Which unit is used to express down pressure on the DHH drill string in hard rock?

Which unit is used to express down pressure on the DHH drill string in hard rock?

• A. Kilograms
• B. Pounds per square inch
• C. Pounds
• D. Pounds per inch of bit diameter

Answer: (D)

In hard rock drilling with a down-the-hole hammer, the important quantity is the axial load (thrust) applied to the bit, and it’s most meaningful when matched to the bit’s size. Expressing this down pressure as pounds per inch of bit diameter normalizes the load by the contact width of the bit, giving a consistent measure of how intensively the bit is pressed into the rock across different bit sizes. This unit directly correlates with rock breakage and tool wear, making it the practical standard for specifying and comparing down thrust in DHH operations. Kilograms would be a mass unit, not the field-standard way to express this thrust. Pounds alone would be total force without accounting for how large the bit is, so it wouldn’t allow fair comparisons across bit sizes. Pounds per square inch is a pressure, not the thrust per bit width, and is not how down pressure is characterized in this context.

In hard rock drilling with a down-the-hole hammer, the important quantity is the axial load (thrust) applied to the bit, and it’s most meaningful when matched to the bit’s size. Expressing this down pressure as pounds per inch of bit diameter normalizes the load by the contact width of the bit, giving a consistent measure of how intensively the bit is pressed into the rock across different bit sizes. This unit directly correlates with rock breakage and tool wear, making it the practical standard for specifying and comparing down thrust in DHH operations.

Kilograms would be a mass unit, not the field-standard way to express this thrust. Pounds alone would be total force without accounting for how large the bit is, so it wouldn’t allow fair comparisons across bit sizes. Pounds per square inch is a pressure, not the thrust per bit width, and is not how down pressure is characterized in this context.