11. Technical Library: Mechanical Fastening
Design & Design Strengths
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Austenitic stainless steels have excellent resistance to corrosion by acetic acid emitted by wood. They are NHBC recommended for a range of fasteners and fixings in timber. For use in immersed timber, the choice of grade depends on the specific water conditions, including chloride level and flow rate. Careful grade selection is also required for fixings in timber in swimming pool buildings.
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The moisture level in timber is the most important factor in determining the incidence of corrosion. Above a threshold moisture level, wood is always acidic due to the breakdown of wood cellulose to acetic acid. Applied chemical treatments to the timber or exposure in a marine environment can increase the corrosion risk.
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This article compares the chemical composition of a number of 316 type grades as covered by the now superseded BS1449 and BS970 and their replacement, BS EN 10088 Parts 2 and 3. Grades covered include 316S11, 316S12, 316S13, 316S16, 316S31, 316S33, and European steel numbers 1.4401, 1.4404, 1.4432, and 1.4436
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Galling, sometimes known as cold welding, is a form of severe adhesive wear which can occur when two metals are in relative motion and under sufficient load to permit the transfer of material. Severe galling can result in seizure of metal components. The key factors affecting the tendency for galling are design tolerances and the surface finish, hardness and microstructure of the metals in contact.
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Galling, sometimes known as cold welding, is a form of severe adhesive wear which can occur when two metals are in relative motion and under sufficient load to permit the transfer of material. Austenitic and precipitation hardening stainless steels have poor resistance to galling. Hardenable martensitic stainless steels have better galling resistance. The galling characteristics of duplex stainless steels is thought to be similar to that of austenitic stainless steels.
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Galling, sometimes known as cold welding, is a form of severe adhesive wear which can occur when two metals are in relative motion and under sufficient load to permit the transfer of material. Lubrication in improves galling resistance. A solid lubrication system such as a PTFE coating gives better galling resistance than greases. Altering the surface characteristics by nitriding or chromium plating also improves wear and galling resistance.
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This article covers how to design stainless steel bolts (fasteners) subject to shear and tension forces. Shear and tension capacities for bolts of property class 50, 70 and 80 from diameter M10 to M24 are given. (207)
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Maximum breaking torque values for property class 50, 70 and 80 austenitic fastener grades A1, A2, A3 A4 and A5 taken from BS EN ISO 3506-1 are shown. A link is provided to the Bufab web site where maximum tightening torque, yield load and failure load for austenitic stainless steel fasteners are tabulated.
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