How does an Axial Thrust or Axial Gap Transmitter work

Axial Thrust or Axial Gap Transmitters are used to find out the Displacement of a Shaft in the axial direction. This helps operators of the equipment understand whether or not their equipment - be it a pump, fan, turbine, etc. is safe in axial tolerances.

Let us try to understand how IRD7100 - our Machinery Protection Transmitter monitors Axial Gap and how does it provide the proportionate 4-20 mA output.

Let us assume that we have an Eddy Current Probe mounted axially to a shaft and connected to our IRD7100 Machinery Protection Transmitter. Let us assume the range of the transmitter to be +/- 1 mm. We want to monitor the gap in our Control Room and thus wish to use the 0-5 V DC output of the Transmitter or the 4-20 mA output of the transmitter. What shall we take as 4 mA and what shall we consider as 20 mA. The answer for this can be clearly seen through the simple sketch below:

Thus, we can see that when the sensor is set at an initial gap from the sensor. This gap should be -10 to -12 V DC. In our example we have considered -10 V DC to be the initial gap. This is denoted as position 0 mm for axial gap measurement. The target (shaft) can move closer or move away from the sensor while running. If it moves closer, then the movement is denoted as negative with -1 mm being the closest it can get for the transmitter. If it moves away, then the movement is denoted as positive with +1 mm being the farthest it can get from the transmitter.

Thus, when axial gap displayed on transmitter is -1 mm, it will mean that the sensor gap voltage is -2 V DC and the current output will be 4 mA. When the axial gap displayed on transmitter will change to 0 mm, it will mean that the sensor gap voltage is -10 V DC and the current output will be 12 mA DC. Last but not the least, when the axial gap displayed on the transmitter will change to +1 mm, it will mean that the sensor gap voltage is -18V DC and the current output will be 20 mA.

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