DTH hammer failure causes in hard‑rock drilling — four major fault categories explained
In hard‑rock drilling with down‑the‑hole (DTH) hammers, high formation hardness, heavy drilling loads, and harsh working environments make a range of faults more likely. These faults directly affect penetration rate, hole quality, and equipment life. Based on the key conditions of hard‑rock drilling, common failures fall into four groups: impact‑performance degradation, mechanical wear/sticking, cuttings‑removal system abnormalities, and power‑transmission faults. The following analyzes each group’s symptoms and root causes.
Impact‑performance degradation Symptom and effect: a sudden drop in rock‑breaking efficiency. Typical signs are reduced impact force and lower impact frequency; the bit can no longer efficiently fracture hard rock and penetration slows dramatically or stops. Main causes include:
1.1 Power‑medium supply problems
For pneumatic (air‑driven) hammers, insufficient compressor pressure (below the 0.6–1.2 MPa range typically required for hard‑rock drilling), unstable airflow, or air leaks/blockages in the supply line reduce the pressure available to drive the piston.
For hydraulic hammers, insufficient pump pressure or hydraulic oil contamination and clogged passages reduce hammer drive force.
Contaminants in the power medium (moisture or dust in compressed air; metal particles in hydraulic oil) accelerate seal wear and further reduce medium efficiency.
1.2 Internal air/flow distribution failures
Valve‑type hammers commonly suffer valve‑plate wear, deformation, or sticking, which disrupts timely valve switching and prevents the piston from achieving high‑frequency reciprocation.
Valveless designs may be affected by wear or clogging of the piston/cylinder flow grooves; buildup of cuttings delays flow reversal, disrupts the impact cycle, and greatly reduces impact power.
1.3 Piston‑to‑bit interface issues
High‑frequency impacts against the bit cause severe wear on the piston face and the bit tail; increased clearance at the contact interface leads to energy loss during transfer.
Eccentric bit installation or loose locating pins can cause off‑center strikes ("misstrikes"), lowering impact efficiency and accelerating localized wear.
Mechanical wear, sticking, and structural failure Symptom and effect: causes of downtime. Under high‑frequency impacts and rotational torque, mechanical parts wear excessively, stick, or break. Common manifestations include piston seizure, drill‑string damage, and wear of front/rear subs, with these root causes:
2.1 Piston seizure and wear
Heavy loads intensify friction between piston and cylinder wall. Inadequate lubrication (e.g., lack of regular lubrication in pneumatic hammers or degraded lubricity in aged hydraulic oil) and ingress of rock cuttings can reduce clearances and cause the piston to seize. Long‑term high‑frequency operation also wears the piston surface and may produce cracks; in severe cases the piston can fracture.
2.2 Drill‑string damage
The drill string must transmit torque and support the hammer’s weight. If the drill‑string material strength is insufficient, threads are loose, or hole deviation occurs, the string may experience additional bending moments leading to buckling, deformation, or thread failure. Abrasion from rock cuttings on the outer wall also accelerates wear and shortens service life.
2.3 Joint and seal damage
Front and rear subs are critical connection components; under intense vibration and torque their threads can strip or deform. Seals (O‑rings, seal rings) are exposed to abrasive power media and elevated temperatures (heat generated at the bit can conduct to the hammer); seals age and crack, causing power‑medium leaks and allowing cuttings to enter internal assemblies, which accelerates wear.
Cuttings‑removal system abnormalities Symptom and effect: interruptions to continuous drilling. Hard‑rock cuttings are typically hard and coarse; when the cuttings‑removal system underperforms, problems such as hole clogging and poor cuttings evacuation occur. Symptoms include cuttings accumulation in the hole, buried bit, sudden rise in drilling resistance, and in severe cases stuck pipe. Key causes:
3.1 Insufficient cuttings‑transport medium
Pneumatic hammers may lack sufficient flushing air volume; hydraulic hammers may have insufficient flushing fluid flow; either condition prevents timely removal of bottom‑of‑hole cuttings. Wear or deformation of transport passages (e.g., the bit center hole, piston center bore) from prolonged abrasive cuttings transport can narrow channels and further reduce evacuation efficiency.
3.2 Mismatch between drilling parameters and cuttings removal
If penetration rate is too high, generated cuttings exceed the capacity of the removal system. Incorrect rotation speed or bit thrust settings can produce overly coarse cuttings that can’t pass through the flow passages, causing accumulation and blockage.
3.3 Hole deviation and cuttings deposition
Hole deviation during drilling creates low spots on the borehole wall where cuttings collect and are not effectively carried out by the flushing medium. Over time these deposits form a packed "cuttings bed" that obstructs hammer advance and bit action.
Power‑transmission faults Symptom and effect: drilling interruptions due to broken torque transmission or failure to deliver impact energy to the bit. These faults commonly occur at drill‑string/hammer and hammer/bit interfaces. Main causes:
4.1 Loose or damaged connections
Loosened or stripped threads between the drill string and the hammer’s rear sub, or wear of locating pins/splines, prevent reliable torque transfer and stop the hammer from rotating with the string.
Wear or deformation of the hammer’s front‑sub/bit connection components (lock nuts, snap rings, etc.) can let the bit mount work loose, producing "dry strikes" where impact energy is not delivered to the rock and synchronous rotation is lost, producing uneven bit tooth wear.
4.2 Bit damage causing transmission failure
Bit cutters (tungsten‑carbide buttons, polycrystalline diamond cutters) in hard‑rock drilling are prone to wear, chipping, or detachment. A damaged bit cannot effectively engage the formation, so impact energy fails to transfer through the cutters to the rock and instead transmits back into the hammer’s internal structure, increasing internal shock loads and causing secondary failures.
Summary and core influencing factors Overall, common DTH hammer failures in hard‑rock drilling stem from the overriding characteristics of the working condition: high loads and harsh environments. The primary influencing factors can be grouped into three categories:
Inadequate equipment compatibility: hammer model, bit type, or component materials not matched to hard‑rock conditions.
Improper operation and maintenance: unrealistic drilling parameter settings, failure to regularly clean and lubricate, and delayed replacement of worn parts.
Poor coordination of auxiliary systems: unstable power‑medium supply, and mismatch between cuttings removal capability and the drilling cycle.
Identifying the root causes of these failure types provides a basis for troubleshooting and preventive measures to ensure continuous, efficient hard‑rock drilling.
