The effect of helical groove geometry on journal abrasive wear

The effect of helical groove geometry on journal abrasive wear

archives of civil and mechanical engineering 13 (2013) 150–157 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/a...

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archives of civil and mechanical engineering 13 (2013) 150–157

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/acme

Original Research Article

The effect of helical groove geometry on journal abrasive wear J. Sep, P. Pawlus, L. Galda Rzeszow University of Technology, Faculty of Mechanical Engineering and Aeronautics, 12 Powstancow Warszawy Street, 35-959 Rzeszow, Poland

ar t ic l e in f o

abs tra ct

Article history:

The article presents an experimental study of slide bearings operating in lubricant

Received 17 July 2012

contaminated by Al2O3 abrasive particles. The aim of this work was the comparison of

Accepted 2 January 2013

wear resistance of slide bearings with different surface geometry on the journal. It was

Available online 8 January 2013

found that helical groove on the journal significantly reduced wear of sliding pairs. The

Keywords:

results of experiments showed that groove cross section area and helical groove lead

Journal bearing

affected abrasive wear. Valuable results concerning the sleeve wear reduction were

Helical groove

obtained with respect to bearings with textured journals.

Contaminant

& 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

Wear

1.

Introduction

Idea of helical or herringbone grooves application on the cooperating surfaces of bearings to produce pressure of the lubricating medium appeared in the 1940s. Firstly it was used in gas-lubricated journal bearings [1], then in bearings lubricated by oils [2–4] and solid greases [5]. Journals with grooves found practical application in highspeed low-loaded bearings because of their higher reliability and more stable operation comparing to conventional and because of the possibility to obtain the leakage-free operation of journal bearings [2,3]. Nowadays such techniques find implementations in bearings of the computer spindles in hard discs [6,7] and in small precise engines, for example in DVD players [8]. They are still studied and developed because of their practical applications [9,10]. Abrasive wear decrease in case of hard particles presence in the oil is the important advantage of implementation of the groove on the journal of slide bearing [11,12]. Such effect

can be reached by making the helical groove on the journal surface. The amount of abrasive wear depends on the helical groove geometry however there is not enough information explaining these dependences exactly. The main aim of the study described in this article is presentation of the effect of parameters characterizing the helical line on journal surface on abrasive wear of sliding pairs lubricated by the contaminated oil. Additionally the investigations of the surface topography of the journal are introduced to draw out conclusions about wear mechanism. Ability of moving wear debris and contaminants out from the contact zone by grooves created on journal surfaces could lead to new practical applications of slide journal bearings.

2.

Experimental procedure

Abrasive wear of slide bearings consisting of journal with the helical groove on its surface was investigated. The classic

Corresponding author. Tel.: þ48 17 865 1904; fax: þ48 17 865 1184.

E-mail address: [email protected] (L. Galda). 1644-9665/$ - see front matter & 2013 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2013.01.001

archives of civil and mechanical engineering 13 (2013) 150–157

Nomenclature b d(r) dg H hmin I1 Iv Ivj Ivs n p R Sa Sal Sf Sk Sp

bearing length journal diameter (radius) groove depth spiral lead of the groove minimum oil film thickness ratio of oil capacities of groove (oil pockets) on journal surface to oil capacity of bearing clearance volumetric wear intensity volumetric wear intensity of journal volumetric wear intensity of sleeve rotational speed of journal nominal pressure sleeve radius arithmetical mean deviation of the surface fastest decay autocorrelation length groove cross section area core depth maximum height of summits

sliding pair with the smooth surfaces of co-operating elements was assumed as the reference point. The scheme of the bearing with the modified journal surface is introduced in Fig. 1. The investigations of wear in the presence of hard abrasive particles in the lubricant were conducted on the ZAN rig at Gdansk University of Technology. This rig is described in the article [13]. Bearing sleeves were made from alloy MB58 (78% Al, 20% Sn, 1% Cu, 1% Mn) and the journals from steel 42CrMo4 (hardness 52 HRC). The tested series of sliding pairs differed with the geometry of the helical groove on journal surface. The possibility of wear reduction by oil pockets creation on journal surfaces was also examined. Oil pockets existence on steel surfaces demonstrated beneficial effects on tribological characteristics [14–16]. On the basis of computer simulations and preliminary investigations [17] it was assumed that the following parameters describe the helical groove:  the groove cross section area Sf¼ (w  dg)/2,  the spiral lead of the groove H. Index I1 [%] being the ratio of oil capacities of groove (oil pockets) on journal surface to oil capacity of bearing clearance was used to characterize both modified journal surfaces. To obtain I1 index of journals with helical groove the formula (1) was used. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi w  dg  ð2prÞ2 þ H2   ð1Þ I1 ¼ 2pH  R2 r2 The capacity of oil pockets on textured journal surfaces was the summation of all separate oil pocket capacities. Single oil pocket capacity was calculated on the basis of surface topography of measured samples. Parameters values describing analyzed journal series in the first stage of examination are presented in Table 1. Tested

Spc Spk Sq Str Sv Svk s t v W w Zv e Y

j C o

151

arithmetical mean peak curvature reduced summit height root-mean-square deviation of the surface texture aspect ratio of the surface maximum depth of valleys reduced valley depth sliding distance duration of the test sliding speed of journal load carrying capacity groove width volumetric wear journal eccentricity coordinate in circumferential direction as measured from the point where bearing interspace thickness is maximum attitude angle radial clearance ratio journal angular velocity

samples (series 3 and 4) were textured by burnishing technique to obtain isolated oil pockets in two different arrangements. The photos of journals with textured surface are presented in Fig. 2 and journals with helical groove in Fig. 3. In this experiment textured journals were characterized by the same value of index I1 ¼ 1.6% as one of chosen journal variant with helical groove. The influences of the groove cross section area Sf and the spiral lead of the groove H in wide variable range on journal and sleeve wear intensity were given into examinations. Evolutionary Operation (EVOP) technique was applied to find the minimum wear intensity of the journal. There were the following geometry and operating conditions of tested journal bearings:

          

journal diameter: d ¼52.7 mm, bearing length ratio: b/d ¼0.5, radial clearance ratio: c¼ 0.002, minimum oil film thickness: hmin ¼0.017 mm, rotational speed of journal: n ¼600 rpm (the sliding speed v¼ 1.65 m/s), nominal pressure: p¼ 1.57 MPa, duration of the test: t ¼20 h (the sliding distance s¼120,000 m), lubricant agent: motor oil SAE 40 with kinematic viscosity 15 mm2/s, contaminant: dust Al2O3, the average diameter of grains 21 mm, concentration of contaminant in oil 0.05 g/l, cleanliness according to ISO 4406 standard: 22/20/18, lubricant temperature: 22 1C.

The wear amount of journal was assessed on the basis of the profiles from the sliding surface in the axial section. Then

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archives of civil and mechanical engineering 13 (2013) 150–157

Fig. 1 – Bearing with journal having helical groove.

Table 1 – Parameters of journals for series 0–4. Series

Sf [mm2]

H [mm]

I1 [%]

0 1 2 3 4

0 0.016 0.025 Textured (a) Textured (b)

0 20 20 8 –

0 1.6 2.5 1.6 1.6

volumetric wear was calculated. The wear measure of the bimetal sleeves was the mass difference before and after the test. The mass loss of the sleeve was used for calculation of volumetric wear. The volumetric intensity of wear Iv [mm3/ km] (2) being the quotient of volumetric wear Zv and the sliding distance s was compared with reference to all tested series. Iv ¼

Zv s

ð2Þ

During investigations wear intensity of the journal Ivj and sleeve Ivs were obtained. The surface topography measurement of worn journals was conducted. The white light interferometer Talysurf CCI Lite was used. The number of measurement points was 1024  1024 in the area of 3.3 mm  3.3 mm. The aim of these investigations was the identification of surface topography parameters which showed substantial correlation with journal wear. Additionally the analysis of height surface parameters in the area of 1.2 mm  1.2 mm were done.

3.

Results and discussion

In the first stage wear intensity of sliding pairs with different surface topographies of the journal was examined. The oil pockets presence in two arrangements created by burnishing method and grooves existence differing with cross section area were taken into consideration.

Figs. 4 and 5 present the effect of the relative oil capacity I1 and the shape of lubricant reservoirs on journal and sleeve wear respectively for series 0–4. The best result of journal wear intensity was obtained for I1 ¼ 2.5% (Fig. 4). Wear was over two times smaller (Ivj ¼1.18 mm3/km) compared to plain series. Volumetric wear of textured journals was smaller than that of plain journals too. When oil pockets were in helical line (H ¼ 8 mm) the decrease of wear was about 19%. For series with oil pockets located in the same distance in line and between pits lines journal wear decreased over 23%. Comparing wear of series with the same I1 index of 1.6% it can be observed that textured samples got smaller values of volumetric wear over 28% and 32% than those with groove. This situation probably took place because depth and width of groove differ from those of pits. Array of oil pockets had negligible influence on wear in these investigations. The great difference in wear intensity of journals was noticed for different groove cross section area. When parameter Sf was equal to 0.016 mm2 the journal wear intensity Ivj was the highest from all tested journals but for the groove cross section area of 0.025 mm2 the wear intensity got the smallest value of 1.18 mm3/km. These results prove that geometry of grooves created on sliding surfaces significantly influence wear. The groove and oil pockets creation on journal surfaces resulted in decrease in wear intensity of the co-operating sleeves (Fig. 5). The smallest wear of sleeve was obtained for bearings with textured journal where oil pockets were uniformly located. Ivs index was over 31% lower than that for bearings with plain journal. Similar results were obtained for the best series from grooved journal. The decrease of wear reached over 24% when oil capacity I1 was 2.5%. Decrease in journal and sleeve wear could be explained by smaller contaminant aggregation in sliding elements and faster wear debris elimination from co-acting surfaces but detailed investigations are needed to explain this phenomenon. The arrangement of oil pockets on sliding surfaces had minor effect on wear in these experiments but geometry of

archives of civil and mechanical engineering 13 (2013) 150–157

153

Fig. 2 – Photos of journals having oil pockets in two arrangements (arrays): in helical line—textured (a) and in the same distance between the nearest pits—textured (b).

Fig. 3 – Photos of journals with helical groove: Sf ¼0.016 mm2; H ¼20 mm (a) and Sf¼0.025 mm2; H ¼20 mm (b). Ivj [mm3/km] 3.5 2.73 3.0 2.41 2.5

1.95

2.0

1.84

1.18

1.5

1.0

- plain journal - grooved journal - textured journal

0.5 0.0 0%

1.6%

textured (a)

1.6%

1.6%

2.5%

textured (b)

I1

Fig. 4 – The influence of relative oil capacity I1 and the shape of lubricant reservoirs on journal wear; textured (a)—oil pockets in helical line, textured (b)—oil pockets in the same distance between the nearest pits.

Ivs [mm3/km] 0.14

0.119

0.113

0.12 0.094 0.10

0.090

0.082

0.08

- plain journal - grooved journal - textured journal

0.06 0.04 0.02 0.00 0%

1.6%

textured (a)

1.6%

1.6%

2.5%

textured (b)

I1

Fig. 5 – The influence of relative oil capacity I1 and the shape of lubricant reservoirs on sleeve wear; textured (a)—oil pockets in helical line, textured (b)—oil pockets in the same distance between the nearest pits.

groove had the substantial influence on abrasive wear. Additionally, process of creating the groove on the journal surface is not complicated and allow to form grooves in wide range of depth and width. In case of producing oil pockets by burnishing technique the special tool is essential and there are some limitations of the application mainly connected with material hardness. Actually, the device used for oil pockets burnishing is able to form pits up to 60 mm of depth on steel journal of over 50 HRC hardness. Basing on obtained results and the limitation of oil pockets creation further investigations were focused on the effect of helical groove geometry on wear intensity of sliding pair operated in contaminated oil. The next stage of this research reveal the influence of the groove cross section area Sf in range of 0.016–0.025 mm2 and additionally the spiral lead of the groove H in range of 3.5–20 mm. Figs. 6–9 present results of grooved journal and sleeve wear with comparison to wear of smooth surfaces (H¼0 mm, Sf¼ 0 mm2). The 95% confidence intervals obtained on the basis of the statistical results analyses are also marked on graphs. Most from examined journals with helical grooves showed smaller wear intensity than the base series. Only three from analyzed series had higher Ivj index. Two worst results of wear were noticed for samples which the spiral lead of the groove H had the highest value of 20 mm but the third case was for H of 3.5 mm. The greatest wear resistance was observed for series characterized by comparatively large groove cross section area Sf of 0.0205 and 0.025 mm2 but the direct relation between the groove oil capacity and wear intensity was not found in the examined parameters range. The results presented in Figs. 6–9 show that wear intensity of the journal is over 10 times greater than wear intensity of the sleeve. It confirms literature data that in case of the substantial difference of hardness between journal and sleeve the hard

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archives of civil and mechanical engineering 13 (2013) 150–157

Sf=0.016

Ivj [mm3/km]

Sf=0.0205

Sf=0.025

3

2

1

H [mm] 0 0

5

10

15

20

Fig. 6 – The influence of spiral lead of the groove on journal wear.

Ivj [mm3/km]

H=3.5

H=11.75

H=20

3

2

particles are embedded in soft sleeve. They do not plunge completely so as the result affect destructive the journal surface. Investigations also show that the parameters of the helical groove influence mainly journal wear. Suitably chosen groove parameters resulted in over two-times decrease in wear intensity, but wear is larger than that for smooth journal in case of inappropriate groove geometry. Wear characteristics presented in Figs. 6 and 7 do not reveal clearly perceptible regularities. However the graph courses show the extreme occurrence. This motivates to investigate deeply examined area to optimize the helical groove geometry. The helical groove application on the journal resulted in decrease in sleeve wear. In these investigations the decrease is over 20% and wear decreases with increase in the spiral lead of the groove and the groove cross section area. It was found from the analysis of surface topography between grooves after wear that the Spc (arithmetic mean peak curvature) parameter showed substantial correlation with journal wear. The value of correlation coefficient is 0.69. Fig. 10 presents dependence between wear and the Spc parameter. Reciprocal of this parameter value determines

1

Sf [mm2]

Ivj [mm3/km] 3

0 0.005

0

0.01

0.015

0.02

0.025

Fig. 7 – The influence of groove cross section area on journal wear.

2.5

2

Ivs [mm3/km]

Sf=0.016

Sf=0.0205

1.5

Sf=0.025

Spc [1/mm]

0.16 1 0.05

0.12

0.1

0.15

0.2

Fig. 10 – The relation between journal wear and mean peak curvature Spc.

0.08

0.04

H [mm] 0 0

5

10

15

20

Fig. 8 – The influence of spiral lead of the groove on sleeve wear.

Spiral lead of the groove H [mm] 20

Ivj [mm3/km] 2.73

1.18

2.85 (6)

(1)

(2)

II phase

16

0.98 (8)

Ivs [mm3/km]

H=3.5

H=11.75

11.75

H=20

I phase

0.16

(7)

(5)

2.04

1.06

0.12

(4)

0.08

3.5

(3) 1.26

2.45

0.04

Sf [mm2] 0 0

0.005

0.01

0.015

0.02

0.025

Fig. 9 – The influence of groove cross section area on sleeve wear.

0.016

0.0205

0.0223

0.025

Groove cross section area Sf [mm2]

Fig. 11 – Journal wear intensity Ivj values in I and II phases of EVOP.

archives of civil and mechanical engineering 13 (2013) 150–157

mean peak curvature. It is evident from Fig. 10 that smaller radii of peak curvature correspond to higher journal wear. According to Evolutionary Operation technique the minimum abrasive wear for grooved journal was investigated. The experiments were based on two-variable EVOP design in which the groove cross section area and the spiral lead of groove were the controlled variables. In the first phase the abrasive wear tests were held for the groove cross section area Sf values of 0.016 and 0.025 mm2 and the spiral lead of groove H: 3.5 and 20 mm and in the center point: Sf of 0.0205 mm2 and H of 11.75 mm. Fig. 11 presents obtained results after the calculation and statistical analyses. In the first phase of EVOP the smallest journal wear intensity was reached in center point (5) for Sf of 0.0205 mm2 and H of 11.75 mm. Similar values of wear were obtained for the largest groove cross section area of 0.025 mm2. Further Sf parameter increasing to take a course on the right from the center of EVOP design was inadequate because it resulted in the load capacity decrease which was found in previous investigations. This was additional reason to change the center point for new one and the range of variables. Values of parameters Sf and H as well as results of wear process from the next phase are presented in Fig. 11. The minimum value of wear intensity of 0.98 mm3/km was found in the center point (8) of second phase. This value was comparable to that got in the first phase (1.06 mm3/km). Obtained results are satisfying and comparing to wear value of other examined grooved journal with H of 20 mm and Sf of 0.0205 mm2 over two times decrease was reached.

155

After abrasive wear tests surface topographies between grooves of the journal with the lowest wear and the base series were compared (Fig. 12). Wear intensity of the analyzed grooved journal Ivj was equal to 0.98 mm3/km so it was two and half times smaller than that of untextured one. The surface topography height of journal with groove is lower than that of the plain one but the difference between height parameters is not significant. Analysis of surface topographies shows that one-directional texture was formed after tribological test. The Str parameter values of 3.94% for untextured and 6.9% for grooved journals are characteristic for anisotropic surfaces after abrasive wear. Spatial parameter Sal is almost two-times greater for considered grooved journal surface (0.0866 mm) than that of plain series (0.0493 mm). That could be connected with higher ability to contaminant removal by the groove and resulted in smaller wear intensity of this series. Values of height parameters are smaller on grooved surface and it corresponds to lower wear too. Additionally there are also visible regions where the surface topography height is much smaller than in other places (see Fig. 13) and those of the base series. Profile height of untextured journal after abrasive tests was two times bigger (about 8 mm on average) than this of analyzed smoother region between grooves. Surface profile of plain journal after wear resistance test differ from the above described because has high peaks and deep valleys on whole assessment length (Fig. 14). This phenomenon is characteristic for worn surfaces operating in oil with contaminants of hard particles especially.

Height parameters Sq = 1.27 μm Sp = 3.85 μm Sv = 5.40 μm Sa = 0.982 μm

Height parameters Sq = 1.16 μm Sp = 3.53 μm Sv = 4.10 μm Sa = 0.920 μm

Fig. 12 – View of surfaces between grooves and height parameters after abrasive wear tests of journals: plain (a) and of the minimum wear intensity (b).

Length = 3.29 mm Pt = 6.36 µm Scale = 10.0 µm

µm 4 2 0 -2 -4 0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

2.75

3

3.25 mm

Fig. 13 – Profile (between grooves) of the grooved journal with the lowest wear intensity.

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archives of civil and mechanical engineering 13 (2013) 150–157

Parameters describing Sk family were also analyzed. Fig. 15 presents material ratio curves of plain and grooved journals (of the best wear resistance). Although reduced summit heights Spk and core depths Sk are quite similar for both cases parameter describing surface valleys Svk is about 23% lower of grooved journal surface than that of plain journal. Selected region (1.2 mm  1.2 mm) of grooved samples with comparatively small surface topography height was subjected

to detailed analysis and compared to the similar region of untextured worn surface. Fig. 16 presents surface topographies and height parameters of selected areas of plain and grooved journal of the highest wear resistance. The relative differences between height parameters calculated for chosen areas are significant: 47% of Sq, 42.2% of Sp, 32.4% of Sv and 53.8% of Sa. Smoother regions on grooved journal surfaces could be explained by faster contaminants elimination from contact zones which reduced abrasive wear substantially.

Length = 3.28 mm Pt = 8.06 µm Scale = 10.0 µm

µm

2 0 -2 -4 -6 0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

2.75

3

3.25 mm

Fig. 14 – Profile of the plain journal after abrasive wear test.

Sk parameters, unfiltered.

Sk parameters, unfiltered. Spk = 0.907µm

Sk = 2.94µm

Spk = 1.04µm

Sk = 3.00µm Svk = 1.91µm Svk = 1.48µm

0

20

40

60

80

Sr1 = 14.5 %

100 %

0

20

40

60

80

Sr1 = 6.31 %

Sr2 = 87.8 %

100 %

Sr2 = 87.9 %

Fig. 15 – Material ratio curves after abrasive wear test of plain (a) and grooved journal of the highest wear resistance (b).

0

0.2

0.4

0.6

0.8

0

1

1.2 mm

µm 6.5

0

0.2

0.4

0.6

0.8

1

1.2 mm

µm

0 4.5

0.1

6

0.1

0.2

5.5

0.2

4

0.3

5

0.3

4.5

3.5

0.4

0.4

0.5

4 3.5

0.5

0.6

3

0.7

2.5

0.8

2

0.9

1.5

1

1

1.1

0.5

1.2

0

mm

3

0.6

2.5

0.7

2

0.8

1.5

0.9

1

1 1.1

0.5

1.2

0

mm

Height parameters Sq = 1.12 μm Sp = 3.27 μm Sv = 3.35 μm Sa = 0.892 μm

Height parameters Sq = 0.762 μm Sp = 2.30 μm Sv = 2.53 μm Sa = 0.580 μm

Fig. 16 – Surface topographies and height parameters of selected regions from plain (a) and grooved journal of the highest wear resistance (b).

archives of civil and mechanical engineering 13 (2013) 150–157

4.

Conclusions [4]

The main conclusions drawn from this investigation are as follows: 1. Wear intensity of the journal is over 10 times greater than wear intensity of the sleeve operating in lubricant contaminated by Al2O3 abrasive particles. 2. The helical groove presence on the journal affects abrasive wear in case of hard particles presence in the oil. In this study suitably chosen shape and lead of the groove resulted in over two-times decrease in journal wear compared to smooth journal. 3. The helical groove application on the journal resulted in decrease in sleeve wear. In these investigations the decrease is about 20% and wear decreases with increase in the spiral lead of the groove and the groove cross section area. 4. Surface texturing is a promising technique which influences abrasive wear. Journal wear of series with oil pockets located in the same distance in line and between pits lines decreased over 23% compared to plain journals. The smallest wear (over 31% lower than for bearings with plain journal) of sleeve was obtained for bearings with textured journal where oil pockets were uniformly located. 5. Examinations focused on the helical groove optimization result in minimum journal wear intensity Ivj occurrence of 0.98 mm3/km at the spiral lead of the groove H of 16 mm and the groove cross section area of 0.0223 mm2.

[5] [6]

[7]

[8]

[9]

[10]

[11]

[12] [13]

r e f e r e n c e s

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