Maintenance
DTH hammers and bits: getting the hours you paid for
Most hammers are killed by air quality, lubrication and a worn bit left in too long — in that order.
4 min read
A down-the-hole hammer is a simple machine in a hostile place: a piston reciprocating at high frequency in an air stream full of rock dust and water, striking a bit that is being asked to break the ground. When it fails early, the cause is almost never the hammer. It is what was going into it.
Air quality is the first thing to fix
The air line delivers everything the hammer sees: pressure, volume, oil, water and whatever the compressor picked up. Two of those are actively destructive.
- Water condensing in the line washes oil off the internals and starts corrosion inside a machine with fine clearances. An aftercooler and a working water separator are not optional in a humid climate.
- Dust drawn through a neglected compressor intake filter is abrasive, and it goes straight into the percussion mechanism.
- Pressure below the hammer’s rating produces weak blows and slow penetration, which crews compensate for with feed force, which damages the bit.
- Volume matters as much as pressure. Enough pressure with insufficient volume will not clear the hole, and cuttings recirculating under the bit destroy gauge buttons quickly.
Lubrication, checked rather than assumed
The lubricator has to be delivering rock drill oil at the manufacturer’s rate for the air volume in use, continuously, for as long as the hammer is running. It is a two-minute check that is skipped constantly, and a hammer run dry for one shift can be finished.
Use rock drill oil, not hydraulic oil. The tackifier that makes it cling to internal surfaces in a high-velocity air stream is the entire point of the product.
The blow-down check
Hold a clean cloth or a piece of card in the exhaust for a few seconds with the hammer off-bottom. A visible oil mist means the lubricator is feeding. Nothing on the card means you have found the problem before it becomes a rebuild.
The bit: pull it before it takes the hammer with it
- Watch gauge. Once the outer row of buttons wears back, the bit no longer cuts full hole size, the hammer starts binding, and the next bit will not go down easily either.
- Regrind buttons on schedule rather than at failure. A bit ground before the carbide is flattened lasts far longer in total than one run until the buttons are gone.
- A lost button is a stop-work. Carbide left in the bottom of the hole will destroy the next bit within metres.
- Check the bit’s shank and splines for wear at every change. A worn shank hammers the driver sub, and that is a hammer repair rather than a bit replacement.
- Record metres per bit against ground type. That number is what tells you whether a different button profile or grade is worth trying.
Operating habits that cost hours
| Habit | What it does |
|---|---|
| Excess feed force to compensate for a dull bit | Overloads the bit face and the shank, and raises the risk of button loss |
| Insufficient feed force | The hammer bounces rather than transmitting energy; wears the internals and drills slowly |
| Rotating too fast for the button profile | Scuffs and rounds the carbide instead of indenting the rock |
| Starting a hole without blowing it clean | Recirculates cuttings under the bit, destroying gauge |
| Collaring hard into broken ground | Deviates the hole and damages the bit before the hole has started |
Stocking for it
- Hold at least one spare hammer per rig on site. A hammer failure with no spare is a full rig standdown, and it is the failure most likely to happen at the worst moment.
- Carry a service kit and the wear parts that go with it — bit retaining rings, chucks, check valves — not just complete replacements.
- Stock bits in the profile the ground actually needs, and keep enough to survive a regrind cycle.
- Never run the last drum of hammer oil down. Reorder at a set level, not at empty.
- Send failed hammers back for assessment rather than scrapping them on site. What killed one will kill the next, and the strip-down report is where you find out what it was.
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