3D SSPP Version 6. ANALYSIS & USE GUIDE For Reactive & Proactive Use

Transcription

1 3D SSPP Version 6 ANALYSIS & USE GUIDE For Reactive & Proactive Use

2 REQUIREMENTS The user must complete the UAW-GM 3D SSPP training course offered through the UAW- GM Center for Human Resources for the Joint Ergonomics Technician Teams (JETT). These courses cover the basics of program features and options, data measurement and input and retrieving results. It is assumed here that the user has an understanding of these basics. The information given here complements and extends the information given in the training courses. INTRODUCTION The 3D SSPP software allows the user to statically model tasks with a human figure using limited female and male anthropometry, descriptions of posture and the loading on the hands. The program estimates the compressive forces on the low back and the required moments around the joints based upon posture, anthropometry and external loads. The program compares the computed moments at the wrist, elbow, shoulder, torso, hip, knee and ankle joints to predicted mean strengths at each of the joints. Strength as expressed by the program is the ability to resist or generate a moment about a joint. The strength prediction equations used by the program are based upon sex (male or female) and joint position and are independent of anthropometry and body weight. The instructions given here provide the user with the following: 1. A consistent basis or starting point for conducting either a reactive (existing job) or proactive (future job) analysis of strength demands. 2. A strategy for interpreting the strength capability/percent of population capable output of the program s Analysis Summary. 3. A recommended method for including the effects of repetition or frequency when interpreting the program results. The overall goal of this guide is to provide users of the 3D SSPP with consistent strategies for conducting an analysis and interpreting the results. The strategies and approaches described here reflect the best current scientific understanding of the program and the experience of the GM users. Users are advised to consult the 3D course manual provided with the UAW-GM 3D SSPP training course (relevant sections are cited in this guide). Users should also consult the 3DSSPP User s Manual published by the University of Michigan Center for Ergonomics and provided with the UAW-GM 3DSSPP training course. The book Occupational Biomechanics (third edition) by Chaffin, Andersson and Martin (NY: John Wiley & Sons, 1999) is the primary reference on the biomechanical theory and research background of the 3D SSPP. 1

3 SELECTING A BASIS FOR CONDUCTING AN ANALYSIS Selecting Anthropometry The program allows a limited selection of anthropometric features based upon matched percentiles of height and weight or direct input of height and weight. The result is an idealized composite of a human form in size, proportion and weight. The user must understand that even if specific height and weight data are entered for a particular individual, the resulting model is still idealized in most physical aspects and predicted strength will reflect only population means. Refer to the 3D-course manual for basic information on selecting and entering model anthropometry. The UAW-GM agreement is to use the anthropometry of a 95 th percentile female using data entry. This benchmark anthropometry is the basis for an analysis of strength and as a limiting factor in drawing conclusions about strength capabilities. Select the 95 th percentile female anthropometry and enter a body weight of 200 lb and a height of 68 (5 ft 8 inches) as the basis for an analysis. The 95 th percentile female anthropometry provides relatively long moment arms and heavier segment weights combined with the limiting female mean strengths. As such, it provides a practical worst case for most analyses (exceptions noted below) and the best basis for making judgments about the acceptability of a job for the worker population overall. Strength Percent Capable Limits Given the 95 th percentile female anthropometry as the basis for the analysis, use 75 percent capable as the minimum for strength acceptability on Percent of Population Capable screen on the Analysis Summary report. If the percent of population capable is 75 or above for the wrist, elbow, shoulder and torso, then assume that the force demands will be within the strength capability of the majority of the work force. If the percent of population capable is below 75, then suspect that the force demands may be excessive for some workers. Given female anthropometry, percent of population capable values under 75 will fall into the yellow zone on the Analysis Summary sheet (see below) and requires intervention as deemed by the UAW-GM agreement. Example of Analysis Summary Screen (Version 6.04) Note: The results for the lower extremities (ankles, knees, and hips) will not be considered in most analyses. 2

4 Other Program Features (Version 6.04 and above) and Use 1. Wrist moments Wrist posture (horizontal and vertical angles per the 3D conventions) may be adjusted to obtain a realistic hand posture as part of an analysis of a job that requires 3D analysis. Do not use 3D to analyze repetitive hand motion activities. Repetitive hand activities should be analyzed using the RFC Upper Extremity section including the HAL/TLV. 2. Lower extremity manipulation and interpretation 3D version 6.04 and above provide for horizontal angles on the lower extremity which can improve the posturing of the manikin. However, the lower extremity horizontal angles are not posture predicted and any adjustment will have to be by the analyst. Also, only flexion and extension moments are produced for the ankle, knee and hip. For these reasons the percent capable strength indications for the hips, knee and ankle should be ignored. 3. Animation Version 6.04 and above provide an animation capability. This capability allows for multiple postures to be stored in one 3D task file and can animate motion between postures using frame by frame interpolation. While the animation feature does allow the analysis in one task file of a task with several static postures, it does not provide a dynamic analysis of the task and should not be used to draw conclusions about the strength demands of jobs. The recommendation is to use inputs from the beginning of the task and from the end of the task if appropriate. Job Analysis Uses 1. Job force demands, worker strength and low-back compressive force - use the 95 th percentile female anthropometry and 75 percent capable criterion for both proactive and reactive applications. Users conducting analyses of existing jobs in response to worker complaints will often attempt to model the anthropometry of the worker performing the job. Due to the limitations and constraints of the model described above, drawing conclusions about strength capability from a specific individual s anthropometry may be very misleading. The UAW-GM agreed approach for both reactive and proactive assessments of forces and strength capabilities is to set the female 95 th percentile anthropometry to a 68 stature and 200 lb body weight with the postural specifics and minimum 75 percent capable criterion to make judgments about the acceptability of the job related to overall worker population strength capabilities. However, the user may wish to also try a 50th percentile female anthropometry in an analysis, especially where a posture may be more extreme for the smaller anthropometry. The more conservative results of either the 95 th or the 50 th percentile anthropometries would then be used as the basis for making judgments about the job. Use posture prediction and direct manipulation supplemented by observations of operator postures in reactive applications to achieve the appropriate posture for the anthropometry used in the analysis. See Low Back Disc Compression Force of the Retrieving Results section of the 3D-course manual for a discussion on interpreting low back compression force results. 2. Job reach requirements - different anthropometry may be selected to identify reach limits. An iterative assessment approach may involve using small female and large male anthropometry to test reach distances and test potential body postures given 3

5 workplace geometry. Use anthropometric criteria consistent with your divisional specifications or local practices. Some user groups successfully assess both reach and strength issues by using a 50 th percentile female anthropometry. However, be aware of the anthropometric limitations of the 3DSSPP model and use it only as an approximate indicator of human variation in size. Other human modeling programs are superior to the 3DSSPP model for modeling reach and fit. Contact your program ergonomist or Workcell Visualization Group or equivalent support activity for assistance in modeling reach and fit problems. If their model suggests that an unsupported and extreme posture would be required due to an extended reach, then the posture angles from their model can be entered into the 3DSSPP for analysis of strength and low back compressive force. Consult the appropriate section in the 3Dcourse manual. In reactive assessments, the use of the small female anthropometry as the assessment basis may show a potential reach issue that may not be a problem with the current worker on the job. The plant JETT may choose to address the potential reach issues at a later time. NOTE: When changing between anthropometry for a modeled task, be sure and choose Maintain Hand Positions in the anthropometry box but check that the resulting posture is still representative for the job being modeled. Manipulation of some body segment angles may be necessary to achieve the representative posture while maintaining hand positions. ACCOUNTING FOR FREQUENCY/REPETITION IN 3D SSPP ANALYSIS Repetition and 3D Modeling The 3DSSPP does not have any built-in provision for assessing the effects of repetition or frequency for a modeled activity. There are classifications of exertion, such as those based upon percent of maximum effort that are used to draw inferences about repetition. Such strategies are appropriate when individual capacities are known and are much less so when related to modeled population strengths. Given the layered protective nature of the anthropometry and strength percent capable limits given here in selecting the basis for an analysis the results of the model should be valid without any modification for repetitions up to and including TWO actions per minute (2.0/min) for most applications. If the repetition of the 3-D modeled exertion equals or exceeds 2.0/min. *, then the analysis must be supplemented with additional information and other analysis tools. The Rohmert-Rose Recovery Tool can assess local muscle fatigue and recovery within the job cycle for single and repetitive static (over 3 seconds) exertions. The required moment and the population strength from the 3D SSPP Strength Capabilities report for the affected joint along with the exertion duration and job cycle time. The Rohmert-Rose Recovery Tool is taught in conjunction with the 3D SSPP. * See Chaffin, D.B., Andersson, G.B.J and Martin, B.J., Occupational Biomechanics, Third Edition, NY: John Wiley & Sons, 1999, page

6 EXAMPLES 1. Suppose that the activity being modeled occurs once per cycle. The cycle time is 40 seconds long. The number of actions per minute would be: (1.0 action 40 sec.) x (60 sec. 1.0 min) = 1/40 sec x 60 sec. = (1 action x 60 sec.) /40 sec. = 1.5 actions per minute. Since 1.5 < 2.0, the results of the 3DSSPP should be valid with no additional analysis. 2. Suppose that the operator performs the activity twice per job cycle and that the cycle time is 25 seconds. The number of actions per minute would be: (2.0 actions 25 sec.) x 60 sec. 1 min. = 4.8 actions per minute. Since 4.8 > 2.0, the activity is more frequent than what the 3DSSPP model should apply to and other tools and information should be used in the analysis. This approach applies for either reactive or proactive situations. For reactive situations, the RFC prescribes an assessment flow process. This process is explained in the UAW-GM Ergonomics Analysis Process, in the 3D-course manual. 5

7 APPENDIX Secondary Analysis Tools, 3DSSPP Limitations and Using Other Strength Data Other tools that may supplement the 3-D analysis and consider frequency, repetition and duration are: NIOSH Lift Equation is the principle tool to analyze two-handed lifting tasks and takes account of task frequency and duration. Energy expenditure prediction analysis predicts worker energy expenditure and assesses the risk of whole-body fatigue due to sustained repetitive exertions and work activities. Snook (Psychophysical) tables for whole-body pushing, pulling or carrying given distance and frequency. HAL-TLV is embedded in the RFC and is the principle tool for the assessment of repetitive hand activity and combines assessments of peak hand-exerted force and hand motion. Other tools and techniques are used on a limited basis to assess repetition, duration and force demands. The above tools and their tie-in to RFC prescribed analysis flows are described in the Second Level Analysis Tools training course offered through the UAW-GM Center for Human Resources. 3D Model Limitations for Addressing Repetition Further limitations of this method for accounting for repetition would be as follows: Method results are applicable only to relatively gross body movements. Finger/hand/wrist repetition should be assessed by HAL/TLV. The 3DSSPP model does not account for the duration of an exertion. Static strength models are strictly applicable to exertions lasting only about 3.0 seconds at most. If the exertion exceeds three seconds, then Rohmert-Rose Recovery Tool is recommended. Using Other Strength Data As noted above, the 3DSSPP does not provide data for the fingers, hand or wrist. It is suggested that a user treat hand, wrist and finger strength data with the same approach as used with 3DSSPP data. The strength data should consist of, as a minimum, a female mean static strength and standard deviation. The equation for the strength limit that allows 75% of the female population to be accommodated is as follows: 75% Accommodated = Mean Strength (0.647 x Strength Standard Deviation). The calculated value can then be compared to force requirements or measurements. 6