MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)because the male and female unthropometric dimensionsoverlap so that all males equal to or below the 95thpercentile and all females equal to or above the 5th percentileare accommodated.Some of the measurements important to maintainability(Ref. 12) are1. Basic body dimensions:a.b.c.d.e.f.g.h.StatureEye heightShoulder heightArm reachElbow-hand lengthKnee height and leg lengthHand sizeBody breadth2. Body mobility3. Dexterity4. Field of vision.Anthropometric data are usually given as nude bodydimensions; however, MIL-STD-1472 (Ref. 2) and MIL-HDBK-759 (Ref. 3) provide data that take into accountarctic clothing, which usually is the worst case from thestandpoint of maintenance accessibility. Nuclear, biological,and chemical (NBC) protective garments are also aproblem because they produce heat stress, reduced vision,reduced tactile sense, and increased breathing effort. Fig.4-15 shows the increased dimensions for a gloved hand.Figs. 4-13 and 4-14 and Table 4-2 illustrate application ofanthropometric data to facilitate maintenance operations.In the application of anthropometric data, both staticand dynamic body measurements must be addressed.Static measurements range from the gross aspects of bodysize, such as stature, to the distance between the pupils ofthe eyes and are measured with the subject in rigid, standardizedpositions. Dynamic body measurements, on theother hand, usually vary with body movement and relatemore to human performance than to human “fit’’ (Ref. 1).Par. 2.3.1 of MIL-HDBK-759 (Ref. 3) gives the ranges forall voluntary movements the joints of the body can make.In summary, the following should be considered wheninterpreting and applying anthropometric data:1. Nature, frequency, and difficulty of related tasks2. Position of body during performance of tasks3. Mobility or flexibility requirements imposed bytasks4. Increments in the design-critical dimensions imposedby protective garments, packages, lines, padding,etc.9-3 HUMAN STRENGTH ANDHANDLING CAPACITYEquipment that is designed to be consistent with aperson’s capabilities permits more force to be exertedwith less fatigue. However, if the demands placed onhuman strength are too high, inefficient and unsafe performancewill result. Conversely, if the designer underestimatesstrength, unnecessary design effort and expensemay be incurred. The proper strength value to use indesigning equipment is the maximum force that can beexerted by a 5th percentile member of the user population.The maximum force that can be applied by a persondepends on many factors such as the position of the body,the body member(s) applying the force, the direction ofapplication, the object to which the force is applied, andwhether or not support is provided. The following conclusionsregarding the application of force should be ofvalue to the designer (Ref. 12):1. The greatest force is developed in pulling towardthe body. An individual using his leg and back musclescan exert a stronger pulling force than a seated individual.2. The maximum force that can be exerted increaseswith the use of the whole arm and shoulder, but use of thefingers alone requires the least energy per unit of forceapplied.3. Push exerts a greater force than pull in side-tosidemotion.Fig. 9-2 (Ref. 3) shows the arm, hand, and fingerstrength of 95% of male personnel for various directionsof movement and body members.Whenever possible, equipment parts should be designedso that one person can lift them. If this is not possible. theparts should be clearly labeled with weight and lift limitations.Table 9-2 (Ref. 3) shows the lifting capacity of 95%of male users. These weight limits should be reduced whendifficult conditions exist, such as1. When the object is very difficult to handle i.e.,bulky. slippery, etc.2. When access and work space conditions arc lessthan optimal3. When the required force must be exerted continuouslyfor more than 1 min4. When the object relist be positioned exactly orhandled delicately5. When the task must be repeated frequently.Par. 2.4.2 of MIL-HDBK-759 (Ref. 3) provided additionalfigures and tables for determining human strengthsand handling capacities.9-4 HUMAN SENSORY CAPABILITIESIndividuals, as part of a total system, possess manyuseful sensors. Of the five major senses sght, hearing,taste, smell, and touch only sight, hearing, and touchwill be addressed since they are the major factors affectingmaintainability engineering.9-4.1 SIGHTIt is estimated that humans acquire 80% of their knowledgevisually (Ref. 7). Thus the maintenance technician'ssight capabilities are imporatnt to the maintainabilityengineer. The typical maintenance technician1. Can distingiush 10 colors, five sizes of figure, fivebrightnesses of light, and two flicker rates2. Can easily read 6-point type with adequate lighting3. Has a visual field of about 130 deg vertically and208 deg horizontally with maximum acuity at the center4. Requires about 0.6 s to change visual fixationfrom near to far5. Takes about 30 min to adapt completely fromdaylight to darkness9-4