Published in the August 2010 Coal USA Magazine
Chain management starts at installation. Maintaining a good straight face line is imperative.
Any deviation in face alignment can result in differential pre-tensions between the face and the gob side chains and result in uneven wear in the chain pairs. This is more likely to occur on the newly established face as the chains run through the bedding-in period.
Once a differential wear pattern is formed, it is virtually impossible to redress. More often than not the differential continues to worsen, resulting in additional wear, creating even more slack.
The adverse effects of running with a poor face line that leads to excessive variations in side for side pre-tensions are illustrated by the following example: A 1000-foot longwall with 42-millimeter chain has approximately 4000 links in each side.
Accepting that interlink wear takes place at both ends of the link, we have 8000 points at which metal is worn away by interlink pressures as the chain moves down the face. The chain under these impact forces and vibrations suffers shock loading and/or is affected by corrosive attack.
For every 1/1000 inches of wear we generate 8in of increase in length. Considering this fact, we can see that any slight variation between face and gob side wear rates, brought about by uneven tensions, quickly multiplies to a major variation in chain lengths.
Two forgings on the sprocket at the same time could lead to undue wear of the tooth profile. This is due to the loss of positive location in the drive sprocket which allows the link to slide on the driving teeth. This sliding action cuts into the link and also increases wear rate on the sprocket teeth. Once established as a wear pattern, it can only accelerate. At the first sign of cutting of the link, the sprockets must be examined and replaced before the damage destroys the chain.
When you consider a 1000ft longwall with a 42mm chain on the AFC and 34mm on the BSL, the overall number of links and single line components approaches 9000.
If chain pre-tension is too high this will also cause excessive wear on both chain and sprocket. Chain pre-tensions need to be established at values that prevent the creation of too much slack chain under full load. Such conditions will allow scraper bars to be “flicked out” and the risk of damage to the tail sprocket caused by chain bunching as it leaves the sprocket. If pre-tensions are set too high there are two obvious dangers:
1. Exaggerated inter link wear on the chain can occur.
2. Exaggerated wear on drive sprockets is likely.
Chain tension can be a killer. The common tendency is to run the chain too tight. The aim should be to regularly check the pretension and to remove slack chain by two link increments.
More than two links would indicate that the chain was too slack or the removal of four links would create too high a pre-tension which would induce heavy interlink wear and would seriously reduce the life of the chain.
Assuming that the face alignment is good, the value of pre-tension in one side should not exceed the value in the other side by more than 1 ton. Good face management should ensure that any differential can be held to no more than 2t throughout the operating life of the chain.
The increase in length due to interlink wear (sometimes referred to wrongly as “chain stretch”) can be allowed to reach 2% and still run with new sprockets.
The degree of interlink wear is not a problem if chain and sprockets wear together, thus retaining their compatibility. However, the interlink wear does result in a reduction in the chain’s breaking load and resistance to shock loads.
A simple method of measuring interlink wear is to use a device like the one Ellton Longwall manufactures. The Ellton Chain Caliper measures in five pitch sections and applies to a chain elongation chart like the one illustrated here.
However, we will go into the full operation of this simple tool later in this program. Chains would generally be considered for replacement when interlink wear exceeds 3%. Some conservative maintenance managers do not like to see their chain exceeding 2% elongation.
The chart shown illustrates data collected at an Australian longwall mine over more than a year tracking elongation data of 42mm AFC chain after it was installed in June 2007. This was the first Chinese-manufactured Parsons chain to operate in Australia after China’s CME bought the Parsons Chain manufacturing plant and intellectual property rights from Parsons Mining Chain UK’s parent company FKI. The data collected revealed that the chain had less than 0.9% elongation, and minimal sprocket cut and vertical wear after conveying more than 5 million metric tons of coal at the mine.
On rare occasions, samples of used chain are found to have increased in overall outside length, not just an increase in the inside (pitch) dimension.
Such an increase can only be caused by a load that exceeds the calibrating load used to fix the links’ final dimensions during manufacture.
Calibration loads are typically in excess of 70% of the minimum breaking load specified and are well in excess of the normal design operating load. Thus any permanent increase in outer link dimensions is an indication of major shock loading. In service monitoring chain wear by measurement – if done correctly – can give a good guide to the total wear, uneven wear and differential wear between the two chain strands.
Good chain management starts at the installation stage. Intensive monitoring with corrections if needed during the bedding-in period will help ensure a long and trouble-free chain life.
* Mark Newton is the managing director of Ellton Longwall in Australia and is the exclusive distributor of Parsons Mining Chain in Australia and the US. George Roman is the US sales manager for Ellton Longwall and Parsons Chain.
