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SLOW speed monitoring with HS-150

GVS Reliability Products – Mechanical Gain HS-150 Series Low Frequency Sensors

Overview

GVS was approached by Tensor Systems Managing Director, John Morey, to supply the HS-150 series 500mV/g sensors for a trial. The purpose of the trial with the HS-150 series 500mV/g was to be conducted on the low speed end of a slow speed gearbox at a particular Gold Mine.

The output shaft speed was very slow at just 0.141Hz and vibration is less than 15 g zero to peak.

The key requirements given by Tensor Systems was to have; ‘a shear mode sensor with a resnonant frequency of approximately 13KHz using a large mass and crystal to maximise output, rather than a standard mass and crystal using electrical gain, which increases noise’.

Notes

Most of the dominant tooth meshing harmonics of the 6 tooth meshing frequencies are contained & trended in the 0.3 to 3 KHz range. At minor level’s they extend across the whole 32 KHz range and appear with ball pass demod data.

The HS-150 500 mv/g sensor “bolted on/direct coupled” resonance is 15,200 Hz, within normal variation, and locally amplifies the acceleration signal at this point.

The linear part of the signal that is NOT dominated by tooth mesh is trended in the 3-10 KHz range.

The resonant part of the signal is trended in the 10-16 KHz range.

This amplifies poor lube, gear mesh and bearing ball pass frequency vibration very usefully in demodulation data.

The sharp peak around 4.8 KHz is a bearing housing “ring resonance” where the mode shape is an “oval” with the max dimension alternating from horizontal to vertical that many times per second.

The tooth meshing profile shape (green line) is the cause of the rising trend. Metal surface to surface contact through the lube film thickness is minimal and constant.

At the 30 Hz input end of the gearbox , a normal 100 mv/g sensor, resonant in the 16-32 KHz range, is used to allow for 100 g zero to peak. The resonant part of the signal is trended in the 16-32 KHz range. This amplifies poor lube, gear mesh and bearing ball pass frequency vibration very usefully in demodulation data.

With the Tensor online system, the 500 mv/g sensor signal is read directly with a 24 bit adc in the dc to 25 volt range ie dc coupled. The dc component is checked for bias range and wandering digitally. Then using a patented method to minimize low frequency errors, it is ac coupled digitally at a very low frequency and digitally integrated to suit each measurement frequency range, in this case 0.35 Hz, which is lower than this particular shaft speed of 0.517 Hz. The TMF is 20 x 0.517 = 10.34 Hz.

Within the online system there is a feature for fast identification with pull down menus showing :-

  • each shaft speed harmonic series with responding colours
  • each tooth meshing frequency harmonic series
  • each bearing frequency harmonic series

Trend parameters generally mean :

1x s (sync) = imbalance and /or misalignment

2x s = misalignment or roots blower porting frequency

3-8x s = roots blower, screw compressor, pump & fan blade pass harm’s and looseness from all shafts

1-8x ns (non sync) = non sync vibes including first few ball pass vibes, & unusual process pulsations

9-256x s = tooth meshing from all gears, rotor bar passing in motors

9-256x ns = ball pass vibes above fundamental from all bearings

The gearbox condition is good and vibration is very low, yet the trends slowly rise with wear.

The harmonics of this shaft supported by brg 15 are highlighted red, as per the table.

Interestingly 3x this shaft speed is dominant, yet there is no kinematic reason for this.

Conclusion

When monitoring assets with slow speeds, choosing the correct accelerometer is key to the success of the monitoring system.

The team at GVS Reliability Products are here to assist you with correct accelerometer selection for many unique applications. If you have any questions or requirements, please don’t hesitate to get in touch.

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