Advance Science. Applied Technology

Main Bearing Analysis in A

Medium Speed Ship Engine GT Users’ Conference, November 9, 2015

Kevin L. Hoag

Institute Engineer

1

Project Background and Objective

New steel-hull ship

– Two, counter-rotating medium speed V16 diesel engines

– Crankshafts mounted on bedplates – lower main bearing shells

– Lower end repairs require cutting the hull

Imperative to ensure virtually zero main bearing wear

– Pre-lube system

– Conservative MOFT

Objectives

– Validate main bearing MOFT under all conceivable conditions

2

Considerations Evaluated

Engine speed and load

Oil temperature and viscosity

Two possible firing orders, with CW and CCW crankshaft

rotation

Crankshaft torsional vibration

Filtration efficiency

Hull deflection – resulting main bearing deflection

Main bearing misaligment

3

GT Crankshaft System Model

• The flywheel and a partial selection of cylinders are illustrated

• At the crank nose is a torsional vibration damper with adjustable damping

• “Block” object is rigid geometry defining main bearing saddle locations

• “Crankconfig” object specifies crank rotational direction and firing order

Crankshaft Models

Initial Approach CADD model with section mass and stiffness input to stiff beam model

Revised Apprach FEA stiffness calculations input to 3-D quasi-static model

Main Bearing 5: Quasi-Static FEA Forces

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4

12

5

11 14

3 6

Main Bearing 5: Beam Forces

Advance Science. Applied Technology

With clockwise crankshaft rotation, the

figures shown are the ‘X’ and ‘Y’ load

components at rod bearings 1 and 9

(upper and lower left respectively), and

main bearing 1 (below)

Advance Science. Applied Technology

Each rod bearing shows the combination of reciprocating and rotating forces, creating repeating load cycles.

Advance Science. Applied Technology

Superimposed on the kinematic forces are the firing forces for cylinders 1 and 9

Advance Science. Applied Technology

The main bearing force components are a summation of those from each adjacent rod. End mains in a vee engine see forces from two rods. Interior mains see those from four rods.

Main Bearing 3

Cylinder 3 fires

Cylinder 11 fires

Cylinder 10 fires

Cylinder 2 fires

Main Bearing 4

Cylinder 3 fires Cylinder 11 fires Cylinder 12 fires

Cylinder 4 fires

Cylinder Pressure

MOFT is most impacted, not from the pressure of an

individual cylinder, but when adjacent combustion

events are “stacked”

With initial firing order all main bearings see stacked

combustion events, and similar MOFT

With revised firing order the 3,5, and 7 main

bearings see stacked combustion events

Effect of Firing Order

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1

2 3

4 5

6 7

8 9

10 11

12 13

14 15

16 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Original Firing Order Revised Firing Order

Arrows indicate consecutive firing events adjacent to a single main bearing

Hull Deflection Several recent publications discuss the impact of hull deflection on

main bearing loads

– Sverko, D., “Effect of the Hull Deflections on Propulsion System Bearing

Loading,” Paper No. 236, CIMAC 2007, Vienna.

– Sugimoto, I., Naka, T., “Optimization of Both Engine and Shaft

Alignment for Hull Deflection,” Paper No. 160, CIMAC 2007, Vienna

– Vartdal, B.J., “The Influence of Hull Deflection and Propeller Loading on

Load Distribution in Engine Bearings,” Paper No. 233, CIMAC 2010,

Bergen.

At large hull deflection magnitude failures are reported. On the order of 7 to 8mm between adjacent main bearings.

Another aspect of this project included block and hull deflection analysis. Even in high seas it was determined not to be a concern. Dynamic main bearing movement was considered but not pursued.

Hull Deflection

From Sugimoto and Naka, CIMAC Paper No. 160, Vienna 2007

Applying Deflection Results from Block FEA

Worked with Gamma Technologies to create a patch allowing the

cylinder block to be deflected in the ‘X’ or ‘Y’ direction as a function

of time.

Work began with accelerations of entire, rigid hull. Because the hull was found to be very stiff, this portion was suspended.

A Further Look at Individual Bearing

Movement

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Modification made by Peter Nguyen based on approach taken with the rigid hull – single, 2D inertia.

Example Results

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Cyclic motion applied in ‘Y’ direction at MB3. Reactions shown at MB3 and MB2

Self-Acting Hydrodynamic Bearings

Bearing geometry creates a wedge. The spinning shaft drags oil into the wedge, where the converging flow area causes pressure to build. The applied load creates a steady-state condition where inward and outward flow are equal and an oil film is maintained.

At moderate misalignments film growth is delayed; at severe misalignments the film does not form.

Two Ways Misalignment Impacts MOFT

Ability to “pick up” a hydrodynamic film – Wedge created by shaft clearance and geometry

necessary to form hydrodynamic film on start-up

– Misalignment can delay or shut off oil pick-up

Film thickness grows at the expense of crankshaft

deflection

Further Notes on Force Magnitude

Vertical bore misalignment is absorbed in two

locations: – Bearing film thickness reduction

– Bending deformation of the beam

At the end mains, 0.1mm of misalignment increased

bearing loads by ~4000 N (+0.8%)

At the center mains, 0.1mm of misalignment

increased bearing loads by ~28000 N (+4.3%)

Oil Film Development

Aligned Bearings

Oil Film Development

Misaligned Bearings

Worst Case Misalignment Combinations

In the following cases, adjacent main bearings were

simultaneously misaligned in opposite directions:

Case 1: – MB 2 vertically misaligned +0.1mm

– MBs 1 and 3 vertically misaligned -0.1mm

Case 2:

– MB 5 vertically misaligned +0.1mm

– MBs 4 and 6 vertically misaligned -0.1mm

Results were nearly linear with misalignment

0.1 at MB 2 -0.1 at MB 1,3

0.05 -0.1 -0.05 Misalignment

MO

FT C

han

ge, m

icro

ns

-0.5

-1.0

0.5

1.0

MB 2 MB 1

Vertical Misalignment of MB 2

MB 3

0.05 -0.1 -0.05 Misalignment

MO

FT C

han

ge, m

icro

ns

-0.5

-1.0

0.5

1.0

MB 5

MB 6

Vertical Misalignment of MB 5

MB 4

0.1 at MB 5 -0.1 at MB 4,6

Further Parameters

Torsional Vibration – In the absence of damping performance data, a

sweep from zero to maximum damping was done

– Angular deflection varied from 0.4 to 1.4 degrees CA

– No measurable difference in MOFT

Filtration Efficiency

– Not directly addressed

– Specifications based on correlation to MOFT

Oil temperature

– Greatest impact

– Monitored at every bearing

Summary

GT Suite used in conjunction with 3-D FEA allowed evaluation

of a wide range of parameters

Further capabilities for time-dependent bearing movement are

being explored

SwRI expresses our thanks to Peter Nguyen and his colleagues

for ideas, guidance, and modifications needed along the way.

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