Ask anyone on a longwall how big their powered supports are, and they may say something along the lines of “the supports are 800 tonners” or maybe “two leg 900 tonners”.

 

In some cases they might even tell you “our supports are rated at 1100 tonnes”. One or two people on the longwall may actually declare: “According to the OEM’s manual, the supports are rated at 1200 tonnes at yield.”

 

I can guarantee that every roof support manual in Australia will have this latter form of description somewhere on the title page. But what does this mean, and is it relevant to holding up the roof?

The answer – of course – is that whilst it tells you a lot about the structural integrity of the actual machinery, it has very little to do with controlling the roof above your head.

 

The following are dictionary definitions of the word “yield”

 

• Verb meaning to move or bend under pressure or with the application of force
• Verb meaning to give way or give up further resistance
• Verb meaning to admit defeat and surrender

In order to effectively control the strata on a longwall, the support must emulate the coal that it has replaced and keep the overlying strata as stiff as possible.

 

To achieve this, the powered support must exert enormous load into the roof and floor, and effectively clamp the strata and prevent any movement. This force is called the SET LOAD and is directly proportional to the PRESSURE that is put into the legs by the hydraulic pumps.

 

However, there is another important consideration and that is the area of roof being supported. The important number, therefore, is the set load divided by the roof area. This is called SUPPORT DENSITY and is usually calculated in tonnes per square metre.

 

Your OEM’s manual will contain a chart showing this figure at various operating heights. In general, the lower you are in the operating range of your support, the lower the support density (this is a function of leg angle), but the stated LOAD DENSITY depends on SET PRESSURE. The required set pressure will be stated in the manual.

 

And the moral of this story?

 

Keep your support SET pressures HIGH and keep the amount of roof exposed LOW to achieve the biggest support density possible.

 

Published in March 2007 Australian Longwall Magazine