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Torque analysis on gear box shaft

Repeated failures of the gear box shaft on a nitrogen compressor unit.
Figure. 1                                             Figure. 2
Some repeated failures occurred on the shaft between gearbox and reciprocating compressor.
Theoretical analysis was conducted in order to find the probable causes of these cracks and an high value of alternating torque was hypothesized.
In agreement with the customer’s maintenance department, it was decided to carry out a torsional measurement campaign in order to verify experimentally the alternating torque on the shaft.
On-Site measurement campaign
After the first measurement campaign, it was decided to repleace the pin flexible coupling with disc flexible one between the compressor and the gearbox.
A new measurement campaign was conducted in order to compare the different behavior.
Figure. 3. Installation Layout 
Practically, the torque analysis were carried out in the following configurations:
  • OLD configuration - pin flexible coupling
  • NEW configuration - disc flexible coupling
The measurement results are summarized in the following figure.
 NEW configuration
OLD configuration
12.6 kNm
6.2 kNm
Figure 4 – Comparative plot
Figure 5 – Torque trend analysis
Figure 5 – Waterfall shut-down analysis 
The data analysis of the original condition confirm the high alternating torque on the gear box shaft during steady state conditions.
The test with the new coupling have shown very lower alternating torque (mean 5.0 kNm, max 6.2 kNm, min 3.6 kNm), comparing with the results of the previous test (mean 5.2 kNm, max 12.6 kNm, min 0.6 kNm).
The new type of coupling hence improved the stress state of the shaft and solved the failure problems.
Thanks to measurement campaign and the analysis carried out, it was possible to verify the real torque on the machine in original condition and consequently, to choose a new solution (change a type of coupling) in order to solve the problem.
  • Reduction the maintenance costs
  • Feedback of data from field measurements to designers can lead to improved design methodology which will help prevent future problems
  • Quick problems troubleshooting thanks to real time data elaboration
  • Easy installation in field
  • Smart design of the test