How Much Weight Should You Put on a DTH Hammer? The Answer Changes With Every Meter You Drill
Ask three drillers how much feed pressure to put on a DTH hammer and you'll get three answers — and all three might be wrong at different depths. Feed pressure on a DTH rig isn't a set-and-forget parameter. It's a moving target that changes with bit diameter, rock hardness, and — critically — with how much drill string is already hanging in the hole.
Here's how to figure out the right number, and why getting it wrong in either direction destroys bits and hammers.
The Job of Feed Pressure: Just Keep It Seated
A DTH hammer does the breaking work with the piston, not the feed system. The feed pressure has one job: keep the bit in firm contact with the bottom of the hole so the piston's energy goes into the rock instead of ringing back through the hammer.
That means the feed force has to be just enough to counteract the rebound of the piston impact — the tendency of the hammer to bounce off the bottom after each blow. Too little feed, and the hammer bounces. Too much feed, and you're fighting the rock and loading components that shouldn't be loaded. The sweet spot is narrow and shifts constantly.
The Rule of Thumb: 500 Pounds Per Inch
For starting a hole — collaring — the industry-standard starting point is simple: 500 pounds of feed force per inch of bit diameter, or 9 kilograms per millimeter.
A 6.5-inch bit: 500 × 6.5 = 3,250 pounds. A 165 mm bit: 9 × 165 = 1,485 kilograms. That's your starting feed pressure.
But here's the part most people miss: that's a starting point, not a setting. It's the right number for the first meter of hole. As the hole deepens, the number changes.
The Correction Nobody Makes: Drill String Weight
The pressure gauge on the rig shows the feed force the machine is applying. But the actual force on the bit at the bottom of the hole is different — because the drill string itself weighs something, and that weight is pushing down on the bit through the hammer.
At the surface, the gauge pressure equals the bit force, because the string isn't hanging yet. At 50 meters, the string — rods plus hammer — might weigh a ton. At 200 meters, several tons. That weight is gravity doing the feed system's job. If you leave the feed setting at the collaring value, the actual bit force is now the collaring value plus the string weight. You're overfeeding by exactly the weight of the string.
The correction is straightforward: reduce the feed pressure as the hole deepens, subtracting the drill string weight from the collaring value. Keep the bit force constant, not the gauge reading. In practice, this means the gauge pressure should come down as the hole goes down.
Why Too Much Weight Is a Problem
The temptation is always to add weight to speed up penetration. It doesn't work that way with a DTH hammer. The piston is doing the breaking, and the piston's energy is fixed by the hammer design. Extra feed force doesn't make the piston hit harder. It just:
Reduces bit life. Excess feed presses the bit inserts harder into the rock during the rotation phase, accelerating gauge-row wear. The inserts are already doing maximum work; extra feed just grinds them harder against abrasive rock.
Loads the splines and chuck. The bit transmits rotation torque through the splines. Extra feed force increases the friction at the bit-rock interface, which increases the torque demand, which loads the splines harder. Spline wear is one of the most common causes of premature hammer retirement.
Bends the string. In hard, uneven rock, excess feed force flexes the drill string. A flexed string under impact load is a string under bending fatigue. The rods and the hammer body both suffer.
Why Too Little Weight Is Worse
If excess feed slowly reduces tool life, insufficient feed causes sudden, dramatic failures. An under-fed hammer bounces. When the bit leaves the bottom between blows:
The hole wanders. A bouncing bit can't track straight. The hole deviates. In deep holes, a few degrees of deviation at the top becomes meters of lateral offset at the bottom.
The piston blank-fires. When the bit is off bottom, the piston's energy has nowhere to go. It rings through the hammer body instead of the rock. Blank firing is the fastest way to destroy a hammer's internal components.
The splines heat up. A bouncing bit chatters in the chuck. The friction between the bit shank and the spline faces generates extreme local temperatures. Heat damage at the splines is irreversible — once the surfaces are heat-affected, they fail regardless of what you do afterward.
The inserts pop out. The vibration and chatter from bouncing hammers the inserts in their seats. Under-fed drilling is a leading cause of insert loss — the inserts don't break, they just fall out because the bit never ran steady enough to keep them seated.
The Practical Routine
Here's the working method:
Start at the rule of thumb. 500 lbs per inch, or 9 kg per millimeter, at collaring.
Watch the behavior, not the gauge. The right feed pressure is the one where the hammer runs smooth, with no bounce and no chatter. If you hear harsh metallic impacts, add feed slightly. If the rotation motor labors or penetration stalls, back off.
Adjust for depth. As the string grows, subtract the string weight. Keep the bit force constant, not the gauge reading.
Adjust for rock. Softer rock wants less feed — the bit penetrates easily, and too much feed packs the annulus. Harder rock wants more feed — but only up to the point where the inserts stop bouncing and start crushing.
Never force it. When penetration stalls, the answer is almost never more feed. Check the bit condition, check the air pressure, check the hammer. Feed pressure is a contact parameter, not a penetration parameter.
The driller who understands feed pressure isn't the one who sets a number and walks away. It's the one who watches the hammer, listens to the sound, and makes small adjustments every few meters. The gauge is a reference. The hammer is the authority.
