Mining of larger coal seams has resulted in a demand worldwide for 1200-1300- ton yield capacity longwall shields while at the same time legs are getting bigger, up to 400mm, compared to 320mm just 10 years ago.

 

In response to this requirement for greater load cycle capacity, Joy redesigned every part of its RS20 shield roof support—from seals, rams and cylinders to virtually every part of the shield—culminating with the recent launch of the new RS20s electronics.

 

“The resultant RS20s roof support is the strongest, most electronically powerful shield in the industry,” said Pete Crossland, Joy’s longwall sales coordinator.

 

Central to the development in roof control technology is Joy’s one-piece extrusion cylinder.

 

Because the welds in a shield are one of the main determining factors in the underground life of a shield, Joy developed a one-piece extrusion cylinder. During the extrusion process, a tool is forced up into the centre of the cylinder, stopping short of passing through it. The result is a solid cap on the cylinder without welding, which makes a significant difference in the life of the shield underground.

 

Joy is now manufacturing its Mark 6 legs, following on from the Mark 1 legs in the late 1980s, and the Mark 3 in the 90s. Each progression has resulted in an increase in life expectancy. The increase in life from Mark 1 to 3 was approximately 300%; the progression from Mark 3 to Mark 6 further doubles life expectancy.

 

The Mark 6 design has now been successfully tested at the 60,000 cycle target, and was still going strong when testing was halted.

 

“The considerable undertaking of research, design and testing shows the effort and commitment Joy put into improving the life expectancy of hydraulic cylinders. Reliability doesn’t just happen, it has to be engineered into the product,” Crossland said.

 

Joy still offers the fabricated legs in developing countries where cost can be the overriding determining factor in equipment selection, but fabricated legs can have half the life of extruded legs.

 

Another aspect of the research and development effort in bringing the Mark 6 to the market was the completion of over 1.5 million test cycles on the various different bearing strip and seal designs.

 

“Bringing the Mark 6 to market was the result of a lot of hard work in the Joy engineering department, an effort that was then carried over to the rest of the cylinders,” Crossland said.

 

Now in its third generation of leg pad castings, Joy’s Crossland said the shield structure was first developed for 15,000 cycles as the number of cycles before crack initiation. The second generation went to 30,000 cycles, and the current third generation to 60,000 cycles.

 

“As with all structural engineering,” Crossland noted, “it’s a matter of fundamental strength coupled with fatigue endurance.”

 

Joy’s RS20s roof supports are designed to US, British, European, and Australian standards - the most rigid standards in the world. Joy can design to whatever component life cycle (confidence level) a customer stipulates. Which means, of course, shields are increasingly heavy and more expensive at the highest confidence level, which may be more than the customer, or available reserves, require.

 

“For instance, a customer looking at longwalling a smaller reserve typically will want to match the operating life of the equipment to that reserve. We can accommodate that by applying the design codes as detailed in the appropriate standards,” Crossland said.

 

On the next edition of International Longwall News, Joy overviews electronics advances with RS20s and the BROADBAND chain.