VIBRATION ISOLATORS FOR PORTABLE VIBRATING TOOLS

Transcription

1 Industrial Health, 1981, 19, 99. VIBRATION ISOLATORS FOR PORTABLE VIBRATING TOOLS PART 6. PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT Toshisuke MIWA,* Yoshiharu YONEKAWA,* Kazuo KANADA* and Kyo KOBAYASHI** *National Institute of Industrial Health, 21-1, Nagao 6-chome, Tama-ku, Kawasaki, 213 Japan **Institute of Agricultural Machinery, 40-1 Isshin-cho, Ohmiya, Saitama, 330 Japan (Received February 12, 1981) Abstract : Multi-point independent simultaneous force measurements were made to investigate the vibration force distribution on the palm and to determine one representative point on it for the purpose of field survey. The phase relationships between the force and the acceleration were also pursued. In connection with vibration force transmission, important effects such as vector, bypassing and mechanical impedance effects were studied with a model hand by using a vibration table. The real force transmission characteristics were examined with the human hand. A force measurement field survey was finally attempted on a grinder, an impact wrench and a riveter. The results observed on the model hand are in good agreement with those derived from the vibration theory for the vector and the bypassing effects. On the other hand, the results on the human hand are extremely complicated, owing to the contact conditions between the palm and the force pickups, and the results could not be fully interpreted. It can be concluded that the vibration force measured at the palm near the hand joint, where the force level usually shows the largest values, is representative. The vibration force on the palm does not represent proportionally the total force transmitted to the hand, but only reflects the characteristics of a visco-elastic material influenced by the vibration force. Key words : Vibration Isolators-Portable vibrating tools-vibration force measurement- Vibration force level Multi-point force measurement INTRODUCTION Since vibration force measurement can connect directly to amount of vibration influencing the human body through the hand, it is considered that force measurement is more effective than vibration acceleration measurement from the viewpoint of prevention 99

2 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI of vibration diseases. A vibration force pickup has been developed by several manufacturers for the purpose of measurements such as mechanical impedance, in which case the force pickup is inserted between two rigid materials (a vibration donor and a vibration acceptor). The usual force pickups are not appropriate for measurement when the vibration acceptor is a soft elastic material because of the occurrence of complex contact conditions. In the field of industrial hygiene, force pickups are beginning to be applied to the measurement of transmission of vibration force between a hand-held vibrating tool and the human hand. Vibration isolators for the hand-held vibrating tools such as grinder1), a rock-drill of leg-type2) and a pneumatic hammer3) have been developed and the vibration isolation gloves4) have also been studied. Moreover, in order to estimate the effectiveness of the vibration isolators, the force measurement was carried out with force pickups having different cross sections in the previous papers5). It was pointed out that there were three effects to be considered in the measurement of vibration force, that is, the bypassing effect, the vector effect and the impedance effect. The vector effect is derived from the incident angle of the vibration force flux into the force pickup. The bypassing effect arises because the vibration force is partially transmitted to the human palm without passing through the force pickup owing to the fact that some part of the human palm is directly touching the vibrating material. The impedance effect means different transmission of the vibration force on the palm in proportion to distribution of the mechanical impedance of the hand. The frequency characteristics on the palm for vibration force transmission were determined namely, it became constant below 50 Hz and decreased at a rate of -10 db/oct from 50 to 300 Hz, where the attenuation reached -30 db in the previous reports5). When a force pickup with large cross-section is used, the three effects mentioned above may be intermingled. Multi-point independent simultaneous force measurements on the palm were attempted in order to observe the distribution of the vibration force and to investigate the three effects separately. The present study was intended to identify a representative measuring point for the vibration force on the palm, to determine the significance of one-point measurement of the force and to determine whether the vibration power can be deduced. In the present work, the vibration force was simultaneously measured at eight positions on the palm by means of a vibration force pickup of piezo-film type. The frequency range measured was limited between 10 and 300 Hz. EXPERIMENTAL PROCEDURES A) Vibration force measurement with a model hand on a vibration table A mechanical model hand which consisted of two masses and a spring was first used in order to simplify the phenomena. The components were a cylindrical brass bar (50 ƒó ~ 50 mm, 800 gr), an aluminum plate (75 ~ 55 ~ 10 mm3, 120 gr) and an elastic cylindrical bar of polyvinyl chloride (50 95 ~ 14 mm, 4 gr) as shown in Fig. 1, a in Part 55). 100

3 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT The mechanical impedance of the model hand nearly simulated that of the human hand. The vibration force was detected by a pickup of disc type (16 ƒó ~ 4 mm, 5.5 gr, Rion Co.) made of piezo-electric poly-vinylidene film. More than three force pickups were placed on the vibration table or on a handle imitating that of a hand-held vibrating tool, which was screwed onto the vibration table. The force pickups were brought into contact with the model hand under pressure from an air cylinder. Sinusoidal vibration (120 db, 1 g) was applied to the vibration table to observe the frequency characteristics of the system. The reference of force level (20 log10f/fref)was taken as fref=f=10-6 N, while that of the acceleration level (20 log10 a/aref) was aref=10-5 m/s2. A. 1) Vector effect : Three force pickups were applied with adhesive tape on the grip of an imitation handle with a leveled surface (Fig. 1, i). The pickups were pushed by 3 kg with the model hand and the air cylinder. The incident angle of the force flux to the force pickups was changed to angles of 45 and 0 by turning the leveled surface of the grip (Fig. l, ii). The air cylinder was held by an iron frame fixed on the vibration exciter. Synthetic foam rubber (thickness of 2 mm) was inserted between the aluminum plate of the model hand and the force pickup in order to avoid contact resonance. A. 2) Bypassing effect : The vibration force bypassing the force pickup and directly transmitting to the hand can be calculated by the law of partition of the vibration force. The four force pickups were placed directly on the vibration table and the urethane foam rubber (10 mm thickness in the uncompressed state) with the same cross-section as the force pickup was applied to them. The urethane foam rubber was also applied on the Fig. 1. Test of the effect of incidence angle of vibration force flux into the force pickup on the force level. 101

4 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI remaining part of the aluminum plate of the model hand, and its thickness was varied from 1 to 2 sheets. The ratio of cross-section of the total force pickups to that of the other parts under the aluminum plate of the model hand was made equal to 1 : 3 and 1 : 1.5. The force pickup was pushed at 3 kg with the model hand and the air cylinder. The height of the urethane rubber decreased greatly from the original value when pressed. The force pickups were increased in number to seven instead of four in some cases. B) Vibration force measurement on the human palm B. 1) On the vibration table: The eight force pickups were set on the vibration table, suitably brought into contact with the human palm and pushed by about 3 kg, as shown in Fig. 2, i. Two conditions were examined, that is, when the palm could touch not only the force pickups but also the vibration table (Fig. 2, ii), and when the palm could only touch the force pickups. For this purpose, aluminum discs with the same diameter as the force pickup (thickness of 5 mm) were placed on the vibration table and then the force pickup was placed on them to increase the height of the pickup (Fig. 2, iii). The vibration force was measured and at the same time the phase angle between acceleration of the vibration table and the force pickup was observed. B. 2) On the handle set on the vibration table: The imitation handle was set on the Fig. 2. ( i ) Positions at which the eight force pickups were placed. (ii) The force pickups were placed on the vibration table, and some parts of the palm directly touched the table. (iii) The height of the eight pickups was increased, so that the palm did not touch the table. 102

5 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT the vibration table ; the grip was leveled at the top and the bottom to apply the force pickup, while the side surfaces parallel to the force flux were not used, because the vibration force there was negligible in principle. Six force pickups were fixed on the handle, and were grasped by the human hand. C) On portable vibrating tools Input force to the human hand was measured with a grinder (USG-7S, Uryia Co.), an impact wrench (UW-16S, Uryu Co.) and a riveter (PB-2, Uryia Co.). The positions of the force pickups on the palm were selected as indicated in Fig. 2, i. As the surface of the grip of the tool had an irregular curvature, while that of the force pickup was flat, the force pickups were first applied on three strips of bakelite plate (100 ~ 20 ~ 3mm3) which were fixed on the grip of the tool by covering them with vinyl tape. Eight channels of output signals from the force pickups were passed through impedance converters made of FET transistors and sequentially selected to provide a single channel by means of a multiplexer controlled by a microcomputer. The selected signal was amplified and rectified by an rms circuit, and then connected to an AD converter. Data processing was carrid out with the microcomputer (M-220, Sord Comp. Co.). The vibration force pickups were calibrated by means of a mass of 160 gr on the sinusoidally vibrated vibration table. The linearity of the pickup was examined by the use of masses of 100, 300 and 600 gr. Good linearity was obtained on all the pickups. As the force pickup had a flange around the flat centre disc, the effect of the flange was checked ; namely, the same mass was separately loaded on the central disc or on the flange using a suitable attachment. The results showsed a gain reduction of about 6 db on the flange loading comparing with that on the central disc loading, and this was considered to be negligible for practical force measurement. RESULTS AND DISCUSSION A. 1) Vector effect: As regards the incident angle of the vibration force flux into the force pickup surface, it can be estimated that the force level at 0 incidence is larger by 3 db than that at 45 incidence (cos 45 =1/ ã2). The average values of three force pickup outputs agree well with the above assumption, as shown in Fig. 3. The variance among the force values measured by each pickup reached about }2 db, as it is difficult to press uniformly three force pickups arranged in a line with the model hand. The frequency characteristics of the force levels observed depend upon the mechanical impedance of the model hand, as already explained in the previous paper5). The rising slope above 250 Hz is due to the foam rubber inserted between the force pickup and the vibration table. A. 2) Bypassing effect: The bypassing effect was examined under the four conditions shown in Fig. 4, a, and the results are shown in Fig. 4, b. (i) The force pickup was touched with the aluminum plate of the model hand via 103

6 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI Fig. 3. Variation of force transmission with incidence angle of force flux into the force pickup (0 and 45. Fig. 4. (a) Experimental conditions. (i) Four force pickups without bypassing. (ii) A layer of urethance foam and a ratio of cross-section of the force pickups to the urethane foam of 1 : 3. (iii) Two layers of urethane foam (1 : 3). (iv) Two layers of urethane foam (1 : 1.5). 104

7 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT Fig. 4. (b) Frequency characteristics of vibration force under each experimental condition. the urethane foam, so that bypassing of the vibration force did not occur (Fig. 4, a i). As the four pickups can be pushed uniformly by the model hand, the variance of force level on each pickup is within }1 db over the whole frequency range. The frequency characteristics up to 63 Hz also accord with the values derived from the mechanical impedance of the model hand (filled circles in Fig. 4, b) and those above 63 Hz are due to the spring constant of the urethane foam. (ii) One layer of urethane rubber was inserted between the aluminum plate of the model hand and the vibration table (Fig. 4, a, ii). The ratio of the total cross sections of the force pickups to the rubber was taken as 1: 3. The gain of the force decreased by about 1 db owing to bypassing of the vibration force to the hand without changing the frequency characteristics as compared with case i (open triangles in Fig. 4, b). The attenuation value of 1 db is determined by the elasticity of the rubber. (iii) The thickness of urethane rubber was varied to two layers without changing the cross-sectional ratio in case ii (Fig. 4, a, iii). The gain decreased by about 10 db over the whole frequency range, but the frequency characteristics were unchanged below 200 Hz (clear rectangles in Fig. 4, b). The attenuation level of about 10 db is also determined by apportionment of the force owing to the elasticity of the material. The gain, moreover, decreases sharply above 200 Hz because the other resonance system is brought into it. (iv) The cross-sectional ratio was changed to 1 : 1.5, but the other conditions were kept constant in case iii (Fig. 4, a, iv). The gain increased by about 6 db as compared with that in case iii (open circles in Fig. 4, b). regard to the cross section, the ratio of transmission If the spring constant is similar without force in case iii to that in case iv is 1/4/1/2.5=2.5/4 (=4 db). It is, therefore, considered that the spring constant changed 105

8 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI with the cross section even though the pushing force was kept constant. With one force pickup, the force level was about 140 db for 1 g driving (Fig. 1, b, in Part 55)), but with four pickups, the level of each was lower by about 10 db under the same conditions (theoretically 12 db). It is remarkable that the frequency characteristic curves all run parallel below 160 Hz, even though the experimental conditions were different. When bypassing occurred, the vibration force level observed clearly decreased. The phase relation between the vibration force and the vibration acceleration observed Fig. 4. (c) Phase difference for condition (i). Fig. 5. Frequency characteristics of vibration force on four pickups, eight pickups and eight pickups with bypassing of vibration force. In the last case, two layers of urethane foam were inserted and the ratio of the cross-section of the total of eight pickups to the urethane foam was 1 :

9 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT on the vibration table was measured when the acceleration was taken as the reference and the results are shown in Fig. 4, c. The results accord with the data derived from the mechanical impedance of the hand. When the number of force pickups was doubled to 8, the output gain of each pickup decreased by about 6 db (open circles with a dot) compared with the case of four pickups (filled circles) as shown in Fig. 5. of the cross sections of the pickup. The results can be confirmed by simple calculation In the case of insertion of two layers of urethane rubber (ratio of cross-sections, 1 : 1.5), the gain of each of the eight pickups is lower by about 7 or 8 db than that in the non-insertion case (open circles). B. 1) In the case of the human hand pressing eight force pickups placed on the Fig. 6. (a) Frequency characteristics of vibration force on the human palm on eight force pickups which were placed on the vibration table. Three subjects were tested the positions of pickups were the same as shown in Fig. 2(i). Fig. 6. (b) Frequency characteristics of vibration force on the human palm on eight force pickups which were placed on the vibration table. Three subjects were tested ; the positions of pickups were the same as shown in Fig. 2(i). 107

10 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI vibration table, the results for three subjects are shown in Figs. 6, a, b and c. The positions of the pickups were the same as indicated in Fig. 2, i. There are many minor resonances on each curve and the resonances readily change in frequency, because of the complexity of the contact conditions (the pressing procedure and differences of mechanical impedance of the human hand). To put the common features of the curves in perspective, they are classified into two groups. The larger level group is that where the force pickups were set on the positions of the center palm (0, 1 and 2), and is larger by more than 10 db than those on other positions. This means that the vibration force Fig. 6. (c) Frequency characteristics of vibration force on the human palm on eight force pickups which were placed on the vibration table. Three subjects were tested ; the positions of pickups were the same as shown in Fig. 2 (i). Fig. 6. (d) Phase between the force and the acceleration on the vibration table plotted against the frequency. 108

11 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT is transmitted mainly by the central palm instead of the other parts. In this case, bypassing of the vibration force occurs, because the height of the force pickup is low (4 mm), so that some parts of the palm directly touch the vibration table. The average phases between the vibration force and the vibration acceleration on the vibration table are plotted for the three subjects in Fig. 6, d. The curves can be classified into three groups. It is quite interesting that the upper curves are derived from the pickups set on the fingers, the middle curves are for the central part of the palm and the lower curves come from the positions near the hand joint. In order to avoid bypassing of the vibration force to the hand without passing through the force pickups, their height was increased. The curves at the positions 0, 1 and 2 show rather similar tendencies in the case of bypassing or non-bypassing. The data at the other positions, however, do not show any common tendency, though it can be said in the case of bypassing that the force levels at positions 3 to 7 are lower than those at 0, 1 and 2. Thus, representative results at position 2 are shown in Fig. 7. The average curves for three subjects were estimated by superposition of the individual graphs. The force level in this case at frequencies below 40 Hz is larger by about 10 db than that in the case including the bypassing effect, and the difference can be explained in terms of the bypassing force. higher frequency range. However, both curves show similar tendencies in the B. 2) In the case of gripping of the imitation handle set on the vibration table, the force level curves for three subjects are shown in Fig. 8, a, b and c. The curves, Fig. 7. Representative results of bypassing and non-bypassing effect with the force pickup at position 2 in the palm. In the case of non-bypassing, the height of the force pickup was increased by means of an aluminum block with the same diameter as the force pickup. These curves were obtained by averaging superposed curves for three subjects. 109

12 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI including the complex ones, can be classified into two groups, that is, the larger force level group and the lower level group by neglecting the minor resonances on each curve. These curves suggest a tendency for transmission of the vibration force to the human Fig. 8. (a) Frequency characteristics of the vibration force on the human palm with eight force pickups set on a handle imitating that of a hand-held vibrating tool. Three subjects were tested and the positions of the force pickups are shown in Fig. 8, a (i) and (ii). Fig. 8. (b) Frequency characteristics of the vibration force on the human palm with eight force pickups set on a handle imitating that of a hand-held vibrating tool. Three subjects were tested and the positions of the force pickups are shown in Fig. 8, a (i) and (ii). 110

13 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT hand. The positions of the central palm (0, 1 and 2) are important in this force transmission, while the force level is considerably lower on the other positions. The phase relations between the vibration force and acceleration on the palm of one subject (A) are shown in Fig. 8, d. The measurements at 3, 4 and 5 were omitted be- Fig. 8. (c) Frequency characteristics of the vibration force on the human palm with eight force pickups set on a handle imitating that of a hand-held vibrating tool. Three subjects were tested and the positions of the force pickups are shown in Fig. 8 a (i) and (ii). Fig. 8. (d) Phase between the vibration force and the acceleration on the imitation handle set on the vibration table plotted against the frequency. 111

14 T. MIWA, Y. YONEKAWA, K. KANADA AND K. KOBAYASHI cause the phase was unbelievably varied in wide range beyond 2ƒÎ as a result of contact condition. These curves show the clear difference from the position on the palm. As compared with the difference of the phases at the positions of 6 and 7 in Fig. 6, d (plane pushing) and in Fig. 8, d (hand gripping), the attention has to be paid to the fact that these two positions are out of phase mutually in both cases, because the direction of the vibration force is inverse. C) In the case of the three kinds of portable vibrating tools, the data on vibration force level (FL) (right and left hands) are listed in Table 1. As compared with the case of the force pickup with the large cross-section (Fig. 9, b, Part 55)), the force pickup in the present paper shows the difference of about 27 db. This fact means the difficulty of force measurement. The variation of the FL value due to the different positions of Table 1. Vibration force levels for the hand-held vibrating tools (averaged on three subjects and 8 positions). 112 Fig. 9. ( ) Positions on the palm where the vibration force was measured. Three strips of bakelite plate were used in applying the force pickup to the curved handle surfaces of the hand-held vibrating tools. (ii) Vibration force level at various positions on the palm (averaged on three subjects).

15 PROBLEMS CONCERNING VIBRATION FORCE MEASUREMENT the force pickups is shown in Fig. 9. The position (0, 1 and 2) gave higher FL values with the hammer and with the impact wrench there were no differences of FL values among the observed positions. Other positions, however, were sometimes found to be important in the field survey ; namely, on the finger of the right hand (6), the FL values with the grinder are as large as those at the central palm positions (0, 1 and 2). CONCLUSIONS 1) In the vibration force measurement, the angle between the incident force flux and vibration force sensing axis of the pickup has to be noticed. 2) The output of each force pickup decreases proportionally to the number of pickups used. The measured vibration force decreases proportionally to the extent of bypassing. 3) The vibration force level in the palm is determined by the mechanical impedance on it and by the closeness of contact in holding the tool. The vibration force levels measured at the positions 0, 1 and 2 were the largest. The data at any one of these three positions can be selected as representative for the palm, which usually shows the maximum value. 4) The vibration power can be calculated for the pure sinusoidal vibration on the vibration table. In the field survey, however, a new data processing method has to be developed for this purpose, because of the complex phase relationship of the force pickup. ACKNOWLEDGEMENTS The authors are indebted to Dr. H. Sakabe (President of the National Institute of Industrial Health) and Dr. S. Koshi (National Institute of Industrial Health) for suggesting this investigation. The authors also wish to express their gratitude to Dr. K. Yamazaki (Railway Labor Science Research Institute) and Dr. Y. Tokita (Kobayashi Institute of Physical Research) for their invaluable advice and encouragement. REFERENCES 1) Miwa, T. et al. (1979). Ind. Health, 17, 85. 2) Miwa, T. et al. (1979). Ind. Health, 17, ) Miwa, T. et al. (1979). Ind. Health, 17, ) Miwa, T. et al. (1979). Ind. Health, 17, ) Miwa, T. et al. (1980). Ind. Health, 18,