The majority of systems are not well represented by a model with two inertias connected by a spring, and as such the system of equations will be much larger. In these cases, computer software is utilized to simulate the torsional response using numerical simulation methods such as Runge-Kutta.
These simulations are used to identify sections of the drivetrain which may be subject to high amplitude torsional vibrations. It allows designers to make modifications to the model, such as changing spring stiffness, adding damping, or changing inertia values, to determine if a positive effect can be realized. Physically, these modifications could include altering the shaft geometry (tubular or solid), utilizing an elastomeric coupling for increased damping, or adding/removing a flywheel to the drivetrain. If modifications to the drivetrain components is not possible, the analysis may still yield information as to the best location to use a torque overload or limiting device. As these types of alterations can pose significant costs in terms of new components and downtime, it is essential that effective and accurate modeling of the torsional dynamics be completed. When possible, it is best to use empirical data from field torque measurements to validate and normalize the torsional model.
Using predictive maintenance methods
The direct measurement of torque is the best method for addressing uncertainty in natural frequency and forced-response calculations, evaluating the effects of operational changes (load and/or speed), and capturing transient events which could reduce system reliability. The location on the drivetrain where torque is measured is important to consider for useful data to be acquired. Large inertias, such as gearboxes, tend to isolate the driving and driven sides of the drivetrain from transient torque events because they act like flywheels. The measurements need to be taken at a location where there is enough angular deflection to produce a signal in the torque transducer; meaning that short, solid shaft sections may not deflect much under loading. Lastly, for torque sensors such as strain gages, the shaft section geometry (and material properties) where the gage is installed needs to be known so that torque can be calculated from the measured strain. Complex geometries will make the relationship between strain and torque difficult to calculate.
Often, electrical torque measured at the motor is used to determine the torque downstream in the drivetrain. While this is usually adequate for steady-state torque, the electrical torque at the motor does not react quickly enough to torque spikes generated at the driven side of the drivetrain. In VFD-controlled motor applications, the source of torsional vibration is commonly generated from the drive. The VFD is used to control the motor speed by manipulating the frequency of their output by rectifying an incoming AC current into DC, and then using various methods (commonly pulse width modulation) to recreate an AC current and voltage output waveform. While newer VFDs produce smoother outputs and reduce the amplitude of the associated torque ripple, it has been seen that tighter speed control can cause other instabilities as the drive tries to make quick corrections due to the excitation of a TNF which has been misidentified by the drive as a load change. Tighter speed control is also problematic when the motors are driving larger inertias (large diameter fans, gearboxes, etc.) which inherently do not react to sudden speed changes because they tend to act like flywheels.
Changes in the output frequency waveform can produce corresponding torque fluctuations. Generally, the torque ripple for smooth operation should be 1% or less. It would seem logical that the feedback from the drive could be utilized to measure the transient torques; however, the torque data is usually only recorded at 10 Hz (samples/ second) which is generally too slow to capture transient events without truncating the waveform peaks.
Transient events such as impacting, start-ups, and process upsets can produce torque spikes whose first (and largest amplitude) cycle of vibration occurs over only several milliseconds. In order to adequately capture the waveform, without truncating peaks, sampling rates typically need to be at least 500 Hz (Samples/second).
Several common technologies capable of measuring transient torque are available including: phase displacement systems, strain gages, and optical encoders. A comparison of the different technologies is shown in Table 1, where an ‘X’ indicates that the technology is suited for the specific criteria or condition listed in the first column. The choice of the particular technology largely depends on the intended use and application.
The digital signal from the transmitter is sent wirelessly (RF, Bluetooth, or wired) to a signal receiver that can provide various analogue outputs (4-20 mA, 0-10 Vdc, etc.) that can be recorded. A gain (multiplier) and offset are applied to the analogue output signal to provide a torque value based on the known material dimensions and mechanical properties of the rotating shaft/coupling.
70 | ISSUE 109 | SEP 2024 | THE REPORT
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