MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)TABLE 9-2. MAXIMUM WEIGHT LIMITS (Ref. 3)lThese weight limits should be used as maximum values in establishing the weights of items that must be lifted.These limits apply to items up to 380 mm (15 in.) long and up to 305 mm (12 in.) high, with handles or grasp areasas shown in Fig. 4-20. These limits should not be used for larger items or for items which must be liftedrepetitively.2These weight limits should not be used if personnel must carry the item more than five steps.3When an item weighs more than the limit for one-man lifting, it should be prorminently labelled with weight andlift limitations, e.g., two-person or mechanical lift. Items to be lifted mechanically should have prominentlylabelled hoist and lift points.6. Suffers discomfort and impaired vision if brightlights or reflections are located within 60 deg of his line ofsight.Sight is stimulated by radiation of certain wavelengths—430 to 690 m (4300 to 6900 A)—commonly called thevisible portion of the electromagnetic spectrum. Themaintenance technician can see all colors of the spectrumviolet through red—while looking straight ahead. However,color perception decreases as the viewing angleincreases. Consequently, if the equipment has colorbandedmeters or warning lights of different colors thatare near the maximum viewing angle limits, the maintenancetechnician may not be able to distinguish one colorfrom another. The color of the illuminating light source isalso an important factor when viewing color-codedobjects. At night, or in any poorly illuminated area, colormakes little difference. Similarly, if the source is distant orsmall—such as a small warning light—blue, green, yellow,and orange are indistinguishable; they all appear tobe white. Another phenomenon of color perception isapparent reversal of color. When an individual stares at ared or green light, for instance, and then glances away, thesignal to the brain may reverse the color. This phenomenonhas caused many accidents. Therefore, colorshould not be relied upon solely when critical operationsmay be performed by fatigued personnel or under circumstanceswhere color perception may not be good. In additionto problems regarding color perception, maintainabilitydesign is concerned with insuring that visual displaysare placed properly for effective use. Fig. 9-3 providesdesign guidance for the horizontal and verticalvisual fields.A technician needs sufficient light to perform tasksproperly; accuracy, speed, and safety suffer if he cannotsee well. Adequate illumination, however, will not alwaysbe available. Accordingly, equipment designers shouldendeavor to develop their designs to permit effectivemaintenance under even the poorest anticipated lightingconditions. To this end, designers should acquaint themselveswith all of the possible circumstances that mayreduce available illumination. For example, if a flashlightis necessary to illuminate the areai to be accessed, theequipment shoud bed designed so that maintenance can beperformed by the light of a flashlight. This assumes thatthe person performing the maintenance has a free hand tohold the flashlight and that there is space available tostand while holding the flashlight.Basic factors that should be considered in the design oflighting systems are1. Suitable brightness for the tash at hand2 Uniform lighting3, Suitable brightness contract between task andbackground4. Lack of glare from light source or work surface.It is difficult to specity exact illumination levels fordesigning an efficient Iighting system, but guidelines areshown in Tables 9-3 and 9-4 (Ref. 3). The examples ofillumination levels expressed in Table 9-3 are presented sothe designer may estimate the illumination levels for thesetasks within the system that are not directly related to thetasks of Table 9-4.The ability of personnel to see cathode-ray tube signals,such as those on oscilloscopes, depends on five mainvisual factors (Ref. 3). namely.1. Size, in visual angle, of the signal2. Brightness of the background (The ambient illuminanceshould not contribute more than 25% of the screenbrightness.)9-6