Published in the September 2014 International Coal News Magazine

Anyone who has managed or supervised a longwall operation will have their own scary stories about armoured face conveyor creep that will bring back bone chilling memories.

I can still recall the horrors of AFC creep from my very first longwall in 1980 as a young under-manager in Yorkshire. I still have the nightmares! So what is it?

You can Google “AFC creep” and you probably will not find anything (until this column gets published).

It’s not in the Oxford English Dictionary either.

However, if you can lay your hands on a copy of the Australian Longwall Consultants Glossary of Longwall Mining Terms (available from yours truly) or for older coal miners dust off a copy of Joe Foy’s IMC Longwall Training Manual, you will find similar answers: “AFC creep – the lateral migration of the AFC towards the maingate or tailgate ends of the longwall away from the correct position”

I have another definition, less technical, but based on actual events at coalmines around the world: “The very small and sometimes unpredictable lateral movement of the AFC panline, that if not checked (and constantly managed), will result in destruction of the maingate drive, iron bound roof supports, toppling roof supports, broken relay bars, damaged electrical cables, loss of roof control, loss of face access, loss of production, which in turn leads to loss of revenue, loss of the longwall and ultimately loss of jobs. Special note: safety hazards increase in direct proportion to worsening creep”

The causes of AFC creep

There are several contributing factors:

1. Gradient: if the longwall is on an incline across the face, then gravity will pull the AFC downhill;

2. Snaking of the AFC: the simple action of advancing the AFC from one end to the other induces a “snaking motion”. In the area along the “snake” the relay bars are imposing a sideways force on the AFC panline in the direction of the snake;

3. Cutting patterns: uni-directional cutting means the snake is always in one direction. Bi-directional cutting means the snake alternates in each direction, reducing this effect;

4. Shearer forces: the shearer uses the AFC as a platform and anchor. The shearer drives itself along the face by pushing against the AFC. This imposes a force on the AFC in the opposite direction to travel;

5. Friction: the friction between the underside of the AFC panline and the floor material plays a part;

6. Relative angle between the AFC and the roof supports during “steering” of the longwall; and

7. Mismanagement of creep: while this may be obvious, it is probably the main cause of creep disasters.

AFC creep gets worse before it gets better

If left unchecked, the AFC will continue to migrate in one direction. However, eventually the relay bars and the supports themselves will turn in the direction of travel of the creep, and follow the AFC.

When this occurs, the supports then push the AFC in that direction and the creep will then accelerate with very serious and immediate consequences.

How to control AFC creep

Creep control is very much like steering a bicycle. If we want to go left, we push the right side of the handlebars forwards. If we want to go right, we push the left side of the handlebars forward.

The same principle applies to steering the longwall. If the creep goes to the left, we must push the AFC back to the right by advancing the LEFT end of the face relative to the RIGHT.

There is no mathematical formula for calculating creep control. It is based on trial and error plus experience, and constant monitoring, along with prompt action.

How creep is measured

Typically, there is a “zero” creep line in the maingate. This is established by the surveyors and tells us where the maingate drive must be relative to the maingate roadway. Most mines will use a large target on the maingate drive that when referenced against the “zero creep line” gives us our creep measurement toward the maingate or tailgate.

This measurement should be taken every time the maingate is pushed over and recorded. It is vitally important to understand the direction the creep is travelling and at what rate.

The actual position is less important than the rate of creep. Many longwall operations will set trigger action response plans that say if the creep is 300mm to the tailgate, take corrective action (push the tailgate forward). However, if the creep is static, I would suggest leaving it where it is and continue to monitor.

Other factors to consider ...

Always keep the face line straight

. A bent face can give erroneous creep readings and may temporarily give the impression that the creep is static. Once that bend is taken out, the experience may be job changing.

Monitor the relay bars

. The relay bars will eventually show a bias towards maingate or tailgate relative to the pontoons. However, this is a consequence of creep and the corrective action should be taken before the relay bars are at an acute angle. Within a few shears you may be setting your supports on top of the relay bars. When that happens … you are in very deep.

Reverse snaking

because reversing the snake can have a minor affect on the creep. However, it is a sticking plaster job. It will not fix the problem, but may buy you some time before the train wreck.

The see-saw phenomenon

, where in some cases, the creep can go from one extreme to the other, going from tailgate to maingate and back again with huge swings in the face line profile (ever had the wobbles on a bike?). In aviation, we call this pilot induced oscillations where the control inputs are too late and too big and fluctuate from one extreme to the other and it all ends in tears. The same goes for creep. Changing the face profile (the lead in one gate) may take time to take effect, and only local experience and monitoring will provide the information required to make those judgments.

Anti-creep rams

: In extreme cases the solution is to install anti-creep hydraulic cylinders across the face. These have been used in steep seam mining for many years around the world. Not recommended for “normal” longwall operations.

Sounds easy? The reality is quite different. If you don’t get the balance right, just like a bike, you fall off. The trouble with creep, however, is the pain never goes away.