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Control of shear cutting noise: Effect of blade profile
Appfied Acousfics, Vol. 54, NO. I, pp. 45-58, 1998 0 1997 Elsevier Science Ltd. All rights reserved
PII:
SOOO3-682X(97)00066-2
Printed in Great Britain 0003-682X/98/$19.00+0.00
ELSEVIER
Control of Shear Cutting Noise: Effect of Blade Profile Ahmad
Bahrami,
Hugh M. Willamson
and Joseph
C. S. Lai
Acoustics and Vibration Unit, Australian Defence Force Academy, University College, The University of New South Wales, Canberra, ACT 2600, Australia (Received 19 May 1997; revised version received and accepted 25 July 1997)
ABSTRACT Among the machines operating in sheet metal industries, usually the major noise source is due to impact. Various noise sources involved in a roll forming production line have been discussed. It is proposed that the acceleration noise and the noise due to cutting metal sheets may be reduced by changing the shear cutting process. This can be implemented through changing of the blade profile. The e$ects of blade angle on the noise radiated from cutting profiled metal sheets have been investigated using an experimental shear. These results were used to design a new blade with a 2” blade angle. A reduction of 6dB(A) in equivalent sound pressure level has been achieved with this new blade under industrial operating conditions. 0 1997 Elsevier Science Ltd. Keywords:
Industrial noise control, shear cutting noise, machinery noise
control.
INTRODUCTION Considerable research has been carried out into the effects of tooling parameters on radiated noise from punching or piercing machines. However, little published work is available on how blade parameters affect radiated noise for sheet metal shearing operations. Since the 1970s advances in instrumentation technology have been used by some researchers such as Sahlin,’ Koss~,~ and Shinaishin4 to show that the major noise source during blanking is fracture of the work material. They discussed the effects of tooling parameters on the force time history and consequently the radiated noise. Shinaishin’s experiments achieved a reduction of 12 dB in radiated noise by 45
46
A. Bahrami et al
applying shear to the punch. Burrows’ and Evensen have experimentally investigated tooling effects such as blade clearances and shearing effect on the radiated noise of a punch press and have achieved a reduction of up to 10 dB(A) for specific operations. Various noise sources involved in a roll former shear, which produces profiled metal sheets and cuts them into specified lengths, have been discussed by Bahrami and Willamson .’ Extensive investigations by Speakman et ~1.~ have shown that the radiated noise is primarily dominated by the ringing noise from the product immediately after blade impact with the product. By applying a sheet damper close to the blade, they achieved a noise reduction of 6dB(A). Consequently, acceleration noise and noise due to fracturing the product have to be reduced if further noise reduction is to be achieved. Acceleration noise and noise due to fracturing the product may be reduced by changing the impact shear cutting process. If the impact shear cutting process is changed so that the initial impact by the blade on the product is reduced, then the ringing noise due to vibration of the product and the shearing machine itself may also be reduced. One way of changing the impact shear cutting process is by changing the profile of the blades. The objective of this study is, therefore, to investigate the influence of blade profiles on the shear cutting noise in a roll former shear. First, the effects of the blade angle of a straight blade on the noise radiated from cutting a flat sheet are considered. Second, the effects of five different blade profiles on the noise radiated from cutting a profiled sheet are presented. Third, the results of implementing one of the blade profiles in an industrial roll former shear are discussed.
OPERATION
OF A ROLL FORMER
SHEAR
Roll former machines in sheet metal industries are used to form metal sheets to the required shape by a series of rollers. Roll former sheet metal products, which come in a variety of profiles, thicknesses and surface coating, are used extensively for roofing, walling and fencing of industrial and domestic buildings. An example of a roll former product with an almost sinusoidal profile from BHP-Building Products, Australia is shown in Fig. 1. The width of the product is almost 850mm. The operation of a roll former shear is shown schematically in Fig. 2. The flat sheet metal is first uncoiled and passed through a series of rollers to form the finished product profile. Once formed, the profiled product is cut to length in a continuous process using a “flying shear”, a shear which moves with the product. The profiled sheet then has to be removed by an operator from the production line and stacked on a pile ready for delivery.
41
Control of shear cutting noise: effect of blade profile
Fig. 1. Approximate
I
I
I
I
I
sinusoidal sheet product of a roll former.
I
I
I
LJ
L-J
Fig. 2. Diagram of a hydraulic roll former shear.
The lower and the upper blades of the shear are profiled to approximately match the profile of the product. The lower blade is fixed and the upper blade is moved vertically by a driver actuated either mechanically, pneumatically or hydraulically (as shown in Fig. 2) to cut through the product. The upper blades of existing shears normally have a rake to facilitate the shearing process. In the existing flying shears, the profile of the upper blade is the same as the lower blade except that it is pivoted at a rake angle and stretched to be aligned with the lower blade, see Fig. 3. These types of profiles are less noisy than those without a rake. The noise radiated at various stages of the operation of a roll former production line can be identified from the noise signature shown in Fig. 4. These include the noise due to roll forming a flat metal sheet into a profiled sheet, cutting the sheet to required length, removing the sheet from the production line and dropping the sheet onto a stack. It can be seen that high impulsive noise levels are produced by the cutting action (fracturing the metal) and the resulting impact induced vibration of the product and the surrounding structure (“ringing” noise). The noises due to removal of the product from the production line and stacking the product may be reduced by changing the operator’s work practice. Reduction of noise using a sheet damper has been considered by Speakman et aL8 The noise due to fracturing the material is the subject of interest here.
48
A. Bahrami
et al
Fig. 3. Existing upper blade profile: comparison of a pivoted profile with a pivoted and stretched profile.
THEORETICAL
CONSIDERATIONS
Since the subject of concern here is the radiated noise due to the fracturing process, the relationship between the induced force in the sheet product or shear structure and the radiated noise is examined. According to Richards9 the total energy (&) which is developed in an impulsive cutting process is composed of, the work done on the product (&,-,rk), the work transferred to
Sound Pressure
0.0
Time
lPascal)
(seconds)
Fig. 4. Typical sound pressure signature during the operation of a roll forming production line.
49
Control of shear cutting noise: effect of blade profile
the ground or foundation structural damping (E ,t,t)
(Esround) the energy dissipated as heat through and the energy radiated as noise (&&.
(1)
Ein = Ework+ &round+ Estruct + &ad
EradandEstruct can be expressed in terms of physical properties of the structure, and the surrounding co
Erad = pca,,ds
1
(2)
(?)dt
0
cc
E struct = 2~frlspmSd
J@W
(3)
0
where pc is the specified acoustic impedance of the surrounding medium (air), S is the surface area of the structure, C&-Jis the radiation efficiency of the structure, (?) is the spatial average of the mean square normal surface velocity, )I~is the damping (structural loss) factor, pm is the material density of the structure, d is the average thickness of the structure and f is the frequency of sound radiation. By substituting S J (2)dt from eqn (3) into eqn (2), we have 0
(4)
The energy terms Erad and Estructare related to the induced forcef( t) and the velocity x’(t) at the point of impact: I’
&ad +
Estruct =
If(t)x’(t)dt = 0
pcorad +I
2nfrlspn-d
>
Estruct
(5)
It can be seen from eqn (5) that in audible frequency range, most of the energy is dissipated as heat energy, Estruct,through structural damping and only a small amount of the transferred energy is converted into noise energy,
50
A. Bahrami et al.
&ad SO that Z,$zzd is much smaller than I and consequently
_&admay be
neglected. Asf(t) is zero except during the impact between t = 0 and t = t’, eqn (5) may be rewritten using the Parseval theorem as: t’
E Struct
=
J
f(t>x ‘(W = i fW
0
where F(f)
‘W = 2 [ 0
--oo
is the complex conjugate of F(f).
response function, H(f), defined as $#,
E struct = 2
By employing a frequency
eqn (6) can be expressed as:
O” Re I E’(f) I*hy [
H(f)]df=
27 Re[l ‘;$,‘;$!“)]d~
J 0
(7)
0
F”U’) For a centre frequency f with a bandwidth where F(f) = W. can be approximated as E struct = %fRe
(6)
RePTfW(f>ldf
I
1
F”(f) I2 H(f) h’C2f27TJy
AA eqn (7)
(8)
By substituting eqn (8) into eqn (4) we obtain E&f
9Al) = $;;y$
( F”(f) I2 Re[y]
For N operations per day, eqn (9) may be expressed in terms of the equivalent A-weighted radiated sound pressure level LAeq at a centre frequency f with a bandwidth Af as LA~~(~, AA = lOlog ) F”(f) )* +lOlog Re - lOlogd+
lOlogN+
PC A lOlog -[ 8rr’%m.f33
(10) In eqn (lo), the first term, F”( f ), relates to the shape of the induced force inside the structure. In a punching or shearing operation, F”(f) will be influenced
Control of shear cutting noise: eflect of blade profile
51
by tooling design parameters, such as blade clearance, blade profile and the speed of the blade prior to impact. The remaining terms in the above equation relate to the physical properties of the vibrating structure of the machine and can be replaced by a constant, C which cannot be readily changed without rebuilding or redesigning the machine. L_&?Jf) = 10 log ( F”(f)
I2 +c
(11)
1 F”(f) I2 is related to its Fourier transform counterpart f” ) (t) as follows (see, for example, Richards and Stimpsonlo): 00 ( F”(f)
12= F”F”*
=
1 f”(+-2=&jt -co
/ f”(t)e21Tif&
(12)
--oo
Extensive experiments by Burrows’ for punch press noise have shown that impulse forces, f”( t) are significant for only a series of short time periods, At, so integral in eqn (12) can be approximated by r that the double _I
(13)
By substituting eqn (13) into (1 l), we have, L Aeq = 10 1%
~tf’(ol~~,+c
(14)
Hence, eqn (14) indicates that if the maximum rate of change of the induced force, Lf’mlax~can be reduced, LAG,, will be reduced. One means of changing [S(t)],,, is to change the blade’ profile which is the prime concern of this paper.
EXPERIMENTAL
SET-UP AND INSTRUMENTATION
To investigate the influence of different blade profiles on radiated noise in shearing, a hydraulically operated experimental shear (shown schematically in Fig. 5) was provided by BHP Building Products, Australia. For all the experiments, the sound pressure level at a height of 1.lO and l.Om from the
52
A. Bahrami et al.
1600 mm
______3 7
1600 mm
LJ
I
Fig. 5. Experimental
shear with straight blade assembly.
centre of the blade directly in front was measured by a Bruel and Kjaer (B&K) type 2231 Sound Level Meter (SLM) equipped with a B&K l/2” condenser microphone type 4155. The blade displacement was measured by using a linear variable differential transformer (LVDT). All these data were recorded simultaneously using a Boston Technology PC30 A/D data acquisition card controlled by a Toshiba T3200SXC laptop computer. Some of the data were processed with appropriate time constant to give the short term A-weighted equivalent continuous sound pressure level, LACK.Both the noise radiated from cutting flat sheets and sheets with almost sinusoidal profiles were investigated using the experimental shear. The effect of installing a new blade in an existing industrial shear on the cutting noise during operation was also studied. Straight flat sheet
To investigate the effect of blade angle on noise radiated from cutting flat sheets, a set of simple straight blades was designed and manufactured (as shown in Fig. 6). According to eqn (14), the cutting noise is dominated by the maximum rate of change of the induced force ([f’(t)],,,) in the shear and the product. By designing a shear such that the fracturing process is promay be significantly reduced. This can be achieved by longed, W(%,x installing the upper blade at an angle to the lower blade as shown in Fig. 6.
53
Control of shear cutting noise: e$ect of blade profile
UPPER BLADE LOWERBLADE
Fig. 6. Views of the manufactured
straight blade assembly.
Sheet with sinusoidal profile To verify the results of straight blades for shearing a profiled sheet, a product of BHP Building Products, Australia with an almost sinusoidal profile, Fig. 1, was used. Instead of straight blades, profiled blades as shown in Fig. 7 were installed. Five different profiled blade sets were designed and manufactured. For each of these blade sets, the lower blade had the same profile as the specimens but the upper blades were designed to have a specific shearing angle. This is to allow progressive shearing across the sheet. The five blade angles used were 0, 0.5, 1, 2 and 4” with respect to the lower blade. In order to illustrate the action of new profiled blade, Fig. 8 shows schematically
Fig. 7. Sinusoidal blade assembly.
A. Bahrami et al.
LOWER
BLADE
LOWER
FIRST STAGE
LOWER
SECOND STAGE
BLADE
LOWRdiiLADE
THIRD STAGE Fig. 8. Four stages of interaction
BLADE
FOURTH STAGE of a set of 2” shear angle sinusoidal
blades.
the various shearing stages of the product with a 2” blade. It can be seen that a fixed shearing angle of 2” is maintained as the upper blade is moving down and the cutting occurs progressively from left to right. Full scale blade
Based on the results obtained in the experiments using straight and almost sinusoidal profiled blades, a set of full scale sinusoidal blades with 2” shearing angle was designed, manufactured and installed on an existing flying shear in the factory. RESULTS In order to investigate the effect of blade shearing angle on the noise radiated from cutting flat sheets, short specimens were used so as to reduce the amount of radiated noise due to sheet vibration. The test specimens used were zinc/aluminium alloy coated steel with dimensions of 20.0 x 100.0 x 1.O mm.
Control of shear cutting noise: effect of blade profile
85’
0
1
3
2
Blade Angle
55
J 4
(degree)
Fig. 9. Effect of blade angle on maximum radiated noise, L,,,,
in a set of straight blades.
The clearance between the blades was fixed at 0.12mm. The speed of cutting was 0.10 m s-i. Figure 9 shows a reduction of 17 dB(A) in the maximum A-weighted sound pressure level, ~~~~~~by increasing the blade angle from 0 to 2”. The maximum A-weighted noise level ~~~~~ radiated from cutting sheets with almost sinusoidal profiles (like that shown in Fig. 1) for five different blade profile angle is shown in Fig. 10. The test specimens used were zinc/ aluminium alloy coated steel with overall dimension 300 x 300 x 0.45 mm. The clearance between the blades was fixed at 0.06mm and the speed of cutting was 0.07m s- l. It can be seen from Fig. 10 that a reduction of 22 dB(A) in LAmaxhas been achieved by changing the angle from 0” to 4”. The effect of alignment of the blades on ~~~~~ was also studied by introducing a misalignment of about 3 mm along the blade direction. Figure 10 shows that for all blade angles tested except o”, ~~~~~ is significantly increased by even a small misalignment. However, for 0” blade angle, a small misalignment can result in a significant reduction in ~~~~~ (approximately 10 dB in this test). This is because by introducing a small misalignment for 0 blade angle, not all the points on the blade are in contact with the sheet simultaneously during the shearing operation so that the rate of change of induced force is reduced. Based on the results shown in Fig. 10, it appears that the larger the blade angle, the lower the radiated noise level from cutting. However, in trying out a new profile blade in an existing shear a blade angle of 2” was chosen. This is because a blade angle of 2” is equivalent to the maximum stroke of the existing shear. The overall {-weighted continuous sound pressure level due to cutting a 850 x 4000 x 0.45 mm profiled sheet (Fig. 1) with an old
A. Bahrami et al.
56
blade and a new blade in an industrial operating environment is shown in Fig. 11. Results show that a reduction of 6dB in ~~~~ over 3 s has been achieved. It should be pointed out that the old upper blade has a stretched profile and is pivoted as described in Fig. 3. As a result, the blade angle roughly
75L 0
0.5
Fig. 10. Effect of shearing
0
1
1.5 3 2 2.5 Blades Profile Angie (degree)
blade angle and its misalignments blade sets.
0.5
1
1.5
2
I 4
3.5
on radiated
2.5
noise in sinusoidal
3
nme (seconds)
Fig. 11. Comparison
of sound pressure
level during cutting
between
existing and new blades.
Control of shear cutting noise: effect of blade profile
51
varies from 0 to 3” along the blade. Hence, Fig. 11 implies that a profile blade with a constant angle is more effective in reducing the shear cutting noise than a profiled blade with varying blade angle along the blade. This is because if the shearing angle changes during the cutting process, the impact due to the blade movement would cause a greater shock in the product and the shear structure and consequently higher noise level. A constant shearing angle during cutting, as implemented in the new blade not only would reduce the cutting force but also would reduce the maximum rate of change of the induced force; hence lowering noise level according to eqn (14).
CONCLUSION The operation of a roll former shear for cutting profiled sheet metal products has been described. The major noise sources during the roll forming process have been identified to be primarily due to cutting, vibration induced in the product and the shearing machine, removal of the product from the production line and stacking the products, It has been shown here that the noise due to cutting is directly related to the maximum rate of change of the induced force in the product. This suggests that a smooth fracturing process would reduce the rate of change of the induced force and hence the radiated noise level during cutting. In order to achieve a smooth fracturing process, a concept which involves changing the blade profiles by incorporating a small blade angle has been investigated using an experimental shear. Results show that the maximum A-weighted sound pressure level decreases as the blade angle increases. A profile blade with a shearing blade angle of 2” installed in an industrial shear has been demonstrated to give a 6dB reduction in LAeq over 3 s during cutting.
ACKNOWLEDGEMENTS The authors would like to acknowledge the financial support of the Australian Postgraduate Research Award (Industry) Scheme and BHP Building Products. As well as financial support, BHP Building Products also contributed valuable practical advice and encouragement and facilitated the carrying out of factory trials. REFERENCES 1. Sahlin, S. and Langhe, R., Origins of punch press and air nozzle noise. Noise Control Engineering
Journal,
1974, 3, 4-9.
A. Bahrami et al.
58
2. Koss, L. L. and Alfredson, R. J., Identification of transient sound sources on a punch press. Journal of Sound and Vibration, 1974,34, 1l-33. 3. Koss, L. L., Punch press load-radiation characteristics. Noise Control Engineering Journal, 1977, 8, 33-39. 4. Shinaishin, 0. A., Impact induced industrial noise. Noise Control Engineering Journal, 1974, 2(l), 3CL-36. 5. Burrows, J. M., The influence of tooling parameters on punch press noise. M.Sc.
dissertation, University of Southampton, 1979. 6. Evensen, H. A., A fundamental relationship between force waveform and the sound radiated from a power press during blanking or piercing. Journal of Sound and Vibration, 1980, 68(3), 451463. 7. Bahrami, A. and Willamson, H. M., Sources of noise in sheet metal shears.
Proceedings of the Australian Acoustical Society Annual Conference, 1996. 8. Speakman, C., Willamson, H. M. and Lai, J. C. S., Retrofit noise reduction techniques applied to a roll former shear. A case study. Technical report AVC no. 9420 for the worksafe Australia research scheme, Acoustics and Vibration Unit, University of NSW, Canberra, Australia, 1994. 9. Richards, E. J., On the prediction of impact noise: Part III, Energy accountancy in industrial machines. Journal of Sound and Vibration, 198 1, 76(2), 187-232. 10. Richards, E. J. and Stimpson, G. J., On the prediction of impact noise: Part IX, The noise from punch press. Journal of Sound and Vibration, 1985, 103(l), 4381.
PII:
SOOO3-682X(97)00066-2
Printed in Great Britain 0003-682X/98/$19.00+0.00
ELSEVIER
Control of Shear Cutting Noise: Effect of Blade Profile Ahmad
Bahrami,
Hugh M. Willamson
and Joseph
C. S. Lai
Acoustics and Vibration Unit, Australian Defence Force Academy, University College, The University of New South Wales, Canberra, ACT 2600, Australia (Received 19 May 1997; revised version received and accepted 25 July 1997)
ABSTRACT Among the machines operating in sheet metal industries, usually the major noise source is due to impact. Various noise sources involved in a roll forming production line have been discussed. It is proposed that the acceleration noise and the noise due to cutting metal sheets may be reduced by changing the shear cutting process. This can be implemented through changing of the blade profile. The e$ects of blade angle on the noise radiated from cutting profiled metal sheets have been investigated using an experimental shear. These results were used to design a new blade with a 2” blade angle. A reduction of 6dB(A) in equivalent sound pressure level has been achieved with this new blade under industrial operating conditions. 0 1997 Elsevier Science Ltd. Keywords:
Industrial noise control, shear cutting noise, machinery noise
control.
INTRODUCTION Considerable research has been carried out into the effects of tooling parameters on radiated noise from punching or piercing machines. However, little published work is available on how blade parameters affect radiated noise for sheet metal shearing operations. Since the 1970s advances in instrumentation technology have been used by some researchers such as Sahlin,’ Koss~,~ and Shinaishin4 to show that the major noise source during blanking is fracture of the work material. They discussed the effects of tooling parameters on the force time history and consequently the radiated noise. Shinaishin’s experiments achieved a reduction of 12 dB in radiated noise by 45
46
A. Bahrami et al
applying shear to the punch. Burrows’ and Evensen have experimentally investigated tooling effects such as blade clearances and shearing effect on the radiated noise of a punch press and have achieved a reduction of up to 10 dB(A) for specific operations. Various noise sources involved in a roll former shear, which produces profiled metal sheets and cuts them into specified lengths, have been discussed by Bahrami and Willamson .’ Extensive investigations by Speakman et ~1.~ have shown that the radiated noise is primarily dominated by the ringing noise from the product immediately after blade impact with the product. By applying a sheet damper close to the blade, they achieved a noise reduction of 6dB(A). Consequently, acceleration noise and noise due to fracturing the product have to be reduced if further noise reduction is to be achieved. Acceleration noise and noise due to fracturing the product may be reduced by changing the impact shear cutting process. If the impact shear cutting process is changed so that the initial impact by the blade on the product is reduced, then the ringing noise due to vibration of the product and the shearing machine itself may also be reduced. One way of changing the impact shear cutting process is by changing the profile of the blades. The objective of this study is, therefore, to investigate the influence of blade profiles on the shear cutting noise in a roll former shear. First, the effects of the blade angle of a straight blade on the noise radiated from cutting a flat sheet are considered. Second, the effects of five different blade profiles on the noise radiated from cutting a profiled sheet are presented. Third, the results of implementing one of the blade profiles in an industrial roll former shear are discussed.
OPERATION
OF A ROLL FORMER
SHEAR
Roll former machines in sheet metal industries are used to form metal sheets to the required shape by a series of rollers. Roll former sheet metal products, which come in a variety of profiles, thicknesses and surface coating, are used extensively for roofing, walling and fencing of industrial and domestic buildings. An example of a roll former product with an almost sinusoidal profile from BHP-Building Products, Australia is shown in Fig. 1. The width of the product is almost 850mm. The operation of a roll former shear is shown schematically in Fig. 2. The flat sheet metal is first uncoiled and passed through a series of rollers to form the finished product profile. Once formed, the profiled product is cut to length in a continuous process using a “flying shear”, a shear which moves with the product. The profiled sheet then has to be removed by an operator from the production line and stacked on a pile ready for delivery.
41
Control of shear cutting noise: effect of blade profile
Fig. 1. Approximate
I
I
I
I
I
sinusoidal sheet product of a roll former.
I
I
I
LJ
L-J
Fig. 2. Diagram of a hydraulic roll former shear.
The lower and the upper blades of the shear are profiled to approximately match the profile of the product. The lower blade is fixed and the upper blade is moved vertically by a driver actuated either mechanically, pneumatically or hydraulically (as shown in Fig. 2) to cut through the product. The upper blades of existing shears normally have a rake to facilitate the shearing process. In the existing flying shears, the profile of the upper blade is the same as the lower blade except that it is pivoted at a rake angle and stretched to be aligned with the lower blade, see Fig. 3. These types of profiles are less noisy than those without a rake. The noise radiated at various stages of the operation of a roll former production line can be identified from the noise signature shown in Fig. 4. These include the noise due to roll forming a flat metal sheet into a profiled sheet, cutting the sheet to required length, removing the sheet from the production line and dropping the sheet onto a stack. It can be seen that high impulsive noise levels are produced by the cutting action (fracturing the metal) and the resulting impact induced vibration of the product and the surrounding structure (“ringing” noise). The noises due to removal of the product from the production line and stacking the product may be reduced by changing the operator’s work practice. Reduction of noise using a sheet damper has been considered by Speakman et aL8 The noise due to fracturing the material is the subject of interest here.
48
A. Bahrami
et al
Fig. 3. Existing upper blade profile: comparison of a pivoted profile with a pivoted and stretched profile.
THEORETICAL
CONSIDERATIONS
Since the subject of concern here is the radiated noise due to the fracturing process, the relationship between the induced force in the sheet product or shear structure and the radiated noise is examined. According to Richards9 the total energy (&) which is developed in an impulsive cutting process is composed of, the work done on the product (&,-,rk), the work transferred to
Sound Pressure
0.0
Time
lPascal)
(seconds)
Fig. 4. Typical sound pressure signature during the operation of a roll forming production line.
49
Control of shear cutting noise: effect of blade profile
the ground or foundation structural damping (E ,t,t)
(Esround) the energy dissipated as heat through and the energy radiated as noise (&&.
(1)
Ein = Ework+ &round+ Estruct + &ad
EradandEstruct can be expressed in terms of physical properties of the structure, and the surrounding co
Erad = pca,,ds
1
(2)
(?)dt
0
cc
E struct = 2~frlspmSd
J@W
(3)
0
where pc is the specified acoustic impedance of the surrounding medium (air), S is the surface area of the structure, C&-Jis the radiation efficiency of the structure, (?) is the spatial average of the mean square normal surface velocity, )I~is the damping (structural loss) factor, pm is the material density of the structure, d is the average thickness of the structure and f is the frequency of sound radiation. By substituting S J (2)dt from eqn (3) into eqn (2), we have 0
(4)
The energy terms Erad and Estructare related to the induced forcef( t) and the velocity x’(t) at the point of impact: I’
&ad +
Estruct =
If(t)x’(t)dt = 0
pcorad +I
2nfrlspn-d
>
Estruct
(5)
It can be seen from eqn (5) that in audible frequency range, most of the energy is dissipated as heat energy, Estruct,through structural damping and only a small amount of the transferred energy is converted into noise energy,
50
A. Bahrami et al.
&ad SO that Z,$zzd is much smaller than I and consequently
_&admay be
neglected. Asf(t) is zero except during the impact between t = 0 and t = t’, eqn (5) may be rewritten using the Parseval theorem as: t’
E Struct
=
J
f(t>x ‘(W = i fW
0
where F(f)
‘W = 2 [ 0
--oo
is the complex conjugate of F(f).
response function, H(f), defined as $#,
E struct = 2
By employing a frequency
eqn (6) can be expressed as:
O” Re I E’(f) I*hy [
H(f)]df=
27 Re[l ‘;$,‘;$!“)]d~
J 0
(7)
0
F”U’) For a centre frequency f with a bandwidth where F(f) = W. can be approximated as E struct = %fRe
(6)
RePTfW(f>ldf
I
1
F”(f) I2 H(f) h’C2f27TJy
AA eqn (7)
(8)
By substituting eqn (8) into eqn (4) we obtain E&f
9Al) = $;;y$
( F”(f) I2 Re[y]
For N operations per day, eqn (9) may be expressed in terms of the equivalent A-weighted radiated sound pressure level LAeq at a centre frequency f with a bandwidth Af as LA~~(~, AA = lOlog ) F”(f) )* +lOlog Re - lOlogd+
lOlogN+
PC A lOlog -[ 8rr’%m.f33
(10) In eqn (lo), the first term, F”( f ), relates to the shape of the induced force inside the structure. In a punching or shearing operation, F”(f) will be influenced
Control of shear cutting noise: eflect of blade profile
51
by tooling design parameters, such as blade clearance, blade profile and the speed of the blade prior to impact. The remaining terms in the above equation relate to the physical properties of the vibrating structure of the machine and can be replaced by a constant, C which cannot be readily changed without rebuilding or redesigning the machine. L_&?Jf) = 10 log ( F”(f)
I2 +c
(11)
1 F”(f) I2 is related to its Fourier transform counterpart f” ) (t) as follows (see, for example, Richards and Stimpsonlo): 00 ( F”(f)
12= F”F”*
=
1 f”(+-2=&jt -co
/ f”(t)e21Tif&
(12)
--oo
Extensive experiments by Burrows’ for punch press noise have shown that impulse forces, f”( t) are significant for only a series of short time periods, At, so integral in eqn (12) can be approximated by r that the double _I
(13)
By substituting eqn (13) into (1 l), we have, L Aeq = 10 1%
~tf’(ol~~,+c
(14)
Hence, eqn (14) indicates that if the maximum rate of change of the induced force, Lf’mlax~can be reduced, LAG,, will be reduced. One means of changing [S(t)],,, is to change the blade’ profile which is the prime concern of this paper.
EXPERIMENTAL
SET-UP AND INSTRUMENTATION
To investigate the influence of different blade profiles on radiated noise in shearing, a hydraulically operated experimental shear (shown schematically in Fig. 5) was provided by BHP Building Products, Australia. For all the experiments, the sound pressure level at a height of 1.lO and l.Om from the
52
A. Bahrami et al.
1600 mm
______3 7
1600 mm
LJ
I
Fig. 5. Experimental
shear with straight blade assembly.
centre of the blade directly in front was measured by a Bruel and Kjaer (B&K) type 2231 Sound Level Meter (SLM) equipped with a B&K l/2” condenser microphone type 4155. The blade displacement was measured by using a linear variable differential transformer (LVDT). All these data were recorded simultaneously using a Boston Technology PC30 A/D data acquisition card controlled by a Toshiba T3200SXC laptop computer. Some of the data were processed with appropriate time constant to give the short term A-weighted equivalent continuous sound pressure level, LACK.Both the noise radiated from cutting flat sheets and sheets with almost sinusoidal profiles were investigated using the experimental shear. The effect of installing a new blade in an existing industrial shear on the cutting noise during operation was also studied. Straight flat sheet
To investigate the effect of blade angle on noise radiated from cutting flat sheets, a set of simple straight blades was designed and manufactured (as shown in Fig. 6). According to eqn (14), the cutting noise is dominated by the maximum rate of change of the induced force ([f’(t)],,,) in the shear and the product. By designing a shear such that the fracturing process is promay be significantly reduced. This can be achieved by longed, W(%,x installing the upper blade at an angle to the lower blade as shown in Fig. 6.
53
Control of shear cutting noise: e$ect of blade profile
UPPER BLADE LOWERBLADE
Fig. 6. Views of the manufactured
straight blade assembly.
Sheet with sinusoidal profile To verify the results of straight blades for shearing a profiled sheet, a product of BHP Building Products, Australia with an almost sinusoidal profile, Fig. 1, was used. Instead of straight blades, profiled blades as shown in Fig. 7 were installed. Five different profiled blade sets were designed and manufactured. For each of these blade sets, the lower blade had the same profile as the specimens but the upper blades were designed to have a specific shearing angle. This is to allow progressive shearing across the sheet. The five blade angles used were 0, 0.5, 1, 2 and 4” with respect to the lower blade. In order to illustrate the action of new profiled blade, Fig. 8 shows schematically
Fig. 7. Sinusoidal blade assembly.
A. Bahrami et al.
LOWER
BLADE
LOWER
FIRST STAGE
LOWER
SECOND STAGE
BLADE
LOWRdiiLADE
THIRD STAGE Fig. 8. Four stages of interaction
BLADE
FOURTH STAGE of a set of 2” shear angle sinusoidal
blades.
the various shearing stages of the product with a 2” blade. It can be seen that a fixed shearing angle of 2” is maintained as the upper blade is moving down and the cutting occurs progressively from left to right. Full scale blade
Based on the results obtained in the experiments using straight and almost sinusoidal profiled blades, a set of full scale sinusoidal blades with 2” shearing angle was designed, manufactured and installed on an existing flying shear in the factory. RESULTS In order to investigate the effect of blade shearing angle on the noise radiated from cutting flat sheets, short specimens were used so as to reduce the amount of radiated noise due to sheet vibration. The test specimens used were zinc/aluminium alloy coated steel with dimensions of 20.0 x 100.0 x 1.O mm.
Control of shear cutting noise: effect of blade profile
85’
0
1
3
2
Blade Angle
55
J 4
(degree)
Fig. 9. Effect of blade angle on maximum radiated noise, L,,,,
in a set of straight blades.
The clearance between the blades was fixed at 0.12mm. The speed of cutting was 0.10 m s-i. Figure 9 shows a reduction of 17 dB(A) in the maximum A-weighted sound pressure level, ~~~~~~by increasing the blade angle from 0 to 2”. The maximum A-weighted noise level ~~~~~ radiated from cutting sheets with almost sinusoidal profiles (like that shown in Fig. 1) for five different blade profile angle is shown in Fig. 10. The test specimens used were zinc/ aluminium alloy coated steel with overall dimension 300 x 300 x 0.45 mm. The clearance between the blades was fixed at 0.06mm and the speed of cutting was 0.07m s- l. It can be seen from Fig. 10 that a reduction of 22 dB(A) in LAmaxhas been achieved by changing the angle from 0” to 4”. The effect of alignment of the blades on ~~~~~ was also studied by introducing a misalignment of about 3 mm along the blade direction. Figure 10 shows that for all blade angles tested except o”, ~~~~~ is significantly increased by even a small misalignment. However, for 0” blade angle, a small misalignment can result in a significant reduction in ~~~~~ (approximately 10 dB in this test). This is because by introducing a small misalignment for 0 blade angle, not all the points on the blade are in contact with the sheet simultaneously during the shearing operation so that the rate of change of induced force is reduced. Based on the results shown in Fig. 10, it appears that the larger the blade angle, the lower the radiated noise level from cutting. However, in trying out a new profile blade in an existing shear a blade angle of 2” was chosen. This is because a blade angle of 2” is equivalent to the maximum stroke of the existing shear. The overall {-weighted continuous sound pressure level due to cutting a 850 x 4000 x 0.45 mm profiled sheet (Fig. 1) with an old
A. Bahrami et al.
56
blade and a new blade in an industrial operating environment is shown in Fig. 11. Results show that a reduction of 6dB in ~~~~ over 3 s has been achieved. It should be pointed out that the old upper blade has a stretched profile and is pivoted as described in Fig. 3. As a result, the blade angle roughly
75L 0
0.5
Fig. 10. Effect of shearing
0
1
1.5 3 2 2.5 Blades Profile Angie (degree)
blade angle and its misalignments blade sets.
0.5
1
1.5
2
I 4
3.5
on radiated
2.5
noise in sinusoidal
3
nme (seconds)
Fig. 11. Comparison
of sound pressure
level during cutting
between
existing and new blades.
Control of shear cutting noise: effect of blade profile
51
varies from 0 to 3” along the blade. Hence, Fig. 11 implies that a profile blade with a constant angle is more effective in reducing the shear cutting noise than a profiled blade with varying blade angle along the blade. This is because if the shearing angle changes during the cutting process, the impact due to the blade movement would cause a greater shock in the product and the shear structure and consequently higher noise level. A constant shearing angle during cutting, as implemented in the new blade not only would reduce the cutting force but also would reduce the maximum rate of change of the induced force; hence lowering noise level according to eqn (14).
CONCLUSION The operation of a roll former shear for cutting profiled sheet metal products has been described. The major noise sources during the roll forming process have been identified to be primarily due to cutting, vibration induced in the product and the shearing machine, removal of the product from the production line and stacking the products, It has been shown here that the noise due to cutting is directly related to the maximum rate of change of the induced force in the product. This suggests that a smooth fracturing process would reduce the rate of change of the induced force and hence the radiated noise level during cutting. In order to achieve a smooth fracturing process, a concept which involves changing the blade profiles by incorporating a small blade angle has been investigated using an experimental shear. Results show that the maximum A-weighted sound pressure level decreases as the blade angle increases. A profile blade with a shearing blade angle of 2” installed in an industrial shear has been demonstrated to give a 6dB reduction in LAeq over 3 s during cutting.
ACKNOWLEDGEMENTS The authors would like to acknowledge the financial support of the Australian Postgraduate Research Award (Industry) Scheme and BHP Building Products. As well as financial support, BHP Building Products also contributed valuable practical advice and encouragement and facilitated the carrying out of factory trials. REFERENCES 1. Sahlin, S. and Langhe, R., Origins of punch press and air nozzle noise. Noise Control Engineering
Journal,
1974, 3, 4-9.
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2. Koss, L. L. and Alfredson, R. J., Identification of transient sound sources on a punch press. Journal of Sound and Vibration, 1974,34, 1l-33. 3. Koss, L. L., Punch press load-radiation characteristics. Noise Control Engineering Journal, 1977, 8, 33-39. 4. Shinaishin, 0. A., Impact induced industrial noise. Noise Control Engineering Journal, 1974, 2(l), 3CL-36. 5. Burrows, J. M., The influence of tooling parameters on punch press noise. M.Sc.
dissertation, University of Southampton, 1979. 6. Evensen, H. A., A fundamental relationship between force waveform and the sound radiated from a power press during blanking or piercing. Journal of Sound and Vibration, 1980, 68(3), 451463. 7. Bahrami, A. and Willamson, H. M., Sources of noise in sheet metal shears.
Proceedings of the Australian Acoustical Society Annual Conference, 1996. 8. Speakman, C., Willamson, H. M. and Lai, J. C. S., Retrofit noise reduction techniques applied to a roll former shear. A case study. Technical report AVC no. 9420 for the worksafe Australia research scheme, Acoustics and Vibration Unit, University of NSW, Canberra, Australia, 1994. 9. Richards, E. J., On the prediction of impact noise: Part III, Energy accountancy in industrial machines. Journal of Sound and Vibration, 198 1, 76(2), 187-232. 10. Richards, E. J. and Stimpson, G. J., On the prediction of impact noise: Part IX, The noise from punch press. Journal of Sound and Vibration, 1985, 103(l), 4381.