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91430 SPS cover edited - Electronic Fasteners Inc

91430 SPS cover edited - Electronic Fasteners Inc

mine the clamping stress

mine the clamping stress at assembly conditions, theelongation should be multiplied by the modulus ofelasticity at room temperature..0033 30.6 10 6 = 101,145 psiThe assembly conditions will be affected by thedifference between th ideal and actual coefficients ofexpansion of the joint. The ideal coeffienct for the fastenermaterial was calculated to be 7.05 but the closestmaterial – H-11 – has a coefficient of 7.1. Since thismaterial has a greater expansion than calculated, therewill be a reduction in clamping force resulting from theincrease in temperature. This amount equals the differencebetween the ideal and the actual coefficientsmultiplied by the change in temperature, the length ofthe fastener, and the modulus of elasticity at 70°F.[(7.1 – 7.05) 10 –6 ][800 – 70][1.333] [30.6 10 6 ] = 1,490 psiThe result must be added to the initial calculatedstresses to establish the minimum required clampingstress needed for assembling the joint at roomtemperature.101,145 + 1,490 = 102,635 psiFinally, the method of determining the clampingforce or preload will affect the final stress in the joint atoperating conditions. For example, if a torque wrench isused to apply preload (the most common and simplestmethod available), a plus or minus 25 per cent variationin induced load can result. Therefore, the maximum loadwhich could be expected in this case would be 1.5 timesthe minimum, or:(1.5)(102,635) = 153,950 psiThis value does not exceed the room-temperatureyield strength for H-11 given in Table 19.Since there is a decrease in the clamping force withan increase in temperature and since the stress at operatingtemperature can be higher than originally calculatedbecause of variations in induced load, it is necessary toascertain if yield strength at 800°F will be exceeded(max stress at 70°F + change in stress) E at 800°FE at 70°F[153,950 + (–1490)] 24.6 10 6 = 122,56530.6 10 6This value is less than the yield strength for H-11 at800°F, Table 19. Therefore, a 1/4-28 H-11 bolt stressedbetween 102,635 psi and 153,950 psi at room temperaturewill maintain a clamping load 1500 lb at 800°F after1000 hr of operation. A cyclic loading of 100 lb, whichresults in a bolt loading between 1500 and 1600 lb willnot cause fatigue failure at the operating conditions.Table 16PHYSICAL PROPERTIES OF MATERIALS USED TO MANUFACTURE ALLOY STEEL SHCS’SCoefficient of Thermal Expansion, µm/m/°K 120°C to 100 200 300 400 500 60068°F to 212 392 572 752 932 1112Material5137M,51B37M 2 – 12.6 13.4 13.9 14.3 14.64137 3 11.2 11.8 12.4 13.0 13.6 –4140 3 12.3 12.7 – 13.7 – 14.54340 3 – 12.4 – 13.6 – 14.58735 3 11.7 12.2 12.8 13.5 – 14.18740 3 11.6 12.2 12.8 13.5 – 14.1Modulus of Elongation (Young’s Modulus)E = 30,000,000 PSI/in/inTable 19 - Yield Strength at Various TemperaturesAlloy–––––––– Temperature (F) ––––––––70 800 1000 1200Stainless SteelsType 302 35,000 35,000 34,000 30,000Type 403 145,000 110,000 95,000 38,000PH 15-7 Mo 220,000 149,000 101,000 –High Strength Iron-Base Stainless AlloysA 286 95,000 95,000 90,000 85,000AMS 5616 113,000 80,000 60,000 40,000Unitemp 212 150,000 140,000 135,000 130,000High Strength Iron-Base AlloysAISI 4340 200,000 130,000 75,000 –H-11 (AMS 6485) 215,000 175,000 155,000 –AMS 6340 160,000 100,000 75,000 –Nickel-Base AlloysIconel X 115,000 - - 98,000Waspaloy 115,000 - 106,000 100,000NOTES:1. Developed from ASM, Metals HDBK, 9th Edition, Vol. 1 (°C = °K for values listed)2. ASME SA5743. AISI4. Multiply values in table by .556 for µin/in/°F.70

CORROSION IN THREADED FASTENERSAll fastened joints are, to some extent, subjected to corrosionof some form during normal service life. Designof a joint to prevent premature failure due to corrosionmust include considerations of the environment, conditionsof loading , and the various methods of protectingthe fastener and joint from corrosion.Three ways to protect against corrosion are:1. Select corrosion-resistant material for the fastener.2. Specify protective coatings for fastener, joint interfaces,or both.3. Design the joint to minimize corrosion.The solution to a specific corrosion problem mayrequire using one or all of these methods. Economicsoften necessitate a compromise solution.Fastener MaterialThe use of a suitably corrosion-resistant material isoften the first line of defense against corrosion. In fastenerdesign, however, material choice may be only one ofseveral important considerations. For example, the mostcorrosion-resistant material for a particular environmentmay just not make a suitable fastener.Basic factors affecting the choice of corrosion resistantthreaded fasteners are: Tensile and fatigue strength. Position on the galvanic series scale of the fastener andmaterials to be joined. Special design considerations: Need for minimumweight or the tendency for some materials to gall. Susceptibility of the fastener material to other typesof less obvious corrosion. For example, a selectedmaterial may minimize direct attack of a corrosiveenvironment only to be vulnerable to fretting or stresscorrosion.Some of the more widely used corrosion-resistantmaterials, along with approximate fastener tensilestrength ratings at room temperature and other pertinentproperties, are listed in Table 1. Sometimes the nature ofcorrosion properties provided by these fastener materialsis subject to change with application and other conditions.For example, stainless steel and aluminum resistcorrosion only so long as their protective oxide filmremains unbroken. Alloy steel is almost never used, evenunder mildly corrosive conditions, without some sort ofprotective coating. Of course, the presence of a specificcorrosive medium requires a specific corrosion-resistantfastener material, provided that design factors such astensile and fatigue strength can be satisfied.Protective CoatingA number of factors influence the choice of a corrosionresistantcoating for a threaded fastener. Frequently, thecorrosion resistance of the coating is not a principalconsideration. At times it is a case of economics. Often,less-costly fastener material will perform satisfactorily ina corrosive environment if given the proper protectivecoating.Factors which affect coating choice are: Corrosion resistance Temperature limitations Embrittlement of base metal Effect on fatigue life Effect on locking torque Compatibility with adjacent material Dimensional changes Thickness and distribution Adhesion characteristicsConversion Coatings: Where cost is a factor and corrosionis not severe, certain conversion-type coatings areeffective. These include a black-oxide finish for alloy-steelscrews and various phosphate base coatings for carbonand alloy-steel fasteners. Frequently, a rust-preventing oilis applied over a conversion coating.Paint: Because of its thickness, paint is normally notconsidered for protective coatings for mating threadedfasteners. However, it is sometimes applied as a supplementaltreatment at installation. In special cases, a fastenermay be painted and installed wet, or the entire jointmay be sealed with a coat of paint after installation.TABLE 1 – TYPICAL PROPERTIES OF CORROSION RESISTANT FASTENER MATERIALSTensile Yield Strength Maximum Mean Coefficient PositionMaterials Strength at 0.2% offset Service of Thermal Expan. Density Base Cost on GalvanicStainless Steels (1000 psi) (1000 psi) Temp (F) (in./in./deg F) (lbs/cu in.) Index Scale303, passive 80 40 800 10.2 0.286 Medium 8303, passive, cold worked 125 80 800 10.3 0.286 Medium 9410, passive 170 110 400 5.6 0.278 Low 15431, passive 180 140 400 6.7 0.280 Medium 1617-4 PH 200 180 600 6.3 0.282 Medium 1117-7 PH 200 185 600 6.7 0.276 Medium 14AM 350 200 162 800 7.2 0.282 Medium 1315-7 Mo 200 155 600 – 0.277 Medium 12A-286 150 85 1200 9.72 0.286 Medium 6A-286, cold worked 220 170 1200 – 0.286 High 771

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