentsPrevents vibration Causes smooth running Reduces load on main bearing

BalancingPrimary Inertia force by Balance Weight

Partial Balance of Primary Reciprocating Force by Revolving Counterweight

Primary Force Balancer

The primary reciprocating force cannot be balanced by a single counter weight It is possible to balance by two such weights rotating in opposite directions

Secondary Force BalancerSecondary reciprocating forces can only be precisely balanced by two counter weights rotating in opposite directions at twice the crankshaft speed. Secondary force is much smaller

in magnitude than primary andproportional to crank throw con-rod length ratio (1/n)

Second Order Moment Compensator

This eliminates the out-of-balance second order moment The forces from the unbalanced reciprocating masses are the only forces that generate second order moments. These forces act only in the vertical direction The Lanchester balancer is a second order moment compensator and it consists of two counter rotating masses running at twice engine RPM These counter rotating masses can be fitted at the aft end of the engine, driven by the camshaft chain the forward end of the engine, driven by auxiliary chain both ends of the engine driven by gears

MAN B&W Second

OrderMoment Compensator

Action of fitting Lanchester secondary balancer to both ends of engine, where engine is installed at a node of hull vibration

Single Lanchester secondary balancer fitted at one end of engine, when node of hull vibration is near the other end of the engine

Electrical Balancer

Combined Balancer arrangement for SulzerRTA-series low speed engines to counteract both first and second order free couples

1.2.

Define the terms static balance and "dynamic balance. List the forces and moments which may come about in a vertical multi cylinder engine.

3.

Explain how the reciprocating motion of the piston and the revolving motion of the crank effect the balancing of a diesel engine. Explain how the effect due to out of balance forces is controlled within acceptable limit. State what do you understand by primary and secondary disturbing forces. Explain how the primary and secondary forces are balanced in a multi-cylinder engine. Explain what do you understand by the statement : an engine is in good balance .

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