Simon Lovegrove* presents an understanding on the use of galvanised chains in Australian longwall mines.

 

Published in March 2007 Australian Longwall Magazine

 

University research undertaken by the author into the use of galvanised chains in Australian longwall mining applications has attempted to reveal the reasons why the industry has such a poor perception of galvanised mining chains, while also considering the engineering, maintenance and scientific aspects associated with the correct application of a galvanised mining chain in Australian longwall conditions.

 

A chequered history

Traditionally, high strength “black” (ungalvanised) chains have been employed on every longwall in Australia. Occasionally over the past 15 years, some longwall mining operations have opted to trial the use of a galvanised chain set in Armoured Face Conveyor applications in the belief there will be benefits to their operation.

 

However, upon the introduction and subsequent use of these galvanised chains, their actual performance has produced a mixed array of results and emotions.

As a result, it appears that the current industry perception is somewhat negative, but do we really know why we choose not to use galvanised mining chains in our own longwall mining operations or are we just going along with what the next man says?

 

Why does the industry say no?

The longwall chains currently in use at many mines do not have the detrimental problems that are specifically related to corrosion damage, and therefore do not need the chain to be galvanised for its own protection.

 

This is not to say that the longwall chains in service at these mines are not corrosively attacked, but simply suggests that the service life of these chains are not being reduced prematurely from chain failure modes that are directly related to corrosion damage.

 

Management and engineering personnel at some of the mines are uncertain and/or confused about the benefits and circumstances surrounding the use of a galvanised chain. Do they need it and how will it help them?

 

Some mine operators who have tried galvanised AFC chains and have encountered a “bad experience” where new galvanised chains have failed in a very short time of service, have perceived the fault to be that of the chain and/or the galvanising process.

 

Metallurgical and mechanical investigations into the cause of these failures often imply a mixed array of mechanical and/or corrosion-related failure mechanisms that when reviewed in the combined light of warranty claims, financial losses, coal production delays, political gain and a multitude of expert opinion, end up producing a clouded view of what the real cause of chain failure actually was.

 

Consequently, the various perceptions on the actual causes of these galvanised chain failures have permeated throughout the longwall mining industry in a form of “Chinese whispers” where the original truths and facts surrounding the use of galvanised mining chains have now been somewhat distorted.

 

When will it work?

For the use of a galvanised mining chain to be warranted, the specific operational and environmental conditions that encourage corrosion-related chain failures on a longwall panel do not need to be particularly aggressive.

 

Corrosion-related chain failures in the form of stress corrosion cracking or corrosion fatigue cracking require only the presence of both very high mechanical stresses within the chain, and a metallic corrosion reaction between the chain and its immediate environment.

 

For example, if the service life of a mining chain at a particular mine is currently being determined by corrosion-related chain link failures, then there may be an opportunity for a galvanised chain to be utilised to extend the service life of that type of mining chain. Of course further metallurgical and engineering investigations of any chain link failures would be required to assist with identifying the actual cause of failure, but if corrosive environmental conditions were being experienced by these mining chains, and the chains themselves not subjected to extremely high operating stresses, then the use of a galvanised chain may be beneficial to the mine’s longwall operations.

 

If, however, the service life of a mining chain at a particular mine is currently being determined by wear characteristics such as chain pitch elongation through interlink wear, wear to the crown ends of the horizontal links from the action of the drive sprockets, or wear to the legs of the chain links thereby reducing the cross sectional area and load-bearing capacity of the chain, then the use of a galvanised mining chain at this particular mine would do little to increase the reliable service life currently being experienced.

 

Under these conditions, the use of a galvanised chain is not recommended, as the rate of chain wear determines the service life of the mining chain and galvanising the chain does nothing to delay the progressive effects of this wear.

 

As it stands for the majority (80%) of Australian longwalls, the effects of interlink wear on a mining chain currently determines that chain’s ultimate reliable service life and therefore most longwall panels wouldn’t actually benefit from utilising a galvanised chain. There are, however, some longwall panels that appear to have environmental and operating conditions that the author believes would make the mining chains utilised on these longwalls susceptible to corrosion-related chain failures that could be overcome by protecting the chains with a galvanised zinc coating.

 

Galvanising is no silver bullet

One of the most important points to realise when considering the various aspects of a galvanised chain is the fact that a galvanised chain has about a 10% reduction in tensile strength when compared to an equivalent black steel chain.

 

Longwall engineers must be careful that in an attempt to overcome corrosion-related chain failures by introducing a galvanised chain into an AFC system, that they do not initiate extremely high tensile chain stresses because of the galvanised chains’ reduced tensile strength and thereby create a situation that will actually assist in generating corrosion-related chain failures.

 

Remember, it is a combination of extremely high chain stresses and a corrosive environment that can initiate and develop chain failure mechanisms like stress corrosion cracking and corrosion fatigue cracking. This situation demonstrates that with a highly stressed mining chain, corrosion-related chain failure mechanisms can still develop even if the chain is zinc galvanised, because it is actually the high level of tensile stress within the chain that is the primary cause of failure, not just external corrosive elements acting on the chain alone.

 

Although galvanised...click here to read on.

 

* Simon Lovegrove completed his Masters of Maintenance Management at Central Queensland University in 2006. He is currently employed at Anglo Coal’s Lake Lindsay Project as a mechanical engineer.