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An experimental investigation of tribological effects on coastdown phenomena in horizontal rotating machinery
Wear, 91 (1983)
25 - 31
25
AN EXPERIMENTAL INVESTIGATION OF TRIBOLOGICAL EFFECTS ON COASTDOWN PHENOMENA IN HORIZONTAL ROTATING MACHINERY
G. SANTHANAKRISHNAN Fibre Reinforced 600036 (India) B. S. PRABHU Department (India) (Received
P&sties
Research
Cent%
Indian
Indian
institute
Institute
of
Technology,
Madras
and B. V. A. RAO
of AppEied November
Mechanics,
of Technology,
Madras
600036
11,1982)
Summary An experimental investigation of the coastdown characteristics of a two-rotor system consisting of a variable-speed d.c. motor driving a flexible rotor on two journal bearings is presented. The characteristics are plotted in terms of decelaration uemus speed for various operating conditions (speed, lubricant type, m~~ment and commutator carbon brushes). The coastdown curve resembles the classical Stribeck friction diagram and hence can be used to evaluate the transition speed of journal bearings of rotating systems. The results confirm that this simple technique provides a measure of the friction present and indicates the mechanical condition of the rotating assemblies.
1. Introduction The continuing search for greater efficiency and producti~ty over the past few decades has led to a marked increase in the use of power-driven high speed rotating machinery in most engineering applications. The proper maintenance of this machinery can lead to efficient and economic operation of the overall system of which they form a part. Bearings, which allow relative motion between loadcarrying surfaces, have a major role in modern engineering, and their failure in service can lead to financial and legal liabilities in addition to loss of efficiency in the system. A reduction in breakdowns in the system as a result of optimum bearing operation is a major consideration in selecting and designing bearings for any plant. The main aim of a good maintenance programme is to achieve optimum bearing life and utility. Hence a thorough study of bearing performance to enable the condition of the bearings to be predicted and the development of failure prevention methods are of prime importance in any maintenance programme. 0043-1648183/$3.00
0 Elsevier Sequoia/Printed
in Tbe Netherlands
26
Post-failure investigations enable the possible causes of bearing failure and satisfactory remedial measures to be identified. This leads to improved bearing design with a consequent increase in reliability and a reduction in the maintenance requirements. Preventive maintenance, in addition to predictive maintenance or condition monitoring techniques, has recently been developed and has revolutionized maintenance practices. The successful and reliable application of the foregoing maintenance techniques requires that some deterioration of the system has already taken place. If a technique that can predict the onset of failure at its initiation could be developed, it would become a vkry powerful maintenance tool. Coastdown time monitoring, which makes use of the coastdown phenomenon, is one such technique that is likely to be developed in the near future.
2. The coastdown phenomenon and coastdown time When the power supply to any rotating system is cut off, the system begins to lose the momentum gained during operation and finally comes to a halt. The behaviour of the rotating system after the power cut-off is called the coastdown phenomenon (CDP) or the rundown phenomenon, and the exact time that elapses between the power cut-off and the system’s coming to a halt is called the coastdown time (CDT) [ 1 - 31 or rundown time [ 4 - 61, i.e. the CDT is the total time taken by the system to dissipate the momentum acquired during sustained operation. The CDP is inherent in any rotating system, and the CDT depends on (i) the inertia of the system components, (ii) the tribological behaviour of machine elements such as bearings, seals and commutator carbon brushes, (iii) the operating conditions and (iv) environmental effects such as fluid drag. During the coastdown period the entire system is completely free from the power source which is an external disturbing parameter. Fluctuations in the power source (voltage, frequency etc.) can have an appreciable effect on the results of experimental investigations. Therefore results obtained during the coastdown period are more reliable and meaningful than those obtained during sustained operation.
3. Literature survey Although the CDP is inherent in every rotating system, does not depend on the power source and is a reliable guide to the condition of the system, it has received little attention. Previous investigators have concentrated on start-up, running-in and continuous operation of rotating systems. Deprez [7], Dettmar [8,9] and Hodgkinson [lo] were the first to use CDP for friction measurements on journal bearings. Vogelpohl [ll, 121 reported the effect of motor commutator carbon brushes on the CDT of a motor-driven horizontal rotor system supported on journal bearings.
27
Mokhtar et al. [13,14] measured the CDT for a horizontal rotating system supported on journal bearings. Xistris and Watson [4 - 61 made extensive use of the CDP in their investigation of vertical rotors supported by rolling element bearings and established CDT monitoring as a health monitoring, quality control and maintenance tool. Subsequently Craig and coworkers [ 1 - 31 investigated similar brushless vertical motors supported by rolling element bearings and proposed a theoretical expression for calculating the CDT from which they were able to predict the condition of the bearings.
4. Experimental equipment The apparatus shown in Fig. 1 was used to investigate the effects on the CDP and the CDT of the speed of the prime mover, the lubricant type, misalignment in the system and external stray friction such as that due to the commutator carbon brushes. The apparatus consists of a prime mover which is a variable-speed d.c. motor (1800 - 5000 rev min-‘) coupled to the test rotor via a flexible coupling. The rotor, which has a mass of 27.8 kg, consists of a shaft 30 mm in diameter and 920 mm long with a disc 258 mm in diameter and 53.6 mm thick welded at the centre. It has a critical speed of about 1850 rev min-’ and is supported by two phosphor bronze partial journal bearings (120” arc) which are mounted in mild steel housings in two halves supported on the
Fig. 1. Schematic diagram of apparatus: 1, rotor with flexible shaft; 2, bearing housing; 3, main frame incorporating oil sump; 4, oil entry nipples; 5, flexible coupling; 6, d.c. motor; 7, adjustable baseplate; 8, vertical adjustment screws; 9, horizontal adjustment screws; 10, support structure; 11, Dodd bar mounting plates; 12, Dodd bars; 13, Ybrackets with probes; 14, targets or indicator blocks; 15, photoelectric pick-up; 16, noncontact sensor for digital tachometer.
28
main frame which also serves as an oil sump. The bearings are lubricated using either a gravity oil feed or a grease pump. The CDT plots are obtained using an x-y recorder in which time is recorded along the x axis and speed along the y axis. A photoelectric pick-up connected to an electronic tachometer is used to record the speed.
5. Experimen$al technique The following experiments were performed using the apparatus described above. (1) CDT plots were recorded with oil lub~cation, with multip~ose grease lubrication and with Servogem grease lubrication. Data were obtained for four cut-off speeds in all cases. Typical CDT plots for oil- and greaselubricated rotors are shown in Figs. 2 and 3 respectively. (2) The above experiments were repeated for various misalignments, each at four cut-off speeds.
6. Interpretation of the coastdown time plots (a) For all three lubricants the rate of decrease in speed with time (deceleration) is initially higb and then becomes lower (Figs. 2 and 3). An 4000
60
\
\
3
300
Fig. 2. CDT plots for oil lubrication at four cut-off speeds: curve 1,151O rev min-‘; 2, 2010 rev min-‘; curve 3,305O rev min-*; curve 4,400O rev min-$.
360
curve
29 4000
3000
E mG ’ 0
2000
: ::
5
1000
0
\ k
0
50 Tim*
15t 1
0
- SCES
Fig. 3. CDT plots for lubrication with Servogem grease at four cut-off speeds: curve 1, 1530 rev min-‘; curve 2, 2050 rev min-*; curve 3, 3000 rev min-‘; curve 4, 4000 rev min-‘.
abrupt increase in the deceleration occurs at a critical speed, after which it decreases and then increases again just before stopping. This behaviour can be seen more clearly in the curves of deceleration uersus time shown in Fig. 4. (b) The rate at which the deceleration decreases with increasing speed before and after the critical speed increases with increasing lubricant viscosity. (c) There is a marked difference in the curves of deceleration uersus speed near the stopping region. For oil lubrication the rate at which the deceleration increases with decreasing speed is small compared with that observed for grease lubrication. This behaviour can be explained by the fact that for oil lubrication the film between the rotor and the bearing starts to squeeze some time before stopping while for grease lubrication squeezing occurs just before stopping. This is because the less viscous oil cannot sustain the load of the rotor once the speed falls below a specific value (the transition speed), whereas the more viscous greases can sustain the rotor load even at low speeds. (d) CDT plots for rotors lubricated with oil and both types of grease show that the CDT decreases with increasing viscosity (Figs. 2 and 3). The effect on the CDT of misalignment [ 151 of the rotor with respect to the motor shaft was studied for one type of oil and one type of grease. In both cases the CDT decreased with increasing misalignment (Table 1). This is
30
6or-
0
loo0
Sped
2000 - RPM
Fig. 4. Effect of the lubricant used on the CDP (cut-off oil; - - -, multipurpose grease; - - -, Servogem grease.
TABLE 1 Effect of misalignment on the coastdown motor-rotor system [ 15 ] numbera
time of the
Lubricant
Alignment
CDT
Oil
MO1 MO2 MO3 MO4 MO5 MO6
269 255 204 174 154 119
Servogem grease
MGl MG2 MG3
154.5 149.0 135.75
Cut-off speed, 3000 rev min-l. aMisalignment with oil, MO1 < MO2 < MO3 < MO4 < MO5 < M06; misalignment with grease, MGl < MG2 < MG3.
speed, 4000 rev min-I):
-,
31
because both the bearing friction and the power loss increase with increasing misalignment. Additional small changes in the critical speeds are also observed under the above conditions.
7. Concluding remarks (1) A study of the CDT plot of a rotating machine can indicate the critical speed of the rotor because the curve of deceleration uersus speed shows a sudden increase in deceleration near this value. (2) ~is~ignment between the rotor and the shaft of the prime mover affects the CDT. It decreases with increasing mis~i~ment and hence can give a measure of the misalignment. (3) The transition speed at which the lubricant film breaks down in a sliding bearing can be determined from a study of the CDT.
References 1 R. J. Craig, Applications of coaatdown monitoring techniques for vertical shaft motors, DTNSRDC Rep. 76-13, September 1976 (David W. Tayfor Naval Ship Research and Development Center, Annapolis, MD). 2 T. L. Daugherty and R. J. Craig, Coastdown time as a mechanical condition indicator, DTNSRDC Rep. 4547, January 1976 (David W. Taylor Naval Ship Research and Development Center, Annapolis, MD). 3 T. L. Daugherty and R. J. Craig, ASLE Trans., 22 (4) (1979) 349 - 357. 4 G. D. Xistris and D. C. Watson, Proc. Mechanical Failures Prevention Group, Gaithersburg, MD, November 1974, National Bureau of Standards, Washington, DC. 5 G. D. Xistris and D. C. Watson, ASME Prepr. 75-DE-6, 1975 (American Society of Mechanical Engineers, New York). 6 G. D. Xistris and D, C. Watson, Mech. Eng., 98 (May 1976) 20 - 23. 7 M. Deprez, C. R. Acad. Sci., 99 (1884) 861 - 864. 8 G. Dettmar, Elektrotech. Z., 22 (1899) 397 - 400. 9 G. Dettmar, Dinglers PO&tech. J., 315 (6) (1900) 88 - 95. 10 F. Hodgkinson, Proc. Inst. Mech. Eng., London, (2) (1929) 843 - 904. 11 G. Vogelpohi, ~o~truktion, 16 (12) (1964) 491 - 496. 12 G. Vogelpohl, Retrieb#ichere Gleitiager, Springer, Berlin, 1966, pp. 133 - 135. 13 M. 0. A. Mokhtar, R. B. Howarth and P. B. Davies, ASLE Trans., 20 (3) (1977) 183 190. 14 M. 0. A. Mokhtar, R. B. Howsrth and P. B. Davies, ASLE Trans., 20 (3) (1977) 191 194. 15 G. Santhanakrishnan, B. S. Prabhu and B. V. A. Rao, Proc. 5th ISME Conf. on Mechanical Engineering, Allahabad, December 23 - 24, 1982, p. C2.
25 - 31
25
AN EXPERIMENTAL INVESTIGATION OF TRIBOLOGICAL EFFECTS ON COASTDOWN PHENOMENA IN HORIZONTAL ROTATING MACHINERY
G. SANTHANAKRISHNAN Fibre Reinforced 600036 (India) B. S. PRABHU Department (India) (Received
P&sties
Research
Cent%
Indian
Indian
institute
Institute
of
Technology,
Madras
and B. V. A. RAO
of AppEied November
Mechanics,
of Technology,
Madras
600036
11,1982)
Summary An experimental investigation of the coastdown characteristics of a two-rotor system consisting of a variable-speed d.c. motor driving a flexible rotor on two journal bearings is presented. The characteristics are plotted in terms of decelaration uemus speed for various operating conditions (speed, lubricant type, m~~ment and commutator carbon brushes). The coastdown curve resembles the classical Stribeck friction diagram and hence can be used to evaluate the transition speed of journal bearings of rotating systems. The results confirm that this simple technique provides a measure of the friction present and indicates the mechanical condition of the rotating assemblies.
1. Introduction The continuing search for greater efficiency and producti~ty over the past few decades has led to a marked increase in the use of power-driven high speed rotating machinery in most engineering applications. The proper maintenance of this machinery can lead to efficient and economic operation of the overall system of which they form a part. Bearings, which allow relative motion between loadcarrying surfaces, have a major role in modern engineering, and their failure in service can lead to financial and legal liabilities in addition to loss of efficiency in the system. A reduction in breakdowns in the system as a result of optimum bearing operation is a major consideration in selecting and designing bearings for any plant. The main aim of a good maintenance programme is to achieve optimum bearing life and utility. Hence a thorough study of bearing performance to enable the condition of the bearings to be predicted and the development of failure prevention methods are of prime importance in any maintenance programme. 0043-1648183/$3.00
0 Elsevier Sequoia/Printed
in Tbe Netherlands
26
Post-failure investigations enable the possible causes of bearing failure and satisfactory remedial measures to be identified. This leads to improved bearing design with a consequent increase in reliability and a reduction in the maintenance requirements. Preventive maintenance, in addition to predictive maintenance or condition monitoring techniques, has recently been developed and has revolutionized maintenance practices. The successful and reliable application of the foregoing maintenance techniques requires that some deterioration of the system has already taken place. If a technique that can predict the onset of failure at its initiation could be developed, it would become a vkry powerful maintenance tool. Coastdown time monitoring, which makes use of the coastdown phenomenon, is one such technique that is likely to be developed in the near future.
2. The coastdown phenomenon and coastdown time When the power supply to any rotating system is cut off, the system begins to lose the momentum gained during operation and finally comes to a halt. The behaviour of the rotating system after the power cut-off is called the coastdown phenomenon (CDP) or the rundown phenomenon, and the exact time that elapses between the power cut-off and the system’s coming to a halt is called the coastdown time (CDT) [ 1 - 31 or rundown time [ 4 - 61, i.e. the CDT is the total time taken by the system to dissipate the momentum acquired during sustained operation. The CDP is inherent in any rotating system, and the CDT depends on (i) the inertia of the system components, (ii) the tribological behaviour of machine elements such as bearings, seals and commutator carbon brushes, (iii) the operating conditions and (iv) environmental effects such as fluid drag. During the coastdown period the entire system is completely free from the power source which is an external disturbing parameter. Fluctuations in the power source (voltage, frequency etc.) can have an appreciable effect on the results of experimental investigations. Therefore results obtained during the coastdown period are more reliable and meaningful than those obtained during sustained operation.
3. Literature survey Although the CDP is inherent in every rotating system, does not depend on the power source and is a reliable guide to the condition of the system, it has received little attention. Previous investigators have concentrated on start-up, running-in and continuous operation of rotating systems. Deprez [7], Dettmar [8,9] and Hodgkinson [lo] were the first to use CDP for friction measurements on journal bearings. Vogelpohl [ll, 121 reported the effect of motor commutator carbon brushes on the CDT of a motor-driven horizontal rotor system supported on journal bearings.
27
Mokhtar et al. [13,14] measured the CDT for a horizontal rotating system supported on journal bearings. Xistris and Watson [4 - 61 made extensive use of the CDP in their investigation of vertical rotors supported by rolling element bearings and established CDT monitoring as a health monitoring, quality control and maintenance tool. Subsequently Craig and coworkers [ 1 - 31 investigated similar brushless vertical motors supported by rolling element bearings and proposed a theoretical expression for calculating the CDT from which they were able to predict the condition of the bearings.
4. Experimental equipment The apparatus shown in Fig. 1 was used to investigate the effects on the CDP and the CDT of the speed of the prime mover, the lubricant type, misalignment in the system and external stray friction such as that due to the commutator carbon brushes. The apparatus consists of a prime mover which is a variable-speed d.c. motor (1800 - 5000 rev min-‘) coupled to the test rotor via a flexible coupling. The rotor, which has a mass of 27.8 kg, consists of a shaft 30 mm in diameter and 920 mm long with a disc 258 mm in diameter and 53.6 mm thick welded at the centre. It has a critical speed of about 1850 rev min-’ and is supported by two phosphor bronze partial journal bearings (120” arc) which are mounted in mild steel housings in two halves supported on the
Fig. 1. Schematic diagram of apparatus: 1, rotor with flexible shaft; 2, bearing housing; 3, main frame incorporating oil sump; 4, oil entry nipples; 5, flexible coupling; 6, d.c. motor; 7, adjustable baseplate; 8, vertical adjustment screws; 9, horizontal adjustment screws; 10, support structure; 11, Dodd bar mounting plates; 12, Dodd bars; 13, Ybrackets with probes; 14, targets or indicator blocks; 15, photoelectric pick-up; 16, noncontact sensor for digital tachometer.
28
main frame which also serves as an oil sump. The bearings are lubricated using either a gravity oil feed or a grease pump. The CDT plots are obtained using an x-y recorder in which time is recorded along the x axis and speed along the y axis. A photoelectric pick-up connected to an electronic tachometer is used to record the speed.
5. Experimen$al technique The following experiments were performed using the apparatus described above. (1) CDT plots were recorded with oil lub~cation, with multip~ose grease lubrication and with Servogem grease lubrication. Data were obtained for four cut-off speeds in all cases. Typical CDT plots for oil- and greaselubricated rotors are shown in Figs. 2 and 3 respectively. (2) The above experiments were repeated for various misalignments, each at four cut-off speeds.
6. Interpretation of the coastdown time plots (a) For all three lubricants the rate of decrease in speed with time (deceleration) is initially higb and then becomes lower (Figs. 2 and 3). An 4000
60
\
\
3
300
Fig. 2. CDT plots for oil lubrication at four cut-off speeds: curve 1,151O rev min-‘; 2, 2010 rev min-‘; curve 3,305O rev min-*; curve 4,400O rev min-$.
360
curve
29 4000
3000
E mG ’ 0
2000
: ::
5
1000
0
\ k
0
50 Tim*
15t 1
0
- SCES
Fig. 3. CDT plots for lubrication with Servogem grease at four cut-off speeds: curve 1, 1530 rev min-‘; curve 2, 2050 rev min-*; curve 3, 3000 rev min-‘; curve 4, 4000 rev min-‘.
abrupt increase in the deceleration occurs at a critical speed, after which it decreases and then increases again just before stopping. This behaviour can be seen more clearly in the curves of deceleration uersus time shown in Fig. 4. (b) The rate at which the deceleration decreases with increasing speed before and after the critical speed increases with increasing lubricant viscosity. (c) There is a marked difference in the curves of deceleration uersus speed near the stopping region. For oil lubrication the rate at which the deceleration increases with decreasing speed is small compared with that observed for grease lubrication. This behaviour can be explained by the fact that for oil lubrication the film between the rotor and the bearing starts to squeeze some time before stopping while for grease lubrication squeezing occurs just before stopping. This is because the less viscous oil cannot sustain the load of the rotor once the speed falls below a specific value (the transition speed), whereas the more viscous greases can sustain the rotor load even at low speeds. (d) CDT plots for rotors lubricated with oil and both types of grease show that the CDT decreases with increasing viscosity (Figs. 2 and 3). The effect on the CDT of misalignment [ 151 of the rotor with respect to the motor shaft was studied for one type of oil and one type of grease. In both cases the CDT decreased with increasing misalignment (Table 1). This is
30
6or-
0
loo0
Sped
2000 - RPM
Fig. 4. Effect of the lubricant used on the CDP (cut-off oil; - - -, multipurpose grease; - - -, Servogem grease.
TABLE 1 Effect of misalignment on the coastdown motor-rotor system [ 15 ] numbera
time of the
Lubricant
Alignment
CDT
Oil
MO1 MO2 MO3 MO4 MO5 MO6
269 255 204 174 154 119
Servogem grease
MGl MG2 MG3
154.5 149.0 135.75
Cut-off speed, 3000 rev min-l. aMisalignment with oil, MO1 < MO2 < MO3 < MO4 < MO5 < M06; misalignment with grease, MGl < MG2 < MG3.
speed, 4000 rev min-I):
-,
31
because both the bearing friction and the power loss increase with increasing misalignment. Additional small changes in the critical speeds are also observed under the above conditions.
7. Concluding remarks (1) A study of the CDT plot of a rotating machine can indicate the critical speed of the rotor because the curve of deceleration uersus speed shows a sudden increase in deceleration near this value. (2) ~is~ignment between the rotor and the shaft of the prime mover affects the CDT. It decreases with increasing mis~i~ment and hence can give a measure of the misalignment. (3) The transition speed at which the lubricant film breaks down in a sliding bearing can be determined from a study of the CDT.
References 1 R. J. Craig, Applications of coaatdown monitoring techniques for vertical shaft motors, DTNSRDC Rep. 76-13, September 1976 (David W. Tayfor Naval Ship Research and Development Center, Annapolis, MD). 2 T. L. Daugherty and R. J. Craig, Coastdown time as a mechanical condition indicator, DTNSRDC Rep. 4547, January 1976 (David W. Taylor Naval Ship Research and Development Center, Annapolis, MD). 3 T. L. Daugherty and R. J. Craig, ASLE Trans., 22 (4) (1979) 349 - 357. 4 G. D. Xistris and D. C. Watson, Proc. Mechanical Failures Prevention Group, Gaithersburg, MD, November 1974, National Bureau of Standards, Washington, DC. 5 G. D. Xistris and D. C. Watson, ASME Prepr. 75-DE-6, 1975 (American Society of Mechanical Engineers, New York). 6 G. D. Xistris and D, C. Watson, Mech. Eng., 98 (May 1976) 20 - 23. 7 M. Deprez, C. R. Acad. Sci., 99 (1884) 861 - 864. 8 G. Dettmar, Elektrotech. Z., 22 (1899) 397 - 400. 9 G. Dettmar, Dinglers PO&tech. J., 315 (6) (1900) 88 - 95. 10 F. Hodgkinson, Proc. Inst. Mech. Eng., London, (2) (1929) 843 - 904. 11 G. Vogelpohi, ~o~truktion, 16 (12) (1964) 491 - 496. 12 G. Vogelpohl, Retrieb#ichere Gleitiager, Springer, Berlin, 1966, pp. 133 - 135. 13 M. 0. A. Mokhtar, R. B. Howarth and P. B. Davies, ASLE Trans., 20 (3) (1977) 183 190. 14 M. 0. A. Mokhtar, R. B. Howsrth and P. B. Davies, ASLE Trans., 20 (3) (1977) 191 194. 15 G. Santhanakrishnan, B. S. Prabhu and B. V. A. Rao, Proc. 5th ISME Conf. on Mechanical Engineering, Allahabad, December 23 - 24, 1982, p. C2.