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The torque requirements of a gear box are normally drilled down from the application, where the gearbox is coupled. For example, in ready made concrete mixer machine, the main torque requirement and speed of the output shaft of the gear box comes from the rotating drum. And depending upon the gear reduction ratio the torque requirement is transferred to the input shaft of the gear box. So, from the above discussion it should be clear that the torque requirement of the gearbox input shaft will be different. And you can find out the torque requirement by two ways: Strain gauge method and Calculation method.
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Strain Gauge Method
If you apply torque to a shaft then the fibers of the shaft will be under torsional stress. Or, in other words, the fibers will be subjected to the tension in one direction and will be subjected to compression to another direction. The fact is true for the gear box input and output shaft as well. The below picture may help you clarify the concept:
The strain gauge transducer has two parts: the strain gauge and the transducer display and processing unit. The strain gauge senses the torque and sends the signal to the transducer signal processing unit, which in turns connected to the digital display unit. The display unit displays the torque value.
So, for finding out the torque requirement of the gear box input and output shaft, you have to place the strain gauge transducer to the shafts.
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You can use the standard equations available for calculating the torque requirement of the gear box shaft. The basic equations used here are:
P (KW) = (2/60000)*Π*N (Rpm) *T (N.m)……………..Eqn.1
N1/N2 = M2 / M1………………..Eqn.2
P – Power in KW
N – Speed in Rpm
T – Torque in N.m
M- Number of teeth of the respective gear.
First thing you have to know is the Power requirement of the application. Take the concrete mixer example again, after you know the power requirement of the rotating drum, you can apply the Eqn.1 and find out the torque requirement (T) of the gear box output shaft.
Next, use the Eqn.2 to find out the speed (N) of the input shaft of the gear box and again use the Eqn.1 and find out the torque requirement of the input shaft of the gear box.
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We have two methods for how to measure torque requirement of gear box: either by using the strain gauge transducer or by the calculation method using the known mechanical design equations. Fitting the strain gauge is a skillful job and requires different installation procedure prescribed by the respective strain gauge manufacturer.