Engine BalancingEngine Balancing

OverviewOverview

Engine balancing is one of the important aspects for Engine balancing is one of the important aspects for smooth smooth

operation of diesel engine. operation of diesel engine.

Balancing helps in reducing engine vibration to an Balancing helps in reducing engine vibration to an acceptableacceptable

limit.limit.

Unbalanced engine is liable to excite severe vibration and Unbalanced engine is liable to excite severe vibration and

cause damage to engine components and ship structure.cause damage to engine components and ship structure.

Learning ObjectivesLearning Objectives

► Meaning of BalancingMeaning of Balancing

► Static BalancingStatic Balancing

► Dynamic BalancingDynamic Balancing

► Partial Balancing of EnginePartial Balancing of Engine

► Complete Balancing of EngineComplete Balancing of Engine

IntroductionIntroduction

Engine BalancingEngine Balancing

► Balancing of Gas PressureBalancing of Gas Pressure

► Balancing of Moving PartsBalancing of Moving Parts

Balancing of Moving Balancing of Moving PartsParts

Force & moment BalancingForce & moment Balancing

Static BalancingStatic Balancing

Dynamic BalancingDynamic Balancing

Static BalanceStatic Balance

When a shaft carries a number of eccentric When a shaft carries a number of eccentric massesmasses

and if the sum of the moments of all the massesand if the sum of the moments of all the masses

about the axis of rotation is equal to about the axis of rotation is equal to ZeroZero for all for all

angular positions, angular positions,

the shaft assembly is said to be in the shaft assembly is said to be in Static Static BalanceBalance

Static BalanceStatic Balance

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

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

Crank spread =Crank spread =

Hence, a 6 Cyl 2-stroke engine has a crank spread of Hence, a 6 Cyl 2-stroke engine has a crank spread of oo

number of degrees in cyclenumber of cylinders

360 60 6

5

16

32

4

1

63

5

2 4

The closed polygon indicates static balance

Dynamic BalanceDynamic Balance

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

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

Dynamic BalancingDynamic Balancing

► Dynamic balancing is a way of controlling vibrationsDynamic balancing is a way of controlling vibrations by by arranging the overall summation of out-of balance forces arranging the overall summation of out-of balance forces

and and couples cancels out or reduced to more acceptable valuecouples cancels out or reduced to more acceptable value

► There are two groups of forces and couples - related to:There are two groups of forces and couples - related to:

revolving massrevolving mass

reciprocating massreciprocating mass

► Rotating mass balance is quite easily obtained by choice Rotating mass balance is quite easily obtained by choice of of

crank sequence and balance weightcrank sequence and balance weight

► Reciprocating mass balance is more complicated.Reciprocating mass balance is more complicated.

Disturbing Forces in Disturbing Forces in EngineEngine

► Inertia force due to reciprocating motionInertia force due to reciprocating motion

Reciprocating motion of piston creates out-of-balance forces Reciprocating motion of piston creates out-of-balance forces actingacting

along the cylinderalong the cylinder

► Inertia force due to rotating motionInertia force due to rotating motion

Centrifugal force of the crankpin rotating about the main Centrifugal force of the crankpin rotating about the main bearingbearing

centre creates a rotating out of balance forcecentre creates a rotating out of balance force

These forces create objectionable vibration and These forces create objectionable vibration and noise innoise in

engine foundation and to the ship structureengine foundation and to the ship structure

Inertia Force – Revolving MassInertia Force – Revolving Mass

Inertia force of rotating mass, Inertia force of rotating mass,

F = MF = Mrotrot x f = M x f = Mrotrot x x 2 2 r r

This force acting on the crankpin will act This force acting on the crankpin will act

radially outwards at all times and has tworadially outwards at all times and has two

components:components:

Vertical component = MVertical component = Mrotrot x x 2 2 r cos r cos

Horizontal component = MHorizontal component = Mrotrot x x 2 2 r sin r sin

These components are first order force These components are first order force

and will act at a frequency of engine RPMand will act at a frequency of engine RPM

Inertia Force – Reciprocating MassInertia Force – Reciprocating Mass

Inertia force of reciprocating mass,Inertia force of reciprocating mass, F = MF = Mrecrec x f x f

Piston acceleration, f = Piston acceleration, f = 2 2 r (cos r (cos + + cos 2cos 2) )

The inertia force is split up into:The inertia force is split up into:

Primary disturbing or 1Primary disturbing or 1stst order force, order force, F Fpp = M = Mrecrec x x 2 2 r cos r cos

Secondary disturbing or 2Secondary disturbing or 2ndnd order force, order force,

F Fss = M = Mrecrecx x 2 2 r r cos 2cos 2

F = FF = Fpp + F + Fss

First order & Second order forces areFirst order & Second order forces aregenerated by acceleration of reciprocatinggenerated by acceleration of reciprocatingmasses of the running gears.masses of the running gears.

n

n

n

Free

Moments

From

Engine

Controlling of First Order Controlling of First Order MomentMoment

First order moments,First order moments, arising out of primary arising out of primary disturbing disturbing

forces, act in both the vertical and horizontal forces, act in both the vertical and horizontal directions.directions.

First order moment is controlled in the following First order moment is controlled in the following way:way:

► StandardStandard Counterweights / Balance weightsCounterweights / Balance weights

► OptionOption 11stst order moment compensator order moment compensator

Counterweight / Balance weightCounterweight / Balance weight

► It’s an integral part of crank web or separate pieceIt’s an integral part of crank web or separate piece

bolted to the web bolted to the web

► Counter-balances out of balance forces / momentsCounter-balances out of balance forces / moments

► Prevents vibrationPrevents vibration

► Causes smooth runningCauses smooth running

► Reduces load on main bearingReduces load on main bearing

BalancingBalancing

Primary Inertia Primary Inertia forceforce

by by

Balance WeightBalance Weight

Partial Balance of Primary Reciprocating Force by Revolving Counterweight

Primary Force BalancerPrimary Force Balancer

► The primary reciprocatingThe primary reciprocating

force cannot be balanced force cannot be balanced

by a single counter weightby a single counter weight

► It is possible to balance byIt is possible to balance by

two such weights rotating two such weights rotating

in opposite directions in opposite directions

Secondary Force BalancerSecondary Force Balancer

Secondary reciprocating forces Secondary reciprocating forces

can only be precisely balanced can only be precisely balanced

by two counter weights rotating by two counter weights rotating

in opposite directions at twice in opposite directions at twice

the crankshaft speed.the crankshaft speed.

Secondary force is much smaller Secondary force is much smaller

in magnitude than primary andin magnitude than primary and

proportional to crank throw proportional to crank throw

con-rod length ratio (1/n)con-rod length ratio (1/n)

Second Order Moment CompensatorSecond Order Moment Compensator

► This eliminates the out-of-balance second order momentThis eliminates the out-of-balance second order moment

► The forces from the unbalanced reciprocating masses areThe forces from the unbalanced reciprocating masses are the only forces that generate second order moments. the only forces that generate second order moments.

► These forces act only in the vertical directionThese forces act only in the vertical direction

► The The Lanchester balancerLanchester balancer is a second order moment is a second order moment

compensator and it consists of two counter rotating compensator and it consists of two counter rotating masses running at twice engine RPM masses running at twice engine RPM

► These counter rotating masses can be fitted atThese counter rotating masses can be fitted at the aft end of the engine, driven by the camshaft chainthe aft end of the engine, driven by the camshaft chain

the forward end of the engine, driven by auxiliary chainthe forward end of the engine, driven by auxiliary chain

both ends of the engine driven by gears both ends of the engine driven by gears

MAN B&W

Second

Order

Moment

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 Sulzer RTA-series low speed engines to counteract

both first and second order free couples

1. 1. Define the terms “static balance” and "dynamic balance”.Define the terms “static balance” and "dynamic balance”.

2. List the forces and moments which may come about in a 2. List the forces and moments which may come about in a vertical vertical

multi cylinder engine.multi cylinder engine.

3. Explain how the reciprocating motion of the piston and the 3. Explain how the reciprocating motion of the piston and the

revolving motion of the crank effect the balancing of a diesel revolving motion of the crank effect the balancing of a diesel

engine.engine.

4. Explain how the effect due to out of balance forces is 4. Explain how the effect due to out of balance forces is

controlled within acceptable limit.controlled within acceptable limit.

5. State what do you understand by primary and secondary 5. State what do you understand by primary and secondary

disturbing forces.disturbing forces.

6. Explain how the primary and secondary forces are balanced in 6. Explain how the primary and secondary forces are balanced in

a multi-cylinder engine.a multi-cylinder engine.

7. Explain what do you understand by the statement : “ an 7. Explain what do you understand by the statement : “ an

engine is in good balance ”.engine is in good balance ”.