Rotational and Torsional Vibration Analysis

What is Rotational and Torsional Vibration

Torsional vibrations are angular vibrations of an object, typically a shaft along its axis of rotation. They are mechanical vibrations caused by time-alternating torques which are superimposed on the otherwise steady running speed of a rotating shaft. In automotive engineering torsional vibration is primarily caused by the fluctuations in engine power output.

Torsional vibrations are evaluated as the variation of rotational speed within a rotation cycle. RPM variations are typically induced by a rough driving torque or a varying load.

Rotational vibration is simply the dynamic component of the rotational speed. If we measure the rotational speed of the shaft with high precision, we will notice that we get a high deviation of rotational 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 rotational speed or the rotation angle

The level of torsional vibration is influenced by a number of parameters, such as material properties and operating conditions such as temperature, load, RPM, etc.

Connect Any Type of Angle Sensor to DAQ

Torsional and rotational vibration can be measured with either an encoder sensor (up to 3600 pulses per revolution) or a special sensor (CA-RIE-360/720) that has a lower resolution (up to 720 pulses per revolution) but is much less sensitive to vibrations that could damage standard encoders.

Some very typical sensors are already predefined in the Dewesoft X DAQ software. However, if your type is not listed, you can define your own sensor in sensor editor. Note, that for the torsional vibration module the sensor has to be either an Encoder or a CDM type.

A variety of angular sensors is supported:

• Encoders
• CDM with/without zero pulses (Geartooth)
• Zebra tape sensor (tacho and auto-gap detection)
• Sensors with missing teeth (e.g. 60-2)

Torsional Calculation Output Channels

Dewesoft X torsional vibration module automatically calculates several different parameters. The calculations can be done either online in real-time or offline on the saved RAW data.

• Rotational angle: filtered angle value of vibration.
• Rotational velocity: filtered velocity vibration value.
• Torsional angle: the Dynamic torsional angle that is the angle difference from sensor 1 to sensor 2.
• Torsional velocity: difference in angular velocity from sensor 1 to sensor 2.
• X-axis reference angle: the reference angle, which is always from 0 to 360 and can be used as a reference in angle based XY diagrams.
• Frequency: in the RPM units.

Reference Curve Configurations

Centered mounting is very important. Off-center mounting and unsteady pulses from the encoder can be compensated in Dewesoft X using the reference curve.

However, it is required that the load must be removed from the engine - it must be free-running. Otherwise, you would also cancel out vibrations you want to analyze. When the machine is running at idle speed with no real torsional vibration, press the “Set” button.