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Measurement of wear in small bores
Wear, 49 (1978) 247 - 252 @ Elsevier Sequoia S.A., Lausanne
MEASUREMENT K. NARAYANASAMY Department (India) (Received
247 - Printed
in the Netherlands
OF WEAR IN SMALL BORES and V. RADHAKRISHNAN
of Mechanical Engineering,
Indian Institute of Technology,
March 14, 1977; in final form August
Madras 600 036
29, 1977)
Summary A detailed experimental investigation was carried out to assess the application of a replica method for the measurement of wear in small bores of diameter not more than 3 mm. The results of the analysis are summarized.
1. Introduction Small bearings and bushes are important in precision engineering and instrumentation. In space technology instruments of very high precision and accuracy are required to ensure the desired reliability. All the parameters of the components used in the assembly of such instruments have to be controlled precisely. Small bearings and bores which form a part of these instruments have to be accurately manufactured since the initial roughness and subsequent wear of these elements affect instrument accuracy and performance. It is thus of interest to study the surface characteristics and the mode of wear of such small bearings and bores. For this purpose it is essential to measure and determine the surface characteristics of the bore.
2. Replica materials and their application The most common technique for surface roughness measurement is to use a stylus instrument to investigate either the whole surface or a section of it. Since most standards of surface finish refer to a profile traced by a stylus, this method of roughness measurement is important. However, profile tracing with a stylus may not be practical for some small bearings and bores because of their small diameter (3 mm or less). In such cases it is difficult to obtain the profile or the roughness values directly without damaging the part under investigation. For such situations a surface replica technique can be used which reproduces the original surface with reasonable accuracy. Such replicas can easily be evaluated by means of conventional instrumentation. Many replica materials in the form of powders and films are available.
248
The powder type of replica material is mixed with a suitable fluid to form a paste which is allowed to set on the surface to produce a replica. Such replicas are not transparent or reflective and cannot be directly observed with an interference microscope. As the surface hardness of the replica is low, the stylus scratches the surface. Measurement of the replica with an interference microscope requires it to be coated with a highly reflective material. Nevertheless this method of replication is suitable for medium roughness reproduction and for flat surfaces or surfaces of large radius of curvature. For small bores and sharply curved surfaces replication by films is more satisfactory [ 11. Of the replica materials investigated, one specific type (Cellon) of thickness 0.07 mm was found to be most suitable because it gave a good reproduction of the surface and was hard enough to withstand the action of the stylus m~~urement. The replica films were cut to size and a small amount of solvent (acetone or methyl acetate) was smeared on the surface of the clean specimen. The film was laid over the specimen and gradual pressure was applied from one end so that trapped air could be removed. After a few minutes the replica was stripped off. It is preferable to make three or four prelimin~ replicas before taking the final replica. Since the film was thin and tended to curl it was kept flat between thin glass plates and allowed to cure for about 30 min in a hot air bath at 80 “C. This gave replicas with few wrinkles. Other replica materials of different thicknesses were also used. Typical profiles of such replicas are shown in Fig. 1.
(b)
(e)
Fig. 1. Profiles of the original surface and replicas taken with different (a) original surface, R, = 0.92 pm, R, = 9 pm;(b) Cellon, 0.07 mm;(c) (d) Cellon, 0.24 mm; (e) aeetylcellulose, 0.034 mm;(f) acetylcellulose,
film materials: Cellon, 0.1 mm; 0.08 mm.
249
Transparent replicas obtained using films could be evaluated by interferometry using a transmission or a reflection interference arrangement. For reflection interferometry the transparent replica was coated with some highly reflective material. From the interference photomicrographs it was possible to measure the peak-to-valley height R, of the surface irregularities
[a* It was found from Talysurf traces of the replicas that surface reproduction by the films was good [3]. This was confirmed by determination of the cross correlation relation between the original profile and the replica profile. Minor distortions caused by curling can be eliminated by filtering the electrical signal representing the profile. For this, profiles need to be digitized and fed into a digital computer.
3. Measurement
of wear in small bores
To assess the replica techniques for wear measurement the experimental set-up shown in Fig. 2 was used. The bearing tested had the following specifications: material, bronze; bore diameter, 3 mm; outer diameter, 10 mm; length, 4 mm. The direct-weight-loaded bearing was rotated at 500 rev min-l .
Fig. 2. Experimental
set-up for wear measurement.
The initial surface profile of the bearing was directly traced with a stylus instrument and R, and R, were noted (R, = 0.254 ,um, R, = 1.51 pm). The shaft ran in the bearing with a fine clearance of 5 pm. The external loading was 100 g. Replicas of the bore were made at regular intervals to obtain periodic measurements of bearing wear. For a quick inspection these replicas were examined by interference microscopy. The interference photomicrographs of the replicas taken at intervals of (a) 30 min, (b) 150 min, (c) 420 min and (d) 510 min are shown in Fig. 3. From the R, values of the replicas measured by interferometry it was observed that the initial rough-
(cl Fig. 3. Interference photographs of the replica of the bearing surface taken at (a) 30 min interval (R, = 0.142), (b) 150 min interval (R, = 0.222), (c) 420 min interval (R, = 0.189) and (d) 510 min interval (R, = 0.162).
1
OLpm 1Opm
(a)
(b) Fig. 4. (a) Initial surface profile of the bearing face taken by means of a replica film.
and (b) the profile
of the worn-out
sur-
ness of the bore first deteriorated and then gradually improved. This was mainly due to scoring of the shaft. The set-up was allowed to run for 900 min and a final replica was taken. This replica was traced with a stylus instrument. When measuring the
251
profile with the stylus instrument both before assembly and at the end of the experiment provision was made for digitizing the traced profiles. Hence punched tapes with an ordinate spacing of 2 pm could be made from the replica. Figure 4 shows the initial and final profiles of the surface.
4. Analysis of the digitized data To elucidate surface variations and modifications due to wear, the digitized surface profiles were analysed using a computer [ 4, 51. Since the initial roughness of the bearing was produced by a honing operation, the profile contained predominantly a spectrum of long and short waves. This could be observed clearly from the spectral density function shown in Fig. 5.
"1wo
100 Wave
10
1
0
0
50
length IA1 -
150
200
250
Log oJml-+
Fig. 5. Power spectrum of the surface profiles -, before wear; - - -, after wear. Fig. 6. Autocorrelation functions and after (- - -) wear.
100
of the surface
taken before
profiles
and after 900 min running:
of the bearing
taken before
(-)
The autocorrelation function plotted in Fig. 6 shows the high randomness exhibited by the profile. Plots of the spectrum and the autocorrelation function of the replica profile at the end of the experiment clearly show the displacement of the spectrum toward longer wavelengths and the smoothing or reduction of fine irregularities. Thus the surface of the bore was modified by wear to have long waves instead of short ones. These long waves were mostly due to the fine projections produced on the surface by the wear. With the digitized surface profile it is possible to determine other statistical parameters and functions which could clearly identify the nature and extent of wear.
252
5. Conclusions The replica method described is a quick and study of wear of small bores. It could be extended any type of surface, e.g. gears, small bearings and direct examination. As the replicas can be traced it is possible to obtain a digitized profile of the method.
convenient method for the to the study of wear of surfaces inaccessible for by a stylus without damage surface by the replica
Acknowledgments The authors thank Prof. Dr.-Ing. H. Shier, Director, Institut fur Feinwerk- und Regelungstechnik, Technical University, Braunschweig, and Dr.-Ing. W. Hillmann, Physikalisch Technische Bundesanstalt, Braunschweig, F.R.G., for permitting them to carry out this work in their laboratories.
References 1 H. Trumpold, Dber die Brauchbarkeit verschiedener Abdruckverfahren fir die Oberflachenmesstechnik”, Feingeratetechnik, 7 (1958) 165 - 171. 2 VDE/VDI - Richtlinien - Raucheitsuntersuchung mittels Interferenzmikroskopie, VDE/VDI 2604, VDI-Verlag, Diisseldorf, 1971. 3 K. Narayanasamy and V. Radhakrishnan, Relative performance of replica materials in surface reproduction, Proc. 7th AIMTDR Conf., Coimbatore, June 1976, P.S.G. College of Technology, Coimbatore, pp. 245 - 248. 4 V. Radhakrishnan, Statistical behaviour of surface profiles, Wear, 17 (1971) 259 - 267. 5 J. Peklenik, Investigation of the surface typology, CIRP, Ann. Int. Inst. Prod. Eng., 15 (1967) 381 - 384.
MEASUREMENT K. NARAYANASAMY Department (India) (Received
247 - Printed
in the Netherlands
OF WEAR IN SMALL BORES and V. RADHAKRISHNAN
of Mechanical Engineering,
Indian Institute of Technology,
March 14, 1977; in final form August
Madras 600 036
29, 1977)
Summary A detailed experimental investigation was carried out to assess the application of a replica method for the measurement of wear in small bores of diameter not more than 3 mm. The results of the analysis are summarized.
1. Introduction Small bearings and bushes are important in precision engineering and instrumentation. In space technology instruments of very high precision and accuracy are required to ensure the desired reliability. All the parameters of the components used in the assembly of such instruments have to be controlled precisely. Small bearings and bores which form a part of these instruments have to be accurately manufactured since the initial roughness and subsequent wear of these elements affect instrument accuracy and performance. It is thus of interest to study the surface characteristics and the mode of wear of such small bearings and bores. For this purpose it is essential to measure and determine the surface characteristics of the bore.
2. Replica materials and their application The most common technique for surface roughness measurement is to use a stylus instrument to investigate either the whole surface or a section of it. Since most standards of surface finish refer to a profile traced by a stylus, this method of roughness measurement is important. However, profile tracing with a stylus may not be practical for some small bearings and bores because of their small diameter (3 mm or less). In such cases it is difficult to obtain the profile or the roughness values directly without damaging the part under investigation. For such situations a surface replica technique can be used which reproduces the original surface with reasonable accuracy. Such replicas can easily be evaluated by means of conventional instrumentation. Many replica materials in the form of powders and films are available.
248
The powder type of replica material is mixed with a suitable fluid to form a paste which is allowed to set on the surface to produce a replica. Such replicas are not transparent or reflective and cannot be directly observed with an interference microscope. As the surface hardness of the replica is low, the stylus scratches the surface. Measurement of the replica with an interference microscope requires it to be coated with a highly reflective material. Nevertheless this method of replication is suitable for medium roughness reproduction and for flat surfaces or surfaces of large radius of curvature. For small bores and sharply curved surfaces replication by films is more satisfactory [ 11. Of the replica materials investigated, one specific type (Cellon) of thickness 0.07 mm was found to be most suitable because it gave a good reproduction of the surface and was hard enough to withstand the action of the stylus m~~urement. The replica films were cut to size and a small amount of solvent (acetone or methyl acetate) was smeared on the surface of the clean specimen. The film was laid over the specimen and gradual pressure was applied from one end so that trapped air could be removed. After a few minutes the replica was stripped off. It is preferable to make three or four prelimin~ replicas before taking the final replica. Since the film was thin and tended to curl it was kept flat between thin glass plates and allowed to cure for about 30 min in a hot air bath at 80 “C. This gave replicas with few wrinkles. Other replica materials of different thicknesses were also used. Typical profiles of such replicas are shown in Fig. 1.
(b)
(e)
Fig. 1. Profiles of the original surface and replicas taken with different (a) original surface, R, = 0.92 pm, R, = 9 pm;(b) Cellon, 0.07 mm;(c) (d) Cellon, 0.24 mm; (e) aeetylcellulose, 0.034 mm;(f) acetylcellulose,
film materials: Cellon, 0.1 mm; 0.08 mm.
249
Transparent replicas obtained using films could be evaluated by interferometry using a transmission or a reflection interference arrangement. For reflection interferometry the transparent replica was coated with some highly reflective material. From the interference photomicrographs it was possible to measure the peak-to-valley height R, of the surface irregularities
[a* It was found from Talysurf traces of the replicas that surface reproduction by the films was good [3]. This was confirmed by determination of the cross correlation relation between the original profile and the replica profile. Minor distortions caused by curling can be eliminated by filtering the electrical signal representing the profile. For this, profiles need to be digitized and fed into a digital computer.
3. Measurement
of wear in small bores
To assess the replica techniques for wear measurement the experimental set-up shown in Fig. 2 was used. The bearing tested had the following specifications: material, bronze; bore diameter, 3 mm; outer diameter, 10 mm; length, 4 mm. The direct-weight-loaded bearing was rotated at 500 rev min-l .
Fig. 2. Experimental
set-up for wear measurement.
The initial surface profile of the bearing was directly traced with a stylus instrument and R, and R, were noted (R, = 0.254 ,um, R, = 1.51 pm). The shaft ran in the bearing with a fine clearance of 5 pm. The external loading was 100 g. Replicas of the bore were made at regular intervals to obtain periodic measurements of bearing wear. For a quick inspection these replicas were examined by interference microscopy. The interference photomicrographs of the replicas taken at intervals of (a) 30 min, (b) 150 min, (c) 420 min and (d) 510 min are shown in Fig. 3. From the R, values of the replicas measured by interferometry it was observed that the initial rough-
(cl Fig. 3. Interference photographs of the replica of the bearing surface taken at (a) 30 min interval (R, = 0.142), (b) 150 min interval (R, = 0.222), (c) 420 min interval (R, = 0.189) and (d) 510 min interval (R, = 0.162).
1
OLpm 1Opm
(a)
(b) Fig. 4. (a) Initial surface profile of the bearing face taken by means of a replica film.
and (b) the profile
of the worn-out
sur-
ness of the bore first deteriorated and then gradually improved. This was mainly due to scoring of the shaft. The set-up was allowed to run for 900 min and a final replica was taken. This replica was traced with a stylus instrument. When measuring the
251
profile with the stylus instrument both before assembly and at the end of the experiment provision was made for digitizing the traced profiles. Hence punched tapes with an ordinate spacing of 2 pm could be made from the replica. Figure 4 shows the initial and final profiles of the surface.
4. Analysis of the digitized data To elucidate surface variations and modifications due to wear, the digitized surface profiles were analysed using a computer [ 4, 51. Since the initial roughness of the bearing was produced by a honing operation, the profile contained predominantly a spectrum of long and short waves. This could be observed clearly from the spectral density function shown in Fig. 5.
"1wo
100 Wave
10
1
0
0
50
length IA1 -
150
200
250
Log oJml-+
Fig. 5. Power spectrum of the surface profiles -, before wear; - - -, after wear. Fig. 6. Autocorrelation functions and after (- - -) wear.
100
of the surface
taken before
profiles
and after 900 min running:
of the bearing
taken before
(-)
The autocorrelation function plotted in Fig. 6 shows the high randomness exhibited by the profile. Plots of the spectrum and the autocorrelation function of the replica profile at the end of the experiment clearly show the displacement of the spectrum toward longer wavelengths and the smoothing or reduction of fine irregularities. Thus the surface of the bore was modified by wear to have long waves instead of short ones. These long waves were mostly due to the fine projections produced on the surface by the wear. With the digitized surface profile it is possible to determine other statistical parameters and functions which could clearly identify the nature and extent of wear.
252
5. Conclusions The replica method described is a quick and study of wear of small bores. It could be extended any type of surface, e.g. gears, small bearings and direct examination. As the replicas can be traced it is possible to obtain a digitized profile of the method.
convenient method for the to the study of wear of surfaces inaccessible for by a stylus without damage surface by the replica
Acknowledgments The authors thank Prof. Dr.-Ing. H. Shier, Director, Institut fur Feinwerk- und Regelungstechnik, Technical University, Braunschweig, and Dr.-Ing. W. Hillmann, Physikalisch Technische Bundesanstalt, Braunschweig, F.R.G., for permitting them to carry out this work in their laboratories.
References 1 H. Trumpold, Dber die Brauchbarkeit verschiedener Abdruckverfahren fir die Oberflachenmesstechnik”, Feingeratetechnik, 7 (1958) 165 - 171. 2 VDE/VDI - Richtlinien - Raucheitsuntersuchung mittels Interferenzmikroskopie, VDE/VDI 2604, VDI-Verlag, Diisseldorf, 1971. 3 K. Narayanasamy and V. Radhakrishnan, Relative performance of replica materials in surface reproduction, Proc. 7th AIMTDR Conf., Coimbatore, June 1976, P.S.G. College of Technology, Coimbatore, pp. 245 - 248. 4 V. Radhakrishnan, Statistical behaviour of surface profiles, Wear, 17 (1971) 259 - 267. 5 J. Peklenik, Investigation of the surface typology, CIRP, Ann. Int. Inst. Prod. Eng., 15 (1967) 381 - 384.