Torsional Vibration is defined as a sequence of slight changes in the rotation speed of an object. Similar to conventional vibration, torsional vibration is also defined by the amplitude and frequency of these changes.
Torsional vibration occurs when the rotating element is subjected to unequal loads during its operation. Common examples are reciprocating compressors and internal combustion engines. Since at one point in the cycle, the torque of one of the connecting rods is greater when air is compressed and decreases when air enters the chamber.
Because of this, the crankshaft reflects variations in the rotation speed that might lead to internal stresses, which in abnormal cases, can produce a rupture of the crankshaft (or shafts, depends on the machine).
The following case study will explore the possibility of using Erbessd – Phantom brand wireless accelerometers to measure torsional vibration. Additionally, we will provide the sensor’s technical data and those of interest to those who wish to advance in this matter.
One of the most common methods used for measuring torsional vibration is the calculation of the variation of the speed using an optical sensor and a piano-type tape. Hence recording the time difference between each step of the tape. There are certain restrictions for the measurement to be valid and reliable and one of them is the number of available samples per revolution since an incorrect sample rate can generate an aliasing effect or a saturation in the signal.
The use of wireless vibration sensors in industry can provide new ways to measure torsional vibrations. We can now measure torsional vibrations directly on the shafts with the same sampling rate and resolution capabilities of current vibration analysis. We can even permanently monitor the condition of a rotating element under regular operating conditions, to visualize the evolution of its torsional vibrations.
We installed a high-range triaxial vibration sensor (PHANTOM EPH-V11) at the crankshaft end of a 3.2 HP rated two-cylinder air compressor for torsional vibration study purposes. This study intends to prove that the sensor is able to send a readable signal while being attached to the rotating shaft and also to verify the presence that of a second harmonic, product of the sum of the vibration by the variation of torque and the vibration by the inertia of the crankshaft (known components of the vibration torsional in this type of machinery).
The nominal speed of spindle rotation is 900 RPM which is powered by a 3600 RPM single-phase electric motor using a 4: 1 ratio pulley.
Phantom EPH-V22 Wireless Accelerometer Features
Figure 1 – Accelerometer’s eccentricity
One of the advantages of PHANTOM for this study is the eccentric placement of one of its sensors, which allows measuring the tangential vibration of the shaft (Figure 1).
Considering that the change in mass in the system could modify the vibration of the compressor, measurements were taken before, and after attaching the sensor to the shaft of the compressor. The result: an increase in the fundamental by 4% (Figures 2 and 3 ) which was considered negligible for this study.
Figure 2 – FFT without Phantom
Figure 3 – FFT with Phantom
I am happy to share with you the simplicity of sensor placement, the only thing I had to do is to screw the sensor axially to the shaft.
On the other hand, data recordings did not have any loss of data, nor corrupt signals. Additionally, we analyze the X-axis of the accelerometer, which is precisely the one that is tangential to the axis at a radius of 9.6 mm (0.380”).
Figure 4 – Waveform and FFT of Torsional Vibration
I am convinced that what I got was the torsional vibration of the compressor, I am sure as well that this tool will be highly valued by analysts in this field. The spectrum shows second harmonics like those described in theory and even more.
We will leave the Phantom EPH-V11 sensor permanently mounted on the compressor to monitor its trend and spectra over time. Additionally, since machine operation is intermittent, we linked PHANTOM vibration sensor to a Phantom EPH-S40 Hall Effect RPM measurement sensor. This way, the vibration sensor will only send meaningful data when the compressor is in operation.
A B O U T T H E A U T H O R
Armando Torres is a Mechanical Engineer from the Universidad Autónoma Metropolitana. He is currently Chief Engineer & Mechanical Designer at Erbessd Instruments®
ERBESSD INSTRUMENTS is a leading manufacturer of Vibration Analysis Equipment and Dynamic Balancing Machines with facilities in Mexico and the United States and representatives around the world. ERBESSD INSTRUMENTS – MASTERS OF MACHINE HEALTH