Presentation on theme: "Eng. Suleiman Daifi. KEY POINTS Fit the workplace to the operator. Provide adjustability. Maintain neutral postures (joints in midrange). Minimize repetitions."— Presentation transcript:
Eng. Suleiman Daifi
KEY POINTS Fit the workplace to the operator. Provide adjustability. Maintain neutral postures (joints in midrange). Minimize repetitions. Use power grips when force is required. Use pinch grips for precision and not force.
Ergonomics: Designing the workplace, tools, equipment, and work environment to fit the human operator This chapter presents the principles of work design and appropriate checklists to facilitate the use of these design principles. This approach will better assist the methods analyst in designing the (1) workplace, (2) equipment, and (3) tools to meet the simultaneous goals of: (1) increased production and efficiency of the operation and (2) decreased injury rates for the human operator.
ANTHROPOMETRY AND DESIGN The primary guideline is to design the workplace to accommodate most individuals with regard to structural size of the human body. Anthropometry : The science of measuring the human body and typically utilizes a variety of caliper like devices to measure structural dimensions, for example, stature and forearm length. The CAESAR (Civilian American and European Surface Anthropometry Resource) project collected over 100 dimensions on 5,000 civilians using three-dimensional body scans. A summary of useful dimensions that apply to the particular postures needed for workplace design for U.S. males and females is given in Table 5.1.
DESIGN FOR EXTREMES Design for extremes implies that a specific design feature is a limiting factor in determining either the maximum or minimum value of a population variable that will be accommodated. For example (Max): clearances, such as a doorway or an entry opening into a storage tank, should be designed for the maximum individual, that is, a 95th percentile male stature or shoulder width. Then 95 percent of all males and almost all females will be able to enter the opening. Obviously, for doorways, space is usually not at a premium, and the opening can be designed to accommodate even larger individuals. Example (Min): added space in military aircraft or submarines is expensive, and these areas are therefore designed to accommodate only a certain (smaller) range of individuals. Reaches, for such things as a brake pedal or control knob, are designed for the minimum individual, that is, a 5th percentile female leg or arm length. Then 95 percent of all females and practically all males will have a longer reach and will be able to activate the pedal or control.
DESIGN FOR ADJUSTABILITY Design for adjustability: it is typically used for equipment or facilities that can be adjusted to fit a wider range of individuals. Example: Chairs, tables, desks, vehicle seats, steering columns, and tool supports are devices that are typically adjusted to accommodate the worker population ranging from 5th percentile females to 95th percentile males. Obviously, designing for adjustability is the preferred method of design, but there is a trade-off with the cost of implementation. (Specific adjustment ranges for seat design are given later in Table 5.2)
DESIGN FOR THE AVERAGE Design for the average: It is the cheapest but least preferred approach. Even though there is no individual with all average dimensions, there are certain situations where it would be impractical or too costly to include adjustability for all features. For example, most industrial machine tools are too large and too heavy to include height adjustability for the operator. Designing the operating height at the 50th percentile of the elbow height for the combined female and male populations (roughly the average of the male and female 50th percentile values) means that most individuals will not be unduly inconvenienced. However, the exceptionally tall male or very short female may experience some postural discomfort.
PRACTICAL CONSIDERATIONS Reasonable effort must be made to accommodate individuals with all abilities. Special accessibility guidelines (U.S. Department of Justice, 1991) have been issued regarding parking lots, entryways into buildings, assembly areas, hallways, ramps, elevators, doors, water fountains, lavatories, restaurant or cafeteria facilities, alarms, and telephones. It is also very useful, if practical and cost-effective, to build a full-scale mock-up of the equipment or facility being designed and then have the users evaluate the mock-up.
PRINCIPLES OF WORK DESIGN: THE WORKPLACE Machines and Equipment Tools
PRINCIPLES OF WORK DESIGN: THE WORKPLACE
1. DETERMINE WORK SURFACE HEIGHT BY ELBOW HEIGHT 2. ADJUST THE WORK SURFACE HEIGHT BASED ON THE TASK BEING PERFORMED 3. PROVIDE A COMFORTABLE CHAIR FOR THE SEATED OPERATOR 4. PROVIDE ADJUSTABILITY IN THE SEAT 5. ENCOURAGE POSTURAL FLEXIBILITY 6. PROVIDE ANTIFATIGUE MATS FOR A STANDING OPERATOR 7. LOCATE ALL TOOLS AND MATERIALS WITHIN THE NORMAL WORKING AREA 8. FIX LOCATIONS FOR ALL TOOLS AND MATERIALS TO PERMIT THE BEST SEQUENCE 9. USE GRAVITY BINS AND DROP DELIVERY TO REDUCE REACH AND MOVE TIMES 10. ARRANGE TOOLS, CONTROLS, AND OTHER COMPONENTS OPTIMALLY TO MINIMIZE MOTIONS
(1) DETERMINE WORK SURFACE HEIGHT BY ELBOW HEIGHT The work surface height (whether the worker is seated or standing) should be determined by a comfortable working posture( وضع ) for the operator. Typically, this means that the upper arms are hanging ( معلق ) down naturally and the elbows are flexed at 90°so that the forearms are parallel to the ground The elbow height becomes the proper operation or work surface height. If the work surface is too high, the upper arms are abducted, leading to shoulder fatigue. If the work surface is too low, the neck or back is flexed forward, leading to back fatigue
(2) ADJUST THE WORK SURFACE HEIGHT BASED ON THE TASK BEING PERFORMED For rough assembly involving the lifting of heavy parts, it is more advantageous to lower the work surface For fine assembly involving minute visual details, it is more advantageous to raise the work surface by up to 8 in (20 cm) to bring the details closer to the optimum line of sight of 15 These principles also apply to a seated workstation. A majority of tasks, such as writing or light assembly, are best performed at the resting-elbow height. If the job requires the perception of fine detail, it may be necessary to raise the work to bring it closer to the eyes. Seated workstations should be provided with adjustable chairs and adjustable footrests (see Figure 5.6). Ideally, after the operator is comfortably seated with both feet on the floor, the work surface is positioned at the appropriate elbow height to accommodate the operation. Thus, the workstation also needs to be adjustable. Short operators whose feet do not reach the floor, even after adjusting the chair, should utilize a footrest to provide sup- port for the feet.
(3) PROVIDE A COMFORTABLE CHAIR FOR THE SEATED OPERATOR The seated posture is important from the standpoint of reducing both the stress on the feet and the overall energy expenditure. Because comfort is a very individual response, strict principles for good seating are somewhat difficult to define.
(4) PROVIDE ADJUSTABILITY IN THE SEAT Seat height is most critical, with ideal height being determined by the person’s popliteal height, A seat that is too high will uncomfortably compress the underside of the thighs. A seat that is too low will raise the knees uncomfortably high and decrease trunk ( جذع ) angle, again increasing disk pressure. Specific recommendations for seat height and other seat parameters (shown in Figure 5.6) are given in Table 5.2.
(5) ENCOURAGE POSTURAL FLEXIBILITY The workstation height should be adjustable so that the work can be performed efficiently either standing or sitting. The human body is not designed for long periods of sitting. An alternate compromise is to provide a sit/stand stool so that the operator can change postures easily.
(6) PROVIDE ANTIFATIGUE MATS FOR A STANDING OPERATOR Standing for extended periods on a cement floor is fatiguing. The operators should be provided with resilient ( مرن ) anti-fatigue mats. The mats allow small muscle contractions in the legs, forcing the blood to move and keeping it from tending to pool in the lower extremities ( أطراف الجسم ).
(7) LOCATE ALL TOOLS AND MATERIALS WITHIN THE NORMAL WORKING AREA LOCATE ALL TOOLS AND MATERIALS WITHIN THE NORMAL WORKING AREA In every motion, a distance is involved. The greater the distance, the larger the muscular effort, control, and time. It is therefore important to minimize distances. The normal working area in the horizontal plane of the right hand includes the area circumscribed by the arm below the elbow when it is moved in an arc pivoted at the elbow (see Figure 5.12). This area represents the most convenient zone within which motions may be made by that hand with a normal expenditure of energy. The normal area of the left hand may be similarly established. Since movements are made in the third dimension, as well as in the horizontal plane, the normal working area also applies to the vertical plane.
(8) FIX LOCATIONS FOR ALL TOOLS AND MATERIALS TO PERMIT THE BEST SEQUENCE Example: In driving an automobile, we are all familiar with the short time required to apply the foot brake. The reason is obvious: since the brake pedal is in a fixed cation, no time is required to decide where the brake is located. The body responds instinctively ( بصورة غريزية )and applies pressure to the area where the driver knows the foot pedal is located. If the location of the brake foot pedal varied, the driver would need considerably more time to brake the car. Conclusion: providing fixed locations for all tools and materials at the workstation eliminates, or at least minimizes, the short hesitations required to search for and select the objects needed to do the work.
(9) USE GRAVITY BINS AND DROP DELIVERY TO REDUCE REACH AND MOVE TIMES The time required to perform both of the transport actions :Reach and Move is directly proportional to the distance that the hands must move in performing these Actions Utilizing gravity bins, components can be continuously brought to the normal work area, thus eliminating long reaches to get these supplies (see Figure 5.15). Likewise, gravity chutes ( مزالق ) allow the disposal of completed parts within the normal area, eliminating the necessity for long moves to do so.
(10) ARRANGE TOOLS, CONTROLS, AND OTHER COMPONENTS OPTIMALLY TO MINIMIZE MOTIONS The optimum arrangement depends on many characteristics: 1. human (strength, reach, sensory) and 2. task (loads, repetition, orientation). The most important, or most frequently used components, should be placed in the most convenient locations. For example, an emergency stop button should be placed in a readily visible, reachable, or convenient position. Similarly, a regularly used activation button, or the most often used fasteners, should be within easy reach of the operator. Once the general location has been determined for a group of components, that is, the most frequently used parts for assembly, the principles of functionality and sequence of use must be considered. Functionality refers to the grouping of components by similar function, for example, all fasteners in one area, all gaskets and rubber components in another area. Since many products are assembled in a strict sequence, cycle after cycle, it is very important to place the components or subassemblies in the order that they are assembled, since this will have a very large effect on reducing wasteful motions. These principles of work design for workstations are summarized in the Workstation Evaluation Checklist (see Figure 5.16). The analyst may find this useful in evaluating existing workstations or implementing new workstations.
PRINCIPLES OF WORK DESIGN: MACHINES AND EQUIPMENT
MACHINES AND EQUIPMENT: 1. TAKE MULTIPLE CUTS WHENEVER POSSIBLE BY COMBINING TWO OR MORE TOOLS IN ONE, OR BY ARRANGING SIMULTANEOUS CUTS FROM BOTH FEEDING DEVICES 2. USE A FIXTURE INSTEAD OF THE HAND AS A HOLDING DEVICE 3. LOCATE ALL CONTROL DEVICES FOR BEST OPERATOR ACCESSIBILITY AND STRENGTH CAPABILITY 4. USE SHAPE, TEXTURE, AND SIZE CODING FOR CONTROLS 5. USE PROPER CONTROL SIZE, DISPLACEMENT, AND RESISTANCE 6. ENSURE PROPER COMPATIBILITY BETWEEN CONTROLS AND DISPLAYS
Advanced production planning for the most efficient manufacture includes taking multiple cuts with combination tools and simultaneous cuts with different tools. Of course, the type of work to be processed and the number of parts to be produced determine the desirability of combining cuts, such as cuts from both the square turret and the hexagonal turret. (1) TAKE MULTIPLE CUTS WHENEVER POSSIBLE BY COMBINING TWO OR MORE TOOLS IN ONE, OR BY ARRANGING SIMULTANEOUS CUTS FROM BOTH FEEDING DEVICES
If either hand is used as a holding device during the processing of a part, then the hand is not performing useful work. Invariably, a fixture can be designed to hold the work satisfactorily, thus allowing both hands to do useful work. Advantage: Fixtures not only save time in processing parts, but also permit better quality in that the work can be held more accurately and firmly. Many times, foot-operated mechanisms allow both hands to perform productive work.. (2) USE A FIXTURE INSTEAD OF THE HAND AS A HOLDING DEVICE
Many of our machine tools and other devices are mechanically perfect, yet incapable of effective operation, because the facility designer overlooked various human factors. Handwheels, cranks, and levers should be of such a size and placed in such positions that operators can manipulate them with maximum proficiency and minimum fatigue. Frequently used controls should be positioned between elbow and shoulder height. Seated operators can apply maximum force to levers located at elbow level; standing operators, to levers located at shoulder height. Hand wheel and crank diameters depend on the torque to be expended and the mounting position. The maximum diameters of handgrips depend on the forces to be exerted. (3) LOCATE ALL CONTROL DEVICES FOR BEST OPERATOR ACCESSIBILITY AND STRENGTH CAPABILITY
Advantage: Shape coding, using two- or three-dimensional geometric configurations, permits both tactual and visual identification. Use: It is especially useful under low-light conditions or in situations where redundant or double-quality identification is desirable, thus helping to minimize errors. Shape coding permits a relatively large number of discriminable shapes. An especially useful set of known shapes that are seldom confused is shown in Figure 5.18: Multiple rotation knobs are used for continuous controls in which the adjustment range is more than one full turn = Fractional rotation knobs are used for continuous controls with a range less than a full turn, while detent positioning knobs are for discrete settings. The surface texture can provide sense ( شعور )for discrimination by touch. Typically, three textures are rarely confused: smooth, fluted, and knurled. However, as the number of shapes and textures increases, discrimination can be difficult and slow if the operator must identify controls without vision. If the operator is obliged to wear gloves, then shape coding is only desirable for visual discrimination, or for the tactual discrimination of only two to four shapes. Size coding, analogous to shape coding, permits both tactual and visual identification of controls. Size coding is used principally where the controls cannot be seen by the operators. Of course, as is the case with shape coding, size coding permits redundant coding, since controls can be discriminated both tactually and visually. (4) USE SHAPE, TEXTURE, AND SIZE CODING FOR CONTROLS
The three parameters that have a major impact on performance are: control size - control-response ratio - and control resistance when engaged. A control that is either too small or too large cannot be activated efficiently. The control-response (C/R) ratio is defined as the amount of movement in a control divided by the amount of movement in the - A low C/R ratio indicates high sensitivity, such as in the coarse adjustment of a micrometer. A high C/R ratio means low sensitivity, such as the fine adjustment on a micrometer. Control resistance is important in terms of providing feedback to the operator. Ideally, it can be of two types: pure displacement with no resistance or pure force with no displacement. The first has the advantage of being less fatiguing, while the second is a deadman’s control, that is, the control returns to zero upon release. (5) USE PROPER CONTROL SIZE, DISPLACEMENT, AND RESISTANCE
Compatibility: It is the relationship between controls and displays that is consistent with human expectations. Basic principles include affordance, the perceived property results in the desired action; mapping, the clear relationship between controls and responses; and feedback,so that the operator knows that the function has been accomplished. (6) ENSURE PROPER COMPATIBILITY BETWEEN CONTROLS AND DISPLAYS
PRINCIPLES OF WORK DESIGN: Tools
(1) USE A POWER GRIP FOR TASKS REQUIRING FORCE AND PINCH GRIPS FOR TASKS REQUIRING PRECISION Prehension الإمساك of the hand can basically be defined as variations of grip قبضة between two extremes: a power grip and a pinch قرصة grip. In a power grip: the cylindrical handle of the tool, whose axis is more or less perpendicular to the forearm ساعد, is held in a clamp formed by the partly flexed fingers and the palm كف. Opposing pressure is applied by the thumb, which slightly overlaps the middle finger.The line of action of the force can vary with (1) the force parallel to the forearm, as in sawing; (2) the force at an angle to the forearm, as in hammering; and (3) the force acting on a moment arm, creating torque about the forearm, as in using a screwdriver. In a pinch grip: the item is held between the distal القاصي ends of one or more fingers and the opposing thumb (the thumb is sometimes omitted). The relative position of the thumb and fingers determines how much force can be applied and provides a sensory surface for receiving the feedback necessary to give the precision needed. There are four basic types of pinch grips, with many variations: (1) lateral جانبي pinch, thumb opposes the side of the index finger السبابة ; (2) two- and three-point tip طرف (or pulp) pinches, in which the tip of the thumb opposes to the tips of one or more fingers (for a relatively small cylindrical object, the three digits act as a chuck, resulting in a chuck grip); (3) palm pinch, the fingers oppose the palm of the hand without the thumb participating, as in glass wind- shield handling; and (4) finger press, the thumbs and fingers press against a surface, as in garment workers pushing cloth into a sewing machine. One specialized grip is an external precision or writing grip, which is a combination of a lateral pinch with the middle finger and a two-point pinch to hold the writing implement.
(2) AVOID PROLONGED STATIC MUSCLE LOADING When tools are used in situations in which the arms must be elevated or the tools must be held for extended periods, muscles of the shoulders, arms, and hands may be statically loaded, resulting in fatigue, reduced work capacity, and soreness وجع.
(3) PERFORM TWISTING MOTIONS WITH THE ELBOWS BENT When the elbow is extended, tendons أوتار and muscles in the arm stretch out and provide low force capability. When the elbow is bent 90° or less, the biceps brachii ( العضلة ذات الرأسين العضدية ) has a good mechanical advantage and can contribute to forearm rotation.
(4) MAINTAIN A STRAIGHT WRIST معصم To reduce this problem, the workplace or tools should be redesigned to allow for a straight wrist; for example, lower work surface and edges of containers, and tilt jigs toward the user.
(5) AVOID TISSUE انسجة COMPRESSION Often, in the operation of hand tools, considerable force is applied by the hand. Such actions can concentrate considerable compressive force on the palm of the hand or the fingers, resulting in ischemia الإسكيمية, which is the obstruction إعاقة of blood flow to the tissues and eventual numbness خدران and tingling وخز of the fingers. Handles should be designed with large contact surfaces, to distribute the force over a larger area or to direct it to less sensitive areas, such as the tissue between the thumb and index finger.
(6) DESIGN TOOLS SO THAT THEY CAN BE USED BY EITHER HAND AND BY MOST INDIVIDUALS Alternating hands allows the reduction of local muscle fatigue. However, in many situations, this is not possible, as the tool use is one-handed. Furthermore, if the tool is designated for the user’s preferred hand, which for 90 percent of the population is the right hand, then 10 percent are left out. Good examples of right-handed tools that cannot be used by a left-handed person include a power drill with side handle on the left side only, a circular saw, and a serrated knife leveled on one side only. Typically, right-handed males show a 12 percent strength decrement in the left hand, while right-handed females show a 7 percent strength decrement. Surprisingly, both left-handed males and females had nearly equal strengths in both hands. One conclusion is that left- handed subjects are forced to adapt to a right-handed world (Miller and Freivalds, 1987). Female grip strength typically ranges from 50 to 67 percent of male strength (Pheasant and Scriven, 1983); for example, the average male can be expected to exert approximately 110 lb (50 kg), while the average female can be expected to exert approximately 60 lb (27.3 kg). Females have a twofold disadvantage: an average lower grip strength and an average smaller grip span. The best solution is to provide a variety of tool sizes.
(7) AVOID REPETITIVE FINGER ACTION If the index finger is used excessively for operating triggers, symptoms of trigger finger develop. Trigger forces should be kept low, preferably below 2 lb (0.9 kg) (Eastman Kodak, 1983), to reduce the load on the index finger. Two or three finger operated controls are preferable
(8) USE THE STRONGEST WORKING FINGERS: THE MIDDLE FINGER AND THE THUMB Although the index finger is usually the finger that is capable of moving the fastest, it is not the strongest finger Where a relatively heavy load is involved, it is usually more efficient to use the middle finger, or a combination of the middle finger and the index finger.
(9) DESIGN 1.5-IN HANDLE DIAMETERS FOR POWER GRIPS Power grips around a cylindrical object should entirely surround the circumference of the cylinder, with the fingers and thumb barely touching. For most individuals, this would entail a handle diameter of approximately 1.5 in (3.8 cm) In general, the upper end of the range is best for maximum torque, and the lower end is best for dexterity and speed. The handle diameter for precision grips should be approximately 0.5 in (1.3 cm).
(10) DESIGN HANDLE LENGTHS TO BE A MINIMUM OF 4 IN For both handles and cutouts (11) DESIGN A 3-IN GRIP SPAN FOR TWO- HANDLED TOOLS
(12) DESIGN APPROPRIATELY SHAPED HANDLES For a power grip, design for maximum surface contact to minimize unit pressure of the hand. Typically, a tool with a circular cross section is thought to give the largest torque. However, the shape may be dependent on the type of task and the motions.
(13)DESIGN GRIP SURFACE TO BE COMPRESSIBLE AND NONCONDUCTIVE For centuries, wood was the material of choice for tool handles. Wood is readily available and easily worked. It has good resistance to shock and thermal and electrical conductivity, and it has good frictional qualities, even when wet. Since wooden handles can break and stain with grease الشحوم and oil, there has recently been a shift to plastic and even metal. Metal should be covered with rubber or leather to reduce shock and electrical conductivity and increase friction. Such compressible materials also dampen vibration and allow a better distribution of pressure, reducing fatigue and hand tenderness
(14) KEEP THE WEIGHT OF THE TOOL BELOW 5 LB The weight of the hand tool will determine how long it can be held or used and how precisely it can be manipulated. For tools held in one hand with the elbow at 90° for extended periods, Greenberg and Chaffin (1976) recommend loads of no more than 5 lb (2.3 kg). In addition, the tool should be well balanced, with the center of gravity as close as possible to the center of gravity of the hand (unless the purpose of the tool is to transfer force, as in a hammer). Thus, the hand or arm muscles do not need to oppose any torque development by an unbalanced tool. Heavy tools used to absorb impact or vibration should be mounted on telescoping arms or tool balancers to reduce the effort required by the operator. For precision operations, tool weights greater than 1 lb are not recommended, unless a counterbalanced system is used.
(15) USE GLOVES JUDICIOUSLY بتروي Gloves are often used with hand tools for safety and comfort. Safety gloves are seldom bulky, but gloves worn in subfreezing climates can be very heavy and can interfere with grasping إستيعاب ability. Wearing woolen or leather gloves may add 0.2 in (0.5 cm) to the hand thickness and 0.3 in (0.8 cm) to the hand breadth at the thumb, while heavy mittens قفاز add 1 in (2.5 cm) and 1.6 in (4.0 cm), respectively (Damon et al., 1966). More important, gloves reduce grip strength 10 to 20 per- cent (Hertzberg, 1973), torque production, and manual dexterity performance times. Neoprene gloves slow performance times by 12.5 percent over bare-handed performance, terry cloth by 36 percent, leather by 45 percent, and PVC by 64 per- cent (Weidman, 1970). A trade-off between increased safety and decreased performance with gloves must be considered.
(16)USE POWER TOOLS SUCH AS NUT AND SCREWDRIVERS INSTEAD OF MANUAL TOOLS Power hand tools not only perform work faster than manual tools, but also do the work with considerably less operator fatigue. Greater uniformity of product can be expected when power hand tools are used shafts.
(17) USE THE PROPER CONFIGURATION AND ORIENTATION OF POWER TOOLS In a power drill or other power tools, the major function of the operator is to hold, stabilize, and monitor the tool against a work piece, while the tools perform the main effort of the job. Although the operator may at times need to shift or orient the tool, the main function for the operator is effectively to grasp and hold the tool.
(18) CHOOSE A POWER TOOL WITH THE PROPER CHARACTERISTICS In general, using electrical tools at lower than normal rpm levels, or underpowering pneumatic tools, results in larger reaction torques and more stressful ratings. Pulse- type tools produce the lowest reaction torques, perhaps because the short pulses “chop up” the reaction torque. Other potential problems include noise from the pneumatic mechanism reaching levels as high as 95 dB(A), vibration levels exceeding 132 dB(V), and dust or oil fumes emanating from the exhaust
(19)USE REACTION BARS AND TOOL BALANCERS FOR POWER TOOLS