By Lars Persson, Technical Manager, Rock Drill Design, Atlas Copco.

Over the years, Atlas Copco has made major contributions to the twin goals of boosting performance while reducing the damage that powerful rock drills can inflict on drill steel.

Today's production drilling demands high performance and high availability of both people and equipment, particularly when drilling long straight holes in all directions and in extreme conditions.

This is where the design of the rock drill becomes a crucial factor. It must be powerful enough to do the job, robust enough to withstand the most rigorous of environments and yet gentle enough to minimise wear on drill steel and other equipment.

Atlas Copco rock drills have certainly come a long way - perhaps further than most - in being able to meet all these criteria. And as drillers will appreciate, it is no easy task.

The COP series of rock drills is based on many years of research and testing and much of this work has been focused on getting the right balance between performance and overall economy.

Impact and shock waves

Our designs centre on the relationship between the percussive impact and the resulting shock waves that are transmitted from the drill's piston through the drill steel and the rock.

As a result, we know that slender pistons transmit the highest possible impact energy more gently than large diameter pistons, and we have arrived at detailed specifications of what the size, length and weight of these pistons should be.

For example, with Atlas Copco's powerful COP 1800 series, this can be up to 25 tonnes per impact. But that's not all. We have also acquired considerable knowledge of how shock waves from the piston are reflected from the bottom of the hole back to the rock drill. And we know that if large reflex shock waves are reflected up the drill steel and into the rock drill, they will then be transmitted to the feed and the boom and all the equipment will suffer serious damage.

The COP 1838ME rock drill: The first rock drill in the Atlas Copco range with the all-important feature of double-dampening.

Double dampeners

The solution is reflex dampening. In Atlas Copco rock drills we have incorporated dampening systems which effectively absorb these shocks. This function also ensures that the joints in the drill string are kept tight and do not overheat - an extremely important factor in long-hole drilling - as well as reducing the level of transmitted noise.

Dampeners not only provide protection for the rock drill, they also safeguard consistent and continuous feeding of the drillsteel which ensures that the bit maintains constant contact with the bottom of the hole. This, in turn, also contributes to increased drill steel life.

Rule of thumb

The rule of thumb we use is that the maximum shock wave imposed on the drillsteel should not exceed the steel's breaking point. This means that the ideal diameter of the piston that transmits the blows should be as close as possible to the diameter of the drill rods. Slender pistons, in general, can strike with greater impact speed and drill faster without exerting undue stress on the drillsteel.

Furthermore, when drilling in softer rock formations which does not require such high impact energy, the piston's impact stroke can be adjusted and the frequency increased to provide optimum drillsteel economy.

This is how the basic reflex dampener works: The system consists of a dampening piston, accumulator and adapter. When the reflected shock wave knocks the damper piston backwards, the pressure rapidly rises in what is known as the "extra" piston. This jets out the oil volume in the piston, which consumes the energy by means of heat. At the same time, the accumulator also absorbs pressure from the damper piston (Fig1).

Fig 1. The traditional shock absorber (damper piston and accumlator) has been integrated with an extra cylinder which is activated by the recoil waves and a separate pressurized air flow.

How dampening works

When the reflected shock wave ceases, the damper piston is then pressed forward again by both the pressure in the accumulator and the locked-in pressure in the extra piston. The damper piston then locates itself over the control hole, where the shank adapter is positioned correctly for the next impact stroke.

If the feed force increases, the damper piston will start to close the control hole and pressure inside the damper chamber will rise. If it decreases, the hydraulic pressure in the damper chamber will move the piston forward, allowing the damper pressure to escape through the control hole.

This means the damper piston position will always be within the position of the control hole and the pressure in the damper is proportional to the actual feed force on the drill string. When the energy from the rock drill's piston passes through the drillstring, it alternately compresses and lengthens the drillsteel.

Other design improvements are also moving the frontiers of rockdrill performance forward. For example, a major contributor to overall productivity and economy in long-hole drilling is a unique back-hammering device which virtually eliminates jamming and the subsequent loss of drill strings.

This so-called extractor unit uses the back-hammering function to loosen joints during downwards-hole extension drilling and to retrieve the drill strings.

Cuts time and wear

By eliminating jammed joints it reduces the time for rod handling between holes and also reduces wear on gripper jaws, shanks and drill tubes. The device is available for almost all COP 1238, COP 1838 and COP 4050 rock drills which have reverse back-hammering and a percussive energy of 20 per cent.

Another example is the COP 4050 which, during upwards drilling, has extra protection to prevent water and grinding paste from entering the system.

But even the best rock drills in the world can still under-perform. In cases where a rockdrill may not be functioning at its full capacity, or where couplings may be overheating, drillers can turn to Diarot, Atlas Copco's expert analysis program which can be used to diagnose problems and recommend optimal settings and drillstring combinations.

By adapting the performance of the rockdrill, the right combination of impact energy, adjustable torque and feed force can keep the force where it is needed - at the bottom of the hole - and drilling costs under control.

Choosing the best match for production drilling

For further information,
please contact Technical Manager Lars Persson:
lars.persson@atlascopco.com

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