THE SCIENCE OF ROTATION: ATTACK ANGLES AND PICK MOVEMENT IN COAL CUTTING SYSTEMS

Does the pick and sleeve actually rotate while the system is under pressure? Common sense suggests that the pick would be compelled to rotate inside the sleeve as it slices through the coal, due to the friction exerted by the coal on the pick's surface.

However, pick rotation at the coal face is hindered by insufficient rotational force, which fails to overcome the friction between the pick and sleeve. It might be feasible, with adequate lubrication, to reduce the friction between these surfaces.

In reality, while the system is under direct pressure, the assembly operates as a cohesive unit and does not rotate when the pick is engaged with the coal. Rotation only occurs once the pick exits the coal face, propelled by the momentum generated upon impact with the coal. Most rotation happens when there is minimal or no direct force on the pick tip, particularly after the pick body comes into contact with the coal.

Another common misunderstanding is that the pick assembly is pushed backwards, away from the drum during cutting. Consequently, pick boxes were installed at an angle to accommodate this anticipated wear pattern, allowing the attack angle of the pick to increase and gradually align with the ideal attack angle as it wears.

In fact, the rotational force of the cutter drum is substantially lower than the traction force propelling it into the coal face during the sumping cycle. It is also much less than the downward force applied by the two hydraulic cylinders pulling the cutter drum into the coal during the shear-down process. The evidence for this can be seen by examining the internal wear on a used pick box.

This reveals that the pick assembly moves inward towards the cutter drum, thereby increasing the pick assembly's attack angle. This suggests that the pick box should be mounted on the drum at an attack angle less than the ideal, allowing the system to wear towards the optimal angle.

In conclusion, our investigation into coal cutting systems clarifies that picks rotate mainly after exiting the coal, not while cutting under pressure. It also shows that picks move inward, not backward, requiring pick boxes to be installed at angles that anticipate wear.