Fault Diagnosis for GAP Mill in Metal Industry

Case Study

Fault Diagnosis for GAP
Mill in Aluminium Industry

1. Abstract

Anode used in an aluminium smelter is made up of a precise combination of coke and coal tar. Coke should be of a specific size range to achieve the appropriate strength in the anode. Ball Mill in a Green Anode Plant (GAP) grinds coke into small particles ranging from 0 to 0.08 mm. This study illustrates methods of identifying defects in gear drive of the ball mill which is single line critical with respect to production.

2. Introduction

The Ball Mill schematic (Make: McNally Bharat, Model: 37TPH) is shown in Figure 1 installed at a GAP. Sensor nodes enabling fault diagnosis for this machine are mounted at positions marked in red dots. These sensor nodes communicate to a cloud dashboard and provided comprehensive vibration signal analysis and fault diagnosis.
Figure 1: Ball Mill Construction & Sensor Positions

3. Background

3.1GAP Mill
Construction

The ball mill contains a hollow cylinder (filled with stainless steel or rubber balls) shell that rotates about its axis for grinding the material to fine form. Ball Mill uses a motor and gearbox to reduce the speed and stable operation to have high reliability and high efficiency over a long period in harsh environments. The final torque from main gearbox is transferred to a pinion and girth gear drive through a cardan shaft. Hence running this equipment under a fault condition is highly dangerous and could damage any drive component.

3.2Sequence of
Events

Figure 2 shows the sequence of events from fault diagnosis to corrective action.

4. Solution

The plots presented in this section validate the presence of the fault as predicted by the system.
  • Figure 4 and Figure 5 show an increase in total acceleration & velocity trends. 
  • Figure 6 indicates the presence of Pinion/Girth gear defect frequencies & its harmonics predominating in the spectrum.
  • Figure 7 shows an improvement in Total Acceleration after correction of root clearance of girth & pinion gear

Advanced Fault Diagnosis in IDAP Platform

A. Increase in total acceleration since 20 November 2021 in GB I/P DE Bearing

Figure 4: Increase in Total Acceleration

B. Pinion DE Bearing High-Velocity Values in all 3 directions from 05 February 2022 to 04 March 2022.
Figure 5: Increase in Velocity Values

C. Shockwave Spectrum – Outer Race defects observed in the spectrum.

Figure 6: Girth & Pinion Gear Mesh Frequencies

D. Decrease in Velocity values at Pinion DE bearing after root clearance adjustment

Figure 7: Trend reduction observed after maintenance

5. Site Inspection and Findings

Girth & pinion gear root clearance was adjusted from 3.8 to 5.5 mm on 04 March 2022 by the maintenance team. The inspection picture is shown in Figure 8

Figure 8: Girth & Pinion Gear

6. Conclusion

Shockwave and total acceleration are critical parameters which can be used to identify defects between pinion and girth gears like misalignment, improper meshing and bearing defects. Machine learning algorithms using these parameters were able to accurately predict gear defects well in advance in the GAP ball mill avoiding an estimated 48 hours of unplanned production downtime

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Case Study

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