Hiroomi Kishikawa*
Jiheye Matsumoto**
Tsuyosh i Kawazoe *The SKF type keyless propeller, as its name implies, requires no key for fastening it on the cone parr of the propeller shaft, Its unique feature therefore is that the propeller torque s taken only by friction between the propeller and propel/er shaft.
lt has the following advantages surpassing the keyed propel/er: Simple Construction.
No key slot and conseqùently no concentration of stresses at the edge of key slot unlike the ordinary keyed propeller. Unifòrm distribution of stresses over the internal surface of the prOpeller boss.
The keyless propeller therefore aroused a great interest among the maritime communities and that demand for it is on the steady increase.
In view of this trend, we carried out the push-up and the slippage tests on a test keyless propeller to evaluate the reliability of the keyless propeller for practical marine application The test key/ass propeller was of 6 8 m in diameter and made of Ni Al bronze identical to the working unit.
The test results revealed that the SKF type keyless propeller was quite sound both in design and Construction for practical marine use, and provided clues to posEible improvements.
1. Preface
The SKF type keyless propeller is of a new fastening system of the propeller and the shaft intending to transmit
torque from the propeller shaft to the propeller merely
through their mutual frictional force by means of
eliminat-¡ng the key from fastening part between the propeller shaft
and the propeller as its name impliès. Adoption of this
system can allow simple construction, no key way and con-sequently no concentration of stresses at the edges of key
way and uniform distribution of stresses over the internal
surface of the propeller boss, and so forth. Therefore
recently a great interest to keyless propeller was aroused among the maritime communities, and the demand for it
is on the steady increase.
In vieW of this trend among the shipbuilding circle, we
carried out the push-up and the torque-slippage tests on a
test keyless propeller to evaluate the reliability of the
keyless propeller for practical marine application. Besides the test keyless propeller was of 6.8 m in diameter and
made of Ni-Al bronze, identical to the workiñg unit. The tests are briefly outlined as follows:
In the push-up test, the relation between contact ratio and frictional coefficient during pushing-up and the confir-matiön of the propriety of the calculation method of hoop
stress effecting on the propeller boss were main subjects of
investigation, moreover the push-up test in case of
inten-tionally making a gap of matching mark of fitting
be-tween shaft and boss, the influence of lubricants for wet fit
applied and so forth were investigated from the wide point
of view. Furthermore in the torque-slip tests the safety
factor was estimated by the frictional coefficient at
slip-page and besides the frictional coefficients in case of
stick-slip carriéd out on normal, reverse directiOns of rota-tion and continuously, were observed.
Thus the tests were carried out to evaluate the
bility of the keyless propeller fòr practical marine applica-tion as well as to obtain necessary data for its planning and manufacturing. As the tests resulted in various data
on the keyless propeller, hereafter they will be described.
Test propeller
The actual propeller (of 6.8 m in diameter and made of
Ni-Al bronze) for the main engine of a rated output 26 100 PS was applied as the test propeller. The principal particu-lars of this propeller and the materiâl of the propeller shaft are shown in Table i and the cross-sectional view of the test
propeller boss, in Fig. 1.
Push-up tests
3.1 Testing method
The push-up tests were executed to push up the shaft hydraulically to the propeller boss by means of hydraulic jacks set between the stern side of the propeller boss and
the propeller nut. Two testing methods were adopted: one
was the dry fit method to push up the shaft merely by
push-up load, and the other was the wet fit method to push
up the shaft by push-up load as well as hydraulically in a
lubricated state through oil grooves inside the
boss and afterwards to remove the oil pressure inside the
boss.
Chief, Propeller Designing Section. Marine Machinery Designing Department, Ñagasaki Shipyard & Engine Works
Ràted output Speed Diameter Pitch (constant) Pitch ratio Develòped area
Developed area ratio Boss ratio
Blade thickness ratio at 0.7R Profile of blade section
Rake Number of blades Direction of rotation Propeller weight Material Material Main engine Propeller
Propé lier shaft
Oil feeder hole for Take-out hole for strain boss expansion gauge leading wire
26 100 PS 122 rpm 6 800 mm 4 300 mm 0.6368 26.0 m° 0.7 159 0.1 765 0.0623 Model M 8° 5 Right hand 36.810 t Ni-AI bronze SF45
Fig. i cross-sectional view of test propeller boss
The push-up travel was measured through reading the
dial gauge set at the boss end and the push-up load through
reading the pressure gauge fitted to the hydräulic pump for pushing up, respectively. Besides the measuring time was fixed for the static state to which the relative move-ment of pushing up between the boss and the shaft f
in-iShed. The push-up travel and load were plotted on a graph
which was previously prepared for indicating the relation among frictional coefficient, push-up travel and its load,
thus being able to determine frictional coefficient.
Moreover, the stresses were measured with the strain gauges attached to each part of the propeller. Besides as for pulling off the propeller a hydrauliô prêssure was im-posed for boss expansion and the taper of contact parts between the propeller boss and the shaft was employed.
Thê push-up test apparatus is shown in Fig. 3.
Dial gauge fer measuring sushup travel
Propeller
Supporting
Hydraulic aok for pushing ap
Ffexiele tobe Pressure Oil pump
gauge For pushing up Hydraulic branch tool
Fig. 2 Sketch of push-up test apparatus
Fig. 3 Push-up test apparatus
Oil feeder hole
for
boss expanuion
Oil pump
For boss
es pa flojee
3.2 Test results and considerations
3.21 Frictional coefficient in push-up process
As seen in Fig. 4, throughout push-up tests the frictional
coefficients at dry fit pointed out about 0.16. This co-incides just with frictional coefficients at dry fit with the
conventional keyed propeller. Moreover, although the fric-tional coefficient at wet fit varies according to the pushup trável and the hydraulic pressure for boss expansion, the
frictional coefficient under the enough high hydraulic pres-sure pointed out 0.02 as shown in Fig. 4.
3.22 Confirmation of frictional coefficient retUrning from wet fit to dry fit
When, as shown in Fig. 5, the hydraulic pressure for boss
expansion was reduced to zero in the state of wet fit, then
left for 5 minutes as it was and afterwards the push-up test
was carried out, it ws confirmed that the frictional
coeffi-cient returned completely to that of dry fit. From this
210 N 1h/2 Tap<'%
j_
-groove 1320 .,nn I 320 320 360Table i Principal particulars of test keyless propeller
10
e
o
o
Contact area ratio About 0%, Machine finished, dry fit
y Contact area ratio 30%, dry fit
o Contact area ratio 60%. dryfit
Contact area fatio 30%. wet fit
Calculated value
2 -3 4 5 6 7 8 9 10
Pushup travel (mm)
Fig. 4 Push-up travel with lOad at dry fit and wet fit
.--- Wet fit
'.o--- Dry fit Calculated value
Contact area ratio 30%
4 6 8 10 12 14
Push-up travel (mrO)
Fig. 5 Frictional coefficient returned, from wet fit to dry fit
reason, as for the installing of the keyless propeller on the
actuál vessél that requires a large pushup travel, it is
con-sidered that wet fit, which can be dOne with a light push-up load, is the most suitable method..
Furthermore, as observed in Fig. 5, thè pushup lad ® at dry fit corresponding to push-up travel of about 5 mm largely surpasses the push-up load ®' corresponding to
about ji = 0.16, and the frictional coefficient. at ® point
approximates to 0.22. The reason causing this large differ-ence. can be probably supposed as follows;
(1) Çpoirit in Fig. 5 isto obtain an instàntaneouspush-up
lOad measured even at very slight mOvement by means of the diàl gauge for measuring push-up travel, i.e. the
maximum static frictional çoefficient còrréspondiñg to push-up travel, while the conventional frictional co-efficient corresponding to ®' point is to measure the push-up travel and loàd in the state of being stably
stuck after occurrence of slippage. (Fig. 6
(2) lt may be considered that as foras point ®, the
inter-fáce between boss and shaft is separated by lubricant
film, but as soon as the hydraulic pressure for boss
expansion rs reduced to zero, lubricant is pushed Out
and the boundary surface is immediately brought to metallic contact quite apart from "affinity".
point
i
/ i
/ i
Maximum statical frictional /
coefficient at push-up / At -stick state point Pushup travel At slip state -Frictional coefficient at push-up M8. IS Measured point
Fig. 6 Push-up frictional coefficient
3.2.3 Contact area ratio and frictional coéfficient
be-twèen thè mating surfaces of boss and
shaft-As seen in Fig. 4, the contact area ratio between boss and shaft without loading (after red lead is transfered to
cellophane tape, thê stained area oñ it i rñeasuèd with
the planimetér) is varied to about 0% (in the fully machined
state of both shaft and boss), 30% and 60%. Under such
conditions push-up tests. ät dry fit were executed, resulting in confirming that the contact area ratio has little influence
on frictional coefficient.
This may be explained in -such manners that because the radial compressive strain between boss and shaft is much larger than the-Sum of maximum surface roughness between
boss and shaft, the microscopic projection on interface is forced to make elastic deformation (partially plastic defor-mation) under push-up state, and the actual contact area
between boss and shaft- becomes cònstant irrespective of contact ratio and surfàce roughness.
Êurther, regarding the relation between the number of pushing up times and contact area ratio any results could not be obtained to report specially1 but this probably re-sulted frOm measurement of cQntact area ratio under
no-load state. That is, under actual push-up no-load, the pushnup chart (curve indicating the relation between push-up travel
and load) is linear so boss and shaft itself at their tapered parts deform elastically leading to a considerably good fit between them-.
Besides, in the regulations of the classification society
NK (Nippon Kaijikyokai) it is specified that with the
sur-veyor's previous consent through submitting push-up test
record chart to prove a linear relation, the push-up test can
be substituted for fitting test. Accordingly it may be said that the possibility of executing push-up work of propeller under machining state of both shaft and boss has arisen.
Fig. 7 shows the states stained with red lead at the contact area ratios of 30% and 60%.
27.2%
Fore end of
boss
62.2%
Contact area ratio was obtained by such treatment that red
lead adhered on the boss inside was transfered to cellophane
tape and its stained area was measured with the planimeter.
Therefore, since red lead can not adhere in spite of any
existence of contact under machining condition, the contact
area ratio was calculated approximately as 0%.
Fig. 7 Contact area ratio between boss and shaft (about 30% and 60%)
3.2.4 Surface roughness and damage
Change of surface roughness (km) (average maximum
axial roughness) before and after push-up test is shown in Fig. 8 and modes of actual surface roughness curves shown in Fig. 9. First, as seen from Fig. 8, the surface roughness of boss and shaft (kmax) before and after test has general-ly a tendency to decrease. This fact displays the collapse of microscopic projection of interface through repeated push-up tests, among them some measured results showing the coarse growth, on the contrary, can be observed. Therefore, as referring to this surface roughness record, the following points should be remarked.
Microscopically, the same position is not always mea-sured before and after test.
The measurement speed is problematical because of operating the feeler needle of roughness indicator by hand.
Besides, as observed in Fig. 9, ,kmax at boss inside be-fore test by contact ratios of 30% and 60% increase
rough-ness further through fitting work with the grinder.
Further-more no surface damage of boss and shaft was observed in the push-up test. Appearances of shaft surface before and after push-up tests are shown in Fig. 10.
3.2.5 Push-up test with gap of circumferential match mark for fitting
Push up tests with gaps of circumferential match marks 90° and 180°, which were scribed at the fore end of
propel-E
Note mae is calculated as a mean value of roughness at total nine positiens:aoial trisection and circumferential trisection.
--- mae at boss inside ' tmao on shaft Surface
Test uoder o state es fresh
machined ouf
(contact rutio of about 0%)
Measuring location: Fore end of propeller boss with a gap ef I 80 el circumferential match mark
Fig. 9 Surface roughness curves before and after push-up test
w
Ip
Ueit
S Test under a state Test under a state
(contact rutio uf 30%) (contact ratio of 60's)
Fig. 8 Change of surface roughness (Rmj before and after push-up test
Propeller boss Propeller shaft
Contact with 4.8,.
Contucf
with 3.S,
area ratio of 0%, before test,
Jdmax
area ratio of D%,af ter test,
Rmao Confect with 6.3u ) Contest with
ares raist of 0%. before
Rmae
erna ratio of 0%,after test,
2. Ip Rmno test. Contact with 4.5p Contact with OSp area ratio Rmae area ratio Rmau of 30%, before test. of 30%,aftor test,
Contact area ratio nf 30%. before test,
with I .2p Rmaa
Custact area rutie of 30%,ufter test.
with I .8p Rmax Ceetact with l.2v, Contact with blp area Rms area Rinsu ratie rutio of 60%, before test, of 60%, after est
Contact area rutio of 60%, before test, with 2.dp Rmau
Contact area ratio of 60%, after test, with 2.4p Rmau
1er boss and shaft, were conducted. As seen from Fig. 11
the result revealed that they had no influence to frictional coéfficient at all. Taking into consideration this fact and
the torque-slip test result later described (as referred to 4.1),
1100 900 700 e 0 500 a-300 100 Before test
Fig. lo Appearance of shaft surface before and after push-up test 4 ,1 o A, 4 6 8 Push-up travel
Contact area ratio 30%
,'
-After test
Fig. 11 Push-up test with gap of circumferential match
mark
means Mitsubishi Diamond Petroleum Series No.
it may be considered that for installing keyless propeller
on the actual vessel in future, exact conformance of match marks with fitting will be not always required.
3.2.6 Influence of lubricants for boss expansión pressure As shown in Table 2, push-up tests were executed with
lubricants forced to feed to the contact part between boss and shaft, varying the kinds (from turbine oil of general use to lubricant with extreme pressure agent). However,
no influence of lubricants to frictional coefficient could
be observed. Accordingly, inexpensive and easily obtáinable lubricant proved to be adoptable.
3.2.7 Pull-off hydraulic pressure
Fig. 12 shows the relation among pull-off hydraulic
pressure, push-up travel and contact pressure at the instant
of pulling off the propeller with imposing hydraülic
pres-sure to contact surface between boss and shaft through oil
groove. lt displays that the ratio of pull-off pressure to
contact surface pressure is within the range of 1 1.2 times and pull-off was done in the state of lubricant spreading
over most part of the interface.
3.2.8 Hoop stress of propeller boss inside
In Fig. 13, shown are the measured values of hoop stress of propeller boss inside at dry fit and wet fit and the calcu-lated values obtained by the M.H.l's design standard. This
chart reveals that regardless of contact area ratio, the
measured values and the calculated values of hoop stresses agree well. Besides in case of wet fit, because of hydraulic pressure for boss expansion acting directly to strain gauges,
Table 2 Lubricants for boss expansion pressure
Fig. 1.2 RelatiOn of pull-off hydraulic pressure, push.up travel and contact pressure
o Dry fit A Wet fit
/
Pall-elf hydrauhc pressure 330I IS
Contact pressare 290 -(with push-up travel of Omm) 4 6 8 10 12 14
Push-vp travel (mm)
630
Diamond No.* 110 120 260 330 (with extreme
L pressure agent Specificgravity 0.867 0.873 0.890 0.879 0.913 Kinematic viscosity (cSt) 33.5 62.3 -366.0 109.9 129 Viscosity index 101 100 95 95 93 o A
Match mark agrees With 90 gap al match mark
With I80 gap el match mark Calculated valve
10
(mm)
the excess amount is rectified.
Next, on studying measured values of hoop stress at each,
measuring location of boss internal strain, differences be-tween the location corresponding to the portion bebe-tween blades and corresponding to directly under blade are ob-served. The latter is small and this can signify relieving
effect of hoop stress- owing to blade root thickness.
Such tendency could be observed similarly in push-up test at the contact area ratio of 60% as well as in one with the gap of circumferential match mark for fitting.
3.2.9 Hoop stress of propeller boss outside
As seen from Fig. 14, the measured value of hoop stress
of propeller boss outside surpasses the calculated one to
Calculated stress
0 Contact area rat,v 30%
} Dry fi Contact area rase 0% machine finished
Contact area ratio 30% 1
Wet fit
/
Contact area ratio 0% machine finished J
-
/
/
/
o //0 / A/
/
/
A -/
//
;t./
,'
-Fig. 13 Hoop stress of propeller boss inside at dry fit and
wet fit o / L,, Calculateed stress 4 6 Push-up travel 8 10 (mm)
some extent. This cause may be that strain measuring
location lies between blades, so concentration of stresses breaks Out. Accordingly, the stress concentration factor (a1) was examined, resulting in the measured value a =
1.3, and by the finite element method a m 1.29. Both agree well.
Then the influence of contact area ratio to hoop stress
of propeller boss outside could not be Observed. Regarding hoop stress of propeller boss outside in push-up test, similar results were obtained at dry fit as well as at wet fit.
-3.2.10 Hoop stress at propeller blade root
Hoop stress of propeller blade root at the boss side end
of blade fillet (R) at dry fit is shown- in Fig. 15 and hoop
stress distribution at the blade root with a push-up travel of
5 mm in Fig. 16. First of äll, ratio of measured hoop stress
- Contact area ratio of abeut 0%
Calculated stress at beso outside
Measuring lscaton Stress at sed of blade- hunt (R) to bous side
3 V V D . V
I
/ I o/
I,
/ V C/
;,
/ - V //*'
/
0 2 4 6 8 Push-up travel (mm) (Pressare side) 10-Test conditien:Cootuct area ratio of 0% et dry fit Measreing location:End of blade fillet (k) te bess side
Hoop stress with push-up travel of 5mm
230 2.03
Fig. 15 Hoop stress of propeller blade root at dry fit
2.36
359
C=0.14 kg/tom2
Every O', with poshup travel of 5mm
-Test cenditien
(Pressurn
c=0.06kg/mm° side) j
Cv = 1.82 kg/mm2
Contact area ratio eh %
at dry ht
- Contact area rutie of 30% at dry fit
Measuring location: Blade side end
or blade fillet (R)
(Suction sidu;
Fig. 14 Hoop stress of propeller boss outside at dry fit Fig. 16 Hoop stress distribution at propeller blade root
3
O Contact area ratio 30%
Centact area ratio 0%. machine finsihed
o
/
I
/
/
/
/
/
/
/
/
E s/
E2I
IoI,
/
/
/
/
/
o,
-I/
O /S,
/
5e, Measurieg lecatiesBoss outside center between bfades
198
2.45 2.58
./,o
1
/
(Measuring location)Bess inside between blades
ho Pushup travel (mm) 6 5 E4
I
2to calculated one as stress concentration factor (a2) was
calculated, obtaining a2 = 1.21 1.90. On the other hand,
by the finite element method, stress concentration factor
at the blade root was calculated as a2 = 1.581.
Next, as observed in Fig. 16, hoop stress at
approximate-ly middle part of bläde chord is indicated high. Besides, hoop stress at blade root of blade side end of blade fillet (R) was approximately zero (0). These factors revealed
that influence of strength from pushing up of keyless propeller could not reach blade part. Moreover, it was confirmed that influence of contact area ratio to hoop
stress at blade root were little.
3.2.11 Stress concentration factor at boss part calcu-lated by finite element method
As already described, at the push-up test of keyless
propeller some difference between measured value of hoop
stress and calculated one by the thick cylinder theory
occurred. TherefOre the stress concentration factor was calculated by the finite element method, resulting in such
values as shown in Fig. 17, which agree with actual stress concentration factors with considerable accuracy.
Propeller blade Propeller boss 0919
DT
1.581 (a2= 1.21-190) 1.290 (at=1.3) Value in parenthesea( ) isstress concentration factor obtained by meaes of actual meaouremeet
Fig. 17 Hoop stress concentration factor by finite element method
3.3 Conclusion on push-up test
The push-up test of keyless propeller has been carried
out and the following conclusion could be attained.
Frictional coefficient at dry fit ¡. = 0.16, which agrees well with one of keyed propeller. Further the frictional
coefficient at complete wet fit ¡.z 0.02.
By reducing hydraulic pressure for boss expansion from wet fit state to zero and leaving for 5 minutes,
frictional coefficient at dry fit returns again completely.
Even if contact area ratio is changed from 0% (both shaft and boss are as machined), to 30% or 60%, it
had no influence to push-up frictional coefficient.
Moreover even with increasing the frequency of
push-up, contact area ratio hardly changes. As the result,
the possibility of applying both shaft and propeller
boss as machined in future is derived.
Regarding surface roughness, it had a tendency to
decrease with push-up, and collapse of microscopic prOjection at the interface can be supposed. Also no
surface damage on shaft and boss could be observed.
In pushing-up with gaps of circumferential match
marks of fitting boss and shaft, 90° and 180°, the fric-tional coefficients in the push-up did not change at all
as compared with one in the case of no gap.
Even when push-up tested by changing kinds of lubri-cants for boss expansion pressure, any special change
could not be observed,- therefore inexpensive and
easily obtainable lubricant could be proved to be
applicable.
Regardless of dry fit, wet fit and contact area ratio,
hoop stress of boss inside agreed well with the calcu-lated values by MHI's designing standard, so its
relia-bility could be fully proved.
Hoop stress concentration occurred at the measuring
lOcation lying between blades. The stress concentration
coefficient calculated by the finite element method
(a2 1.29) agreed very well with actual data.
Hoop stress concentration occurred also at the blade root with boss side end of blade fillet (R), obtaining
a2 = 1.21 1.90, where the influence of push-up to
blade strength could be little observed.
4. Torque-slip test.
4.1 Testing method
On this test, the propeller was fixed, the shaft pushed up at dry fit and wet fit to the propeller boss and then the
propeller shaft was given torsional torque by means of the
hydraulic jack. The torque as slip occurred was measured to obtain the frictional coefficient. Slippage was detected with differential transformers which were set on the fore
and aft ends of the propeller boss. Also push-up travel was measured by the dial gauge.
For certain reasons of preparing the testing apparatus the torque-slip test was executed under the condition with gap of circumferential match mark for fitting (shaft turned by about 90° to boss in turning direction). Contact area
ratiO between boss and shaft was fixed to 70%. A sketch of
torque-slip test apparatus is shown in Fig. 18 and the test
apparatus in Fig. 19.
4.2 Test results and considerations 4.2.1 Safety factor agáinst torque slip
Fig. 20 shows the relation between push-up travel and torque at slippage with parameter of torque-slip frictional
coefficient j.z. This reveals that 0.17'0.30 with
push-up travel of about 1 '3 mm regardless of dry fit and wet fit, and the average frictional coefficient at slippage with
normal turning (the mean value at the initial slip of normal
turríing corresponding to each push-up travel points out
0.218
correspond-Guide plate (hued to propeller b Propeller Supporting block
t
p serve Pressure gaugeo
--Retuinnr(for nermal refuting)
A,,, pushup travel)
Lubricant feeder hole fer boss eepanson flydrashc brunch tool
Fleoble tubo Hydraulic branch tool
Fig. 18 Skétch of torque-slip test apparatus
'I
Hand oil pump for torsional Slip
Hand Oil pump for bons eopansion pressure Boss push-up jaok
Propeller nul
Fig. 19 Torque-slip test apparatus
¡ng to MCR (maximum continuous rating) of the test propeller, a push-up travel of about 2.4 mm will be
re-quired as shown in Fig. 20. Besides, in case of replacing this propeller to keyless propeller, push-up travel of about
13 mm is needed, therefore the safety factor against torque-slip amounts to approximately 5.4 times.
Further, according to the regulations of each
classifica-tion society on fricclassifica-tional coefficient of keyless propeller, it is prescribed as (p) = 0.12 by LR and (p) = 0.16 by NK
etc. Results of these tests prove it lies in the side safer than
these values.
From these facts security against torque-slip of keyless propeller was confirmed. Saying for reference, in a moment
when torque-slip broke out, no axial shifting of shaft
oc-curred.
4.2.2 Torque slip test at dry fit and at wet fit
This test was carried out separately by the push-up
method at dry fit and at wet fit (as left for 5 minutes after
Jack for torsional slip
Rotainer(for rnoerse rotating)
Hand oil pump for boso push-up Differential transformer for slip detecting
E u, 250 200 5 150 loo 50 o 400 E 300 E 500 200 100
Frictional coefficient of normal torque slip at dry fit u Frictional coefficient of renerso torque-slip at dry ht Frictional coefficient of normal torqoe-slip at wet fit o Frictional cuefficient of reoersn torque.slip at wet fil
Figure:Order nf torsion at each push-up (racel
Torgun equivalent to MCE
el test propeller (I 53tm)
1
2 3
Axial push-up travel (mm)
Fig. 20 Results of torque-slip test
pushing up). However, almost no difference of (Pt) between both cases was observed. This.fact confirmed at the push-up test as described above, can be explained from that, in case
of pushing up propeller at wet fit where hydraulic pressure for boss expansion is reduced to zero and then left for 5
minutes, the frictional coefficient returns completely to
that at dry fit.
4.2.3 Stick-slip tests at normal and reverse turning Slip tests were carried out separately with normal
twist of propeller shaft (as conforming to normal propeller
rotating direction) and reverse twist, confirming that
little difference between both Pr occurred.
Further, in case of bringing successively to stick-slip under normal turning sequentially as (1) - (2) -+ (3), the
frictional coefficient showed a trend of decreasing gradually with constant push-up travel. [Here (1) denotes normal slip
which occurs first as shaft is twisted in normal direction,
(2) does normal slip which occurs as imposing torque again
in the former sticking state and (3) does normal slip which is successive to (2).]
For example, as seen in Fig. 20 of the test result as a
push-up travel of approximately 2.8 mm, it is decreasing as
Pt = 0.218 0.190 0.185. The reason can be derived
from that because of constant push-up travel the contact
surface between boss and shaft can be well matched
circum-ferentially at every slip. This can be allowed from the fact that even with a slight change of push-up travel Pt returns
to increasing. In a word, it is the reason that the slight change of push-up travel produces a new contact surface having no "affinity" circumferentially between boss and
shaft.
(-o Torsion bar
(fined to shaft)
Supporting
4.2.4 CompariSon of frictional coefficient of push-up test with that of torque-slip test
As already described, while frictional coefficiént by
push-up test at dry fit is as /1 = 0.16, the mean frictional
coefficiènt by torque-slip test = 0.2 18.
The difference comes from measuring time during
stick-slip motiön when the necessary parameter may be
mea-sured. That is, while frictional coefficient at torque-slip
is required asthe maximum statical frictional coefficient at a moment of starting to slip from the statical state, the
frictional coefficient of push-up test is not maximum
static one as described in paragraph 3.2.2.
Accordingly, when the maximum static frictional
co-efficient of push-up test is obtâined utilizing the time point when frictional coefficient at wet fit returns to that at dry
fit as shown in Fig. 5 and arranged, considerably close values äre indicated as in Table 3 . However, some
differ-ence bétween /.Lm and ¡i still exists, this fact may be
caused as follows: that is, in push-up test push-up at dry fit is successively repeated so that the push-up affinity between
propeller boss and shaft is formed to some extent, while
in torque-slip test the degree of affinity is low.
Generally speaking, difference between both frictional
coefficients is small and in the stage where torque-slip grows, ¡i will lower close to the frictional coefficient of
push-up test.
Besides, as a reason of difference between both cases the influence of surface roughness also can be considered.
However, as shown in Table 4, a considerable difference of average roughness kmax as axial and circumferential can not be observed. Therefore this can not be assumed as su it-able explanatory reason.
4.25 Contact area ratio, surface roughness and surface
damage
The contact area ratio before and after test was exam-ined, resulting in almost not variéd values of 70.3 - 70.9%. Next, the measured value of surface roughness is shown in Table 4. While before and after torque-slip test the
circum-Table 3 Comparison of maximum static frictional
coefficiè-nt of torqùe-slip test with push-up test
Table 4 Surface roughness before and after torque-slip test
ferential roughness generally decreased, the axial surface
roughness increased to some extent, though no surface damage on boss and shaft was observed.
4.3 Conclusion on torque-slip test
Following conclusion was derived from the torque-slip
test results of keyless propeller.
With push-up travel of about i - 3 mm, frictional
co-efficient of torque-slip amounts to 0.17 0:30 (average
0.218). When the actual push-up travel is assumed
ap-plying this relation, regarding this propeller, safety factor for torque corresponding to maximum
con-tinuous rating (MCR) amounts to approximately 5.4 times, thereby proving scurity against torque-slip of
keyless propeller.
Little difference'of frictional coefficient between at
dry fit and at wet fit (as left for 5 minutes after
push-ing up) is observed, therefore even execution of actual push-up at wet fit can sufficiently fulfill the purpose. Even at tests separately executed ¡n normal direction
(identical to propeller rotating direction) and reverse,
the frictional coefficient displayed approximately
similar values. Further, in the comparison of maximum
static frictional coefficient (mean value of 0.19) at
axial push-up with that at circumferential torque slip (mean value of 0.22), the latter became rather high.
This can be caused by différence of "affinity" of
contact surface between boss and shaft.
BefOre and after torque-slip test, little change of
con-tact area ratio was observed, and no surface damage was. Regarding surface roughness, the increase of axial
one (maybe due to circumferential "ploughing") was
observed. Although circumferential surface roughness
Fig. 21 View of propeller being iñstalled to actual ship
Maximum static
frictional coefficient Mini. Average Max.
At torque-slip (ALt) 0.205 ab.0.22 0.300
At push-up (Mmax) 0.178 ab. 0.19 0.216
Unit (,) Before test After test
-Axial average Rmax
Shaft Boss Shaft Boss
-
-2.7 6.1 2.9 6.7
Circumferential
-decreased to some extent, no damage could be ob-served.
5. Installation of keyless propeller to actual ship
To our Nagasaki Shipyardès Ship No. 1732 "TEXACO ITALIA" and Ship No. 1736 "CHAM BORD" respectively, each keyless propeller of 8.7 m in diameter, which was designed and manufactured basing on test data as above
described, was installed just as planned. A view of the pro-peller being installed to the actual ship, S No. 1736 "CHAM
BORD" (delivered date; May, 1974) is shown in Fig. 21. Thereafter, about 20 large keyless propellers of SKF type
have already delivered as of April, 1976.
6. Closing remarks
By execution of push-up test and torque-slip test of
propeller utilizing the actual propeller of 6.8 m in diameter,
variöus data could be secured and its adaptability to the actual ship also confirmed. However, without closing only with the tests lately finished, accumulating further tech-nical improvements, effort for attaining the mcst reliable
keyless propeller shall be continued.
Ref érence
(1) Bowden & Tabor, Friction and Lubricatiôn betwèen Solid
