Specifications for mining equipment should be practical. They should be written aimed at achieving a goal which delivers safe, reliable, equipment, but without an unrealistic burden on the supplier. On the other hand, a suitable safety factor should be included. It is this safety factor that defines the range between the ideal, as specified, equipment, and equipment that deviates from specification, yet is still fit for service. This is not so easy to do. Mining equipment specifications have only developed since the early 1980’s, and have changed over time, always trending towards the more stringent. Yet there is plenty of equipment, of earlier vintage, that would not meet today’s specifications, but still had/has long successful operating lives. Sometimes that older equipment is resold, and operates at loads not planned in the original design, and still is successful.

Quality assurance is involved in finding whether a written technical specification has been met. If the specification has not been met, it does not automatically mean that the equipment is not fit to operate successfully for many years. The evaluation of fitness for purpose is rather difficult, especially when restricted solely to theoretical methods. Often experimental testing will reveal very interesting results. An example of this is described below, for mill head castings[1].

Large mill head castings very often have entrapped shrink areas. Specifications require theoretical evaluation of these for acceptability. Thus, the shrink volume is measured, and a crack is introduced into the theoretical calculations, of the size of the overall shrink dimensions. This forms the theoretical pass/fail test. However, a shrink volume is made up of many, unconnected, small shrink pits. Many years ago we gathered ductile iron plates, containing shrink patterns, from various foundries, for testing. The initial goal was to define how long a life these plates would have, when subjected to the stress amplitude defined as the limiting design value, in specifications. The results were completely unexpected. ALL the specimen reached a (mill) 20 year operating life without failure. Furthermore, when these specimen were examined by ultrasonics, x-ray, or magnetic particle, none showed any evidence of crack growth connecting the individual shrink pits. We then tested the specimen at higher levels of stress to determine the safety factors, which were found to be quite large.

To contrast this to evaluation by theory, we note that the theoretical evaluation calculations STARTED with introducing a crack, the full size of the determined shrink zone. The testing program showed that, at maximum limit design stress range, and 20 year life, NO cracking had yet even started, pit to pit, in the shrink zone. Thus, testing can show the conservativeness of the theory assumptions. Similarly, for welds, mill instrumentation has shown that full contour grinding results in a higher safety factor than the “rule of thumb” 30%, allocated by weld codes for all types of toe dressing, and that under overall oven stress relief, welds with higher compression than tension in the stress range, are safer than the weld code discussions indicate.

Cosmetic appearance of castings is certainly important, but one should also remember that without stresses there will be no failure. So when faced with a casting of poor cosmetic appearance (which certainly indicates flaws), one should still remember to ask the following:

What is the fatigue stress range in this area?
How does this casting compare with other cosmetically poor castings, which have operated, trouble-free, for many years?

If the answers to the above questions are favorable, i.e., low stresses, and no documented earlier failures, then the decision to accept/reject the casting should not be made strictly for cosmetic reasons.

We continue to be the only small, mill evaluation, consulting company that funds independent testing out of it’s own profits.

[1] Svalbonas, V., et al, "Mill Head Castings – Educating Opinions”, SME Annual Meeting, Feb. 2009.