Karts are made of steel tubes with different compositions of Carbon, Nickel, Chrome and Vanadium. Such tubes have particular characteristics that determine the functioning of the chassis. Elasticity of the material, diameter of the tubes and thickness of tube walls determine the capacity of the chassis to bend, absorb bumps in the track and generate vertical forces on tyres as to create downward forces that are the base for grip generation between tyre tread and asphalt. All these situations are possible only because steel tubes are linked together in a certain way. This “way” is welding of the tubes one to the other.
What is a welding?
A welding is a volume of material that has been fused and solidified by the heating with different systems of a volume of material. This volume usually is composed by the sum of parts of two different elements that have to be joined one to the other and some additional material added and mixed with the two parts. In karting weldings are used for unifying together the tubes of the chassis and positioning parts such as rear axle bearing mounts. The way the weldings are realized determines much of the quality of the chassis in terms of resistance, flexibility and deformation.
Kart chassis are welded using a welding machine that creates a high tension between a cathode and an anode and, through the passage of energy between the two, generates very high temperatures. This high temperature melts the metal of an electrode and of the tubes of the chassis. These metals mix and solidify together creating a unique metallic element that links the tubes together.
These electrodes are made of iron or, for better quality, of steel. Finally the welded parts will show an area of melted and solidified material that, in ideal conditions and perfect welding procedure, will have almost the same strength and characteristics of the tubes themselves. In reality though, air and high temperature determine oxidation of melted metals and change in the microscopical structure of metals. This can bring to weakness of the welding.
Welding machine and process
A welding machine has the capacity of generating a tension between two electric lines that end one with a clamp, that has to be positioned on the chassis, and one with a changeable electrode that is mounted on a second clamp held by the welder. The welding machine has a scale that regulates the tension between the clamp, and the chassis to which the clamp is linked, and the electrode. When this element is positioned close to the clamp and/or the chassis the high tension between the two elements creates a passage of electricity and the creation of great heat between the parts even though they are not touching each other. The greater the tension regulated on the machine and the earlier and with greater distance between the two elements we will have a passage of electricity and the melting of both the electrode and the area of contact between the electrode and the chassis. This will lead to a fusion between the metal of the tubes in contact and the material from the electrode.
Welding is a complicated process that gives good results only if the welder has the capacity and experience to obtain the right fusion and melting of material from the electrode and the tubes. In fact if tension is regulated too weak no melting effect will be generated and we will only obtain a continuous and repetitive sticking of the electrode to the tubes with difficulty to take it off once it is stuck. On the other hand a too strong tension will determine an excessive fusion of material with holing of the tubes and weakening of the entire structure. The choice of tension must be done based on the thickness of the electrode and the thickness of the elements joint together (walls of the tubes).
The welding done by the welder manually usually is completed in more phases. After each weld, along the line of contact of the two tubes, the area, that has partially cooled down and from an orange-red colour given by incandescence has become grey, must be checked to see what material has really generated a good and strong link of the tubes, and which is instead only laid over the tubes and has no real joining effect. The first “good” weld shows a silver shiny area, the second is mainly a grey unsmooth surface. Best check is made by hitting hard the welded area with a metal hammer and a metallic brush (often given with the welding machine as kit, together with eye protector glasses). If the material is correctly hardened it will not come off even with the hardest hammering you can do, otherwise metal parts will come off, which means they would not anyway resist more then a few minutes when the kart is running and bending.
Practical welding hints
Let us see how to proceed when welding two parts of a chassis. First of all be sure to position and fix well the two parts that need to be joint together. The surfaces that are in touch must be in good contact and well blocked. Then position the clamp on one of the two parts. Be sure the clamp is in good contact with the metal. If the clamp is positioned on a chassis tube verify that paint does not isolate the chassis from the clamp, in fact there must be contact metal to metal to obtain tension transmission to the area that must be welded.
Now try to set voltage on the welding machine starting from a low value, usually indicated on the instruction sheet of the machine depending on the thickness of the electrode used. To warm up the electrode touch quickly the clamp that is surely the element with better tension transmission compared to the chassis of the kart. After the electrode has lit up producing some sparks it is ready for welding. Always cover eyes with welding mask or glasses!!! This secures from sparks in the eyes and even the strong light coming from the lit electrode must be absolutely avoided since it can harm seriously your eyes and anyway partly blinds you for some time.
Start touching quickly the area to be welded and see if sparks are produced. The quickness in doing this is motivated by the fact that if the movement is too slow the electrode sticks to the metal tubes and is very difficult to pull off. The real welding must be done positioning the electrode at a certain distance from the area that has to be treated. Also the electrode must not be vertical respect to the surface to be welded, but inclined. This should permit a continuous movement of the electrode on the surface with production of a strong light and no sticking of the electrode to the metal. If this does not occur and the electrode goes on sticking to the metal we can try to increase the tension on the welding machine and this will help avoid the problem. After each weld hit hard the area with a metal hummer so all the waist material comes off and leaves a good clean surface on which to work on.