Repair parts - 20/04/2022
Thanks to the combined action of a helical compression spring, cylinder and tie-rod, the recoil spring in tracked vehicles acts as a protection against overloads caused by impacts or rough terrain.
The coil spring absorbs the shocks applied to the front idler when the machine meets an obstacle. In addition, it corrects the tension on the chain by injecting grease into the cylinder. This occurs through a control valve that allows the recoil spring to move forward thanks to the hydraulic pressure. Lastly, the tie-rod keeps the spring constantly pre-loaded, keeping it joined with the cylindrical body. But let's see in detail the characteristics of each of these components.
The spring is produced from circular section wire, according to specific design objectives, including a good quality/price ratio and the ability to be placed in the available space. The latter is a constraint that governs the dimensional limits of the spring's operating length, in addition to its internal and external diameter. These measurements establish the spring tension along with other requirements that need to be known beforehand: load, deflection, and maximum packed length.
When a compression coil spring is loaded, the wound wire is torsionally loaded. The tension is therefore greatest on the surface of the wire and on the inner edge of the coil. The range of tensions governs the life of the spring: the greater it is, the lower the maximum tension must be to have a comparable life.
The choice of materials suitable for the production of compression coil springs follows the dictates of the standards on spring steels. In addition, the materials chosen must meet other requirements such as:
In spring steels, the required properties are obtained through high carbon contents and constituents such as silicon, manganese, chromium, molybdenum and vanadium, and through heat treatments such as tempering.
The importance of the production process
The quality and efficiency of the springs depend on their production process, typically characterized by three phases: forming, heat treatment and post treatment.
There are various spring forming methods:
There is a heat treatment for each type of spring that serves to give it strength.
The small and medium-sized springs, which will not undergo high stress during use, need a stabilization treatment in small ovens that operate at temperatures between 300 and 400°C.
The large springs, which during use will be subject to high stress, need both a hardening and tempering treatment.
The hardening consists in the rapid heating of the springs in an oven at 860°C with consequent sudden cooling in an oil bath. It is used to obtain a good compromise between toughness and hardness.
A tempering treatment follows in order to achieve a good compromise between the hardness, strength and toughness of the steel.
After the heat treatment, the spring is subjected to the presetting process which consists in winding the spring up to a free length greater than the specified free length.The spring at room temperature is compressed to its solid length or to a specific preset length to generate yield.
Possible problems with coil springs
Like any other component, coil springs can also face some problems. The main ones concern breaking, which can happen for various causes including:
This is a high-strength component because when the spring is deflected a load greater than the preload acts on it. The load applied to the tie-rod is of two types:
Load caused by the impact of the nut against the yoke when the compressed spring returns from the stroke end position (for example, the same load value at the stroke end);
Load present if there is offset between the axis of the spring and the axis of the front idler wheel, or the bending load caused by the bending moment together with the end-of-stroke load.
Being subjected to such stresses, the tie-rod must be produced from a steel alloy, and tempering steel is most widely used by virtue of its good strength, forgeability and machinability. Furthermore, it must be suitable for all hot uses up to 500°C to resist creep plasticity. It can also be surface hardened or nitrided to make it more resistant to wear and fatigue for parts not subject to too high specific pressures.
By injecting grease into the cylinder through the check valve, the recoil spring moves forward and tensions the chain. Vice versa, letting the grease out through the valve loosens the chain causing the recoil spring to retract.
When assembling the cylinder, a pump sends grease to the cylinder via the valve. This grease moves the piston which pushes the entire track tensioner. The control valve, on the other hand, must be designed and applied taking into account the maximum operating pressure. The surface must be chrome-plated for greater resistance to wear and corrosion.
The material chosen for the production of the cylinder is usually a classic tempered steel containing a percentage of carbon such as to obtain maximum toughness.
The cylinder must necessarily be connected to the tie-rod inside the recoil spring. This tie rod-cylinder connection can be made through different configurations such as:
Below we illustrate the various types of recoil springs supplied for some of the most important brands on the market.
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