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Workstation evaluation and_design

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work station or environment evaluation prevents hazards attained from the work place. this evaluatiion is important to avoid any type of work related hazards; musculoskeletal, neurological etc.

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Workstation Evaluation and Design By: Radhika Chintamani Introduction: Process of evaluation of existing workstations and their design in order to minimize the risk of potential ergonomic problems. Background and significance to occupational ergonomics: Ergonomics or the ‘law of work’ by definition describes about physical aspects of work such as force and energy requirements. Defining only the physical force and energy requirements depending on the type of work the individual performs is not enough, in fact good workstation design provides optimal work capacity to the individual. Poor workstation increases the energy spent to manage the station more than to perform the task. Sometimes workstation design and evaluation can be thought of as ‘‘applied anthropometry’’. Models for workstation evaluation and design: A. Human-Machine interface model: To properly appreciate the importance of the design of the workstation, a ‘’system’’ must be defined that includes not only the hardware but also the worker and the interface between the two. Human:  Can play multiple roles in system, which can be arranged dimensionally.  No action takes place without intimate human involvement. Role of human can vary from one extreme to another in managing the work such as, that of supervisor to that of controller.  Physical work: degree to which the worker is required to exert force or expend energy for the process to continue properly. At one instinct human may not be required to provide any significant force, such that zero-force and/or zero- displacement controls, but not every-time.
 Mental process such as decision making and information processing do not significantly increase energy expenditure.  More likely the scenario of combination of subtask involving both extremes, mixed with those in which the worker provides some of the force or energy necessary to get the process moving, such as through the use of mechanical aids, servomechanism, or simply letting gravity take over. Machine: the concept of machine, as used here, is quiet broad and includes not only hardware of the process itself, also includes tool, raw materials, finished work and by- products. Interface: it is the interface between the human and the machine. This can be viewed as either the physical characteristics of the connection between the two or the process of establishing, maintaining and terminating such a connection. Physical characteristics include: location, size, shape, texture, color, force and movement. The connection process characteristics include: regularity and forcibility (i.e. operation vs service vs supervisor tasks) frequency (including repetititveness), duration and competing tasks (i.e., primary vs secondary tasks). Environment:  Plays an important role in success of the system.  Difficult to control sue to both technical and non-technical issues.  Environmental issues include: noise and vibration, temperature and humidity, and light and other radiation. B. The Input-Process-Output Model: The basic components of this models are: a. The inputs: system consists of i. Information ii. Raw material: to be transformed to end product or provides the dynamic instruction as to how transformation are to take place. iii. Energy: non-human energy powers the transformation of the raw materials to end product either totally or in concert with human efforts. b. The process: components consists of i. Workers: control the transformation at different levels. ii. Methods: these are the static instructions as to how transformation are to take place. iii. Tools: hardware used in the transformation and can include machines, jigs, fixtures, measurement devices, and manual and powered hand tools. iv. Work-in-process: altered state of process inputs prior to becoming process outputs. c. The output: consists of : i. Finished work: may be input to next station or the final end product of the work.
ii. By-products: either the waste product of the work, or the extra product formed in congestion with end product. C. Object oriented process model: each requirement of the work, the designer must consider all the appropriate ergonomic factors, both individually and in combination with others, to reduce the possibility of undue strain. D. The ergonomics stressor model: here the individual looks for the source of stress, i.e. the cause of the strain. Some of the factors causing the stress are: Physical stressors Psychophysical stressors Psychological stressors Include: force, movement and repetitions Include: Display and other stimuli, information processing and decision making, an controls Include: non-occupational and occupational stressors, motivation and reward system, and environment. Physical stress: Force: can be exerted either statically or dynamically. In static force: force generation is often localized, isolated muscle groups may be working very hard while others are not. Muscle gets easily fatigued. In case of dynamic: force generation is often generalized, and both the groups of muscles must be working enough to maintain the joint in dynamic movement. Here the contralateral muscle contracts alternatively to the ipsilateral muscle contraction. Movement: moving a mass as a result of application of force is the definition of work. In task involving significant worker movement, it can often be shown that the most of the energy expended goes to just moving the worker around. Movement here does not only mean, worker moving around, it may also mean the worker using energy in order to move any object or system. The long-term concern would be energy expenditure especially over the course of work-days. Repetitions: whenever the worker must repeat the same force or movement, especially within short time frames, this repetition can become a primary concern for the musculoskeletal injury. This can be either described as cumulative trauma disorder, or repetitive motion disorder. Eg: carpal tunnel syndrome, tendinitis, and epicondylitis. Psychophysical stress: includes the ‘input-output’ or ‘stimulus-response’ loop: displays and other stimuli information and decision making, and controls. Displays and other stimuli: parisomy or KISS [Keep It Simple yet Sufficient] is applicable here. The stimuli must be such that it must seek sufficient attention, discriminability and appropriateness. Information processing and decision making: Useful stimuli received by the worker Processed=decision making [objective decision rule] Decision reaching appropriately Response.
Worker’s education, training, and experience as well as the use of visual controls in the workstation greatly affect this part. Controls: defined as discrete mechanism provided in the workstation designed to cause the system to operate in a specific way. Psychological stress: includes non-occupational vs occupational stressors, motivation and reward systems and the environment. Non-occupational vs occupational stressors: these are the different types of stress on the individual. Occupational stress are the tension taken by the individual in relation to work like, workload, non satisfaction, wages, deadline, etc. but non-occupational stressors are the tension taken by the individual in relation to any other things except work like, personal life, environmental factors, etc. Evidence: a study done by Alberta et al, on the occupation and non-occupational risk factors causing hindrance of work capacity concluded that type A personality subjects on psychological assessment, had lots of problem completing a given task in a noisy environment. Thus personality of the subject also matters while completing the given task. Motivation and reward system: these can become a psychological support to the individual at work. Individual is mentally stressed when there is no support from his collegues or spouse in work. But by motivation and reward system, individual is cheered up n works more efficiently. Environment: just outside the workstation is the environment, which indirectly stresses the worker. Eg: heat or cold environment, noise, etc. E. The product process model: this model is basically more into the product design and the process of making it. The definition of this design can be as follows: Type fundamental nominal Discretionary Definition Cannot be changed without an overall change in design Can be changed with localized change in design Can be changed without any significant change in design Product example Screw-fastened cover Self-tapping screws Torx head screws Process example One piece flow Automatic part unloading Job rotation F. The life-cycle task model:
Criteria for workstation evaluation and design: Criteria development: Practitioner desires fairly specific criteria for the evaluation and design of a workstation. Applied workstation evaluation and design model: i. Environment: it can act on motivation and capability of workers to perform their tasks. Feeling of isolation/sense of community, privacy as well as communication when needed, assess to general environment, senses like vision and hearing or overwhelmed or not, make a important aspect of the environment. These factors if are exaggerated then may cause the hazard, and if lessened also may cause the hazard. ii. Method: a well designed method includes prescribed methods, where possible, the factors like general skills, specific skills, and training of the worker in that particular field, readily accessible information within the workstation, does the design of the product and process provide sufficient control as to method, workers span of control, frequency of fundamental movements, variance in the work, matters in optimal performance. iii. Information handling: it is the stimulus in the environment that provides necessary data about how to task must be carried out. This includes static information about the products and process and dynamic information about their current status. iv. Materials handling: materials include any object that flows into, are consumed within, or flow out of the workstation. v. Tools: defined broadly as portable and fixed tools, equipments, and machinery, being more or less assigned to workstation which help in reduction of the excessive energy spent during work and also refining the work. vi. Work envelope: it is the complete information available on all the tasks, materials and tools used to complete the tasks. Design for extremes: Design to fit: Dimensions for the largest i.e 95th or 99th percentile are used to determine clearance dimension: - 95th or 99th percentile for largest individuals and 5th or 1st percentile for smallest individuals. - Minimum height for over head conveyors or top of doorway. - Width for asiles. - Size of access opening for machine repair. - Clearance between chair seat and bottom of table or workbench. Design to reach: dimensions of smallest person i.e 5th or 1st percentile are used to determine reach dimension:
- Maximum height of shelves and controls. - Location of parts bins in the workplace. - Height of non-adjustable chair seats. Design for the average: In fact an average dimension will lead to discomfort in a large part of the workplace. Smaller person in the workplace will not be able to reach a part at the distance of the average person, this may cause excessive bending which may lead to back pain. Whereas, the larger person in the workplace for the average person may not fit in. this may cause excessive energy expenditure to manage things around and trying to fit in, leading to fatigue. Provide for Adjustability: This means adjustable workplace. The advantage of this approach is that large proportion of the workplace can be accommodated. But proportion of population accommodated must be determined, typically range of adjustment include the middle 90% or 95% of the population. Obtaining the required percentile dimensions from a typical anthropometric table, accommodating the 5th percentile female through 95th percentile male, might require the following range of adjustment: - Female 5th percentile= 96.8cm - Male 95th percentile= 118.1cm - Range of adjustment= 21.3cm To accommodate a different percentage of population might require using the appropriate Z value from a normal probability table and the dimension’s mean and standard deviation from a typical anthropometric table. Disadvantage includes higher design and manufacturing costs, time of adjustment, and greater likelihood of the adjustable component breaking. Alternatives to Adjustment: Appropriate placement of workers with respect to the tasks accomplishes the fixed height of workstation. But, designing the appropriate work height to fit the individual may be combursoem, if the individual is too big or too small. To fit a smaller individual adjustable platforms can be moved in to the workplace when needed, and to fit a larger individual chair size and the design to fit must be changed accordingly. Job rotation may also help in change in the posture attained while working constantly. B.. Examples of specific design criteria: 1. Sitting Workstation: Advantage:  Energy expenditure is approximately 20% less than standing.  Provides high degree of body stability  Reduces fatigue when work periods exceed 2hr.  Workers can use feet for control actions. Indications and Contraindications:  All items needed to perform the task can be easily placed within the seated workplace
 Frequent reaches above the surface are not required.  Tasks requiring fine manipulative hand movements are predominant.  The handling of parts or application of force does not exceed 45N.  There is no interference with leg clearance. Criteria:  Reach envelope consists of that spatial volume that can be reached by the worker while remaining seated i.e. static in a normal chair without undue stretching, bending, or twisting.  Work surface height: work height is often a trade-off between: i. Close view of the work and the need for precise movement. ii. Freedom to perform gross movement and ability to generate force. Normal work height for seated tasks is 5cm below elbow height. This clearance provides reasonably close view of the work while permitting movement uninterfered, for precise physical and visual tasks, for increased force and movement capabilities. May also provide elbow support useful in prolonged tasks. In case of computer keyboards, home row of the keyboard surface be at elbow height to facilitate the relatively precise movements required. Also, the J sign of the keyboard…  Reach dimensions: reaches should be kept in certain range depending upon the frequency of the reach. The majority of work must be performed in the 15cm of the work surface. Reaches above shoulder level and behind the body must be minimized. Repetitive work must be performed with shoulders and upper arm relaxed and elbows bent at 90degrees. Padded forearm rest should be provided for precision work.  Clearances: clearance below the work surface for legs and toes is necessary. Sufficient thigh clearance (25-30cm) should be provided between the seat pan and the underside of the workbench. Sufficient clearance also permits the worker to vary posture to reduce fatigue.  Seat dimensions: the seat area should be large enough to permit movement, to relieve pressure points. The seat pan and back rest should be padded and covered with a permeable material that will allow absorption of sweat. The back support should adjust both horizontally and vertically.

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Workstation evaluation and_design

  • 1. Workstation Evaluation and Design By: Radhika Chintamani Introduction: Process of evaluation of existing workstations and their design in order to minimize the risk of potential ergonomic problems. Background and significance to occupational ergonomics: Ergonomics or the ‘law of work’ by definition describes about physical aspects of work such as force and energy requirements. Defining only the physical force and energy requirements depending on the type of work the individual performs is not enough, in fact good workstation design provides optimal work capacity to the individual. Poor workstation increases the energy spent to manage the station more than to perform the task. Sometimes workstation design and evaluation can be thought of as ‘‘applied anthropometry’’. Models for workstation evaluation and design: A. Human-Machine interface model: To properly appreciate the importance of the design of the workstation, a ‘’system’’ must be defined that includes not only the hardware but also the worker and the interface between the two. Human:  Can play multiple roles in system, which can be arranged dimensionally.  No action takes place without intimate human involvement. Role of human can vary from one extreme to another in managing the work such as, that of supervisor to that of controller.  Physical work: degree to which the worker is required to exert force or expend energy for the process to continue properly. At one instinct human may not be required to provide any significant force, such that zero-force and/or zero- displacement controls, but not every-time.
  • 2.  Mental process such as decision making and information processing do not significantly increase energy expenditure.  More likely the scenario of combination of subtask involving both extremes, mixed with those in which the worker provides some of the force or energy necessary to get the process moving, such as through the use of mechanical aids, servomechanism, or simply letting gravity take over. Machine: the concept of machine, as used here, is quiet broad and includes not only hardware of the process itself, also includes tool, raw materials, finished work and by- products. Interface: it is the interface between the human and the machine. This can be viewed as either the physical characteristics of the connection between the two or the process of establishing, maintaining and terminating such a connection. Physical characteristics include: location, size, shape, texture, color, force and movement. The connection process characteristics include: regularity and forcibility (i.e. operation vs service vs supervisor tasks) frequency (including repetititveness), duration and competing tasks (i.e., primary vs secondary tasks). Environment:  Plays an important role in success of the system.  Difficult to control sue to both technical and non-technical issues.  Environmental issues include: noise and vibration, temperature and humidity, and light and other radiation. B. The Input-Process-Output Model: The basic components of this models are: a. The inputs: system consists of i. Information ii. Raw material: to be transformed to end product or provides the dynamic instruction as to how transformation are to take place. iii. Energy: non-human energy powers the transformation of the raw materials to end product either totally or in concert with human efforts. b. The process: components consists of i. Workers: control the transformation at different levels. ii. Methods: these are the static instructions as to how transformation are to take place. iii. Tools: hardware used in the transformation and can include machines, jigs, fixtures, measurement devices, and manual and powered hand tools. iv. Work-in-process: altered state of process inputs prior to becoming process outputs. c. The output: consists of : i. Finished work: may be input to next station or the final end product of the work.
  • 3. ii. By-products: either the waste product of the work, or the extra product formed in congestion with end product. C. Object oriented process model: each requirement of the work, the designer must consider all the appropriate ergonomic factors, both individually and in combination with others, to reduce the possibility of undue strain. D. The ergonomics stressor model: here the individual looks for the source of stress, i.e. the cause of the strain. Some of the factors causing the stress are: Physical stressors Psychophysical stressors Psychological stressors Include: force, movement and repetitions Include: Display and other stimuli, information processing and decision making, an controls Include: non-occupational and occupational stressors, motivation and reward system, and environment. Physical stress: Force: can be exerted either statically or dynamically. In static force: force generation is often localized, isolated muscle groups may be working very hard while others are not. Muscle gets easily fatigued. In case of dynamic: force generation is often generalized, and both the groups of muscles must be working enough to maintain the joint in dynamic movement. Here the contralateral muscle contracts alternatively to the ipsilateral muscle contraction. Movement: moving a mass as a result of application of force is the definition of work. In task involving significant worker movement, it can often be shown that the most of the energy expended goes to just moving the worker around. Movement here does not only mean, worker moving around, it may also mean the worker using energy in order to move any object or system. The long-term concern would be energy expenditure especially over the course of work-days. Repetitions: whenever the worker must repeat the same force or movement, especially within short time frames, this repetition can become a primary concern for the musculoskeletal injury. This can be either described as cumulative trauma disorder, or repetitive motion disorder. Eg: carpal tunnel syndrome, tendinitis, and epicondylitis. Psychophysical stress: includes the ‘input-output’ or ‘stimulus-response’ loop: displays and other stimuli information and decision making, and controls. Displays and other stimuli: parisomy or KISS [Keep It Simple yet Sufficient] is applicable here. The stimuli must be such that it must seek sufficient attention, discriminability and appropriateness. Information processing and decision making: Useful stimuli received by the worker Processed=decision making [objective decision rule] Decision reaching appropriately Response.
  • 4. Worker’s education, training, and experience as well as the use of visual controls in the workstation greatly affect this part. Controls: defined as discrete mechanism provided in the workstation designed to cause the system to operate in a specific way. Psychological stress: includes non-occupational vs occupational stressors, motivation and reward systems and the environment. Non-occupational vs occupational stressors: these are the different types of stress on the individual. Occupational stress are the tension taken by the individual in relation to work like, workload, non satisfaction, wages, deadline, etc. but non-occupational stressors are the tension taken by the individual in relation to any other things except work like, personal life, environmental factors, etc. Evidence: a study done by Alberta et al, on the occupation and non-occupational risk factors causing hindrance of work capacity concluded that type A personality subjects on psychological assessment, had lots of problem completing a given task in a noisy environment. Thus personality of the subject also matters while completing the given task. Motivation and reward system: these can become a psychological support to the individual at work. Individual is mentally stressed when there is no support from his collegues or spouse in work. But by motivation and reward system, individual is cheered up n works more efficiently. Environment: just outside the workstation is the environment, which indirectly stresses the worker. Eg: heat or cold environment, noise, etc. E. The product process model: this model is basically more into the product design and the process of making it. The definition of this design can be as follows: Type fundamental nominal Discretionary Definition Cannot be changed without an overall change in design Can be changed with localized change in design Can be changed without any significant change in design Product example Screw-fastened cover Self-tapping screws Torx head screws Process example One piece flow Automatic part unloading Job rotation F. The life-cycle task model:
  • 5. Criteria for workstation evaluation and design: Criteria development: Practitioner desires fairly specific criteria for the evaluation and design of a workstation. Applied workstation evaluation and design model: i. Environment: it can act on motivation and capability of workers to perform their tasks. Feeling of isolation/sense of community, privacy as well as communication when needed, assess to general environment, senses like vision and hearing or overwhelmed or not, make a important aspect of the environment. These factors if are exaggerated then may cause the hazard, and if lessened also may cause the hazard. ii. Method: a well designed method includes prescribed methods, where possible, the factors like general skills, specific skills, and training of the worker in that particular field, readily accessible information within the workstation, does the design of the product and process provide sufficient control as to method, workers span of control, frequency of fundamental movements, variance in the work, matters in optimal performance. iii. Information handling: it is the stimulus in the environment that provides necessary data about how to task must be carried out. This includes static information about the products and process and dynamic information about their current status. iv. Materials handling: materials include any object that flows into, are consumed within, or flow out of the workstation. v. Tools: defined broadly as portable and fixed tools, equipments, and machinery, being more or less assigned to workstation which help in reduction of the excessive energy spent during work and also refining the work. vi. Work envelope: it is the complete information available on all the tasks, materials and tools used to complete the tasks. Design for extremes: Design to fit: Dimensions for the largest i.e 95th or 99th percentile are used to determine clearance dimension: - 95th or 99th percentile for largest individuals and 5th or 1st percentile for smallest individuals. - Minimum height for over head conveyors or top of doorway. - Width for asiles. - Size of access opening for machine repair. - Clearance between chair seat and bottom of table or workbench. Design to reach: dimensions of smallest person i.e 5th or 1st percentile are used to determine reach dimension:
  • 6. - Maximum height of shelves and controls. - Location of parts bins in the workplace. - Height of non-adjustable chair seats. Design for the average: In fact an average dimension will lead to discomfort in a large part of the workplace. Smaller person in the workplace will not be able to reach a part at the distance of the average person, this may cause excessive bending which may lead to back pain. Whereas, the larger person in the workplace for the average person may not fit in. this may cause excessive energy expenditure to manage things around and trying to fit in, leading to fatigue. Provide for Adjustability: This means adjustable workplace. The advantage of this approach is that large proportion of the workplace can be accommodated. But proportion of population accommodated must be determined, typically range of adjustment include the middle 90% or 95% of the population. Obtaining the required percentile dimensions from a typical anthropometric table, accommodating the 5th percentile female through 95th percentile male, might require the following range of adjustment: - Female 5th percentile= 96.8cm - Male 95th percentile= 118.1cm - Range of adjustment= 21.3cm To accommodate a different percentage of population might require using the appropriate Z value from a normal probability table and the dimension’s mean and standard deviation from a typical anthropometric table. Disadvantage includes higher design and manufacturing costs, time of adjustment, and greater likelihood of the adjustable component breaking. Alternatives to Adjustment: Appropriate placement of workers with respect to the tasks accomplishes the fixed height of workstation. But, designing the appropriate work height to fit the individual may be combursoem, if the individual is too big or too small. To fit a smaller individual adjustable platforms can be moved in to the workplace when needed, and to fit a larger individual chair size and the design to fit must be changed accordingly. Job rotation may also help in change in the posture attained while working constantly. B.. Examples of specific design criteria: 1. Sitting Workstation: Advantage:  Energy expenditure is approximately 20% less than standing.  Provides high degree of body stability  Reduces fatigue when work periods exceed 2hr.  Workers can use feet for control actions. Indications and Contraindications:  All items needed to perform the task can be easily placed within the seated workplace
  • 7.  Frequent reaches above the surface are not required.  Tasks requiring fine manipulative hand movements are predominant.  The handling of parts or application of force does not exceed 45N.  There is no interference with leg clearance. Criteria:  Reach envelope consists of that spatial volume that can be reached by the worker while remaining seated i.e. static in a normal chair without undue stretching, bending, or twisting.  Work surface height: work height is often a trade-off between: i. Close view of the work and the need for precise movement. ii. Freedom to perform gross movement and ability to generate force. Normal work height for seated tasks is 5cm below elbow height. This clearance provides reasonably close view of the work while permitting movement uninterfered, for precise physical and visual tasks, for increased force and movement capabilities. May also provide elbow support useful in prolonged tasks. In case of computer keyboards, home row of the keyboard surface be at elbow height to facilitate the relatively precise movements required. Also, the J sign of the keyboard…  Reach dimensions: reaches should be kept in certain range depending upon the frequency of the reach. The majority of work must be performed in the 15cm of the work surface. Reaches above shoulder level and behind the body must be minimized. Repetitive work must be performed with shoulders and upper arm relaxed and elbows bent at 90degrees. Padded forearm rest should be provided for precision work.  Clearances: clearance below the work surface for legs and toes is necessary. Sufficient thigh clearance (25-30cm) should be provided between the seat pan and the underside of the workbench. Sufficient clearance also permits the worker to vary posture to reduce fatigue.  Seat dimensions: the seat area should be large enough to permit movement, to relieve pressure points. The seat pan and back rest should be padded and covered with a permeable material that will allow absorption of sweat. The back support should adjust both horizontally and vertically.