Cadia Ball Mill Gear “Anniversary”

A gear is aligned in the field, in steps. First a “satisfactory” geometric alignment is achieved, and then it is modified, in steps, relying on mesh temperature, and temperature distribution along the tooth face width. Any static alignment position determines the initial tooth interaction pressure pattern, which then self adjusts, in the running condition, depending on tooth deformation and temperature in the mesh. If this results in a running thermal equilibrium condition, which is within the manufacturer’s guidelines, the alignment is accepted. If a thermal equilibrium condition is not achieved, or if it is outside the manufacturer’s guidelines, a further pinion position adjustment is usually made.

There are other items which influence this process. The tooth profile may have been modified in anticipation of some deflection such as pinion “windup”. Lubricant viscosity and quantity will also affect the amount of deflection “tolerated”. Of course, the overall gear structure also influences tooth deflection. If we want to analyze mesh behavior, we must realize that each pinion position in the alignment process represents a starting point for a new analysis. The unsatisfactory earlier ones should produce a continually increasing thermal solution, whereas the final running position should produce a satisfactory thermal equilibrium.  A paper such as [1] merely scratches the surface of this complex analysis, and omits thermal considerations entirely. It is useful, however, in identifying the overall behavior of different gear structures. It was prepared as a study of the original Cadia ball mill gear.

When the Cadia ball mill was started up in 1997, the initial gear alignment attempt produced damage at one edge of the tooth face. In analyzing the the causes of this, it was noted that the lubricant being used was not among those recommended by the gear fabricator. A new “in spec.” gear lubricant was obtained, and a new alignment produced a satisfactory running gear set for seventeen years. Analytical studies funded at that time, by Cadia, however, put the blame on the unsymmetric geometry of the rim in the Y shaped gear [2].  This geometry was deemed a “design flaw”, regardless of the fact that the gear was running successfully using a more viscous, fabricator recommended, lubricant. The research in [1] was done to counter that argument. In the process, we studied the one item that was “novel” in the Cadia gear/pinion system – the smaller diameter to face width ratio of the pinion, for this range of motor power. As shown in the study, the substitution of a theoretical, larger diameter pinion, reduces the flexibility of the mesh, and thus may be expected to result in an easier process to achieve satisfactory alignment.

In the last quarter of 2014, the original Y gear was replaced with a spare, procured quite some time ago. The new gear was a perfectly symmetric T shape, and was fabricated with a forged rim, rather than being cast, as the original. During the initial alignment/start up, this gear also suffered damage, on the drive end of the tooth face. Since the lubricant was acceptable, alignment continued and achieved the now satisfactory running condition with this gear.

What can be inferred from this recent occurrence? With the complete change in the gear shape surely any hypothetical “design flaws” associated with the rim non-symmetry were removed. Yet similar damage occurred. This just demonstrates the difficulty of aligning a flexible mesh. In both cases there are pinion position starting points which lead to unsatisfactory results. In both cases, the diligent care of the field personnel, combined earlier with a higher viscosity lubricant, which acts as a safety factor against metal-to-metal tooth contact, solved the problem. In both cases, according to [1], a larger diameter pinion would have had a significant beneficial effect in easing the work.

[1] Fresko, M, et al, “Use of finite element analyses in understanding alignment and load distribution in large grinding mill gear and pinion stes”, SME Annual Meeting, Feb. 2004.

[2] Meimaris, C., Duncan, M., and Cox, L., “Failure Analysis of Ball Mill Gears”, SAG Conference 2001, Vancouver.