ArticleTorsional Vibration Of A Methanol Compressor Drive Train
by Neville F. Rieger
This brief paper gives an account of how the author identified and solved a torsional vibration problem with a methanol compressor. The problem symptoms and diagnosis are described along with the final solution which involved a better set of gears and most importantly a different coupling. A new elastomeric coupling replaced the original gear coupling – this reduced the torsional stiffness as well as added torsional damping to the system. Both were positive changes to the machine which reduced its vibration levels and extended its life.
“The methanol compressor drive train shown in Figure 1 and Figure 2 was exhibiting the following symptoms:
• excessive gearbox noise of 103 to 105 dB (plant background noise, 90 dB)
• intermittent sharp metallic knocks within gearbox
• rapid erosion of gear surface at pitch line, evident as a bright band after 72 hours of operation
In addition, severity of noise and knocks increased as process loads were increased. One gear pair had been removed from service due to pitch-line wear six months after start-up.
Specifications for the machine are given in the table. Gear-tooth couplings were in use. The compressors were rigidly secured to a poured concrete foundation. No special provisions had been made for thermal growth.
Preliminary diagnosis of the symptoms suggested that impacts of gear teeth as a result of torsional vibration were the most likely cause of the problem. It was concluded that the bright metal surface at the pitch line was attributable to plastic flow.
The torsional natural frequencies and mode shapes shown in Figure 3 were calculated using a mathematical model and the Holzer method. The second torsional natural frequency of 1728 CPM and the motor speed of 1780 RPM are so close that a once per revolution gear excitation could have strongly excited the second mode; see Figure 3 (b). In addition, because the gearbox is located so close to a node of the system Figure 3 (b) — relatively small excitations could cause a large amplitude response.
The most probable source of excitation, a once-per-revolution excitation front the lose-speed gear, could have arisen from machining tolerances or front non-concentric mounting of the gear on its shaft. The motor was also a possible source of once-per-revolution torsional excitation due to air gap and nonuniformities of the electrical waveform.
The remedy involved manufacture of new gears with a pitch of approximately AGMA 6 to 10 grade. The new pitch probably reduced excitation and increased resistance date to wear. In addition, an elastomeric coupling was inserted between the drive motor and the gearbox to detune the second torsional mode from resonance with rotational speed. The elastomer also provided a small degree of torsional damping; a value for torsional stiffness seas included in data from the manufacturer.
A plot of natural frequencies of the system vs coupling stiffness (Figure 4) was prepared to assess the effectiveness of the remedy. One stiffness was selected from a range of available coupling options. No torsional vibration problems had recurred 12 months after installation of the new coupling.”
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