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Shaft surface manufacturing methods for rotary shaft lip seals
FEATURE
Shaft surface manufacturing methods for rotary shaft lip seals By Dipl.-Ing. Thomas Kunstfeld and Professor Dr.-Ing. Werner Haas – IMA Institut für Maschinenelemente, University of Stuttgart, Germany Manufacturing methods such as hard turning are of interest to produce the counter face for rotary shaft lip seals to provide an economical replacement for plunge grinding. Currently applications are restricted due to lack of experience with such shaft surfaces. This article describes research work to investigate the suitability of several manufacturing processes by studying the pumping rate of the seals. The work shows that several surface finishing methods – including axial, plunge and tangential turning – can be used.
Introduction Rotary lip seals (Figure 1) are used in many applications where a rotary shaft passes through a housing wall and has to seal a fluid at zero pressure. You can find this, for example, in combustion engines, vehicle or wind force transmissions. The sealing system always consists of at least three components: the oil which has to be sealed, the seal which is bought from a supplier, and the shaft surface, which is usually produced by the manufacturer. The current requirements to reduce production costs have created attention on the shaft surface manufacturing method. Until now, the shaft surface has been plunge ground according to technical standards such as those in References 8 and 9. The roughness characteristic values used to describe the requirements for the shaft surfaces have been Rz = 1 to 5 µm or Ra = 0.2 to 0.8 µm, Rmax < 6.3 µm, measured in the axial direction. These are combined with the requirement for a lead-free machining treatment. These characteristic roughness values are not sufficient to describe the shaft surface used in sealing technology.[1] Some seal manufacturers recommend additionally a bearing area ratio, Mr, from 50% to 70% on a cut depth of 25% Rz.[3, 4] Expensive machinery and a long operating time, typically 20 to 30 seconds, cause high production costs, so industry is interested in alternative, more economical manufacturing methods. Hard-turning is one
Sealing Technology July 2005
possibility. The hardened work is completely finished in one setting. Re-clamping and an expensive grinding process are omitted. However, there is disagreement in the industry on the functionality of hardturned shaft surfaces for use with radial shaft seals. Unexplained failures in field tests, ignorance concerning the manufacture parameters which can be used, and lack of experience with these shaft surfaces lead
to the fact that this manufacturing method is not widely accepted. There is therefore some resistance to wider use. For applications with moderate requirements, non-hardened shaft surfaces are also possible. The advantage of this method is the reduced production costs, because the hardening process is avoided. The situation is similar to that of hard-turned shaft surfaces – there is also no experience available.
Investigations To estimate the suitability of the different manufacturing methods for use as a counter face for radial shaft seals, shaft surfaces manufactured by different processes were tested to assess their pumping characteristics. In addition the entire sealing system, sealing ring and shaft surface, was tested to evaluate sealing and pumping behavior.
Figure1. Radial lip seal.
5
FEATURE
Figure 2. Surface topography of the shafts tested.
Sealing investigations To evaluate the sealing behavior, a 240 hour sealing test in accordance with DIN 3761[9] was performed. In addition, for field use, relevant examples were selected and tested for 1000 hours. The focus was on the traverse turning and tangential turning processes. The respective investigation parameters are shown in Table 2. The seal testing was carried out in both directions of rotation of the shaft. Part of the investigations of the research project was carried out in the laboratories of Freudenberg Dichtungs- und Schwingungstechnik KG. Figure 3. Schematic of pumping rate evaluation.
Shaft surfaces considered The emphasis of the investigations was on the hard turning process, plunge turning and tangential turning.[6] Non-hardened shaft surfaces were also investigated. In order to be able to compare the results with the current state-of-the-art, plunge ground shaft surfaces were also included as a control. One representative combination of manufacturing parameters for each manufacturing method was used, and these are presented in Table 1. All shaft surfaces were, with the exception of the plunge ground and the tangential turned shafts, manufactured on the same machine, in order to exclude different machine influences. The machine that
6
Pumping rate measurement was used to produce the shaft surfaces was especially optimized for the hardturning process. The guidance and the spindle of this machine are all located by hydrostatic bearings. Figure 2 shows the surface topographies of the different manufactured shaft surfaces. The illustration shows in each case a surface range of 750 × 750 µm. With the exception of the non-hardened shaft surface, the axis scale in the z-direction is identical. The large deviation of the profile geometry between the axial direction, which is the standard measuring direction, and the circumferential direction of the topography being studied is especially interesting. This is reflected in the roughness parameters (Table 1).
A substantial component of the sealing system is the shaft surface. In order to be able to estimate its influence on the whole system, the pumping rate of the shaft surface was measured. The shaft pumping rate, when added to the pumping rate of the seal, affects the sealing performance of the whole system. The best case would be a nonpumping shaft surface. The procedure to determine the pumping rate of the shaft surface is shown in Figure 3. The seal is installed in the reverse of the intended installation direction, with the sump on the base side. The pumping mechanism of the seal continuously pumps oil outwards, which is measurable as leakage. In the first step of the pumping rate evaluation, the combined pumping rate of the
Sealing Technology July 2005
FEATURE seal, FW-RWDR, and the pumping rate of the shaft surface being investigated, FWWOF, are measured. Subsequently, a nonpumping shaft surface is installed. The resulting leakage is exclusively caused by the seal, FW-RWDR. The difference in the measured values is the pumping rate of the shaft surface being tested. At least four specimens of each different shaft surface were considered to carry out the mean pumping rate.
Pumping rate measurement of the entire sealing system in operation The pumping ability of the lip seal is essential for the function of the sealing system. Depending on the operating conditions in the sealing contact, the pumping ability is generated by the tribological contact with the shaft surface. The shaft surface has a considerable effect on the tribology in the sealing contact, and thus the development of the pumping ability of the lip seal. If one observes the development of the pumping ability of sealing systems with different shaft surfaces over a long period, the positive or negative influence of the shaft surface on the sealing system can be evaluated. The comparison of the shaft surfaces, manufactured by different methods, against each other is as interesting as the comparison with the standard plunge ground shaft surface. The main feature of the measuring method used in these tests is that the sensitive sealing system does not have to be disassembled, and no changes or other interference are made to the seal. The pumping rate of the sealing system has to be measured directly on the long-term test rig. This is carried out according to the procedure presented for the first time in Reference 7, by means of the modified twochamber method. This procedure is represented schematically in Figure 4. The basic idea is to flood the secondary chamber, arranged on the ‘air-side’ of the sealing system, with oil only for the short time interval of the pumping rate measurement, and empty it thereafter. Thus the disturbing influence on the sealing system – for example, the much-improved lubrication with the flooded air-side – is minimized. This does not impair the quality of the pumping rate measurement. If leakage occurs during the sealing investigation between the pumping rate measuring intervals, it can still be measured. Measurements using this large-scale investigation method were only carried out for selected shaft surfaces. Four specimens
Sealing Technology July 2005
Figure 4. Schematic drawing of the pumping rate measuring arrangement.
Figure 5. Pumping rate of the shaft for the different manufacturing methods.
Figure 6. Sealing system pumping rate for the different manufacturing methods.
7
FEATURE
Manufacturing method:
Material (number)
Cutting tool material
Plunge ground DIN 3761
Traverse turned, non-hardened
Traverse turned, hardened
Plunge turned
Tangential turned
100Cr6
Ck 45
20MnCr5
20MnCr5
20MnCr5
(1.2067)
(1.0503)
(1.7147)
(1.7147)
(1.7147)
CBN
CBN
Cemented
Cubic boron
carbide
nitride (CBN)
Cutting speed, m/min
300
160
160
150
Feed, mm/rev
0.29
0.10
0.01
0.30
Cutting depth, mm
0.25
0.10
0.10
0.05
Radius of cutting edge, mm
1.20
1.20
Rz axial direction, µm
2.10
9.90
2.29
0.69
1.94
Rz circumferential
0.49
0.77
0.56
0.54
0.55
direction, µm Table 1. Specification of shaft surfaces tested.
each of the plunge ground shaft surface, the baseline for the comparison, hardened and non-hardened traverse turned shaft surfaces were examined by means of this method. The first pumping rate measurement was carried out after 200 h operation, and afterwards at intervals of approximately 100 h.
Results The results of the sealing and pumping rate measurements for the different shaft surfaces will now be discussed. Based on these results, their suitability as a counter face in radial shaft seals is then evaluated.
Pumping rate measurement All the shaft surfaces considered show a pumping ability, even those manufactured without axial feed, plunge ground, tangential and plunge turned. If we accept the model conception, which represents the feed spiral as a ‘pumping thread’, one would expect the traverse turned shaft surfaces to have a positive pumping rate with clockwise rotation, and a negative pumping rate with counter-clockwise rotation. However, the pumping rate of the traverse turned shaft surfaces is to a large extent the opposite of the expected pumping direction. The hardened traverse-turned shaft surfaces and the plunge ground shaft surfaces show comparable pumping rates (Figure 5). There is no connection between the direction of the machining feed spiral and the pumping direction of the shaft surface. The non-hardened shaft surfaces show a clearly higher pumping rate in the clockwise rotation; however, with both clockwise 8
Operating time
240 hours
1000 hours
Shaft diameter, mm
50
50
Seal type
BAUM 4X7
BAUM 4X7
Seal material
FPM
FPM
Oil
SAE 80
SAE 80
Temperature, °C
80
80
Sliding velocity, m/s
3.90
7.80
Dynamic eccentricity, mm
0.10
<0.02
Static eccentricity, mm
0.10
<0.02
Stationary every 24 hours, h
4
1
Table 2. The leakage test parameters.
and counter-clockwise rotation the pumping rate is positive. Tangential turned and plunge ground shaft surfaces generally cause low pumping rates. The direction of rotation does not have an influence on the development of the pumping direction. The pumping ability of the shaft surfaces is smaller than the pumping ability of the lip seal in all cases.
Sealing investigations All the sealing systems completed the sealing investigation successfully. The air side of the sealing rings was not moistened with oil. The sealing rings did not exhibit unusual wear. The wear tracks on the shaft surfaces were to a large extent smoothed, but the essential structure was
still recognizable. The wear width of the seal does not show any dependence on the direction of rotation. Even during the 1000 h sealing investigation, no drops of leakage could be observed, nor could any oil or oil mist be seen on the air side of the seals. None of the seals showed unacceptable wear width. The maximum wear width occurred in combination with the plunge ground shaft surface, with a wear width b = 0.47mm. The wear width did not depend on the rotation direction. Shaft surface wear was to a large extent not measurable. In a few cases small run-in tracks were evident, but there was no observed connection with the rotation direction or the shaft surface used. Table 3 summarizes the results of the sealing investigations.
Sealing Technology July 2005
FEATURE Manufacturing method:
Plunge ground DIN 3761
Traverse turned, non-hardened
Traverse turned, hardened
Plunge turned
Tangential turned
Sealing investigation, 240 hours
No leakage
No leakage
No leakage
No leakage
No leakage
Sealing investigation, 1000 hours
No leakage
No leakage
No leakage
—
No leakage
Table 3. Results of the leakage tests.
Pumping rate measurement of the entire sealing system All the sealing systems showed a decreasing pumping rate with increasing operation time. Figure 6 shows a typical example of the pumping rate development for each shaft surface type. The pumping rates of the non-hardened traverse turned shaft surfaces are on the right-hand secondary axis. The pumping rate of the plunge ground shaft surfaces showed very little fluctuation. The initial pumping rate was in the range between 0.10 and 0.23 ml/h. The mean decrease in the system pumping rate was between 0.04 and 0.10 ml/h per 1000 hours. The direction of rotation of the sealing systems tested did not have a discernible influence on the pumping rate level at the beginning or on the decrease of the pumping ability during operation. The initial pumping rate of the sealing systems with hardened traverse turned shaft surfaces was comparable with those of the plunge ground shaft surfaces. Even the mean decrease in the system pumping rate, between 0.02 and 0.10 ml/h per 1000 hours, was comparable with the plunge ground shaft surfaces. It was not possible to associate the initial pumping rate or the development during operation with the direction of rotation. The sealing systems with non-hardened traverse turned shaft surfaces clearly showed a higher initial pumping rate, whereby the height of the initial level did not compare with the direction of the machining feed spiral. However, the decrease in the system pumping rate was approximately 10 times faster in the counter-clockwise rotation direction, turning spiral in the ‘sealing direction’, than in the clockwise rotation, turning spiral in the ‘leakage direction’. The decrease in the pumping rate was between 0.05 and 0.50 ml/h per 1000 hours.
Conclusions The investigation methods discussed here make it possible to establish the relevant sealing characteristics of shaft surfaces. The pumping rates determined for the shaft
Sealing Technology July 2005
surfaces permit a rapid pre-evaluation of the suitability of a shaft surface as a counter face for lip seals. The observation of the system pumping rate during operation without interference to the tribological system – for example, by disassembly and renewed installation of the seal – provide further evaluation possibilities. The influence of the different shaft surfaces on sealing security and reliability of the sealing system can be determined. The selection of suitable manufacturing processes and parameters is made easier. Several manufacturing methods were checked for their suitability for the counter face in shaft seals. It is shown that the machining feed spiral that is created on the shaft surface is not the reason for the pumping ability of the shaft surface. With appropriate manufacturing, both hardened and non-hardened traverse turned steel shaft surfaces can be sealed by means of suitable lip seals. The direction of rotation is not crucial. This is proved by the sealing tests that were successfully completed. The turning methods without axial feed, such as tangential turning and plunge turning, also produce suitable counter faces for radial lip seals.
References 1. H.K. Müller, W. Haas and G. Wüstenhagen: Gegenlaufflächen für Radial-dichtungen, Tribologie und Schmierungstechnik, May/June 1996. 2. H. Bodschwinna: Rauheitsmessung zur Bewertung der Funktionseigenschaften technischer Oberflächen. Prüfen und Bewerten von Oberflächenschutzschichten, VDI-Gesellschaft für Werkstofftechnik, Germany, 1988. 3. Busak+Shamban: Dichtungen und Führungen. Firmenschrift Busak+Shamban GmbH, Stuttgart, 1995. 4. Parker Hannifin: Dichtungshandbuch. Firmenschrift Parker Hannifin GmbH, Bietigheim-Bissingen, Germany, August 1996. 5. T. Kunstfeld and W. Haas: Erfassung und Beschreibung von Wellenoberflächenstrukturen aus der zeitgemäßen und zukünftigen Fertigung und deren Einfluss auf die Dichtqualität von Radial-Wellendichtungen. Abschlussbericht, FKM Forschungshefte No. 279, Frankfurt, Germany, 2004.
6. J. Schneider and L. Schreiber: Mit dem Tangentialdrehen zu drallfreien Oberflächen, Werkstatt und Betrieb, 6/2002. 7. T. Kunstfeld and W. Haas: Standzeitprognose durch Förderwertmessung während Langzeitversuchen. Tagungsband 5. Hamburger Dichtungs-technisches Kolloquium Dynamische Dichtungen, 3–4 June 2004. 8. Deutsches Institut für Normung: DIN 3760, Radial-Wellendichtringe. 9. Deutsches Institut für Normung: DIN 3761, Radial-Wellendichtringe für Kraftfahrzeuge, Anwendungshinweise. Contacts:
Dipl.-Ing. Thomas Kunstfeld, IMA Institut für Maschinenelemente, Universität Stuttgart, Pfaffenwaldring 9, D-70569 Stuttgart, Germany. Tel: +49 711 685 6170, Email: [email protected], Web: www.ima.uni-stuttgart.de
Professor Dr.-Ing. Werner Haas, IMA Institut für Maschinenelemente, Universität Stuttgart, Pfaffenwaldring 9, D-70569 Stuttgart, Germany, Tel: +49 711 685 6170, Email: [email protected]
This article is an English translation based on a paper presented in German at the 13th ISC – International Sealing Conference, Sealing Systems for Fluid Power Applications, which took place in Stuttgart, Germany in October 2004. The ISC was organized by the Fluid Power Association of the VDMA, the German Engineering Federation. For more information or copies of the 13th ISC proceedings (140), contact Ralf Stemmjack at the VDMA: Tel: +49 69 6603 1318, Fax: +49 69 6603 2318, Email: [email protected], Web: www.sealing-conference.com. The 14th ISC will be held in Stuttgart on 10–11 October 2006. Editor’s comment: I am very pleased to be able to publish an English version of this paper. From my rather inadequate understanding of the German version, it appeared to contain some interesting work. The authors’ conclusions concerning the effect of the different surface textures will be of considerable interest to a number of readers. Some may well have different opinions and experience, so further contributions on this subject will be very welcome. 9
Shaft surface manufacturing methods for rotary shaft lip seals By Dipl.-Ing. Thomas Kunstfeld and Professor Dr.-Ing. Werner Haas – IMA Institut für Maschinenelemente, University of Stuttgart, Germany Manufacturing methods such as hard turning are of interest to produce the counter face for rotary shaft lip seals to provide an economical replacement for plunge grinding. Currently applications are restricted due to lack of experience with such shaft surfaces. This article describes research work to investigate the suitability of several manufacturing processes by studying the pumping rate of the seals. The work shows that several surface finishing methods – including axial, plunge and tangential turning – can be used.
Introduction Rotary lip seals (Figure 1) are used in many applications where a rotary shaft passes through a housing wall and has to seal a fluid at zero pressure. You can find this, for example, in combustion engines, vehicle or wind force transmissions. The sealing system always consists of at least three components: the oil which has to be sealed, the seal which is bought from a supplier, and the shaft surface, which is usually produced by the manufacturer. The current requirements to reduce production costs have created attention on the shaft surface manufacturing method. Until now, the shaft surface has been plunge ground according to technical standards such as those in References 8 and 9. The roughness characteristic values used to describe the requirements for the shaft surfaces have been Rz = 1 to 5 µm or Ra = 0.2 to 0.8 µm, Rmax < 6.3 µm, measured in the axial direction. These are combined with the requirement for a lead-free machining treatment. These characteristic roughness values are not sufficient to describe the shaft surface used in sealing technology.[1] Some seal manufacturers recommend additionally a bearing area ratio, Mr, from 50% to 70% on a cut depth of 25% Rz.[3, 4] Expensive machinery and a long operating time, typically 20 to 30 seconds, cause high production costs, so industry is interested in alternative, more economical manufacturing methods. Hard-turning is one
Sealing Technology July 2005
possibility. The hardened work is completely finished in one setting. Re-clamping and an expensive grinding process are omitted. However, there is disagreement in the industry on the functionality of hardturned shaft surfaces for use with radial shaft seals. Unexplained failures in field tests, ignorance concerning the manufacture parameters which can be used, and lack of experience with these shaft surfaces lead
to the fact that this manufacturing method is not widely accepted. There is therefore some resistance to wider use. For applications with moderate requirements, non-hardened shaft surfaces are also possible. The advantage of this method is the reduced production costs, because the hardening process is avoided. The situation is similar to that of hard-turned shaft surfaces – there is also no experience available.
Investigations To estimate the suitability of the different manufacturing methods for use as a counter face for radial shaft seals, shaft surfaces manufactured by different processes were tested to assess their pumping characteristics. In addition the entire sealing system, sealing ring and shaft surface, was tested to evaluate sealing and pumping behavior.
Figure1. Radial lip seal.
5
FEATURE
Figure 2. Surface topography of the shafts tested.
Sealing investigations To evaluate the sealing behavior, a 240 hour sealing test in accordance with DIN 3761[9] was performed. In addition, for field use, relevant examples were selected and tested for 1000 hours. The focus was on the traverse turning and tangential turning processes. The respective investigation parameters are shown in Table 2. The seal testing was carried out in both directions of rotation of the shaft. Part of the investigations of the research project was carried out in the laboratories of Freudenberg Dichtungs- und Schwingungstechnik KG. Figure 3. Schematic of pumping rate evaluation.
Shaft surfaces considered The emphasis of the investigations was on the hard turning process, plunge turning and tangential turning.[6] Non-hardened shaft surfaces were also investigated. In order to be able to compare the results with the current state-of-the-art, plunge ground shaft surfaces were also included as a control. One representative combination of manufacturing parameters for each manufacturing method was used, and these are presented in Table 1. All shaft surfaces were, with the exception of the plunge ground and the tangential turned shafts, manufactured on the same machine, in order to exclude different machine influences. The machine that
6
Pumping rate measurement was used to produce the shaft surfaces was especially optimized for the hardturning process. The guidance and the spindle of this machine are all located by hydrostatic bearings. Figure 2 shows the surface topographies of the different manufactured shaft surfaces. The illustration shows in each case a surface range of 750 × 750 µm. With the exception of the non-hardened shaft surface, the axis scale in the z-direction is identical. The large deviation of the profile geometry between the axial direction, which is the standard measuring direction, and the circumferential direction of the topography being studied is especially interesting. This is reflected in the roughness parameters (Table 1).
A substantial component of the sealing system is the shaft surface. In order to be able to estimate its influence on the whole system, the pumping rate of the shaft surface was measured. The shaft pumping rate, when added to the pumping rate of the seal, affects the sealing performance of the whole system. The best case would be a nonpumping shaft surface. The procedure to determine the pumping rate of the shaft surface is shown in Figure 3. The seal is installed in the reverse of the intended installation direction, with the sump on the base side. The pumping mechanism of the seal continuously pumps oil outwards, which is measurable as leakage. In the first step of the pumping rate evaluation, the combined pumping rate of the
Sealing Technology July 2005
FEATURE seal, FW-RWDR, and the pumping rate of the shaft surface being investigated, FWWOF, are measured. Subsequently, a nonpumping shaft surface is installed. The resulting leakage is exclusively caused by the seal, FW-RWDR. The difference in the measured values is the pumping rate of the shaft surface being tested. At least four specimens of each different shaft surface were considered to carry out the mean pumping rate.
Pumping rate measurement of the entire sealing system in operation The pumping ability of the lip seal is essential for the function of the sealing system. Depending on the operating conditions in the sealing contact, the pumping ability is generated by the tribological contact with the shaft surface. The shaft surface has a considerable effect on the tribology in the sealing contact, and thus the development of the pumping ability of the lip seal. If one observes the development of the pumping ability of sealing systems with different shaft surfaces over a long period, the positive or negative influence of the shaft surface on the sealing system can be evaluated. The comparison of the shaft surfaces, manufactured by different methods, against each other is as interesting as the comparison with the standard plunge ground shaft surface. The main feature of the measuring method used in these tests is that the sensitive sealing system does not have to be disassembled, and no changes or other interference are made to the seal. The pumping rate of the sealing system has to be measured directly on the long-term test rig. This is carried out according to the procedure presented for the first time in Reference 7, by means of the modified twochamber method. This procedure is represented schematically in Figure 4. The basic idea is to flood the secondary chamber, arranged on the ‘air-side’ of the sealing system, with oil only for the short time interval of the pumping rate measurement, and empty it thereafter. Thus the disturbing influence on the sealing system – for example, the much-improved lubrication with the flooded air-side – is minimized. This does not impair the quality of the pumping rate measurement. If leakage occurs during the sealing investigation between the pumping rate measuring intervals, it can still be measured. Measurements using this large-scale investigation method were only carried out for selected shaft surfaces. Four specimens
Sealing Technology July 2005
Figure 4. Schematic drawing of the pumping rate measuring arrangement.
Figure 5. Pumping rate of the shaft for the different manufacturing methods.
Figure 6. Sealing system pumping rate for the different manufacturing methods.
7
FEATURE
Manufacturing method:
Material (number)
Cutting tool material
Plunge ground DIN 3761
Traverse turned, non-hardened
Traverse turned, hardened
Plunge turned
Tangential turned
100Cr6
Ck 45
20MnCr5
20MnCr5
20MnCr5
(1.2067)
(1.0503)
(1.7147)
(1.7147)
(1.7147)
CBN
CBN
Cemented
Cubic boron
carbide
nitride (CBN)
Cutting speed, m/min
300
160
160
150
Feed, mm/rev
0.29
0.10
0.01
0.30
Cutting depth, mm
0.25
0.10
0.10
0.05
Radius of cutting edge, mm
1.20
1.20
Rz axial direction, µm
2.10
9.90
2.29
0.69
1.94
Rz circumferential
0.49
0.77
0.56
0.54
0.55
direction, µm Table 1. Specification of shaft surfaces tested.
each of the plunge ground shaft surface, the baseline for the comparison, hardened and non-hardened traverse turned shaft surfaces were examined by means of this method. The first pumping rate measurement was carried out after 200 h operation, and afterwards at intervals of approximately 100 h.
Results The results of the sealing and pumping rate measurements for the different shaft surfaces will now be discussed. Based on these results, their suitability as a counter face in radial shaft seals is then evaluated.
Pumping rate measurement All the shaft surfaces considered show a pumping ability, even those manufactured without axial feed, plunge ground, tangential and plunge turned. If we accept the model conception, which represents the feed spiral as a ‘pumping thread’, one would expect the traverse turned shaft surfaces to have a positive pumping rate with clockwise rotation, and a negative pumping rate with counter-clockwise rotation. However, the pumping rate of the traverse turned shaft surfaces is to a large extent the opposite of the expected pumping direction. The hardened traverse-turned shaft surfaces and the plunge ground shaft surfaces show comparable pumping rates (Figure 5). There is no connection between the direction of the machining feed spiral and the pumping direction of the shaft surface. The non-hardened shaft surfaces show a clearly higher pumping rate in the clockwise rotation; however, with both clockwise 8
Operating time
240 hours
1000 hours
Shaft diameter, mm
50
50
Seal type
BAUM 4X7
BAUM 4X7
Seal material
FPM
FPM
Oil
SAE 80
SAE 80
Temperature, °C
80
80
Sliding velocity, m/s
3.90
7.80
Dynamic eccentricity, mm
0.10
<0.02
Static eccentricity, mm
0.10
<0.02
Stationary every 24 hours, h
4
1
Table 2. The leakage test parameters.
and counter-clockwise rotation the pumping rate is positive. Tangential turned and plunge ground shaft surfaces generally cause low pumping rates. The direction of rotation does not have an influence on the development of the pumping direction. The pumping ability of the shaft surfaces is smaller than the pumping ability of the lip seal in all cases.
Sealing investigations All the sealing systems completed the sealing investigation successfully. The air side of the sealing rings was not moistened with oil. The sealing rings did not exhibit unusual wear. The wear tracks on the shaft surfaces were to a large extent smoothed, but the essential structure was
still recognizable. The wear width of the seal does not show any dependence on the direction of rotation. Even during the 1000 h sealing investigation, no drops of leakage could be observed, nor could any oil or oil mist be seen on the air side of the seals. None of the seals showed unacceptable wear width. The maximum wear width occurred in combination with the plunge ground shaft surface, with a wear width b = 0.47mm. The wear width did not depend on the rotation direction. Shaft surface wear was to a large extent not measurable. In a few cases small run-in tracks were evident, but there was no observed connection with the rotation direction or the shaft surface used. Table 3 summarizes the results of the sealing investigations.
Sealing Technology July 2005
FEATURE Manufacturing method:
Plunge ground DIN 3761
Traverse turned, non-hardened
Traverse turned, hardened
Plunge turned
Tangential turned
Sealing investigation, 240 hours
No leakage
No leakage
No leakage
No leakage
No leakage
Sealing investigation, 1000 hours
No leakage
No leakage
No leakage
—
No leakage
Table 3. Results of the leakage tests.
Pumping rate measurement of the entire sealing system All the sealing systems showed a decreasing pumping rate with increasing operation time. Figure 6 shows a typical example of the pumping rate development for each shaft surface type. The pumping rates of the non-hardened traverse turned shaft surfaces are on the right-hand secondary axis. The pumping rate of the plunge ground shaft surfaces showed very little fluctuation. The initial pumping rate was in the range between 0.10 and 0.23 ml/h. The mean decrease in the system pumping rate was between 0.04 and 0.10 ml/h per 1000 hours. The direction of rotation of the sealing systems tested did not have a discernible influence on the pumping rate level at the beginning or on the decrease of the pumping ability during operation. The initial pumping rate of the sealing systems with hardened traverse turned shaft surfaces was comparable with those of the plunge ground shaft surfaces. Even the mean decrease in the system pumping rate, between 0.02 and 0.10 ml/h per 1000 hours, was comparable with the plunge ground shaft surfaces. It was not possible to associate the initial pumping rate or the development during operation with the direction of rotation. The sealing systems with non-hardened traverse turned shaft surfaces clearly showed a higher initial pumping rate, whereby the height of the initial level did not compare with the direction of the machining feed spiral. However, the decrease in the system pumping rate was approximately 10 times faster in the counter-clockwise rotation direction, turning spiral in the ‘sealing direction’, than in the clockwise rotation, turning spiral in the ‘leakage direction’. The decrease in the pumping rate was between 0.05 and 0.50 ml/h per 1000 hours.
Conclusions The investigation methods discussed here make it possible to establish the relevant sealing characteristics of shaft surfaces. The pumping rates determined for the shaft
Sealing Technology July 2005
surfaces permit a rapid pre-evaluation of the suitability of a shaft surface as a counter face for lip seals. The observation of the system pumping rate during operation without interference to the tribological system – for example, by disassembly and renewed installation of the seal – provide further evaluation possibilities. The influence of the different shaft surfaces on sealing security and reliability of the sealing system can be determined. The selection of suitable manufacturing processes and parameters is made easier. Several manufacturing methods were checked for their suitability for the counter face in shaft seals. It is shown that the machining feed spiral that is created on the shaft surface is not the reason for the pumping ability of the shaft surface. With appropriate manufacturing, both hardened and non-hardened traverse turned steel shaft surfaces can be sealed by means of suitable lip seals. The direction of rotation is not crucial. This is proved by the sealing tests that were successfully completed. The turning methods without axial feed, such as tangential turning and plunge turning, also produce suitable counter faces for radial lip seals.
References 1. H.K. Müller, W. Haas and G. Wüstenhagen: Gegenlaufflächen für Radial-dichtungen, Tribologie und Schmierungstechnik, May/June 1996. 2. H. Bodschwinna: Rauheitsmessung zur Bewertung der Funktionseigenschaften technischer Oberflächen. Prüfen und Bewerten von Oberflächenschutzschichten, VDI-Gesellschaft für Werkstofftechnik, Germany, 1988. 3. Busak+Shamban: Dichtungen und Führungen. Firmenschrift Busak+Shamban GmbH, Stuttgart, 1995. 4. Parker Hannifin: Dichtungshandbuch. Firmenschrift Parker Hannifin GmbH, Bietigheim-Bissingen, Germany, August 1996. 5. T. Kunstfeld and W. Haas: Erfassung und Beschreibung von Wellenoberflächenstrukturen aus der zeitgemäßen und zukünftigen Fertigung und deren Einfluss auf die Dichtqualität von Radial-Wellendichtungen. Abschlussbericht, FKM Forschungshefte No. 279, Frankfurt, Germany, 2004.
6. J. Schneider and L. Schreiber: Mit dem Tangentialdrehen zu drallfreien Oberflächen, Werkstatt und Betrieb, 6/2002. 7. T. Kunstfeld and W. Haas: Standzeitprognose durch Förderwertmessung während Langzeitversuchen. Tagungsband 5. Hamburger Dichtungs-technisches Kolloquium Dynamische Dichtungen, 3–4 June 2004. 8. Deutsches Institut für Normung: DIN 3760, Radial-Wellendichtringe. 9. Deutsches Institut für Normung: DIN 3761, Radial-Wellendichtringe für Kraftfahrzeuge, Anwendungshinweise. Contacts:
Dipl.-Ing. Thomas Kunstfeld, IMA Institut für Maschinenelemente, Universität Stuttgart, Pfaffenwaldring 9, D-70569 Stuttgart, Germany. Tel: +49 711 685 6170, Email: [email protected], Web: www.ima.uni-stuttgart.de
Professor Dr.-Ing. Werner Haas, IMA Institut für Maschinenelemente, Universität Stuttgart, Pfaffenwaldring 9, D-70569 Stuttgart, Germany, Tel: +49 711 685 6170, Email: [email protected]
This article is an English translation based on a paper presented in German at the 13th ISC – International Sealing Conference, Sealing Systems for Fluid Power Applications, which took place in Stuttgart, Germany in October 2004. The ISC was organized by the Fluid Power Association of the VDMA, the German Engineering Federation. For more information or copies of the 13th ISC proceedings (140), contact Ralf Stemmjack at the VDMA: Tel: +49 69 6603 1318, Fax: +49 69 6603 2318, Email: [email protected], Web: www.sealing-conference.com. The 14th ISC will be held in Stuttgart on 10–11 October 2006. Editor’s comment: I am very pleased to be able to publish an English version of this paper. From my rather inadequate understanding of the German version, it appeared to contain some interesting work. The authors’ conclusions concerning the effect of the different surface textures will be of considerable interest to a number of readers. Some may well have different opinions and experience, so further contributions on this subject will be very welcome. 9