1. Method of Thermal Extension: This method is also called thermal expansion or hot alignment method. It requires to heat the shaft and its bearings to a pre-determined temperature value, usually 250℃ to 350℃ , and then measure the distances between the shaft and bearings with a suitable shaft protrusion indicator. One of the advantages of this method is that the brinell hardness or surface irregularities have no influence on the measurement. This method is very precise, but it is difficult to apply when measuring the shafts with length over 500mm, especially when the shafts are vertically mounted on the wall.
2. Method of Optical Measurement: This method uses an optical tool known as “Optical Geometrical Set”- primarily comprising of a telescope and a graduated scale to measure the distance between the shaft and bearings precisely. This set uses the difference in angular divergence of two lines drawn on the rotating shaft and its bearings when the axis is moved in and out of alignment to take the measurement. To ensure high accuracy of the readings, the reference lines should be as long as possible and free from scratches, stains or other faults that may affect their straightness.
3. Method of Magnetic Sensing: This method applies a permanent magnetic field that is used to actuate the shaft or bearing. An electronic sensor is used to measure the response of the shaft or bearing frequency in presence of the magnetic field. In this technique, the mounted shaft is opened to the magnetic field, and the response is measured by the sensor, which gives the necessary information about the clearance. Magnetic sensing is capable of detecting even the smallest of gaps. The main advantage of this technique is that its readings are not affected by the surface irregularities, as it is based on detecting the gap between two parts.
Each of these techniques has its own advantages and applications. The choice of the method to be used is dependent upon the design and capacity of the shaft, the amount of material to be taken up, the accuracy and precision required, etc. Moreover, the selection also depends upon the labour costs, time and money that are involved in a particular method. The selection of the appropriate technique should therefore be based on the particular requirements of a particular application.
