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The engineers, technicians and designers know what the KEY is for, has the function of joining two mechanical elements in order to transmit torque, for example, the shaft-to-gear coupling, with couplings and etc..
Recalling an accident that occurred in the decade of 90 in assembling a Porteiner, in which keyway failure caused the boom to fall (key connecting the reducer shaft with the gear), the energy generated by the fall of the boom when hitting the end of the course, caused the rear legs of the machine to be lifted twice until the swing stopped, causing damage to various parts of the machine frame, miraculously there were no fatalities, but it could have happened as there were many assemblers on the machine, I remember an assembler that stuck to the leg of the machine, which only came out after a lot of psychology.
Due to this accident that was not our technical responsibility, because the reducer was from the supplier, and the same was given all the necessary information so that this would not happen, I was interested in deepening the calculations, and it was possible to observe that in certain cases, the sizing of the key should be designed taking into account several factors, how will i inform in this article.
Key sizing is very simple, involves two criteria, learned in college:
1 – Key Shear – T = Force / Key Area < Allowable T of key material
2 - Side crushing of the key with the mechanical elements (axle, gear and etc.) – S = Force / Lateral area < S allowable of key material or mechanical element
What changes?
In calculating lateral crush, pressure should take into account the following multiplicative factors:
1 – Fs – Service factor = f1 x f2 (f1 - operating factor and f2 - triggering factor). For equipment that follows the ABNT NBR Standard 8400, Porteiner's case, the Fs factor can vary between 1,00 a 1,25, mechanical group function.
2 – Fc – Charge concentration factor (One key - 1,0 x Fc; Two Keys - 0,6 x Fc). The Fc value is a function of the input load distribution on the mechanical element and on the output through the key, where the load concentrates, in addition to the ratio of key length to shaft diameter, and by the average diameter of the mechanical element, surrounding the outer diameter, hub diameter, key width and mechanical element width. This value can vary between 1,05 a 1,7.
The allowable voltage must also be taken into account., the following multiplicative factors the yield stress of the key material or mechanical element:
1 – Adjustment factor (friction) – fA - Factor that takes into account the effect of the pressed adjustment in reducing the "fretting corrosion" (corrosion produced by relative micro displacements between two surfaces in contact. For example between key and hub or key and shaft), this value is a function of the maximum drive torque and the minimum torque of the pressed adjustment, and lies between 0,5 a 1,0)
2 – Factor fC - Factors that take into account the numbers of cycles (pulsating or reversible), during the useful life. Those load peaks where the moments clearly exceed the Fs x Nominal Torque value. This value can vary between 0,4 a 1,6.
concluding, I would like to point out that the simple mechanical element like a key, in specific cases, the dimension considered theoretically correct in the Faculty, does not produce a reliable result in practice, needing to include all those factors mentioned above that aim to bring theory closer to reality.
Alert to newly graduated Calculating Engineers, when key break occurs, do not say: "I calculated as I learned at Faculty and, therefore, there should be no problem". Always remember that theories were developed for an ideal world, and we live in a world with many variables, why we, calculating engineers, we have important relevance in society, it's not simple formula applications, that any computer does better than us, it is necessary to know how to assess each situation, all have their particularities, that only the calculating engineer can correctly define, as long as they feel like (passion) to continually improve, the good engineer is an eternal student and humbles to learn from the rare specialists in Brazil, that are in extinction!
Check out other materials in FP for Project, calculation and selection of keys!