MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)engineer to engage in a meaningful dialogue with theproduction engineer relative to trade-offs. If any of severallabeling methods will satisfy the labeling requirement,the method chosen should be the one that simplifiesthe production task; conversely, if only a particularmethodwill satisfy the requirement, the maintainability’engineer must insist on the method regardless of theincreased production effort.6-4.1 INK STAMPINGThis method is widely used, primarily for piece parts. Inthis technique ink is applied with a rubber stamp. Watererpetroleum-soluble inks should not be used. Ink stampingby hand can be awkward, especially when the lengthof the stamp is much greater than the height.The chief production advantages of this method arethat it is easy to apply and can be changed during manufactureor in the field. The maintainability advantages arethat the process does not alter the surface finish, and it canbe used in small, restricted areas. The disadvantages arechiefly lack of durability, the characters may not besharply defined, the stamp does not conform to unevensurfaces, ink is subject to smudging, and ink can be appliedunevenly.6-4.2 STEEL STAMPINGThis method of marking provides identification ofmechanical parts at a low cost; it is permanent and notsubject to deterioration in any environment. However,care must be taken to assure that the stamping proceduredoes not damage the part. Steel stamping will penetrateanticorrosion coatings and painted surfaces and willrender them ineffective where the item is stamped. It mayalso produce sites susceptible to stress corrosion to whichthe material may otherwise be immune. This methodcannot be used on surfaces that are subject to wear or onitems that are installed through a gland or packingbecause the roughened surface will cause damage.One of the more common uses of steel stamping is toadd specific identification data to preprinted metal identificationplates, which are attached subsequently to productionrun hardware. The plates may be brass or stainlesssteel (avoid bare, dissimilar metals in contact witheach other because of galvanic action), printed or reverseetched,with spaces allocated for stamping the part identificationinformation on as one of the final steps in themanufacturing process.6-4.3 ENGRAVINGEngraving is the act of cutting characters into the surfacewith a tool; it has the same wearing qualities as steelstamping. However, the cost of engraving is high andproduction is slow. Engraving will penetrate anticorrosion-coatedsurfaces and painted surfaces and will renderthem ineffective where the item is engraved. this methodalso may produce sites susceptible to stress to which thematerial may otherwise be immune. It can be applied tononwearing surfaces that are too thin for steel stamping.The "electric pencil" marking device is one type of engravingtool that is often used for marking metal parts. Thismethod is not generally accepted for formal identificationpurposes because the resulting label may be illegible.6-4.4 MOLDING-INFor flat surfaces requiring no additional marking orwork after casting, molding-in is an effective and inexpensivemethod of permanently identifing a part. Theidentification is permanent. When used with sand castings,care must be exercised to ensure that the identificationis not washed out in the molding process. A washoutmay make numerals such as 0, 6, and 8 indistinguishablefrom each other or completely eliminate the partidentifier.6-4.5 DECALCOMANIA TRANSFERDeclacomania transfer (decals) are a printed design orcharactes on thin plastic material that is mounted on apaper backing for handling and storage. It is applied bysoaking in water until the transfer is loosened from thepaper and then carefully sliding the transfer off the paperand into place on the item. The transfer is held on thesurface by a lacquer-type adhesive. some decals are of theopen-letter type, i.e., individual letters and numerals,which apermit on-the-spot composition of lebels. An overcoatingof lacquer or clear varnish—compatible with thefinish coating on the part—usually is applied to add stabilityto the composed label. Decals are not wear resistantand are not acceptable for areas subject to repeated handlingor to any type of abrasion. They can be applied tometal, glass, plastic, or organically finished surfaces.Decals with water-soluble coatings should not be used.Additional advntages of decals are that they can be ofany size and can be multicolord.6-4.6 STENCILINGStenciling is applying ink to an item through an outlineof the characters the stencil. It is it common method formarking the outside of shipping containers and is oftenused for vehicle identification. Stencils are made of heavy,treated paper or thin plastic, and the characters arepunched out by a stencil-cutting machine. Stenciling is arelatively slow process, but stencils can be applied touneven surfaces and to any material if the stenciling ink iscompatible with the surface finish.If stenciling is to be applied to plastic, a cover coatingallowing the stencil to take usually may be omitted providedthe stenciling ink meets the requirements of FederalSpecification TT-1-1795 (Ref. 13). For some plastics.however, that exhibit a slick surface , e.g., polyethylene. acover coating is necessary.6-6
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.6-4.7 PHOTOETCHINGPhotoetching—a photomechanical process by photographingan image on a metal plate and then etching—canbe applied to metal surfaces. Reverse photoctching isnormally specified for metal marking. In this process thecharacters are printed or drawn to an enlarged scale and aphotographic reproduction, reduced to proper size, ismade on the surface to be marked. The characters arethen treated to make them resistant to the etching agent,acid. When acid is applied, the unprotected area (background)is etched out, which leaves raised characters. Thephotoetching process is frequently used for makingstandard identification plates that are later stamped toinsert information. for the specific item to which it isattached.6-4.8 METAL PLATESMetal plates are generally used to label vehicles, tanks,aircraft, and ground support equipment. The plates aremarked with the categories of required identificationdata, and spaces are left for inserting the data for thespecific item of equipment. The metal plate can be attachedwith screws, rivets, or adhesive, as desired. Thechosen method of attachment should be compatible withthe operational environment anticipated for the item. Theequipment, plate, and fasteners should be of the samemetal unless they have a protective coating to preventcorrosion resulting from galvanic action.6-4.9 TAGSTags are pieces of paper, plastic, or metal that areattached to an item when it is not possible or convenientto apply information directly to the item. Tags are oftenused for attaching shipping information to a loose item,and they can be used to carry imprinted identificationinformation when the item is too small to accept thenecessary characters on its body (see par. 6-3.5). Usually,the tag is used only during stocking and shipping and isremoved at the time of installation. It is common practiceto mark failed items by attaching a rejection tag thatremains attached until the item is repaired. The disadvantagesof tags are that they can get in the way of work, canbecome tangled with other items, and can be easily separatedfrom the item.6-4.10 PHOTOCONTACTThe photocontact process should be used where precisionmarkings are required, e.g., on dials. This processexposes a sensitized surface by means of a photonegativecontaining the desired legend. The exposed surface isprocessed to develop the image. This process can be appliedto metallic and nonmetallic materials. An advantageis that a lot of information can be displayed in a smallspace.DOD-HDBK-791(AM)6-4.11 SCREEN PRINTINGScreen printing (silk screening) is a relatively inexpensivemethod for marking parts. It can be applied to a widevariety of materials and items, and almost any size of itemcan be marked. It is particularly suitable and economicalfor small and medium production runs. Screen printing isa paint process and, therefore, should not be used on thehandling or wear surfaces of controls. This process isuseful for applying function data to valves, and controland dial labels to control panels.6-4.12 ADHESIVE-BACKED LABELSAdhesive-backed plastic or metal labels are a commonmethod for marking items for function and for markingpart locations on equipment. Photoetched metal platesused for part identification are often applied with anadhesive, rather than with screws, whenever the environmentallows this method. Adhesive-backed labels may beapplied on painted surfaces and directly on metal orplastic.6-5 LABELINGLabels are lettered or diagrammatic indications of thename, identifying number, and function of equipment;they are affixed on or near the relevant equipment orfunction. They may include lettered warning signals andabbreviated instructions (both lettered and diagrammatic)relating to the operation or maintenance of the equipment.Operation and maintenance instruction manualsare not often readily available at the equipment, andmeaningful labeling can be a satisfactory substitute inmany instances. When evaluating the need for and contentof labels, it is usually better to overlabel than tounderlabel. The characters, markings, and symbols onlabels and signs should remain sharp, have high contrast,and be resistant to wear.The subparagraphs that follow present recommendationsfor organization and wording, displacement andpositioning, size, and color of labels. The discussion onplacement and positioning also presents some recommendationsfor the application of specific types oflabels--nomenclature, warning, instructions, and identificationof specific subsystems.6-5.1 LABEL CONTENTThe label content must be consistent with the criticalityof the information that it attempts to convey—i.e., partidentification, function of the equipment, or hazards asindicated in par. 6-2. The label content also will be determinedby equipment function, i.e..1. Describe in terms that the typical observer, operator,or technician understands. Engineering characteristics,nomenclature, or other terminology should only beused when a commonly understood term does not exist.6-7
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