What down pressure is required on the drill string to operate the DHH drill in hard rock per inch of bit diameter?

• A. 100 to 300 pounds per inch
• B. 2000 to 3000 pounds per inch
• C. 50 to 150 pounds per inch
• D. 500 to 1,500 pounds per inch of bit diameter

Answer: (D)

In hard rock, the drill needs enough weight-on-bit to drive the percussion action into the rock while keeping the bit in solid contact. When you express this as weight per inch of bit diameter, it normalizes for bit size so you can compare and set the correct pressure across different bits. For a DHH drill in hard rock, the recommended range is about 500 to 1,500 pounds of down pressure per inch of bit diameter.

So, for example, with an 8-inch bit, you’d be looking at roughly 4,000 to 12,000 pounds of weight on the bit. This range provides enough force to fracture rock effectively without overloading the drill string or causing excessive wear. Lower ranges wouldn’t sustain the necessary bit engagement, while ranges much higher than 1,500 pounds per inch of diameter can lead to equipment stress and inefficiency.