Exposure to forceful exertions and vibration in a foundry

Transcription

1 International Journal of Industrial Ergonomics 3 (22) Exposure to forceful exertions and vibration in a foundry Thomas J. Armstrong, Matthew M. Marshall*, Bernard J. Martin, James A. Foulke, D. Christian Grieshaber, Gwendolyn Malone University of Michigan, Center for Ergonomics, Ann Arbor, MI, USA Received 13 March 21; accepted 28 March 22 Abstract This paper describes the results of a study that investigated the exposure of workers in a foundry to vibration and related physical stresses. The primary objective of this research was to show that use of vibrating tools in the foundry involves exposure not only to vibration, but also to physical stresses both while the tool is running and while it is not running. Four types of tools were investigated: (1) small handheld grinders, (2) scaling hammers, (3) inline hammers, and (4) chipping hammers. The paper describes the characteristics of these tools with respect to physical work factors such as muscle load, upper extremity posture, hand repetition, and exposure to hand/arm vibration. The analysis is based on the use of instrumentation as well as the use of observational methods to quantify the physical work elements. With the exception of the grinders, all the tools investigated in this study produced vibration levels that limited their time of acceptable use, according to published guidelines. In addition to vibration exposure, workers experienced high peak muscle loading, medium to high levels of hand repetition, and extreme or awkward posture of the elbow and shoulder. All tools met or exceeded published recommendation limits for hand activity level. Relevance to industry The use of vibrating tools is common in many industrial settings. This paper describes how, in addition to the stress imposed by hand/arm vibration, workers who use these tools are exposed to other physical stresses such as hand repetition, forceful exertions, and extreme or awkward posture. r 22 Elsevier Science B.V. All rights reserved. Keywords: Force; Vibration; Electromyography; Repetition; Foundry 1. Introduction The term work-related musculoskeletal disorders, WMSDs, is widely used as a family name for disorders that involve common risk factors and *Corresponding author. Present address: Department of Industrial and Systems Engineering, Kate Gleason College of Engineering, Louise M. Slaughter Building, 81 Lomb Memorial Drive, , Rochester, NY, USA. Tel.: ; fax: address: mmmeie@rit.edu (M.M. Marshall). common intervention strategies associated with work (National Research Council, 1999, 21). WMSDs may involve bones, joints, muscles, tendons, nerves, or blood vessels. Specific examples of WMSDs include tendinitis, synovitis, epicondylitis, and carpal tunnel syndrome. Hand Arm Vibration syndrome or HAVS also is widely used to refer to these and other conditions when work history involves use of vibrating tools. While there are some conditions, e.g., vibration induced white finger (Bovenzi, 1998a, b) that /2/$ - see front matter r 22 Elsevier Science B.V. All rights reserved. PII: S (2)98-7

2 164 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) appear to be unique to vibration exposure, there may be some overlap between the use of these terms. The purpose of this paper is: (1) to show that use of vibrating tools involves exposure to not only vibration, but also ergonomic stresses while the tool is running and not running and (2) to provide information about the range of exposures associated with the use of pneumatic tools used for parts finishing in a foundry. Frequently cited work or ergonomic factors of WMSDs include: repeated and sustained exertions, forceful exertions, contact stresses, certain postures, low temperatures, vibration, and psychosocial stresses. A model describing the relationship between work activities, ergonomic stresses, and various physiological and biomechanical responses was provided by several investigators and is shown in Fig. 1. An important feature of this model is that it shows how multiple exposure factors interact with personal and environmental factors to produce a cascading series of events in which one response is an exposure factor for another response. There is not a one to one relationship between each exposure factor and each response. For example, tendon loads are related to the position of the hand and the intensity of hand force (Armstrong et al., 1982; Dennerlein et al., 1998). It has been shown that Fig. 1. Conceptual model of the relationship between external physical and social factors and internal responses. The model shows how multiple exposure factors interact with personal and environmental factors to produce a cascading series of events in which one response is the dose for the next and accounts for the progression of transient to long term responses (adapted from Armstrong et al., 1993). vibration impairs sensory function and stimulates muscles to exert more force than is necessary, which also contributes to tendon loads (Radwin et al., 1987; Martin et al., 1996). Vibration exposures are always accompanied by exposure to other physical stresses because vibration exposure occurs when a worker grips something that vibrates. Gripping, by definition, requires exertion of force and may be performed for large portions of the work shift. In addition, vibration exposure also may be accompanied by other factors (e.g., posture, contact stresses and low temperatures). Hand arm vibration disorders have been studied extensively and yet recommended exposure limits (ISO, 1986; ANSI, 1986; ACGIH Worldwide, 22) explicitly consider only vibration frequency, amplitude, and duration. It may be that in some cases limiting exposures as per the vibration standard is sufficient to also protect workers from over exposure to ergonomic stresses; however, in other cases there may be significant exposure to ergonomic stressors associated with other job activities. Even holding large or heavy tools that are inoperative may be a significant burden for some persons. Occupations in which pneumatic tools such as reciprocating hammers and handheld rotary grinders are often associated with significant hand arm vibration exposures and ergonomic stress. These jobs may also involve ergonomic stresses associated with other work elements such as holding the tool, assembly, inspection, and material handling activities. Application of existing vibration standards would not specifically consider these other stresses. Several investigators have recommended average force exposure limits for preventing fatigue. These limits are summarized in Fig. 2 and range from % to 15% maximum voluntary contraction (%MVC) for continuous exertions and from 15% to 21%MVC for intermittent exertions. Additionally, these jobs may be associated with high levels of hand activity. ACGIH Worldwide (22) has proposed a threshold limit value (TLV) for preventing WMSDs associated with monotask handwork that considers hand activity level and normalized peak force. Distinguishing between stresses associated with vibration and those imposed by the ergonomic

3 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Fig. 2. Recommended average workload limits for continuous and intermittent work. An example EMG signal obtained during this project for a subject using a chipping hammer has been overlaid for comparison. Source: Bystr.om and Kilbom (199), Bystr.om and Fransson-Hall (1994), Rhomert (1973), Bystr.om and Fransson-Hall (1994), Bj.orksten and Jonsson (1977) and Sjgaard et al. (1986). stresses requires assessing the overall pattern of exertion for a given job and then separating those that occur when the tool is running and producing vibration from those that occur when the tool is not running and is not producing vibration. The purpose of this study was to evaluate the exposure of workers in the Finishing Department of a foundry to ergonomic risk factors as well as hand arm vibration. This paper describes the characteristics of the tools and operations investigated with respect to their physical work factors, including forceful exertions, upper extremity posture, hand repetition, and exposure to hand/arm vibration. 2. Methods 2.1. Description of jobs and tools The jobs analyzed were selected to provide a representative sample of operations in the Finishing Department of a large automotive foundry in the Midwestern United States. As the result of the sand casting process, the castings are formed with excess material that needs to be trimmed. For example, fins are formed from the molten metal that seeps between the adjoining halves of the mold. Also excess material is formed from metal that gets into vent openings and voids in the mold. Workers use pneumatic power tools to remove this excess material from castings as the castings are conveyed down the assembly line. In some cases, castings are conveyed continuously as workers perform the necessary operations directly on the assembly line. In other cases, the castings are diverted to a workstation off the main line as workers perform the finishing operations. Workers at each station are assigned to finish a specific area of the casting with their respective tool. The tools used were classified into four categories: (1) grinders, (2) scaling hammers, (3) inline hammers, and (4) chipping hammers. Grinders are characterized by their high-frequency rotational cutting action while the three types of hammers are percussive tools that reciprocate at a lower frequency. Drawings of these tools and how they are handled are shown in Figs. 3 and 4. This classification of tools will be used as the framework for discussing the results of the study Grinders The abrasive wheel on the handheld grinders removes excess material from the casting. The grinders analyzed in this study were all activated using a bar trigger, as shown in Fig. 3a. No overhead balancers were used to support the weight of these tools.

4 166 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) (a) (b) (c) (d) Fig. 3. Schematic drawings of the tools used in this study: (a) grinder, (b) scaling hammer, (c) inline hammer, and (d) chipping hammer Scaling hammers Scaling hammers are handheld impact hammers used in the foundry. The tool is operated primarily such that its long-axis is in the horizontal plane. To activate the tool, the worker pushes against the end of the tool once the bit is engaged with the casting. The tools typically are equipped with a T handle for the worker to push against, but in some cases the T handle is not used and the worker pushes against the elbow connector of the airline. These hammers are used to remove material from smaller castings and from tight spaces. No overhead balancers were used to support the weight of these tools Inline hammers These hammers are used to do the same types of operations as the scaling hammers and are activated using a bar trigger that is inline with the long axis of the tool. The tool is operated primarily in the vertical plane. No overhead balancers were used to support the weight of these tools Chipping hammers Chipping hammers refer to the large impact hammers used to remove material from the castings. The function of these tools depends on the type of bit used. While most bits are used to knock off large fins from the casting and have a Fig. 4. Illustration of hand placement and forces acting on tools. F R represent the reaction forces on the tool, F W represents the weight of the tool, and F TB represents the supporting force supplied by the tool balancer. The shaded ellipses depict the approximate location of the hand(s) in using the tools. The grinder (a) is the only tool held with one hand and is activated by squeezing its bar trigger. The handle of the scaling hammer (b) is held with the dominant hand, but is operated by pushing on the end of the tool. The opposite hand supports and directs the tool. The inline hammer (c) is gripped with both hands and activated by squeezing its bar trigger with the right hand. The handle of the chipping hammer (d) is held and activated with the dominant hand while the operator supports and directs the main shaft of the tool with the opposite hand. Only the chipping hammer is used in combination with a tool balancer. flat, squarish bit, others are used to bore out holes. These hammers were the heaviest of all the tools and were used in combination with a variety of overhead tool balancers to support the weight of the tools Data collection and analysis The data collection at the foundry took place over a period of 4 days using six subjects, all of whom had volunteered to participate. Three subjects were female and three were male, and all were right handed. Because of variability in worker experience and time limitations, it was

5 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) not possible to balance subjects, tools, and jobs, so a statistical analysis of the data collected was not performed. Thus, the results of this study should be interpreted with caution and should not be used to make broad conclusions about the specific ergonomic risk of the tools and processes analyzed. Data collection consisted of three methods: electromyography of the forearm muscles to measure the forceful demands of the jobs, vibration analysis to assess the vibration levels of the tools, and observational analysis to assess all ergonomic exposures. Each of these is described in more detail below Electromyography A method described by Armstrong et al. (1982), Jonsson (1988), and Martin et al. (1996) was used to assess muscle activity as workers performed their jobs. EMG electrodes were placed on the participants forearms in the vicinity of the flexor digitorum superficialis (FDS) and extensor carpi radialis (ECR) muscles. EMG signals were normalized with respect to the EMG elicited during participants MVC. The relationship between force and EMG is not always linear and may be affected by the movement speed. Therefore EMG is, at best, an estimate of force. EMG data were collected using a computer and LabVIEW software. Data were collected at a sampling rate of 1 Hz while workers performed the study jobs. EMG data were summarized using amplitude probability distribution (APD) calculations, as specified by Jonsson (1988). As a practical matter, because of the stochastic nature of EMG, the 95th percentile was treated as peak to exclude spurious localized maximums. The EMG data were examined for both tool running and tool not running conditions to examine the effect of tool operation on the EMG/force profile Vibration measurement A Bruel & Kjaer triaxial accelerometer (Type 4321) was used to measure the vibration level at the hand for each tool. Vibration measurements were obtained in both the field and the laboratory. Data from the field were sampled at a rate of only 1 Hz and were primarily used to determine durations of when the tool was in use or not in use. A detailed analysis of the vibration levels was performed in the lab using the actual tools and castings from the foundry. Calibration of the accelerometer was recorded before and after acquisition of the data. Depending on the size and shape of the handle, the accelerometer was mounted to it with either a metal hose clamp or nylon cable ties, but in a position as close to the hand as possible without interfering with how the tool was handled by the worker. As specified by ISO (1986), the data were recorded using highspeed acquisition software written in LabVIEW. In the laboratory, the vibration signal was sampled at 1 khz over a 1-s cycle of continuous tool use. Two low-pass filters were used to filter out frequencies above 5 khz. A spectral analysis was performed on the vibration data collected in the lab, from which a daily exposure rating, Að8Þ; was calculated for each type of tool as specified by ISO (1986). The Að8Þ rating is calculated using the following equation: rffiffiffiffiffi T i Að8Þ ¼a w ; T o where a w is the frequency-weighted (ISO) acceleration level (from Hz) for the axis having the highest vibration level; T i is the cumulative exposure time as measured in the field; and T o is the reference duration of 28,8 s (8 h). Root mean square (RMS) vibration data acquired in the field were used to determine when the tool was running to calculate T i in the equation above and to stratify the EMG data by tool status. The daily vibration exposure rating was then compared to the daily exposure limits using ACGIH threshold limit values (TLVs) published in their guidelines Observational analysis Observational analysis was used to assess the peak and average ergonomics stressors (excluding vibration) of the jobs such as upper extremity force, posture, contact stress and hand repetition using the methodology described by Latko et al. (1997). A series of ten-point scales was used for rating hand/wrist repetition, hand force, upper

6 168 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) body joint postures, hand, wrist and forearm contact stresses. The scales are designed such that corresponds with the lowest possible exposure and 1 corresponds to the greatest exposure possible. Studies by Latko et al. (1999) show that repetition ratings are highly correlated with the prevalence of tendon and nerve symptoms in the upper limb. While ratings are subject to inter-rater variability, they provide a structure for assessing the relative contribution of all ergonomic stressors present in a given job. Jobs were rated independently by a team of 3 5 people, after which the ratings were discussed to reach consensus, defined as 7.5 points on the ten-point scale. 3. Results First, general results about exertion patterns and vibration exposure are presented, followed by a more detailed discussion of the individual tools. Tables 1 4 summarize the data obtained in the field and in the laboratory for all the tools Electromyography General results For the entire work cycle (tool in use and not in use), time-weighted average EMG activity ranged from 8 16% of maximum (%MVC) for the flexor muscles and 6 21%MVC for the extensor muscles (Tables 1 and 2). In all the jobs studied, the average right hand EMG activity exceeded that of the left hand (by an average of 52%), consistent with the fact that all the subjects were right handed and, therefore, operated the tools accordingly. Thus, the analysis will primarily focus on EMG from the right hand, as it represents the highest level of muscle activity. For the tools we studied, only the average EMG activity from the wrist extensors (Table 2) among workers using the inline chipping hammers (21%MVC) met or exceeded the guidelines for intermittent work specified in Fig. 2. However, 95th percentile values were very high for all of these tools. Use of the inline hammers was associated with the highest muscle load for both the FDS and ECR (53% and 66%MVC, respectively). The chipping hammers were associated with the next highest (44% and 51%MVC) followed by the scaling hammers (28% and 37%MVC), then the grinders (22% and 25%MVC). For all the tools and each of the two muscles analyzed, the average EMG activity when the tool was running and vibrating was higher than when the tool was not running. For the grinder and scaling hammer, this difference was very small (range: 4 9%MVC). For the inline hammer and chipping hammer, however, this difference was much larger (range: 16 2%MVC). Table 1 Means and standard deviations for flexor digitorum superficialis EMG Tool No. of jobs Hand Finger flexor EMG (%MVC) Avg.: tool not in use Avg.: entire cycle Avg.: tool in use 95th percentile tool in use Grinder 4 R L Scaling hammer 6 R L Inline hammer 6 R L Chipping hammer 5 R L Pooled 21 R L

7 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Table 2 Means and standard deviations for extensor carpi radialis EMG Tool No. of jobs Hand Wrist extensor EMG (%MVC) Avg.: tool not in use Avg.: entire cycle Avg.: tool in use 95th percentile tool in use Grinder 4 R L Scaling hammer 6 R L Inline hammer 6 R L Chipping hammer 5 R L Pooled 21 R L Table 3 Mean and standard deviation tool utilization rates and vibration ratings Tool % time tool in use Að8Þ rating (m/s 2 ) Grinder Scaling hammer Inline hammer Chipping hammer Probability density analysis Fig. 5 shows sample APD plots of right hand FDS EMG activity for each of the four types of tools. The curves have been separated to show the distribution of muscle activity for when the tool is in use and when the tool is not in use. The extent to which the curve stretches to the right reflects the magnitude of the peak EMG and the exertion intensity. The vertical extent of the APD curve corresponds with the cumulative duration of exertion at a given level of force. As shown by the EMG data in Table 1, the inline and chipping hammers were associated with the highest 95th percentile FDS EMG levels (44% and 53%MVC, respectively), which is the reason the APD curve of the inline and chipping hammers stretches the farthest to the right on the %MVC axis. As would be expected, EMG activity increases when the tool is activated versus when it is idle. The inline and chipping hammers had the largest increase in finger flexor EMG from when the tool is not in use to when it is in use, increasing from 9% to 27%MVC and from 9% to 25%MVC, respectively. This is seen in Fig. 5 by the distinct bimodal pattern of the APD curves. For the grinder and to some extent, the scaling hammer, there is not as distinct a difference between when the tool is in use and when it is not in use, so the bimodal pattern is not as prevalent. Thus, the incremental stress when the grinding and scaling hammers are running would be due almost entirely to vibration. The incremental stress when the chipping and inline hammers are running is due both to muscle load and vibration. Further discussion of EMG results for each type of tool follows Vibration analysis Table 3 summarizes the mean daily vibration exposure, Að8Þ; vibration ratings calculated for these tools. Tool utilization values, which are needed to compute Að8Þ ratings, were obtained from accelerometer readings collected in the field over the four day data collection period. The grinders produced the lowest level (3.3 m/s 2 ) and the inline hammers produced the highest (11.8 m/s 2 ). ACGIH Worldwide (22) has published TLVs for exposure to hand/arm vibration. The grinder was the only tool with a mean rating that did not exceed the TLV for exposure of the hand to vibration specified by ACGIH (maximum TLV

8 17 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Table 4 Mean (7standard deviation) ergonomic ratings for the four types of tools analyzed Hand activity Contact stress Posture Forearm Elbow Shoulder Neck Wrist rad/ uln Wrist flex/ ext Wrist/ palm Peak/avg. Repetition Force Fingers/ hand Tool No. of jobs Grinder 4 P A Scaling 6 P Hammer A Inline 6 P Hammer A Chipping 5 P Hammer A of 4 m/s 2 ). For the chipping hammer, the vibration level (7.2 m/s 2 ) is acceptable for use between 1 and 2 h (maximum TLV of 8 m/s 2 ). The vibration levels for the scaling hammer (8.5 m/s 2 ) and inline hammer (11.8 m/s 2 ) are acceptable only for use during a time period less than 1 h (maximum TLV of 12 m/s 2 ). For the chipping, scaling and inline hammers, typical durations of tool use for the foundry workers we studied significantly exceeded the recommended daily exposure Grinder EMG With one exception, the grinders studied were operated exclusively with the right hand (Fig. 4). The left hand was used primarily to manipulate and orient castings moving down the line, so overall average activity from the left hand for both muscles was less than 1%MVC (Tables 1 and 2). EMG activity was slightly higher in the right hand, with overall average FDS and ECR activity of 14% and 13%MVC, respectively, when the tool was in use. Ninety-fifth percentile EMG activity did not exceed 25%MVC for either muscle. A sample plot of the vibration and the simultaneous finger flexor muscle activity of the grinder is shown in Fig. 6. The EMG activity closely matches the vibration profile of the grinder. The operator squeezes the trigger while holding the tool as well as when grinding, as indicated by the sustained EMG activity. When the trigger is released, EMG drops off to a level produced only in holding the tool Ergonomic ratings Despite relatively low force requirements, the grinders were associated with medium-high levels of repetition (average 7/1), as listed in Table 4. This is consistent with the duration of tool usage (Table 3 and Fig. 5a) in that the grinder was in use about 5% of the time. Even when the tool was not grinding, the workers had to continually rotate their forearm and flex/extend their wrist to position the wheel of the grinder with the work surface of the castings. Besides adding to the overall hand activity level, this positioning of the grinder required medium to high (average 5/1)

9 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Amplitude Probability Distribution Curves Probability Probability.16 Grinder (a) Tool Off Tool On Total RFF Tool Off RFF Tool On RFF Total Tool Off: 8 3 Tool On: 14 2 Total: 11 2 Inline Hammer Tool Off: 9 2 Tool On: 27 9 Total: 16 5 Scaling Hammer Tool Off: 9 2 Tool On: 15 4 Total: 11 3 (b) Chipping Hammer Tool Off: 9 2 Tool On: 25 1 Total: %MVC (c) %MVC (d) Fig. 5. Sample amplitude probability curves of right hand FDS EMG activity for each of the tool groups: (a) grinder, (b) scaling hammer, (c) inline hammer, and (d) chipping hammer. Each plot is a representative sample from one subject. The means and standard deviations listed have been pooled from all the subjects/tools analyzed. Table 1 contains the summary statistics pertaining to each tool. levels of forearm pronation and supination as well as wrist flexion and extension (average 5/1) to orient the wheel with the surfaces to be grinded. This was particularly true for crankshaft castings, which required grinding surfaces that were not easily accessible Scaling hammers EMG To use the tool, the operator must push on the handle when the bit is engaged with the casting (Fig. 4b). While the operator must still grasp the handle, the push force to activate the tool comes primarily from muscles in the upper arm and shoulder. Squeezing the handle is not required to activate the tool, so finger flexor activity was relatively low. The right and left hands averaged 15% and 13%MVC when the tool was in use. For the ECR, these values were 17% and 16%MVC. Unlike the grinders, the operator uses his or her left hand to hold and direct the tool, accounting for slightly elevated EMG values. The 95th percentile FDS EMG was 28%MVC and 29%MVC for the right and left hands, respectively. Ninety-fifth percentile ECR values were somewhat higher at 37% and 38%MVC for the right and left hand. A sample plot of the vibration and the simultaneous finger flexor muscle activity of the scaling hammer is shown in Fig. 7. Unlike the grinder, the vibration and EMG patterns do not closely correspond with one another. The force

10 172 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Vibration and EMG - 4 -Inch Grinder 1 m/s %MVC % 1 5% 5% Seconds Fig. 6. Representative plot of EMG activity of right hand FDS (%MVC) and vibration associated with using 4-in grinder. Percentiles drawn on graph have been calculated with respect to the entire cycle. Vibration and EMG _ Scaling Hammer 1 m/s 2 1 % MVC Seconds Fig. 7. Representative plot of EMG activity of right hand finger flexors (%MVC) and vibration associated with using scaling hammer. Percentiles drawn on graph have been calculated with respect to the entire cycle. 95% 5% 5% used to activate the tool is not provided entirely by the finger flexors for this tool, so this pattern is not surprising Ergonomic ratings The scaling hammers were predominately used at jobs with medium-high levels of repetition (average 7/1). The scaling hammers were also associated with moderate to extreme wrist radial and ulnar deviation (average 5.5/1) as well as moderate to extreme elbow postures (average 6/ 1). The awkward elbow postures are partly the result of the length of the tool bit, as shown in Fig. 1. Though the locations of the castings were

11 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) generally at about waist height, the length of the tool bit forced workers hands to be located at approximately chest height while they operated the tool. This posture typically required high levels of elbow flexion Inline hammers EMG To use the inline hammers, the operator grips the shaft of the tool with the right hand and squeezes the bar trigger to activate the tool (see Fig. 4). The operator also grips the tool near the bit with the left hand to support and direct the tool. The average right FDS EMG values were higher than the left, particularly when the tool was in use and the operator had to both squeeze the bar trigger and apply downward force with the tool to remove material (27% and 15%MVC). Ninety-fifth percentile FDS EMG for the right and left hands were 53% and 29%MVC, respectively. The same pattern was found for the ECR. EMG activity averaged 31% and 16%MVC for the right and left hand when the tool was in use. Ninetyfifth percentile values were 66% and 31%MVC, respectively. The higher EMG values observed for the inline hammers may also be the result of the fact that they were used without any mechanical assist such as a tool balancer. The operators had to support the entire weight of the heavy tool themselves. A sample plot of the vibration and the simultaneous finger flexor muscle activity of the inline hammer is shown in Fig. 8. Like the grinder, the EMG activity closely matches the vibration profile of the hammer. The operator squeezes the trigger throughout the duration that the tool is used, as indicated by the sustained EMG activity. When the trigger is released, EMG drops off to a level produced only in holding the tool Ergonomic ratings The inline hammers were associated with medium to high repetition levels (average 5/1) and extreme elbow postures (average 6.5/1). The operators elbows approached full flexion while using the tool. Again, the excessive length of the chisel bit contributed to the elbow flexion observed. Though the locations of the castings were generally at about waist height, the length of the tool bit forced workers hands to be located at approximately chest height while they operated the tool. This posture typically required high levels of elbow flexion. Vibration and EMG _ Inline Hammer 1 % MVC m/s Seconds 95% 5% 5% Fig. 8. Representative plot of EMG activity of right hand FDS (%MVC) and vibration associated with using an inline hammer. Percentiles drawn on graph have been calculated with respect to the entire cycle.

12 174 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Chipping hammers EMG To use the chipping hammer, the operator holds the tool by the handle with the right hand and lightly grips the tool bit (or near the bit) with the left hand. Like the inline hammers, the right hand EMG was higher than the left, averaging 25% and 2%MVC when the tool was in use. Ninety-fifth percentile values were 44% and 33%MVC, respectively. The same pattern was found for the EMG activity of the ECR, averaging 26% and 17%MVC for the right and left hand, respectively, when the tool was in use. Ninety-fifth percentile values were 51% and 3%MVC. A sample plot of the vibration and the simultaneous finger flexor muscle activity of the chipping hammer is shown in Fig. 9. Like the grinder (Fig. 6) and the inline hammer (Fig. 8), the EMG activity closely matches the vibration profile of the hammer. The operator squeezes the trigger throughout the duration that the tool is used, as indicated by the sustained EMG activity. When the trigger is released, EMG drops off to a level produced only in holding the tool (9%MVC) Ergonomic ratings Similar to the other tools, the chipping hammers were associated with medium levels of repetition (average 5/1). High ratings were also given with respect to elbow and shoulder postures (average 6/ 1 and 7/1, respectively), as the length of the tool and the tool bit forced the operators to work in awkward postures, as shown in Fig. 1. Figs. 11 and 12 illustrate how orientation of the tool affects the location of the tool handle and, as a result, the worker s upper extremity posture. As the tool moves from a horizontal orientation to one that is more vertical, the vertical location of the hand grasping the handle increases approximately 75 cm. This requires the worker to fully flex their elbow and/or flex their shoulder to grasp the handle and the trigger at its most vertical position (Fig. 12). Peak shoulder posture was highest for the chipping hammers due to the fact that both the tool and tool bit were the longest of all the tools we observed. 4. Discussion The purpose of this study was to evaluate the exposure of workers in the Finishing Department of a foundry to ergonomic risk factors as well as their exposure to hand/arm vibration. We observed a high association between vibration levels and exposure to forceful exertions. The inline hammers produced the largest vibration levels Vibration and EMG _ Chipping Hammer 1 m/s 2 1 % MVC Seconds Fig. 9. Representative plot of EMG activity of right hand finger flexors (%MVC) and vibration associated with using chipping hammer. Percentiles drawn on graph have been calculated with respect to the entire cycle. 95% 5% 5%

13 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) Fig. 1. Two examples of how the length of the tool bit required the operator to work in extreme shoulder or elbow postures. The picture on the left is of a chipping hammer and the picture on the right is of a scaling hammer. 75cm 53cm Fig. 11. Illustration of how vertical location of the handle of the chipping hammer increases as the orientation of the tool changes from a horizontal orientation (far left) to vertical (far right). While the vertical orientation requires the worker to use awkward elbow and shoulder postures, the weight of the tool contributes to the force needed for material removal. (Að8Þ rating of 11.8 m/s 2 ) as well as the highest muscle load (FDS average and 95th percentile of 16%MVC and 53%MVC, respectively). Conversely, the grinders had the lowest vibration levels (Að8Þ rating of 3.3 m/s 2 ) and smallest muscle load (FDS average and 95th percentile of 11%MVC and 22%MVC). Rotary tools such as the grinders are inherently smoother than the low-frequency reciprocating hammers. The grinders produce their modal operating frequencies for the higher frequency octaves (>1 Hz), which are heavily discounted by the ISO guidelines in computing the Að8Þ rating. For example, the weighting factor for the 1 Hz octave band is.16, decreasing to.125 for the 125 Hz band. While isolation devices may help reduce the low frequency components of vibration for the reciprocating tools, it is doubtful that the levels can be controlled below what are considered acceptable exposure limits. Ideally, the quality of casting operations such as these could be controlled to the extent that the finishing operations would not be necessary. In the absence of such quality control, rotary tools such as the handheld grinders appear to expose the worker to less detrimental vibration levels in comparison to the alternative reciprocating tools. For all the tools and each of the two muscles analyzed, the average muscle load when the tool was running was higher than when the tool was not running (Figs. 1 and 2). For the grinder and scaling hammer, this difference was very small (range: 4 9%MVC). For the inline hammer and chipping hammer, however, this difference was much larger (range: 16 2%MVC). The increase in muscle load may partially be explained by what is understood about the relationship between vibration and the force that a muscle produces when it is exposed to vibration. The tonic vibration reflex causes an involuntary increase in contraction of the muscles exposed to vibration. Radwin et al. (1987) found that EMG increased

14 176 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) (e) (a) (c) (f) (b) (d) (g) 3cm Fig. 12. Approximated postures required to hold chipping hammer for average link lengths of a tall male (187 cm) and short female (153 cm). (Top) Horizontal orientation of hammer for male (a) and female (b). (Middle) Hammer held at 451 from horizontal. (Bottom) For the vertical orientation of hammer, tall male workers (e and f) must hold the hammer with a fully flexed elbow and may also flex their shoulder (f). Smaller female workers (g) must work with a flexed shoulder and have very little leverage. 12% for forearm flexor muscles and 32% for forearm extensors in a forearm gripping a handle with vibration compared with that obtained without vibration. Furthermore, the effect of vibration on the EMG was largest for low vibration frequency levels (2 4 Hz), which is approximately the modal operating frequency of the inline and chipping hammers in this study, which are the tools that produced the largest increase in EMG from when the tool is not running to when it is running. It has also been shown that vibration reduces tactility and that tactility affects the amount of force exerted to hold or manipulate a given object (Westling and Johansson, 1984). These two factors combine to cause an increase in muscle load when exposed to vibration. The grinder, inline hammer, and chipping hammer all require the operator to firmly grip the tools handle (chipping) or shaft (grinder and inline) to squeeze a trigger and activate the tool, which facilitates the transfer of vibration from the tool to the forearm muscles. Since the grinder produces significantly lower vibration than the chipping and inline hammers for octave band frequency ranges from 6.3 to 63 Hz, the effects of vibration on the grip force (and therefore muscle activity) are less profound. Conversely, the higher vibration levels of the chipping and inline hammers may cause a significant involuntary increase in the force exerted by the worker on the tool. In the case of the scaling hammer, the worker is not required to tightly grip the tool s handle since it is activated by pushing and not gripping. The worker only needs to lightly grip the handle to hold the tool, as opposed to squeezing a trigger. Thus, even though the scaling hammers produced the second highest vibration level of all the tools we studied (Að8Þ rating of 8.5 m/s 2 ), the hand is not as tightly coupled to the vibrating tool, which would explain why little increase in muscle load was observed when the tool was in use. When the tools were not running, the muscle load was much lower in comparison to when the tools were running, but the EMG values were still greater than zero. Average levels of 9%MVC (Table 1) and 11%MVC (Table 2) were observed for the flexors and extensors respectively, values

15 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) that are sufficiently high to induce muscular fatigue according to the recommendations for continuous work provided in Fig. 2. What can be concluded from this is that simply holding and supporting the weight of the tool requires a significant amount of muscle load. The use of mechanical assists such as tool balancers would provide some support for those tools that are not already used with such a device (i.e., grinder, scaling hammer, and inline hammer). However, it is worth noting that the position and tension of the tool balancer are very important. Even with overhead tool balancers, the heavy chipping hammers exhibited an average tool-off EMG level of 9%MVC for the finger flexors. This tool balance did not support the entire weight of the chipping hammer. Most of the tool balances used were simple springs from which the tool was suspended. More sophisticated devices may be used that enable the tension of the balancer to be adjusted to better support the weight of the tool. Though the inline hammers were the only tools whose use produced average EMG values that exceeded recommended limits summarized in Fig. 2, the 95th percentile levels were high for all the tools and these forces were combined with other stressors such as extreme or awkward upper extremity vibration and exposure to hand/arm vibration. Simultaneous measurement of EMG and vibration is useful in understanding, qualitatively, where peak muscle activity occurs relative to the work cycle of the tool. Figs. 6, 8 and 9 illustrate for the grinder, inline hammer, and chipping hammers how elevated EMG levels of the finger flexor muscles occur at the same time as the tool is activated and vibrating. For the scaling hammers (Fig. 7), the finger flexor EMG and vibration levels do not correspond as well with one another, primarily due to the fact that the tool is push-activated and finger flexor contraction is not necessary to activate the tool. Only two muscles were monitored in this study. Many other muscles of the upper extremity are involved in using these tools. The upper arm and shoulder, for example, are used with all the tools to engage the tool bit with the casting. This was particularly true for the scaling hammer, which required a push force just to activate the tool. Indeed, the energy needed to remove the excess metal comes not only from the vibrating tool, but also from the worker applying manual force to the casting with the tool. A comprehensive analysis of muscle load would include monitoring the EMG of upper arm, shoulder, and back muscles, including the biceps, deltoids, and trapezius. Besides forceful exertions, the jobs on which these pneumatic tools were used were associated with medium to high levels of repetition (average ratings 5 7/1). The tools were used to remove material from castings that move down the conveyor. While some of these operations allow the worker to work offline as castings arrive to their station, this is most often not the case and these tasks are largely machine-paced. While machine-paced jobs do not automatically mean that the musculoskeletal system is exposed to high levels of repetition, these jobs required steady motion with only occasional breaks to keep pace with the line, resulting in medium to high repetition ratings according to the rating system developed by Latko et al. (1997). Irrespective of vibration exposure, these hand activity levels meet or exceed levels that have been associated with elevated incidents of cumulative trauma disorders according to the ACGIH Worldwide (22) TLV. With the exception of the grinder (peak muscle load of 22%MVC and average HAL rating of 7/1), which exceeded only the Action Limit, all the tools exceeded the TLV for hand activity. While some of the hand activity for these jobs is used to manipulate and position the tools when they are not engaged with the casting and vibrating, 37 53% (Table 3) of the hand activity takes place simultaneously with exposure to hand/ arm vibration, imposing additional physical stress on the workers. The jobs we studied also placed postural stress on the workers. In particular, workers were observed using extreme or awkward elbow and shoulder postures for jobs that involved use of the reciprocating tools (scaling, inline, and chipping hammers). For these jobs, peak elbow postures averaged 6/1 (see Table 4). For chipping hammer jobs, the shoulder posture averaged 7/1. While some of this is attributable to the layout of the workstations and the work itself (i.e., conveyor

16 178 T.J. Armstrong et al. / International Journal of Industrial Ergonomics 3 (22) heights, casting heights, etc.), at least some of the blame should be placed on the tool. On multiple occasions workers were observed maintaining extreme shoulder or elbow postures because the length of the tool bit was so long that the worker had to grip the handle of the tool at a location much higher off the ground than would be necessary with a shorter tool bit. Two examples of this are pictured in Fig. 1. Much like the repetition and the forceful exertions required of these jobs, the postural stress is very closely linked with exposure to hand/arm vibration of the tool. When the worker assumes these awkward postures it is while he or she has the vibrating tool engaged with the casting. While a horizontal orientation of the chipping and scaling hammers would have some effect in minimizing the awkward elbow and shoulder posture, the tool s weight would no longer contribute to the force component necessary to remove material from the casting, as it does with a more vertical orientation (Fig. 11). With a horizontal orientation of the tool, the worker must provide all the push force to remove the material. Thus there exists a tradeoff between the worker s posture and the force he or she must produce to perform the finishing operation. Incorporating adjustability into the workstations may alleviate some of the postural stress experienced in using these tools. 5. Conclusions The primary objective of this research was to demonstrate that use of vibrating tools involves exposure to not only vibration, but also to ergonomic stresses while the tool is running and not running. All of the reciprocating tools investigated in this study produced vibration levels that exceeded guidelines of acceptable use, according to the ISO and ACGIH published guidelines. In addition to the vibration exposure, workers at the foundry experienced high peak muscle loading, medium to high levels of hand repetition, and extreme or awkward posture of the elbow and shoulder. All tools were associated with jobs that met or exceeded ACGIH recommendations for repetitive handwork. Since the study was conducted in the field using volunteers from the workforce, it was limited by the small number of subjects run and the lack of balance in experimental design between subjects and tools. One subject had not used one of the tools prior to study. Consequently, these results should be interpreted with caution and should not be used to draw specific conclusions about the ergonomic risk imposed on the worker for any one risk factor (e.g., vibration exposure or muscle load). Nevertheless, the results provide valuable insight into the range of physical exposures experienced by workers in the finishing department of the foundry. The results of this analysis emphasize the need to be comprehensive in evaluating risk factors associated with a job. For instance, even though the grinders had relatively low levels of vibration and muscle load, workers still had to maintain a high level of hand repetition and had to use awkward or extreme elbow and shoulder postures to use the tool. Simply using recommended vibration exposure limits might not take into account exposure to these other ergonomic risk factors. Finally, this study may help manufacturers of these types of tools to better understand the exposure patterns that workers who use the tools face. Acknowledgements The work described in this article was supported by a major automotive manufacturer and its associate bargaining unit. References American National Standard Institute, Guide for the measurement and evaluation of human exposure to vibration transmitted to the hand. American Institute of Physics for the Acoustical Society of America, ANSI S ACGIH Worldwide, 22. TLVs and BEIs threshold limit values for chemical substances and physical agents biological exposure indices. ACGIH Worldwide ISBN: Armstrong, T., Foulke, J., Joseph, B., Goldstein, S., Investigation of cumulative trauma disorders in a poultry processing plant. American Industrial Hygiene Association, Journal 43 (2),

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