Navigation:  Tutorials > Dynamic signal analysis tutorials > DEWESoft 7.0  data acquisition, processing, analyzing and storage software Torsional Vibration

It is important to know that we actually have two different parameters that can be measured with the torsional vibration module: rotational vibration and torsional vibration. What is the difference between them?

Rotational vibration is simply the dynamic deviation of the rotation speed. If we measure the rotation speed of the shaft with high precision, we will notice that we get a high deviation of rotation speed in some regions of the run up . This is caused by the angular vibration crossing the angular natural frequency of the shaft. It is calculated by cutting off the DC component of the rotation speed or rotation angle. We can see an enlarged section of the run up in the graphs below. The high deviation of this frequency can be seen, while the yellow curve shows the deviation in degrees, which is actually the rotation angle vibration.

 

Torsional vibration is an oscillation of angular motions (twist) which occur along rotating parts, such as gear trains, crank shafts or clutches. We need two encoders to measure the torsional vibration, so the torsional vibration is actually a difference between angles of the two encoders. The torsional vibration also measures the static twist of the shaft with higher RPMs. The graph below shows the run up and coast down where we can nicely see the static twist of the shaft, and when passing natural frequency, two high angular vibration of the shaft.

 

To measure torsional or rotational vibration, we need an Orion card with counter expansion, because all other methods do not have the required precision needed to do this.

Torsional and rotational vibration can be measured with either an encoder (up to 3600 pulses per revolution) or a special sensor that has less resolution (up to 720 pulses per revolution) but is much less sensitive to vibrations that could damage standard encoders.