Machinery's Handbook, 31st Edition
Spring Materials 309 and therefore do not undergo variations in spring stiffness because of modulus changes due to temperature differentials. They also have low hysteresis and creep values which makes them preferred for use in food-weighing scales, precision instruments, gyro- scopes, measuring devices, recording instruments and computing scales where the tem- perature ranges from − 50 to +150°F. These materials are expensive, none being regularly stocked in a wide variety of sizes. They should not be specified without prior discussion with spring manufacturers because some suppliers may not fabricate springs from these alloys due to the special manufacturing processes required. All of these alloys are used in small wire diameters and in thin strip only and are covered by US patents. They are more specifically described as follows: Elinvar * : (nickel, iron, chromium) This alloy, the first constant-modulus alloy used for hairsprings in watches, is an austenitic alloy hardened only by cold-drawing and cold- rolling. Additions of titanium, tungsten, molybdenum and other alloying elements have brought about improved characteristics and precipitation-hardening abilities. These im- proved alloys are known by the following trade names: Elinvar Extra, Durinval, Modul var and Nivarox. Ni-Span C * : (nickel, iron, chromium, titanium) This very popular constant-modulus alloy is usually formed in the 50 percent cold-worked condition and precipitation-hard ened at 900°F for 8 hours, although heating up to 1250°F for 3 hours produces hardnesses on the Rockwell C scale of 40 to 44 RC, permitting safe torsional stresses of 60,000 to 80,000 pounds per square inch. This material is ferromagnetic up to 400°F; above that temperature it becomes non-magnetic. Iso-Elastic † : (nickel, iron, chromium, molybdenum) This popular alloy is relatively easy to fabricate and is used at safe torsional stresses of 40,000 to 60,000 pounds per square inch and hardnesses of 30 to 36 RC. It is used principally in dynamometers, instru ments, and food-weighing scales. Elgiloy ‡ : (nickel, iron, chromium, cobalt) This alloy, also known by the trade names 8J Alloy, Durapower, and Cobenium, is a non-magnetic alloy suitable for sub- zero temperatures and temperatures up to about 1000°F, provided that torsional stresses are kept under 75,000 pounds per square inch. It is precipitation-hardened at 900°F for 8 hours to produce hardnesses of 48 to 50 RC. The alloy is used in watch and instrument springs. Dynavar § : (nickel, iron, chromium, cobalt) This alloy is a non-magnetic, corrosion- resistant material suitable for sub-zero temperatures and temperatures up to about 750°F, provided that torsional stresses are kept below 75,000 pounds per square inch. It is precipitation-hardened to produce hardnesses of 48 to 50 RC and is used in watch and instrument springs. Spring Stresses Allowable Working Stresses for Springs.— The safe working stress for any particular spring depends to a large extent on the following items: 1) Type of spring — whether compression, extension, torsion, etc. 2) Size of spring — small or large, long or short 3) Spring material 4) Size of spring material 5) Type of service — light, average, or severe 6) Stress range — low, average, or high *Trade name of Soc. Anon. de Commentry Fourchambault et Decazeville, Paris, France. †Trade name of John Chatillon & Sons. ‡Trade name of Elgin National Watch Company. §Trade name of Hamilton Watch Company.
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