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Use of a small scale model to predict steam hammer noise
Journal
qf Sound and Vibration (1978) 58(4), 587-59 I
LETTERS USE OF A SMALL
SCALE
TO THE EDITOR
MODEL
TO PREDICT
STEAM
HAMMER
NOISE?
1. INTRODUCTION
The usefulness of physically scaled models is accepted in auditorium acoustics and is becoming recognized in studies of sound propagation outdoors. Their value in reproducing the noise of complex machinery within enclosed industrial environments on the other hand remains undetermined. A preliminary assessment is presented here of results obtained from a model of one of the noisiest industrial processes-the forging of metal billets [l-3]. Forming normally occurs through impacts from an accelerated or freely falling body called a tup or ram. Although the billet is usually preheated, cold, flat faced billets have been employed commonly in full scale experiments primarily to emphasize the impact noise produced by the striking bodies [2, 31. Consequently the latter situation where easily subdued, “extraneous” noises from gas exhausts and the like have been silenced, were considered exclusively. 2.
THE MODEL AND MEASURING
EQUIPMENT
An accelerated tup is probably more difficult to simulate than a freely falling one. On the other hand it generates impact forces and, hence, dynamic responses with a potentially wider range of amplitudes for a fixed maximum drop height. This last factor becomes especially important for smaller sized models where the height is restricted proportionally. On balance the additional flexibility offered by an accelerated tup seemed worthwhile. Therefore a model was constructed of one of the more popular machines in this category, a 10 cwt Rigby pattern, Ross type steam hammer. A typical foundation was represented as far as practicable by seating the model on a wooden baseplate isolated by rubber from a solid supporting structure. The complete model, isolated from peripheral reflecting surfaces, is displayed from the side in Figure 1. An impression of the model’s dimensions may be gained by comparison with the coin. The universally employed 1/12th size reduction was about the largest consistent with maintaining uniform response of the transducers to the correspondingly higher frequency motions expected [4]. A schematic outline of the model’s ancillary control unit is illustrated in the upper part of Figure 2. The tup is accelerated by the sudden release of air through a piston valve inside the cylinder. Various acceleration levels are possible by regulating the pre-released air at different pressures. Activation of the process is controlled for greater consistency by a solenoid rather than a hand lever as employed normally in practice. After the initial acceleration, the tup drops essentially freely before impacting with a square sectioned, 6.3 + 0.5 mm billet having a length of 22.0 ? 0.5 mm. The billet is stationed in a 1 mm deep locating slot to ensure reasonably constant positioning. Constraints are avoided on the billet by providing a nominal clearance of 0.1 mm from the sides of the slot. A major factor in the choice and location of measuring equipment was the need to compare model with full scale results. Unfortunately, the detailed behaviour of a full scale Rigby steam hammer is unknown and could not be determined due to scheduling difficulties. However information has been published relating the near field Lpear to the maximum of the peak vertical acceleration or deceleration of the tup and anvil for different drop hammers 7 An early version of this letter giving more tentative results was presented at the 18th International Machine Tool Design and Research Conference, London, September 1977. 587 0022-460X/78/06224587 $01.00/O ? 1978 Academic Press Inc. (London) Limited
588
LETTERS TO THE EDITOR
I
Valve chest Steam
Inlet
Cylinder
Mouthpiece Column
!Tup
Pallets
Hand lever Anvi I
Baseplate
Figure 1. The model.
[l.-31. These variables are relatively insensitive to structural “ringing” after the impact P -41 and, presumably, to the specific configuration of a hammer. Therefore comparisons With this data should be reasonably valid. Silencer, Maxam Type x)OP99325
Silencer, Maxam Type 7COlP99325 Exhaust
II Solenoid wrated valve, Maxam Type
Model steam hammer
-
Lubricator, Maxom Type A606
Pressure regulator, Moxam Type Al 19
Accelerometer, BBKf Type 4344 Microphone, BBK’ Type 4133 Sound level meter, B 81 K+ Type 2209
Tmnslent recorder, Catalab Type DL90
Charge ampl ifler, _ Klstler Type 5001
Tmnwnt wonder, Datalob Type DL905
Oscilloscope, Tektronix
Oscilloscope, Brad I ey Type I56
tBrijel and Kjoer
Figure 2. Equipment.
Filter, Maxam Type A602
Air supply 100 psi
LETTERS
TO THE EDITOR
589
Accurate measurement of maximum peaks accentuates the usual requirement in model work for the resolution of high frequencies. Upper limits used, nominally 40 and 125 kHz, respectively, for the sound and accelerations, were the highest attainable with readily available commercial transducers. Details of the transducers are given in the lower part of Figure 2. Transducer signals were amplified, sampled digitally at a frequency not less than 200 kHz, stored and displayed on an oscilloscope for analysis. The beginning of each signal was captured by using the pre-triggering facility of the transient recorders. Calibration of the complete measurement system was performed normally by using oscillatory signals of known amplitude. An accelerometer was stud-mounted on the anvil’s pallet as shown in Figure 1. Lack of space prevented direct observation of the tup’s acceleration. Fortunately the accelerating force on the tup of a steam hammer is proportional to the pressure of the pre-released air which was determined from a gauge. Finally L,,,, was measured to the side of the hammer and in the plane of impact at a distance of 31.25 mm from the closest edge of the billet. This distance, when scaled, is comparable to that used in practice at close work stations. 3.
EXPERIMENTAL
PROCEDURE
Initial experiments indicated the possibility of highly fluctuating maximum peak values for model tests with supposedly constant conditions. Trial and error investigations suggested that the alignment between the tup and billet was critical. Spacers between the cylinder and tup’s collar and the realignment, after each test if necessary, of the friction restrained pallets alleviated the problem. With the modification the peak levels in the time histories still changed quite appreciably despite a close similarity in the form of the histories. Therefore the value of the maximum peak is somewhat meaningless if obtained from a single record. Conversely the mean of such values observed in ten nominally identical tests was found to be fairly stationary and, hence, repeatable. (Specifically, for reasonably assumed random variations [5], this mean was not significantly different at the 95% confidence level from the mean of ten previous tests [6].) Consequently ten tests were performed consecutively at any one condition to enable the mean and standard deviation of the maximum peak values to be calculated. The pressure in the cylinder was changed in increments of 10 psi from 20 to 60 psi and the exercise repeated. The possibility that a large number of repeated loads might produce an extraneous effect upon a billet then arose. However, no anomalous behaviour was detected from sound or acceleration measurements in a separate investigation involving substantially more loads cycled between their extreme levels. Therefore the number and pattern of loads appeared unimportant providing pallets were aligned correctly. 4.
RESULTS
A comparison of model and full scale results is presented in Figures 3 and 4. Two materials, steel and brass, were used for the model billets to expand the number of data points limited by the restricted range of cylinder pressures. The densities of these materials are almost alike so that their peak dynamic behaviours should be virtually identical [7]. Information regarding the scatter of constant condition, full scale experiments is scant although, by inference from reference [l], Lpeakmight vary by as much as 8 dB. Figure 3 indicates that on average Lpeak is almost directly proportional to the air pressure in the model’s cylinder. This behaviour is consistent with full scale situations where the billet remains elastic [l-3]. Corresponding trends between Lpear and the anvil’s peak acceleration shown in Figure 4 also agree reasonably well although there is a magnitude difference in levels.
590
LETTERS TO THE EDITOR
Air
pressure
(psi)
Figure 3. Near field peak sound pressure levels for different model hammer pressures and billets. -, Steel billet (E.N. 24); ----, brass billet (vertical extensions show + one standard deviation).
Model I
/
103 Ful I xde
102 Peak acceleration
/
104
measured on anvil (g)
Figure 4. Comparison of results for the model steam hammer and a full scale CECO 60FD drop hammer [2]. -, Steel model billet (E.N. 24); ----, brass model billet (vertical and horizontal extensions show + one standard deviation); ?? , full scale drop hammer.
5.
DISCUSSION
The preliminary comparisons presented suggest that trends observed for isolated full scale drop hammers might be duplicated by geometrically scaled and carefully operated models. Further research is required to determine if the correspondence for AC,,,, extends, for example, to L,,,? which might be more appropriate for the semi-reverberant conditions typically encountered in industry. The possibly important effects of ringing noise, which depend upon a particular hammer’s configuration [24], must be useful in this context.
t L,, is the standard notation for the energy mean level.
LETTERSTO THE EDITOR
591
ACKNOWLEDGMENT
The third author acknowledges gratefully the travel fellowship awarded by the National Research Council of Canada which enabled him to assist in this work whilst on sabbatical leave at Sheffield Polytechnic. Department of Mechanical and Production Engineering, Sheffield City Polytechnic, Sheffield Sl 1 WB, England
G. J. MCNULTY
Department of Mechanical Engineering, University of Manitoba, Winnipeg, Canada R3 T 2N2
N. POPPLEWELL
J. HOLDING
(Received 16 January 1978)
REFERENCES 1. J. R. BAILEY,J. A. DAGGERHART and N. D. STEWART1975 Design Engineering Technical Conference of the American Society of Mechanical Engineers, Washington D.C. A systems approach for control of punch press noise. 2. H. W. LORD, H. A. EVENSENand A. A. HENDRICKSON1974 4th Quarterly Progress Report by The Noise Project Task Force, Forging Industry Educational and Research Foundation, Cleveland, Ohio. Forge hammer noise study. 3. H. A. EVENSEN,C. W. FRAMEand C. J. CROUT 1976 International Conference on Forging Noise Control, Atlanta, Georgia. Experiments in forge hammer noise control. 4. E. J. RICHARDS1976 in An Advanced Course in Noise and Vibration, University of Southampton.
Mechanical noise sources. 5. L. L. KOSS and R. J. ALFREDSON1974 Journal of Sound and Vibration 34, 11-33. Identification of transient sound sources on a punch press. 6. K. C. CRANDALLand R. W. SEABL~~M1970 Engineering Fundamentals in Measurements, Probability, Statistics and Dimensions. McGraw-Hill Book Company, Inc. 7. D. C. HODGSONand J. E. BOWCOCK1975 Journal of Sound and Vibration 42, 325-335. Billet expansion as a mechanism for noise production in impact forming machines.
qf Sound and Vibration (1978) 58(4), 587-59 I
LETTERS USE OF A SMALL
SCALE
TO THE EDITOR
MODEL
TO PREDICT
STEAM
HAMMER
NOISE?
1. INTRODUCTION
The usefulness of physically scaled models is accepted in auditorium acoustics and is becoming recognized in studies of sound propagation outdoors. Their value in reproducing the noise of complex machinery within enclosed industrial environments on the other hand remains undetermined. A preliminary assessment is presented here of results obtained from a model of one of the noisiest industrial processes-the forging of metal billets [l-3]. Forming normally occurs through impacts from an accelerated or freely falling body called a tup or ram. Although the billet is usually preheated, cold, flat faced billets have been employed commonly in full scale experiments primarily to emphasize the impact noise produced by the striking bodies [2, 31. Consequently the latter situation where easily subdued, “extraneous” noises from gas exhausts and the like have been silenced, were considered exclusively. 2.
THE MODEL AND MEASURING
EQUIPMENT
An accelerated tup is probably more difficult to simulate than a freely falling one. On the other hand it generates impact forces and, hence, dynamic responses with a potentially wider range of amplitudes for a fixed maximum drop height. This last factor becomes especially important for smaller sized models where the height is restricted proportionally. On balance the additional flexibility offered by an accelerated tup seemed worthwhile. Therefore a model was constructed of one of the more popular machines in this category, a 10 cwt Rigby pattern, Ross type steam hammer. A typical foundation was represented as far as practicable by seating the model on a wooden baseplate isolated by rubber from a solid supporting structure. The complete model, isolated from peripheral reflecting surfaces, is displayed from the side in Figure 1. An impression of the model’s dimensions may be gained by comparison with the coin. The universally employed 1/12th size reduction was about the largest consistent with maintaining uniform response of the transducers to the correspondingly higher frequency motions expected [4]. A schematic outline of the model’s ancillary control unit is illustrated in the upper part of Figure 2. The tup is accelerated by the sudden release of air through a piston valve inside the cylinder. Various acceleration levels are possible by regulating the pre-released air at different pressures. Activation of the process is controlled for greater consistency by a solenoid rather than a hand lever as employed normally in practice. After the initial acceleration, the tup drops essentially freely before impacting with a square sectioned, 6.3 + 0.5 mm billet having a length of 22.0 ? 0.5 mm. The billet is stationed in a 1 mm deep locating slot to ensure reasonably constant positioning. Constraints are avoided on the billet by providing a nominal clearance of 0.1 mm from the sides of the slot. A major factor in the choice and location of measuring equipment was the need to compare model with full scale results. Unfortunately, the detailed behaviour of a full scale Rigby steam hammer is unknown and could not be determined due to scheduling difficulties. However information has been published relating the near field Lpear to the maximum of the peak vertical acceleration or deceleration of the tup and anvil for different drop hammers 7 An early version of this letter giving more tentative results was presented at the 18th International Machine Tool Design and Research Conference, London, September 1977. 587 0022-460X/78/06224587 $01.00/O ? 1978 Academic Press Inc. (London) Limited
588
LETTERS TO THE EDITOR
I
Valve chest Steam
Inlet
Cylinder
Mouthpiece Column
!Tup
Pallets
Hand lever Anvi I
Baseplate
Figure 1. The model.
[l.-31. These variables are relatively insensitive to structural “ringing” after the impact P -41 and, presumably, to the specific configuration of a hammer. Therefore comparisons With this data should be reasonably valid. Silencer, Maxam Type x)OP99325
Silencer, Maxam Type 7COlP99325 Exhaust
II Solenoid wrated valve, Maxam Type
Model steam hammer
-
Lubricator, Maxom Type A606
Pressure regulator, Moxam Type Al 19
Accelerometer, BBKf Type 4344 Microphone, BBK’ Type 4133 Sound level meter, B 81 K+ Type 2209
Tmnslent recorder, Catalab Type DL90
Charge ampl ifler, _ Klstler Type 5001
Tmnwnt wonder, Datalob Type DL905
Oscilloscope, Tektronix
Oscilloscope, Brad I ey Type I56
tBrijel and Kjoer
Figure 2. Equipment.
Filter, Maxam Type A602
Air supply 100 psi
LETTERS
TO THE EDITOR
589
Accurate measurement of maximum peaks accentuates the usual requirement in model work for the resolution of high frequencies. Upper limits used, nominally 40 and 125 kHz, respectively, for the sound and accelerations, were the highest attainable with readily available commercial transducers. Details of the transducers are given in the lower part of Figure 2. Transducer signals were amplified, sampled digitally at a frequency not less than 200 kHz, stored and displayed on an oscilloscope for analysis. The beginning of each signal was captured by using the pre-triggering facility of the transient recorders. Calibration of the complete measurement system was performed normally by using oscillatory signals of known amplitude. An accelerometer was stud-mounted on the anvil’s pallet as shown in Figure 1. Lack of space prevented direct observation of the tup’s acceleration. Fortunately the accelerating force on the tup of a steam hammer is proportional to the pressure of the pre-released air which was determined from a gauge. Finally L,,,, was measured to the side of the hammer and in the plane of impact at a distance of 31.25 mm from the closest edge of the billet. This distance, when scaled, is comparable to that used in practice at close work stations. 3.
EXPERIMENTAL
PROCEDURE
Initial experiments indicated the possibility of highly fluctuating maximum peak values for model tests with supposedly constant conditions. Trial and error investigations suggested that the alignment between the tup and billet was critical. Spacers between the cylinder and tup’s collar and the realignment, after each test if necessary, of the friction restrained pallets alleviated the problem. With the modification the peak levels in the time histories still changed quite appreciably despite a close similarity in the form of the histories. Therefore the value of the maximum peak is somewhat meaningless if obtained from a single record. Conversely the mean of such values observed in ten nominally identical tests was found to be fairly stationary and, hence, repeatable. (Specifically, for reasonably assumed random variations [5], this mean was not significantly different at the 95% confidence level from the mean of ten previous tests [6].) Consequently ten tests were performed consecutively at any one condition to enable the mean and standard deviation of the maximum peak values to be calculated. The pressure in the cylinder was changed in increments of 10 psi from 20 to 60 psi and the exercise repeated. The possibility that a large number of repeated loads might produce an extraneous effect upon a billet then arose. However, no anomalous behaviour was detected from sound or acceleration measurements in a separate investigation involving substantially more loads cycled between their extreme levels. Therefore the number and pattern of loads appeared unimportant providing pallets were aligned correctly. 4.
RESULTS
A comparison of model and full scale results is presented in Figures 3 and 4. Two materials, steel and brass, were used for the model billets to expand the number of data points limited by the restricted range of cylinder pressures. The densities of these materials are almost alike so that their peak dynamic behaviours should be virtually identical [7]. Information regarding the scatter of constant condition, full scale experiments is scant although, by inference from reference [l], Lpeakmight vary by as much as 8 dB. Figure 3 indicates that on average Lpeak is almost directly proportional to the air pressure in the model’s cylinder. This behaviour is consistent with full scale situations where the billet remains elastic [l-3]. Corresponding trends between Lpear and the anvil’s peak acceleration shown in Figure 4 also agree reasonably well although there is a magnitude difference in levels.
590
LETTERS TO THE EDITOR
Air
pressure
(psi)
Figure 3. Near field peak sound pressure levels for different model hammer pressures and billets. -, Steel billet (E.N. 24); ----, brass billet (vertical extensions show + one standard deviation).
Model I
/
103 Ful I xde
102 Peak acceleration
/
104
measured on anvil (g)
Figure 4. Comparison of results for the model steam hammer and a full scale CECO 60FD drop hammer [2]. -, Steel model billet (E.N. 24); ----, brass model billet (vertical and horizontal extensions show + one standard deviation); ?? , full scale drop hammer.
5.
DISCUSSION
The preliminary comparisons presented suggest that trends observed for isolated full scale drop hammers might be duplicated by geometrically scaled and carefully operated models. Further research is required to determine if the correspondence for AC,,,, extends, for example, to L,,,? which might be more appropriate for the semi-reverberant conditions typically encountered in industry. The possibly important effects of ringing noise, which depend upon a particular hammer’s configuration [24], must be useful in this context.
t L,, is the standard notation for the energy mean level.
LETTERSTO THE EDITOR
591
ACKNOWLEDGMENT
The third author acknowledges gratefully the travel fellowship awarded by the National Research Council of Canada which enabled him to assist in this work whilst on sabbatical leave at Sheffield Polytechnic. Department of Mechanical and Production Engineering, Sheffield City Polytechnic, Sheffield Sl 1 WB, England
G. J. MCNULTY
Department of Mechanical Engineering, University of Manitoba, Winnipeg, Canada R3 T 2N2
N. POPPLEWELL
J. HOLDING
(Received 16 January 1978)
REFERENCES 1. J. R. BAILEY,J. A. DAGGERHART and N. D. STEWART1975 Design Engineering Technical Conference of the American Society of Mechanical Engineers, Washington D.C. A systems approach for control of punch press noise. 2. H. W. LORD, H. A. EVENSENand A. A. HENDRICKSON1974 4th Quarterly Progress Report by The Noise Project Task Force, Forging Industry Educational and Research Foundation, Cleveland, Ohio. Forge hammer noise study. 3. H. A. EVENSEN,C. W. FRAMEand C. J. CROUT 1976 International Conference on Forging Noise Control, Atlanta, Georgia. Experiments in forge hammer noise control. 4. E. J. RICHARDS1976 in An Advanced Course in Noise and Vibration, University of Southampton.
Mechanical noise sources. 5. L. L. KOSS and R. J. ALFREDSON1974 Journal of Sound and Vibration 34, 11-33. Identification of transient sound sources on a punch press. 6. K. C. CRANDALLand R. W. SEABL~~M1970 Engineering Fundamentals in Measurements, Probability, Statistics and Dimensions. McGraw-Hill Book Company, Inc. 7. D. C. HODGSONand J. E. BOWCOCK1975 Journal of Sound and Vibration 42, 325-335. Billet expansion as a mechanism for noise production in impact forming machines.