Measurement of propeller shaft stress in service condition

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30 Measurement of Propeller Shaft Stress

In Service Condition

Outline

With regard to the fine cracks in circular direc-tion caused by bending at the big end of propeller

shaft taper even if the cracks were eliminated a

prediction is difficult of a relapse of these cracks sand the possibility of their propagation so that an

apprehension is always entertained by users.

According to the investigation Of actual results

made hitherto, cracks of this kiñd are

liable to appear ii-respective of type and size of ships,. kind of main engines, number of propeller . blades, etc.;

and particularly, since the individual character is

not found out all ships are considered to have a

possibility of occurrence of these cracks For this

reason, the people concerned and always interested

in the above fact, have carried out a close

inspec-tion of nondestructive inspecinspec-tions such as magnetic

particle detection and others, whenever an inspec-tion of propeller shaft is undertaken, to discover such fine cracks at an early stage and to take proper treatments for them.

Accordingly, it has become quite seldom to cause

5j accident, such as missing in sea,

Of a shaft

together with the propeller due to propagation of

racks However, if the appearance of fine cracks

jis overlooked, then the cracks

will proceed con-siderably deep in three years before the following inspection time; and some times the shaft is com-pelled to be changed to a new one.

To cope with such actual circumstances, it is

necessary to study countermeasures to prevent an appearance of cracks of this kind simultaneously with the early discovery of initial fine cracks

The big end of propeller shaft taper is affected

by the external force, such as the bending moment due to propeller weight and the fluctuating bending

moment due to eccentric thrust of. propeller. In addition the fatigue strength of bending is reduced by the press-in of propeller, añd fretting corrosion

cracks are liable to appear in the big end of pro

peller shaf taper. As mentioned aboye, the big end of propeller shaft taper from viewpoints of external

pforcêworked

and structüral field, is umider qualita. May 19.68

By uro Hoshino, Dr. Eng., Chief,. Engine Section, Technical Research Laboratory

Hiroshi Kume, Technical Staff, Head Office

Nippon Kaiji Kyokai

tively disadvantageous condition, but its quantita-tive study is being delayed.

'king these considerations into account, The

Shipbuilding Research Association of Japan, with

subsidies from Japan Shipbuilding Industry Founda-tion, has conducted experiments for two years from 1964, i.e.: The experiment under fluctuating load

condition, by.. the use of a klarge shaft model to

ascertain

the fatigue strength of bending at

the

big end of propeller shaft taper the measureilient of shaft stress in service condition to ascertain the bending stress actually generated, and the experiment to ascertain the effects of induction hardening to

increase he fatigue strength of bending, etc The author was in charge of the measüremeht of propeller shaft stress in service condition of the

above mentioned research and study and the results are described in the following:

Particülars of Ship and

Outline of Measuremcñt

Particulars of Ship

Ship's Name: "Ösumi Maru", owned by Nippon

Yusen K. K. and built by Mitsu bishi Heavy Industries, Ltd at its

Kobe Shipyard Pi-incipal Particulars:

Pellet carrier, 56,100 D.W.T.

211,.0i.rnx31.8Brnx 17;5Dm (11.7dm)

Particulars of Main Engine:

-stroke cycle single acting diesel engine, 13,8OÓHPx1I9RPM Particulars of. Shafting:

See Figs.

I an4 2.

Contents and Outline of Measúremeút Coñtents of Measurement

1) Measurement of two points of the bending

stress in way of the big end of propéller shaft taper [Measured by setting strain gauges on

Points (1) and (2) f Fige 1]

(2)

(in mm)

Propel er (21.1 Tons) (5 Bladed Solid Type)

710 230v Obb-IUU.1,IiJb

Measurement at two points of the bending,

stress in way of the coupling of propeller shaft

'arid intermediate shaft [Measured by setting strain gauges on Points (3) and (4) of Fig. 1]

Measurement at one point of the torsional stress of intermediate shaft [Measured by setting a strain gauge on Point (5) of Fig. 1]

Measurement at one point of the

thrust of intermediate shaft [Measured by setting a strain gauge on POint (6) of.Fig. 1]

Angles of pitching and rolling of hull. Gauges añd Wiring Arrangement

I) The FM Radio Link Strain Telemeter developed

by Primo Co., Ltd., in Tokyo and Nippon Kaiji

Kyokai's Technical Research Laboratory was

used for each strain measurement.

2)

Angles of pitching and rolling of hull were

measured by gyro rocking. meter.

24

Proe)ler Shaft (16.2 Tons)

Intermediate Shaft (13.4 Tons)

7,270 _9,000

0. -o.

245"--

-Bearing Clearance:2Omm

Fig. 2 Stern Arrangement

410 125 125 642.5

Fig. i .Deta!Is of Shafting and Set-Points of Strain Gauges

'o n, 30 Groove on Boss Width 10 Lead Wires ' Center Hole 25 1,080

ri;4r4

Gaugés fr Bending Strain (2 Sets, Phase Angle 90) Groove on Key -P 3010 10 5,2-70

Fig. -3 Sethng of Strain Gaugis and Lead Part of Propeller Shaft

410 125 Wires oD CoDe

The wiring arrangement 'of lead wires from the

bending gauges of measuring Points (1) and (2) at

the big end of propeller shaft taper is - shown in

Fig. 3.

-Furthermore, the arrangement sketch of setting

apparatus on shaft is shown -in Fig. 4, and thel actual

thing is- shown in Photo 1. Photo 2- shows the FM

receiver and recording apparatus installed jn the engine control room. The measured values .f the

above-mentioned 8 points are recorded in a» series on the same recording paper.

Date of Meajurement - - -

-At sea trial:

-18 and 20 August, 1965

During navigation on Northern Pacific Ocean

(japanLos Angelesjapan):

26 NOvember, 1965

to i Janual,

1966:

-Japan Shipbuilding & Marine Engineering To FM Transmitter

I

Detail ® Detail ®

(6): ®

See from After End

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I

II

Photo i Test Apparatus

Results of Measurement

Measuring Records at Sea Trial

Figs. 5-10 were extracted from measuring records at sea trial, indicating fully the phases of a gradual

change in the wave profile from low revolution

number to high revolution number. As the sea con-dition was very calm, these record&-can be regarded as representing changes in the wave profile due to changes of the revolution number. Furthermore, the sea trial was taken place in light loaded condition at

displacement of 32,900 tons and of stern draft of

i.93m. May 1968

Photo 2 FM Receiving Apparatu5

tiluminating Lamp Beam CeII.

FM Transmi ter

FM Transmitting Antenna

(Fitted to Shaft)

(1) (2), (3), (4): Gauge fur Bending Strain : Gauge for Twisting Strain

: Gauge for Thrust Strain

FM Receiving Antenna

(Fitted to Bearing)

Aftermost

Bearing Ç

Fig. 4 Sketch of Test Apparatus Fitted on Intermediate Shaft

Fig. 5 Measuring Records at Sea Trial (RPM: 31.6) 25

Extracts from Measuring Records Symbols used herein are as follows:

oi: Bending stress of measuring Point (1) of Fig. 1

Bending stress of measuring. Point (2) of Fig. i

Bending stress of measuring Point (3) of Fig. i

s

ç4: Bending stress of measuring Point (4) of

Fig. i

T : Torsional stress of measuring Point (5) of

Fig. i

t : Thrust of measuring Point (6) of Fig. i

P : Pitching angle of hull

(4)

I-o o -E

-

be

to

f O o_o -E to o I-- o o -E to t=0 E to E b No.31 56 R.P.M.

-No.33 71.9 R.P.M. 1 sec-V Ib 1 rev. rev.

Fig. 8 Measuring Records at Sea Trial (RPM: 90.5)

I-. o o -E be 0=0 T0 E

.6

No.43 119,2RPM. 1 sec i rev.

Measuring Records at Service. Condition

Figs. 1 1-16 were extracte4- from measuring

rec-ords during navigation on Northern Pacific Ocean. Various conditions (navigation route, sea condition draft, etc.) corresponding to these sketches werç shown together in the table to follow.

Fig.

11 shows an example in the case of an

extreme change

of mean torque and

thrust at

ballast condition.

Fig. 12 is a measuring record at service con4i

ijon in. the case of extreme racing of propeller, and the maximUm total amplitude reached 12 kg/mm2

In this measurement it was found that in tle case of racing of propeller, the larger becomes the

in-crease of revolution number óf shaft, the larger will be. the bending stress of propeller shaft Therefore,

in order to find the effect of change of the setting point of majn engine governor response upOn the1

Japan Shipbuil4ing & Mariñe Engineering

Fig. 6 Measuring Records at Sea Trial (RPM 56) Fig. 9 Measuring Records at Sea TriaI (RPM: 106)

Fig. 10 MeasUring Records at Sea Trial (RPM: 119.Z Fig. 7 Measuring Records at Sea Trial (RPM: 11.9)

(5)

r=O

Table i Sea and Ship Conditiöns at Which Records in Figs. 11-16 and rigs. 23-26 Were Taken

Fig. 11 Measuring Records at Service

Conditioñ (In thè Case of Extreme Change of Mean Torque and Thrust at Ballast

Condition) P

PvwvJw

+ '1!

rnmm

I I J

No.55-i 84 130 R.P.M. Ballast Condition

Condition (In the Case of Extreme Racing of Propeller)

May 1968 27

so n.i . Swell Draft

C o e o eu o _-- 'O Displace- For-E e ° E O _E

:2

e ment Z bC s

.!°

--o., o'

C'

oe.

Aft ward Sea Route

E 0 . V u -

.

-- Beaufort Scale -ou _(m) _(Ton) 11 _2nd

551

13:40 46°N

23 Dec. 930 11.3 WNW 8 8 W Heavy 6.71 9.67 45, 391 From

169°W . 'apan 57-4 15 1965 14:38 I To 12 64 4th 10:42

44°N Mod. Los Angeles

24 Dec. 101° 11.3 W 7 7 W 673 9.61 45,273 (Ballast 65-4 13 1965 _10:55 154°W Long Condition) 25 14 From

221

29th 10:55

31°N 280° 14.,7 ESE 6 6

-

Los Angeles

26 Dec. 11.60 11.83 66,594 ;tcan 148°W . (Full Load 21-4 16 1965 21:10 277° 13.3 SW 7 7 SW Heavy Condition) n E 1 sec I I E

e

I

i rev. rev. o o E ta E E to e e i rev. No64 88 125 R.P.M. Ballast Condition

I I I I I

1\j\

(6)

A=o

R=oi

r=oiT

t=O

Condition (In the Case of Extreme Racing

Of Propeller, Changing GovernOr Respoñse

Set-Point against the Case of Fig. 12) t=o

Fig. 15 Measuring Records at Service Condition (Ballast)

bending stress of propeller shaft, at almost the same time with the -measurement of -Fig. 12, .a record was

obtained by lowering the setting point of the göv-ernor alone, without chauging. the maximum fuel

injection quantity which is shown in Fig. 13. These three examples of Figs. 11-13 were

meas-ured at ballast condition, and on the other hand

i sec I i I I I-1 rev. F: I I rev. t t 00 o E

t

E - i-. o o

it was confirmed that at full loaded condition, though the hull pitches were heavy when caught in a storm, the load will not decrease as the propeller

immersion is deep so that the torque thrust and

bending stress will not fluctuate widely.

Fig. 14 is an example of the laximurn clange

of stress at full loaded condition.

28 _{Ja pan Shipbuilding & Marine Engineering}

t t Q-E E E E 00 o E -E I-o o E E E 00 E E 00 No.22-I ..t t i t t

...

,gj g - i, rev.

!VV

1

4

bec

rvfl!!fl!!r

TíiiTi;i:1: fl:,

:::: ::::

::

i rev, ree.

No.65-4 Ballast Condition _i _rev.

o E E t _:00 t t q. o _{Fig. 14} _Measuring RécirdS at ServiCe Condition (In the Case of Extreme Change

of Bending Stress at Full LOaded

E Condition)

o-

I.-o o

(7)

p

u, 1 p=o a T0 t=0 <C .

-

-- - ---o-During Turning

o o o iò o ¿o ¿o o ¿o ¿o lÒoliOl0l30

R.P.M.

Fig. 17 Measured Bending Stress at Point (1)

0.4 0.3 0.2 E na. o' o o. o o. o. E ea° o. During Turning

..»-._

o 10 20 30 40 50 60 70 80 90 ioo 110 120 130 R.P.M.

R.P. M.

May 1968 o 1-o o

J-6 o 5 toE . E 4

r;

e

Fig. 16 Measuring Records at Service Condition (Full Loaded)

R.P.M

Fig. 21 Measured Twisting Stress at Point (5)

These sketches show measuring records of various

values developed over scores of seconds, and the magnified records per second are shown in Figs.

15 and 16. Fig. 15

is an example of records at

ballast condition, and Fig. 16, at full loaded

con-dition.

Results of Measurement

Results of Stress Measurement at Sea Trial

As the sea condition was calm and the same

draft was maintained, at sea trial, representing each

measureci value against the revolution number of shaft, is shown in Figs. 17-22.

Figs. 17-20 show the bending stress of shaft; Fig. 21, twisting stress; and Fig. 22, thrust force, respec-tively.

2.

No.21-4

A'd.

i rev. . - -

j

- rev. -3.7X (--'' Curve₁₁₃

Resonance (6th Order Harmonic)

.11 \_____

O 10 20 fl ¿U ÇU ri in RU ¿A

4

During Turning

2L

'

..

-U lA nfl nfl .'n çA ¿ri nri ari ¿n , ,in , n-, ,n 1

e na 7 6 10 20 30 40 50 60 70 80 90 100110120130 R.P.M. 3 2 During Turning g > o u 4 3 +1 o 10 20 30 40 50 60 70 80 90 100110120130 R.P.M 2 Fig. 18 Measured Bending Stress at Point (2)

=1

I

o o.' E 3

2-. During Constant Running During Variable Revolution

During Steering

ob

o

(8)

L

30 s 5 î 4 - &--H .3 2 100 80 E 60 40 20 cL ₂ 4. 2 12 10 130 120 110 100 90 80 0 o 7 6 o 10 20 30 40 50 60 70 80 90 100110120130 R.P.M. Mean Torque R.P. M. o 90 Mean Thrust 80 = 60 i 40 20 o 4, 2' -f 0'

Total Amplitude of. Bending Stress on Point (3)

Tdtal Amplitud f Biridiñj Stress dn Point (1)

Propeller Shaft Revolution

Fig. 23 Analysis of Record Shown in Fig. 11

40 1 E E 00 13 12 11 10 9 B Wean Torque 1 i Mean Thrust

-u.

r

Pitching Angle ( + Stern Up)

Total Amplitude of Bending Stress on Point (3)

Total Amplitude of Bending Stress on Point (1)

7° 60 50 40 30 20 60 50 40 I-30 20 4 00 0 0' 2' 4 ' Total Amplitude of

.0 _{Totl Arilit(i} _(if

10

80'

o

Mean Torque

Propeller Shaft Revol ution

Changing of Stern Draf,t 1.77m/deg.

"T

Iv

V

Mean Thrust

..

.... _._

Pitchiñg Angle (+ Stern Up) Changing of Stern Draft

1.77m/deg.

u...

Bending Stress on Point (3)

._L-4-Bding Stress on Point

Fig. 25 Analysis of Record in Fig. 13

Ja pan Shipbuilding & Marine

EñgzneerinI

e. t Revoluti&l

JÌh.!Ii'

-nrî"u

-. Propeller Shaft

-..

Resonance (6th Order Harrnoni c)

A

f

0'

..:, O 10 20 20 AO 50 50 70 20 00 flflnfl'00' 110 100 90 o 80 70 L 70 60 .1

t

-l-I 60 . 50 I-40 30 20 10

P tching Angle (+ Stéri Up) Changing of Stern Draft

1 77m/deg

w

10 20 30 " '40 Sec 10 20 30 Sec E 8 E ₆ 00 s 2 o 2 Sec 30 40

°I

Fig. 22 Measured Thrust Force at Point (6) Fig. 24. Analysis of Record in Fig. 12

8 2 o 120 110 d 100 go

(9)

70 Mean Thrust Rolling Ang e 4. , 2

n.

8 6 4 o Mean Torque

Uïiï

(± Port Up) 110 Propeller

'oo

90 o

iiì AS

.. r i

-

4

2 Pitching Angle (+ Stern Up)

1

o

-

2 2

2

-u-

Total Amplitude of Bending Stress on Point (3) of Bending Stress on Point (1)

uiuuuu u

uuuuuu

UULU A A

Shaft Revolution

Changing of Stern Draft 1.77rti/deg.

A

w

r

F

u...

TotaI Amplitude 10 20 30 Sec

Fig. 26 Analysis of Record in Fig. 14

40

Results of Stress Measurement during Navigation

In. the case of the ballast condition, if the

pro-peller immersion is shallow and the immersion

varies with the time, the bending stress of shaft as

well as

the shaft revolution number, thrust and

torque change with the time; Especially, in the

case of racing by exposure of part of propeller,

the amplitude of fluctuation becomes larger. There-fore, from the records of Figs. I F--l4, the fluctuating

phases against the time elapsed of these

meas-ured values were analyzed, examples of which are shown in Figs. 23-26..

Fig. 23 is an example of extreme changes of mean

torque and mean thrust, corresponding to the above-mentioned Fig. li.

Figs. 24 and 25 áre examples compared to

investi-gate the degree of effect of the set-point of main engine governor on the change of shaft revolution number by the racing of propeller, and the degree of effect on the change of various stresses, corres-ponding to Fig. 12 and Fig. 13, respectively.

At full loaded condition, if the propeller im-mersion is deep, the shaft revolution number and various stresses do not change remarkably even if

the pitching is large in a stormy weäther.

Fig. 26 shows an example of analyses at the time

when the maximum bending stress of shaft was measured at full loaded condition in a rough weath-er, corresponding to the above-mentioned Fig. 14.

/Iay 1968

-60

Study of Results of Measurement

The main object of measurement conducted so far, was to find the actual condition of the extent

of amplitude especially in a rough weather and

also of the changing phases, concerning the bending

stress of the big end of propeller shaft taper. The

measuring, records have reached an enormous volume.

Although they are still in the course of analysis,

some of the important records were selected to

describe herein. Results of measurement can be

inferred in the following:

I) The maximum amplitude of the nominal

bend-ing stress in way of the big end of propeller

shaft taper in a calm sea condition is regarded,

without making any special measurement, as about ±3 kg/mm2 for a ship of normal structure.

2) During navigation,

the amplitude and wave

profile of the bending stress of shaft are always changed due to waves and swell. However, with the racing of propçller especially at ballast con-dition, the amplitude becomes extremely large,

and the maximum stress, of about ±6 kg/mm2

was measured in this measurement.

Whereas, it was found that in full loaded

condition, even if in the same rough weather, the amplitude of the stress was not specially

increased, because the propeller immersion was

deep and the change of load was little.

As the length of the ship is 211 m and there

was hardly any

swell of the. corresponding

length, the large racing did not appear.

How-ever, ships of short length are affected by swell

especially in

light loaded condition, and the

bending stress exceeding ±6 kg/mm2 is supposed

to occur frequently by the racing of propeller.

According to results so far obtained of fatigue'

test of bending of a big press-fitted shaft, the

limit of stress causing initial fine cracks is about

±4 kg/mm2, and the limit of fatigue leading

to break-off is about ±9 kg/mm2. The bending

stress at the big end of propeller shaft taper in

service condition is about ±3 kg/mm2 as a minimum, and over this minimum value 'by the effect of wave condition. Therefore, occúrrence of fine cracks of shaft and development of such defects are determined by the integral frequency

of high stress. Accordingly, if

a method of

estimation of bending stress applicable to all

conditions and a method of calculation of

frequency of such occurrence can be

investi-gated, the probability of existence of cracks of

propeller shaft is to be estimated.

3) It was found that when the number of revolu-tion rapidly increases due to the racing 'of, pro-peller, the bending stress of propeller shaft will also increase, and the fluctuating curves of both 31

(10)

are relatively similar. From this fact,

if both

relevant coefficients were obtained, the bending stress of shaft would be estimated by recording the fluctuation of the number of revolutions of shaft. If this method becómes applicable to áll

ships, the extent of the bending stress of shaft may be estimated, but the results Of fürther

study are expected. Furthermore, it is

con-sidered that in a rough weather when the racing of propeller appears, basing on above-mentioned relations of the number of shaft revolutions and the bending stress, it will, be helpful tO decrease the amplitude of the bending stress of propeller

shaft employing such a siep 'as preventing the

increase of revolution number of shaft by

lowering the set.point of governor.

4) In case of pitching of a ship, even if the stern risès, the propeller immersion will not always

become shallow dtie to relations with swelling 'of wäves. Therefore, the extreme racing of pro-peller is caused by a periodical coincidence of the top of' pitching and the bottom, of swelling,

and the frequency of such extreme racing is

comparatively rare. For this' reason, the

maxi-mum value 'of bending stress is not always

pro4uced at the time of the stern risen to its

highest point. These aspects

are shown in

Figs. 23'-2'6.

5) In a rough weather, especially in light loaded

condition, the amplitude of the bending stress of propeller shaft fluctuates greatly. This is

considered to be attributed to the large fluctua-tiòn of magnitude and eccentricity of'the thrust

of propeller, due to

the large 'fluctuation of

wake distribution at stern. The fluctuation and eccentricity of propeller thrust are affected mostly

by the number of blades of propeller, and also by, draft condition and stern arrangement, but this 'theory holds good only under' the condi-tion of constant dtaft. In service condition of

a real ship, a periodical fluctuation of draft

should be taken into account, 'which affects

more' than' the number of propeller blades. Accordingly,, in the case of estimation' of the bending stress such as the causing of cracks of

propeller shaf,t,' and of the frequency 'of such

occurrence, the figure of the components of

the bending momçnt due, to

the fluctuating

thrust of propeller should have been obtained as a function ok periodical fluctuatiqn of

pro-peller jmmersio in addition' to the number

of propeiler: blades. Furthermore, the increasing

degree of the number of revolutions Qf shaft

depending on the set-point of governor should he taken into account 'as a function. Since the fluctuationi of the bending moment of shafting due to the eccentricity of propeller thrust affects on the fluctuation of reaction, force of bearing

and vibratiOn of hull, the analysis of the

magnitude and eccentricity o'f propeller thrust

applicable to a real ship in' service condition,

is an important subject for study in the future.

) It was fou'nd that if the stern túbé bearing is

' iot in a condition bf svere wear, the amplitude of bending stress of shafting in the engine roomi is less than ±1 kg/mm2 even in a rough weather, showing ' sO ' smäll a figure with almost no

dif-ference in a calm weather.

ships of the present shafting arrangement

aremeeting with the opportunity of the

bend-ing stress causbend-ing fine

cracks 'at the big end

of propeller 'shaft taper. In other words, as to

whether-or not' the cracks will appear in fäct, it is determined' by the integral frequency meeting

with the opportunity of causii the benthn stress of over about ±4 kg/mm2. This defines

that particularly the frequency of meeting with

a rough weather in light loaded condition or

handling condition of the main engine in a

rough weather is a factor influencing upon the

occurrence of cracks of propeller shaft. However, it is difficult to draw a design of shafting by

estimating the

frequency of meeting with a

rough weather for individual ships. For this

reason, as preventive measures against Occurrence of cracks, the following steps are considered to

be taken: '

i) To thickèn only the diameter of propeller shaft, leaving the present shafting arrange; ment as it is.

ii), To improve the fatigue strength by surface

treatment in way of the big end of

pro-peller shaft taper, leaving the present

shaft-ing arrangement as it is.

To reconstruct radically the fitting arrange'

ment of propeller shaft and propeller. To find the relations as to whether ot not

the 'technique of press-in of propeller affects

on the occurrence of cracks, and if such

relations exist, to improve the techni4ué of press-in.

As for the measures of these four items, i)

has been put in practice by' giving allowance

to the required diameter and in the case of iii

the roller hardening is' sometimes adopted in a real ship. As to iii), many plans have beçn made

and some of them are in a stage of 'adoption

in a real ship.

Lastly, iv)

is in the course of

study at present.

Among these measures, which one should be adopted as' an ultimate -preventive measure, is in abeyance pending the results of further study and actual results in the future. No 'matter what

it' may be, the present time is in a transitional

'period of undergoing changes of arrangement Of

shaftiii, and a further study is expected to be

made fòi the improvement of the reliability.

It is firmly believed that results of

measure-ment 'Of propeller shaft stress in service

con-ditiön performed this time will be instrumental to the above-mentioned purpose as an essential

basis.