| Aerospace compression springs. | | | | coating. This not only improves corrosion resistance |
| When you consider aerospace compression springs, | | | | but also improves the fatigue performance; the |
| whatever calculation method is used the spring | | | | soft-metal coating reduces the surface roughness |
| designer does have to make a decision as to what | | | | which may arise when drawing, and from what I |
| stress to use, and this depends on the material. And | | | | have said already you will appreciate that any surface |
| that, in turn, means "what we can get". Some | | | | defect on a coil spring wire is highly undesirable. (Even |
| desirable types are just not available in reasonable | | | | a thin coating of rust!) |
| quantities and others need heat treatment after | | | | Hard-drawn stainless steel springs are a very useful |
| winding. Most springs are wound cold, from hard wire. | | | | material, especially for situations where the |
| That hardness is achieved by repeated drawing | | | | temperature may be high or there is risk of corrosion |
| through dies, and each drawing operation increases | | | | - it can be worked up to 300 deg. C, whereas carbon |
| both the shear and tensile strength-it is work | | | | steel wire is a little unhappy above 125 deg. C. It has |
| hardened. So, the shear stress we can use does | | | | an elastic limit in shear very slightly higher than regular |
| depend on the wire size - in general the smaller the | | | | carbon steel. (The working stress must, of course, |
| diameter the higher the allowable stress. This can be | | | | be reduced when applied in hot environments). This |
| surprisingly high to those used to normal working | | | | material is expensive and not too easy to wind. Hard |
| stresses - the torsional YIELD point - elastic limit - of | | | | drawn phosphor-bronze also is non-corrosive so far |
| 10 thou. piano wire, for example, is nearly 200,000Ibf | | | | as steam/water is concerned, but normally |
| sq.in. - about 1400 Newton/ sq.mm - and falls only to | | | | recommended for continuous use only below about |
| 140,0001bf/ sq.in. at 0.080in. die. For most duties we | | | | 110 deg. C. It is relatively easy to obtain in a wide |
| have to keep below this elastic limit and the usual rule | | | | range of gauges or to metric dimensions. |
| for aerospace compression springs is that when it is | | | | Aerospace nickel alloy springs. |
| compressed so that all coils are touching, the stress | | | | Nickel alloy springs have corrosion resistance plus |
| should lie just below this elastic limit. | | | | superior strength and heat resistance. This grade of |
| The "regular" spring wire we get is what is known as | | | | wire possesses high elastic qualities similar to music |
| "Patented carbon steel spring wire". The word | | | | wire while maintaining the corrosion resistant qualities |
| "patented" does not refer to the patent office, but | | | | of standard stainless. Nickel alloy springs are an |
| to a process applied to the steel to make it easier to | | | | excellent material for all kinds of springs where long |
| draw into smaller gauges. It is typically between 0.65 | | | | life is required under severe service conditions, |
| and 0.75% carbon with perhaps 0.75% of manganese | | | | providing excellent fatigue properties. |
| but no alloying content. It is available in the usual | | | | The higher prime spring material, chrome especially, |
| Standard Wire Gauge size but is also to be had in | | | | vanadium steel, for example, must be heat treated |
| metric diameters. | | | | after winding, as must beryllium-copper - i.e. the |
| Also fairly readily available is "Piano" or "Music" wire | | | | spring is wound in the "soft" condition and then |
| which, as its name implies, is intended for use in | | | | hardened. Monel can be wound hard drawn and will |
| stringed instruments. It has a higher carbon content - | | | | safety withstand both sea-water and corrosion and |
| 0.85-0.95% - to give a higher tensile strength, for, in | | | | temperatures up to 225deg.C.The shear elastic limit is |
| pianos especially, the wires are very tightly stretched. | | | | about the same as carbon phosphor-bronze. Finally, |
| The shear strength is correspondingly increased. It | | | | hard-drawn 70/30 brass is a very cheap spring |
| has the advantage (apart from higher permissible | | | | material for cases where mild conditions apply. It has |
| stresses) that it is available in many more sizes which | | | | strength properties about two-thirds of phosphor |
| come in between the s.w.g. diameters, but the | | | | bronze and should not be used above 80 deg. C but, |
| disadvantage is that the higher tensile strength | | | | oddly enough, can be used at low temperatures. The |
| makes it rather more difficult to wind. Both of these | | | | most usual application of "spring brass", however, is |
| carbon steel wires can be had either zinc or cadmium | | | | for flat springs, especially where sharp bends may be |
| coated (not electroplated) before the final drawing | | | | needed. |
| process - the wire is drawn through the dies after | | | | |