Springs in agriculture: mechanical components and working parts
The springs are common application in the technic, and are used for very different purposes. In agricultural mechanics they play a fundamental role, also as working tools. In this case, the wear to which they are subjected can be minimized thanks to thanks to special treatments of the steel with which they are manufactured
In the framework of agricultural mechanics, particularly the traditional one, springs are very commonly used, not only as mechanical parts with obvious elastic functions, but also as real working parts, of the ground or of different agricultural parts, such as the teeth of some harrows, or balers pick-up, side-delivery rakes, tedders, etc. From this point of view, springs can be used in many different ways, and consequently, their shape can be extremely different from one another. As a general rule, the springs applied to agricultural machinery are differentiated mainly in terms of their function, namely: torsion (single or double): the wire, with a diameter that is commensurate with the compressive force to be exerted, is wound around itself; the two terminal points are the ones subjected to force. The main difference between single or double torsion springs lies in the number of gimped bodies; tensile stress: they work by extension, that is to say opposing resistance to the force applied along the main axis, which causes a more or less accentuated elongation. The danger, in this case, is the yielding, that is to say, the overcoming of a determined elongation limit value, which determines a plastic or permanent deformation; compression: like the previous ones, these types of springs also work with forces applied along the main development axis, but in this case, they are subject to a shortening, which reduces the distance between the turns (sometimes up to cancel it).
In addition to the springs working in the equipment following the modalities described, in agriculture is now a consolidated experience the adoption of springs which form the elastic working parts of some operating machines, including tedders, hay rakes, windrowers, packing machines, self-loading wagons and machinery for soil preparation, such as elastic tines harrows. Although apparently different in terms of shape, ruggedness, and mode of action, they all share a twist and are therefore characterized by a spiral body and two external arms, on which the forces to which they must react are applied.
Typical are the working parts of the elastic tine harrow and of the rake: in the first case it is a device that, partially sunk into the ground, makes the strip of steel metal material (or its alloys) work in torsion. The torsion force is practically developed by the resistance opposed by the soil to the sliding of one of the ends of the spring, while the machine continues to travel linearly. This causes the crushing of the clods previously produced by the plowing of the soil. The opposite (upper) end of each spring/tooth is instead bolted to the harrow frame.
Despite working differently, the working part of the rake has a similar shape of the teeth which, although with a decidedly lower strength than those of the harrow due to the lower resistance opposed by the processed product (i.e. forage instead of soil). They are, however, made with steel strips or alloys with circular section wires. In this case, the teeth consist of a spring working with a double twist (and therefore is composed of two spiral bodies), which in order to move the mowed material efficiently have both ends oriented downwards, so that the spring is specular on the horizontal plane. In detail, the spring-working part is inserted by sliding in its seat and then fixed with a bolt.
The types of steel
The materials used for the construction of the springs must be commensurate with the intended use (iron, steel, alloys, etc.). For springs for strictly component use, carbon steel wire is widely used, in compliance with the UNI EN 10270 standard. These materials are characterized by different performances, according to the denomination (SM, SH or DH) and the diameter of the wire considered, usually between 1 and 20 mm. In greater detail, the SM marked wire is suitable to withstand medium-high static or rare dynamic stresses, the SH one is fine for higher static loads or more dynamic stresses, while the DH is indicated when the static inputs are high, and the dynamic ones are mid-level. Essentially, the chemical composition of the steels is similar, with a little increase in carbon and a decrease in phosphorus, sulfur, and copper for the better performing alloy.
Finally, the same standard also normalizes stainless steel springs, which are advantageous to use when exposure to corrosive agents and significant temperature variations is expected.
In this case, with a slightly lower elasticity than the types already described, the chromium content in the alloy is logically remarkable (from 16 to 19%), with also a significant presence of nickel (6-9.5%), as well as of silicon and manganese (2% both).
Wear
When used as working or machine parts, springs are usually subject to abrasive, erosive and corrosive wear.
The first two types mainly relate to the working parts coming into contact with the ground or with “tenacious” materials, thus producing an intense friction due to sliding, which turns out to increase as the silica component of the material increases.
Abrasive wear, caused by the impact and movement of the tool, consumes, by thinning it, the working part, and it is not possible to exclude the unexpected damage due to accidental impact with stones of considerable size. Instead, erosive wear takes place in the opposite way: if the abrasive wear is caused by the impact of the working part against the material to be processed, in this second case it is the abrasive substance that hits the object.
This is the case, for example, with the teeth of self-loading wagons, rakes, etc. Indeed, it is less wearing than abrasion, but if undervalued, it can cause injury and breakage of the working parts.
Finally, the corrosive wear is chemical rather than physical and is caused by prolonged contact with substances reacting with metal, resulting in a weakening of its structure. Corrosion can involve indifferently both the working parts and the internal springs and depends on the contact with chemical agents of very different nature, such as mineral fertilizers, pesticides, livestock manure, washing liquids, or more simply, water itself, both of meteoric and anthropic origin.
A further type of wear, similar to the corrosive one, is the oxidative one, for which the damaging agent is the oxygen of the atmosphere, which harmful effect is enhanced by conditions of high humidity.
Spring treatments
To ensure greater resilience, a longer life and a pleasant aesthetic appearance, the materials used for the construction of the springs are subjected to different treatments, aimed at increasing the strength of the finished product, protecting it from corrosion and more generally minimizing the possible damage caused by an environment hard from the environmental point of view, as it typically is the agricultural one. However, the main treatments are corrosion protection. In detail: browning: it gives an artificial coloring of the metal surface, also preventing oxidation; zinc coating: applying a thin layer of zinc over the entire surface of the wire before the spring is made it also prevents corrosion (at least partial) from corrosive agents; the so-called “Geomet”: commercial definition of a process aimed at the non-galvanic coating of metal based on zinc and aluminum, with an anticorrosive effect. It is used for elements that can come into contact with aggressive chemicals, such as e.g. acids; phosphating: in addition to a higher resistance to corrosion, it represents a valid substrate for subsequent varnishing; tumbling: it is a finishing of printed or cast metal, made with abrasive elements. It is produced rolling inside a gyratory sifting; shot peeling: it is a finish by hardening of cast or pressed metal parts, carried out by impact with spherical pellets, kept in whirling motion with centrifugal impellers or compressed air flows; powder coating: the metal is coated with a film of paint, which adheres to the surface in an electrostatic manner. In addition to improving the aesthetic appearance of the finished product, it limits its sensitivity to aggressive agents, and therefore to corrosion; cataphoresis treatment: anti-corrosion treatment of metal elements, by depositing epoxy or acrylic resins.
Special springs
In addition to the more typical springs working by traction, compression or torsion, there are also numerous other types of springs (so-called “special”) that differ in shape, operating mode and field of use. Without pretending of being exhaustive, (the subject is undoubtedly very vast), we can mention: the belt springs: flat section and shaped in concentric windings; Belleville washers: they work with compression, and have a typical disc shape, like a truncated cone-shaped washer; leaf springs: they also work with compression, and are the result of the assemblage of a number of metal plates; volute springs: typically used on shears and similar work tools, they are made with a metal band whose windings, instead of being flat, as in the band springs, are precisely arranged, giving the device a conical or biconical shape; constant force springs: these are very compact springs, made up of a ribbon rolled onto a drum, which, after reaching the maximum tension load, exert a constant force; balance/setting springs: mainly used in watchmaking, they have a typical oval or elliptical shape and their function is to keep the resonance frequency constant while the hands turn.
Another special type of spring is the nautical one, used for boat mooring; its characteristic structure allows the spring to work simultaneously in tension and compression.
