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CIE ergo notes - Ergonomics Review Lectures
Industrial Engineering (ERGO1)
University of the Assumption
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ERGONOMICS
Human‐System Interface Technology As a science, ergonomics is concerned with developing knowledge about human performance capabilities, limitations and other characteristics as they relate to the design of the interfaces between people and other system components, As a practice, ergonomics concerns the application of human‐system interface technology to the analysis, design, and evaluation of systems to enhance safety, health, comfort, effectiveness and quality of life.
Definition
Ergonomics (or human factors) is the scientific discipline concerned with the understanding of the interactions among humans and other elements of a system, and the profession that applies theoretical principles, data and methods to design in order to optimize human well being and overall system performance.
Practitioners of ergonomics, ergonomists, contribute to the planning, design and evaluation of tasks, jobs, products, organizations, environments and systems in order to make them compatible with the needs, abilities and limitations of people.
Physical ergonomics is concerned with human anatomical, anthropometric, physiological and biomechanical characteristics as they relate to physical activity.
Cognitive ergonomics is concerned with mental processes, such as perception, memory, reasoning, and motor response, as they affect interactions among humans and other elements of a system.
Organizational ergonomics is concerned with the optimization of sociotechnical systems, including their organizational structures, policies, and processes.
WORK ENVIRONMENT DESIGN
- Illumination
The basic theory of illumination applies to a point source of light of a given luminous intensity. Light emanates spherically in all directions from the source with 1- candela (cd) sources emitting 12 lumens (lm) (as determined from the surface area of a sphere). The amount of light striking a surface, or a section of this sphere, is termed illumination or illuminance and is measured in foot-candles (fc). The amount of illumination striking a surface drops off as the square of the distance (d) in feet from the source to the surface:
Illumination = intensity / d 2
Some of that light is absorbed and some of it is reflected. The amount reflected is termed as luminance and is measured in foot-lamberts (fL). It is determined by the reflective properties of the surface known as reflectance (a unitless proportion and ranges from 0 to 100%; high quality white paper has a reflectance of about 90% and matte black paint has 5%).
Luminance = illuminance x reflectance
Illumination is typically measured with a light meter (similar to one found on cameras, but in different units), while luminance is measured with a photometer. Reflectance is usually calculated as the ratio between the luminance of the target surface and the luminance of a standard surface of known reflectance placed at the same position on the target surface.
Visibility – refers to the clarity with which the human sees something. - has 3 critical factors: visual angle, contrast, and illuminance. - Visual angle is the angle subtended at the eye by the target - Contrast is the difference in luminance between a visual target and its background - Visual angle is usually defined in arc minutes (1/60 of a degree) for small targets by
Visual angle (arc min) = 3,438 x h/d
Where h =height of the target d = distance from the target to the eye
Contrast (unitless) = (Lmax – Lmin) / Lmax
Where L = luminance
Light Source and Distribution
Two important parameters related to artificial lighting:
- Efficiency – light output per unit energy (lm / w or lumens per watt)
- Color rendering – relates to the closeness with which the perceived colors of the object observed match the perceived colors of the same object when illuminated by standard light sources.
Glare – the excessive brightness in the field of vision.
Direct glare can be reduced by:
- Using more luminaires with lower intensities
- Using baffles or diffusers on luminaires
Process of hearing:
Pneumatic Pressure Waves Mechanical Vibrations
Electric Impulses Mechanical Vibrations Hydraulic waves
Decibel Scale (dB) – the logarithmic ratio of the actual sound intensity at the threshold of hearing of a young person. Thus, the sound pressure level L in decibels is given by
L = 20 log 10 Prms / Pref
where Prms = root-mean-square sound pressure [microbars (μbar)] Pref = sound pressure at the threshold of hearing of a young person at 1,000 Hz (0 μbar)
Since sound pressure levels are logarithmic quantities, the effect of the coexistence of two or more sound sources in one location requires that a logarithmic addition be performed as follows:
LTOT = 10 log 10 (10L 1 /10 + 10L 2 /10 + ....)
where LTOT = total noise L 1 and L 2 = two noise sources
Loss of hearing is a result of loss of receptors in the inner ear which then fail to transmit the sound waves to the brain. As the frequency approach 2,400 – 4,800 Hz, the chances of hearing loss increases.
Noise control is done by:
- Reducing the noise level at its source
- Investigating opportunities to isolate the equipment
- Wearing hearing protection
Noise Dose
OSHA uses the concept of noise dose, with the exposure to any sound level above 80 dBA causing the listener to incur a partial dose. If the total daily exposure consists of several partial exposures to different noise levels, then the several partial doses are added to obtain a combined exposure:
D = 100 x (C 1 / T 1 + C 2 / T 2 + .... + Cn / Tn) ≤ 100
where D = noise dose C = time spent at specified noise level (h) T = time permitted at specified noise level (h)
The total exposure to various noise levels cannot exceed a 100 percent dose.
Permissible Noise Exposures Duration per day (h) Sound level (dBA) 8 90 6 92 4 95 3 97 2 100 1 102 1 105 0 110 0 or less 115
A computational formula can be used for intermediate noise levels:
T = 8/2(L – 90)/
Where L = noise level (dBA)
The noise dose can also be converted to an 8-h time-weighted average (TWA) sound level. This is the sound level that would produce a given noise dose if a worker were exposed to that sound level continuously over an 8-h workday. The TWA is defined by
TWA = 16 x log 10 (D/100) + 90
Example:
A worker is exposed to 1 h at 80 dBA, 4 h at 90 dBA, and 3 h at 96 dBA. The worker is permitted 32 h (computed using the same formula shown below) for the first exposure, 8 h for the second exposure, and
T = 8/2(96-90)/5 = 3.
hours for the third exposure. The total noise dose becomes
D = 100 x (1/32 + 4/8 + 3/3) = 139.
If earplug is considered or used, its effectiveness is measured quantitatively by a noise reduction rating (NRR). Thus, the equivalent noise exposure for the listener is equal to
TWA + 7 – NRR
- Implementing engineering controls or modifying the environment
- Modifying administrative controls
Cold stress can be reduced by wearing body warmers or by installing heaters in the work area.
Clo – the amount of thermal insulation in clothing needed to maintain comfort for a person sitting in a normally ventilated room at 70⁰F and 50% relative humidity.
Ventilation - provided to dilute contaminants, exhaust the stale air and supply fresh air.
- Radiation
In choosing personal protective equipment for radiation exposure, a specialist must be consulted. A large and dangerous dose of radiation amounts to 100 rads (1 rad = 0 Joule per kilo).
Shiftwork and Working Hours
Long hours increase the risks of accidents, causes exhaustion, threaten health physically and mentally. Changes in length of workday have a direct effect on work output. The implementation of shiftwork causes stress on circadian rhythms, which are the roughly 24-h variations in bodily functions in humans. The length of the cycle varies from 22 to 25 h. Work overtime causes low productivity and the more strenuous the work is, the greater is the decrease in production.
Guidelines for overtime:
- Avoid overtime for heavy manual work.
- Re-evaluate machine-paced work for appropriate rest periods.
- Rotate the work among workers.
- It is better to extend work hours during the workdays that extending the workweek.
Guidelines for shiftwork:
- Avoid shiftwork for workers older than 50.
- Use rapit rotations as opposed to weekly or monthly cycles.
- Schedule as few night shifts in succession as possible.
- Limit the total number of working shifts in succession to 7 or less.
- Include free weekends.
- Schedule rest days after night shifts.
Problems:
What is the combined noise level of two sounds of 86 and 96 decibels?
In the Dorben Company, an industrial engineer designed a workstation where the seeing task was difficult because of the size of the components going into the assembly. The desired brightness was 100 foot Lamberts and the workstation was painted a medium green having a reflectance of 50 percent. What illumination in foot candles would be required at this workstation to provide the desired brightness?
In the Dorben Company, an industrial engineer was assigned to alter the work methods in the press department to meet OSHA standards relative to permissible noise exposures. The IE found a time weighted average sound level of 100 db. The 20 operators in this department wore earplugs provided by Dorben with an NRR value of 20 dB. What improvement resulted?
In the Dorben Co., an all-day study revealed the following noise sources: 0 hrs., 100 dBA; 1 hr. less than 80 dBA; 3 hrs., 90 dBA; 3 hrs., 92 dBA. Is this company in compliance? What is the dose exposure? What is the TWA noise level?
In problem #4, consider that the last exposure is in press room which currently has five presses operating. Assuming that Dorben Co. can eliminate some of the presses and transfer production to the remaining presses, how many presses should Dorben eliminate so as not to exceed 100% dose exposure for the workers?
What is the illumination on a surface 6 inches from a 2 candela source?
What is the luminance of a surface having a 50% reflectance and 4 foot candle illumination?
ENGINEERING ANTHROPOMETRY
Anthropometry
- It is the science of measurement and the art of application that establishes the physical geometry, mass properties, and strength capabilities of the human body.
- It is derived from anthropos (human) and metrikos (pertaining to measurement).
Uses
- Design for Fitting Clothing, Tools, Workstations, and Equipment to body
- Evaluate postures and distances to reach controls
- Specify clearances separating the body from hazards such as surrounding equipment
- Design for Motion
- Identify objects or elements that constrict movement
- Assist in the biomechanical analyses of forces and torques
What to Measure?
- Body segment lengths and height
- Breadth, depths and circumferences
- Surface contours
- Body volumes, densities and areas
CIE ergo notes - Ergonomics Review Lectures
Course: Industrial Engineering (ERGO1)
University: University of the Assumption
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