Published in March 2005 Australian Longwall Magazine

To maintain a competitive edge the pressure is always on to improve efficiency or reduce risk. One perceived way to achieve this is to purchase lager equipment or equipment with higher power ratings in order to increase production or reduce downtime from equipment overload failures. However, this solution doesn’t always work.

Borehole casing

The mode of collapse of borecasing varies depending on its size and thickness. For small diameters of casing, a higher factor of safety against collapse can be purchased by using higher strength steel. In the lower range it is the steel strength that has the major influence on the factor of safety against collapse. There comes a point, however, when it is more cost effective to buy a lower grade steel casing with a larger wall thickness. In this range the geometrical properties are the factors that have the most influence on the issue of safety.

Correctly purchasing either steel grade or steel quantity (higher wall thickness), depending on the bore size, can minimise the long-term risks associated with the borehole, while also minimising cost.

Gearcase gearing

It is expected that a new gearcase with a higher power rating or designed with a higher application factor would be under less stress, and thus have greater ability to resist failure from unexpected impacts. If existing equipment is rated to an American Gears Manufacturers Association (AGMA) gear standard and new equipment is rated to the new International Standards Organisation (ISO) standard, then this may not be the case.

In the article ISO 6336 Versus AGMA 2001 Gear Rating Comparison for Industrial Gear Applications, Falk Corporation’s Glen Cahala compared the rating of some industrial gears to AGMA and ISO standards.

“The strength ratings using AGMA 2001 are substantially less compared to the ISO 6336 strength ratings. An increase of not less than 42% is achieved for the entire sample population using the ISO rating standard, while several of the reducers have a rating increase over 80% and as high as 100%,” he said.

“The higher strength gear ratings of ISO 6336 can potentially lead to a significant increase in bending failures. In addition, the ISO strength rating will reduce the temporary overload capacity of the gearing, potentially resulting in permanent plastic deformation, and, eventually, tooth breakage. The ISO strength ratings are aggressive when compared to AGMA. Inversely, it could be said that the AGMA ratings are too conservative.”

Cahala also made similar comments with respect to pitting ratings.

A check on some gears used in longwall equipment from various manufacturers shows they too have significant variations in ratings to the different standards (in line with Cahala’s findings).

When comparing ratings from various manufacturers, it is crucial the same standard with the same application factors are used in all cases. This also applies to old equipment. If old successful equipment with known operating conditions is re-rated to the new standards then a benchmark can be determined that may indicate the relative rating of the new equipment compared to the old. The change in standards requires a change in mindset to ensure equivalent or superior performance is gained from equipment.

Gearcase bearings

Usually a failure of one bearing produces a failure of the whole gearcase. The life of the whole gearcase is termed composite life and is substantially less than the life of individual bearings in the gearcase. The L10 composite life of a gearcase is required so different gearcases can be compared.

Risk of failure can also be computed from a list of individual bearing lives. This is probably more meaningful for comparing different items of plant. The risk to operation can be determined for a certain period – say two years of operation for a main conveyor gearcase.

Condition monitoring is a means of reducing the risk of failure once a piece of plant is installed. The calculated risk of failure is a means to compare plant, or can be used as a guide to the condition monitoring strategy.

Individual components

Most standards require a “minimum” factor of safety or compliance with a probability of failure-based analysis. Some manufacturers may rate their product to an “average” factor of safety. The average implies that some components do not meet the minimum requirements. A supplier who guarantees a minimum factor of safety may need to use more material in their product to achieve a consistent rating.

If the decision to purchase is made on cost, then components with some increase in risk may inadvertently be chosen over components with consistent factors of safety. Tenders need to clearly state that the minimum factors of safety need to be listed if equipment ratings are to be adequately compared.

* Ian Dunn is a director at IMD & Associates.