Understanding RC Hammers

This brief summary is designed to give geologists and drillers knowledge of the RC system they use, as background information to relate to when analyzing the systems overall performance.

Hammer Principles
Conventional and Reverse Circulation (RC) hammers are simple air motors that are powered by high pressure air. This is feed via an intake/ valving/timing system that controls the piston that delivers the energy to the percussion bit. The air exhaust system is controlled by the percussion bit and the air passages to the surface. As with any non-electrical motor, any exhaust restriction will reduce the motors power. In the case of a DTH Hammers air exhausting from the hammer is also used to carry the rock cuttings to the surface. Any restrictions that limit the flow of exhaust air and cuttings to the surface restricts the power of the hammer.

Conventional Hammers vs Reverse Circulation Hammers
Conventional Hammers exhaust the high pressure air through the holes in the percussion bit into the drill hole and to the surface up the outside of the hammer. Reverse Circulation Hammers exhaust the air down the outside splines of the percussion bit and then up the holes of the percussion bit and the internal diameter of the hammer and the drill string to the surface.

RC Drill Rods
RC drill rods are essentially two drill rods of different diameter with the smaller drill rod contained inside the large one. The high pressure air is passed down the ID of the larger Rod past the OD of the smaller rod with the sample and exhaust air going up the ID of the smaller Rod. Drilling with incorrect drill Rod sizes will restrict the exhaust air and thus reduce the power output of the hammer.

Piston cycle rates
At boosted pressures of up to 1000psi the cycle rate of the piston can be up to 2400 cycles per minute which equates to the piston striking the percussion bit 30-40 times per second.

Bit Interaction with Sample
The oscillating, cutting action of the rotating percussion bit occurs at a rate of up to 40 times per second creating a crushing, grinding rotary action on any chips at the face of the percussion bit. This grinds the sample into a fine dust which can reduce the energy of the piston. The aim is to effectively clear the sample chips from the face of the bit quickly thus reducing the chance of hammer power loss. To achieve this the driller must maintain correct down pressure and not flood the hole with water.

Limitations on Reverse Circulation Drilling
With the advent of higher pressure boosters in the drilling industry other components have not kept pace with the changing technology. Drill Rods today are basically the same as was used 25 years ago with Rotary Mud Drilling. These aged designs have small annuluses designed to accept 500cfm-900cfm but are used with large 1200cfm-220cfm compressors exhausting air at 150psi to 200psi pressures into a small chamber created by the annular restriction. The higher exhaust pressure created helps the mass flow of air up the smaller inner tube, this also creates very high turbulent back pressures in the chamber around the percussion bit. Hammer drilling performance is reduced as air and trapped particles swirl around trying to find their way to the face holes.