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Rotating shafts have applications as power transmitting devices everywhere around us. Car engines, aeroplane engines, fans,etc use a rotating shaft to transmit power. These shafts can experience whirling if they are not properly machined or installed. An unbalance in the shaft, misalignment of bearings or an external impact affects the life of the shaft. It will fail if it reaches its critical speeds.
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To avoid this, theoretical calculations are used for finding the critical speed of a particular shaft. In this project, you will device an experimental setup for finding the critical speeds of the shaft while it is rotating. The challenge here is creating a disturbance in the shaft without any contact.
Arrange an electromagnet for creating an impact on a rotating shaft. Get an AC-DC power supply of the same voltage as the electromagnet and a DC motor for rotating the shaft.
Setup the shaft and motors using bearing and an appropriate coupling. Get the shaft turned before fixing in the setup to eliminate any unbalance.
Connect the electromagnet to the power supply and use a Gauss meter to check its magnetic field strength. This helps to find the distance within which the magnet has an effective magnetic field.
Once you know the distance, place the electromagnet in the setup below the shaft. Make sure to fix it in place. Choose a strong electromagnet and a shaft of equivalent diameter. If the shaft is too heavy then it will be difficult to reach high rpm of rotation. Electromagnet won’t create any disturbance on a heavy shaft.
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Start the motor and check for any whirling in the shaft or vibrations in the setup. If any exist then there is some error in the setup. Eliminate any existing causes for whirling of shaft vibrations in setup.
Once the shaft is stable, turn on the electromagnet for a fraction of a second. To give an impact force the time of application should be very small.
Use a potentiometer to change the rpm of the motor and perform the same experiment at different rpm. For acquiring the whirling speed of the shaft use a DAQ. The displacement of the shaft will be in order of micro-meters. So keep the magnet very close to the shaft.
DAQ gives the results on a computer monitor. Use these results to plot a Critical Speed Map. Its a plot between the whirling speed on y-axis and rotation speed on x-axis. Record the readings at different rotating speeds. Plot whirling speeds at each rotating speed and in the end join them through a line.
Depending on the different impact forces given, plot a straight line on the graph. The point of intersection of this line and the whirling speed line is the critical speed of the shaft.
Experiment different rpm of motor applying different impact forces. You will obtain various critical speeds for a shaft. Avoid these critical speeds in any application of the shaft.
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