engine top bracing

8/20/2019 Engine Top Bracing.

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Engine Top BracingEngine Top Bracing


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Hull structure

Top bracing

Engine


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Learning ObjectivesLearning Objectives

Understand the purpose of engine top bracing arrangementUnderstand the purpose of engine top bracing arrangement

Describe the mechanical top bracing arrangementDescribe the mechanical top bracing arrangement

Explain the functioning of mechanical top bracing arrangementExplain the functioning of mechanical top bracing arrangement

Describe the hydraulic top bracing arrangementDescribe the hydraulic top bracing arrangement

Explain the functioning of hydraulic top bracing arrangementExplain the functioning of hydraulic top bracing arrangement

Describe the periodic maintenance to be carried outDescribe the periodic maintenance to be carried out


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OverviewOverview

Engine top bracings are used to connect upper end of the engineEngine top bracings are used to connect upper end of the engine

to a hull structure for damping lateral vibration generateto a hull structure for damping lateral vibration generated by slowd by slowspeed main enginesspeed main engines

Bracing bolts play an important role for satisfactory operatiBracing bolts play an important role for satisfactory operati

on ofon of

slow speed diesel enginesslow speed diesel engines

Prolonged running with slack bracing bolts may cause seriousProlonged running with slack bracing bolts may cause serious

vibration and damage to engine structurevibration and damage to engine structure

Proper maintenance of engine top bracing arrangement isProper maintenance of engine top bracing arrangement is

essential at regular intervals.essential at regular intervals.


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Forces

Acting on

X-Head

Guides

Due to

Con-rod

Angularity


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Guide Force MomentGuide Force Moment

Forces on the guides will cause the guide force moment betweenForces on the guides will cause the guide force moment between

guide shoe axis and c/shaft axisguide shoe axis and c/shaft axis

Guide force moments are caused by the gas force on the pistonsGuide force moments are caused by the gas force on the pistonsand by the inertia forcesand by the inertia forces

The moments may excite engine vibrations by moving the engineThe moments may excite engine vibrations by moving the engine

top athwart shipstop athwart ships

It may cause a rocking (excited by HIt may cause a rocking (excited by H--moment) or twistingmoment) or twisting

(excited by X(excited by X--moment) movement of the engine.moment) movement of the engine.

Engines with less than seven cylinders, this guide force momentEngines with less than seven cylinders, this guide force momenttends to rock the engine in transverse directiontends to rock the engine in transverse direction

Engines with seven cylinders or more, it tends to twist the engiEngines with seven cylinders or more, it tends to twist the enginene


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Transverse Vibration of EngineTransverse Vibration of Engine


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Transverse Vibration

Due to

Guide

Force

Moments


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To counteract the impact of guide force momentTo counteract the impact of guide force moments

the installation of a set of top bracings isthe installation of a set of top bracings is

recommended between the upper gallery of therecommended between the upper gallery of the

main engine and the hull structure.main engine and the hull structure.


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Top BracingTop Bracing

FunctionFunction

To detune lateral vibration of main engine by increasingTo detune lateral vibration of main engine by increasingthe natural frequency of engine vibrating system to suchthe natural frequency of engine vibrating system to such

an extent that resonance occurs above the runningan extent that resonance occurs above the running

range of engine speed.range of engine speed.

If vibration level due to guide force moment is low andIf vibration level due to guide force moment is low and

within acceptable limit, top bracing may be eliminated.within acceptable limit, top bracing may be eliminated.

Vibration level normally to be checked by measurementsVibration level normally to be checked by measurements

with and without top bracing during sea trial.with and without top bracing during sea trial.


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Top Bracing ArrangementTop Bracing Arrangement

Mechanical Friction TypeMechanical Friction Type

Hydraulic TypeHydraulic Type


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Top bracing is installed between

the engine's upper platformbrackets

Top bracing is normally placed onthe exhaust side of the engine

Top bracing can alternatively be

placed on the camshaft side


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Mechanical Top Bracing Arrangement


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Mechanical Top Bracing ArrangementMechanical Top Bracing Arrangement


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Hydraulic Top BracingHydraulic Top Bracing The hydraulically adjustable top bracing is anThe hydraulically adjustable top bracing is an

alternative to the mechanical top bracingalternative to the mechanical top bracing

It is intended for application in vessels where hullIt is intended for application in vessels where hull

deflection is foreseen to exceed the usual leveldeflection is foreseen to exceed the usual level

oror

In cases where the detuning effect from the topIn cases where the detuning effect from the top

bracings is only required in part of the engine speedbracings is only required in part of the engine speed

range.range.


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Hydraulic

top bracingarrangement


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Hydraulic Cylinders And Pump Station


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HydraulicHydraulic

TopTopBracingBracing

ArrangementArrangement


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Hydraulic TopHydraulic Top

Bracing ArrangementBracing Arrangement


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w


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Hydraulic

Top-Bracing


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Hydraulic TopHydraulic Top--BracingBracing

Working principleWorking principle

The hydraulic topThe hydraulic top--bracing consists of a singlebracing consists of a single--acting hydraulic cylinder andacting hydraulic cylinder anda hydraulic controla hydraulic control--unit mounted directly on the cylinder housing.unit mounted directly on the cylinder housing.

The topThe top--bracing is self bracing is self --adjusting, using the vibration energy emitted byadjusting, using the vibration energy emitted by

the engine.the engine.

When the engine is running, the unit builds up a pressure in proWhen the engine is running, the unit builds up a pressure in proportionportion

to the vibration level.to the vibration level.

This detunes the engineThis detunes the engine’’s natural frequencies and accordingly lowers the impact ofs natural frequencies and accordingly lowers the impact of

the vibrations.the vibrations.

When the engine is at standstill, the pressure is then released.When the engine is at standstill, the pressure is then released.


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When the distance

between the hull and

engine increases, oil flows into the cylinder

under pressure from the

accumulator

Working Principle


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When the distance between

hull and engine decreases,

a non return valve prevents

oil from flowing back to the

accumulator and the pressure

rises.

If the pressure reaches a preset

maximum value, a relief valve

allows the oil to flow back to

the accumulator, maintaining

the force on the engine and

hull structures below the

specified level.

Working Principle


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The top-bracing is automatically

controlled by the RPM signal fromthe propulsion control-system.

The system is active as standardwhen the engine is running, but a

programmable switch makes itpossible to choose an RPM rangewithin which the top-bracing systemis active.

For service purposes, manual controlis also possible.

Operation


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MaintenanceMaintenance

&&

InspectionInspection


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Mechanical Top BracingMechanical Top Bracing Relative movements between top bracing andRelative movements between top bracing and

fastening plate to be checked with dial gaugefastening plate to be checked with dial gauge

Accordingly tightening of bracing bolt to be carriedAccordingly tightening of bracing bolt to be carried

out with specified torqueout with specified torque

Tightening to be checked with torque wrench orTightening to be checked with torque wrench or

by hydraulic tensioningby hydraulic tensioning

Checking should be carried out once or twice a year.Checking should be carried out once or twice a year.


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If relative

movementlarger than

+/- 0.02 mm,

increase

tightening

torque


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Hydraulic Top BracingHydraulic Top Bracing Oil pressure of the hydraulic cylinders are to beOil pressure of the hydraulic cylinders are to be

compared with each other at regular intervalscompared with each other at regular intervals

If one of the cylinders shows a considerable lowerIf one of the cylinders shows a considerable lower

pressure, the opposite hydraulic cylinder must bepressure, the opposite hydraulic cylinder must be

released of its pressure until the problem has beenreleased of its pressure until the problem has been

remediedremedied

Oil leakages to be checkedOil leakages to be checked

Hydraulic cylinder to be overhauled in case of leakageHydraulic cylinder to be overhauled in case of leakage

engine top bracing

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