Published in the June 2010 Australian Longwall Magazine
Moranbah North was one ambitious project for original equipment manufacturer Joy. To overcome major yielding issues and to cope with the deep and geologically problematic Goonyella middle seam, mine ownerAnglo American Metallurgical Coal put in an order for super chocks for the Queensland mine.
The specification was for the supply of 143-odd face supports and eight-odd gate end supports set at 2.05-metre centres to work an extraction height of 3.5-4.5m for a 305m-long coal block. The supports were to provide a roof load density pre-cut of 148 tonnes per square metre with an 85% set-to-yield ratio.
A lower, advance and set cycle time of 14.5 seconds was specified and the supports had to be capable of being operated from the front and rear walkways.
Total transportation height, including transporter, was limited to 2.7m and support weight to 62t.
To achieve this tricky specification, Joy proposed a 2 x 1750t support with 480mm-bore double-telescopic constant yield legs and 300mm-bore stabilising ram with an open height of 5m and closed height of 2.4m.
Anglo accepted the proposal.
The supports were also to be equipped with a 200mm-bore advancing ram, a 170mm-bore base lift ram, 130mm-bore base pusher ram, integral two-stage flipper and hydraulic shielding to both sides of the support. Full electro-hydraulic control of the support was achievable from both the front and rear walkways.
Joy powered roof supports and armoured face conveyors product director Clive Hibbert told Australian Longwall that to successfully build the world’s highest powered roof supports, Joy had to overcome three major design challenges.
The OEM needed to produce a high-rated, high-density support with a high life-cycle capability at an acceptable weight and cost.
It needed to produce fabrications which could be manufactured, handled and assembled in a safe and efficient manner.
And lastly, it needed to produce an ergonomically friendly product that ensured the safety of mine personnel.
Hibbert said to keep the weight of the supports down to a manageable level, Joy had used extensive finite element analysis-based fatigue analysis throughout the design of the major fabrications. This achieved the best sections for the components to keep theweight of the fabrications to a minimum.
To counter any safety issues with theamount and pressure of fluid being pumped through the massive hydraulics, Joy subjected the designs to risk assessments. During prototype build, it paid particular attention to the mounting of valve gear and routingof hydraulic hoses. Burst protection and positive retention was provided on all hoses which, where possible, were routed away from the operator.
Extensive shield fatigue testing was also carried out.
“Where typical shield fatigue testing requirements range between 30,000 and 60,000 cycles, the final fatigue checking on the Moranbah shields successfully achieved 90,000 cycles – something unheard of in the industry,” Hibbert said.
He said it was acknowledged that the testing of a single prototype was not statistically valid to confidently generate crack-free structures for the full duration of in-service life. Anglo’s specification was for the support to have an in-service life of 45,000 cycles with a guarantee that no more than 12% (18 shields) would suffer serious enough damage to require their removal from service for repair.
In the opinion of the consultants commissioned by the customer, this required a 90,000-cycle structural test program.
Joy’s decision to increase the leg diameter from 400mm to 480mm required innovative engineering in the design of the cylinders, seals, bearing materials and feed pipes.
“The challenge with the 480-millimetre bore leg was to provide a crack-free 90,000-cycle design. Early design work coupled with FEA and fatigue analysis indicated that, to achieve this, we could not have any weldments on the outer cylinder in the main tube area that is subjected to yield pressure,” Hibbert said.
“In addition, the hole sizes required to provide a high-flow, rapid-yield capability to the leg precluded the use of deep hole drilling.
“A twist-lock feed pipe coupling was designed that removed the requirement for welding in this critical area, at the same time providing a robust and reliable means of fluid transfer from valves mounted part-way up the leg cylinder. The bottom leg retention lugs were also made integral with the cylinder base pressure block.
“Joy’s experience in bearing and seal technology and testing over many years required only larger seals to be produced and no new innovations were required.”
Joy has already been able to use the innovations from the Moranbah job on other installations, including methods of handling major structures, base lift mounting, twist-lock feed pipe and several new design hydraulic valves.
Moranbah North now has its super chocks up and running after surface compatability trials in March last year, training of longwall crews mid-year and production ramp-up in August.
The supports have been teamed with a new system consisting of two Joy 7LS6 shearersand two matching Joy 2.05m-wide AFCsrated at 4500 tonnes per hour with 50mm Broadband chain.
The AFC is powered by three 1000-kilowatt maingate and tailgate drives, mated with Joy BSL/crushers and a pair of Longwall Hydraulics pump stations, and is backed by a Macquarie Manufacturing monorail.
