Causes & Solutions for Low Impact of Rock Drill, Quick Repair

When a hydraulic rock drill loses impact power, the first component that gets pulled and replaced on most sites is the hydraulic pump. That's usually the wrong call. The pump generates flow, not pressure—pressure in a hydraulic system is resistance to flow, and a genuine pump failure typically shows up as insufficient flow at rated speed, not low percussion energy alone. Replacing a working pump wastes a full shift and leaves the actual fault unfixed.

Low impact in a hydraulic rock drill is a symptom, not a fault. The fault is almost always one of four things: accumulator pre-charge pressure outside specification, percussion circuit pressure set below the drifter's rated value, bypass leakage across a worn percussion piston seal, or a partially blocked control valve reducing the oil flow rate to the impact cylinder. Each of these produces a similar surface symptom—the drill sounds duller, penetration drops, the pressure gauge pointer wanders—but they require different diagnostic steps and different fixes.

Fault 1: Accumulator Pre-Charge Out of Specification

The high-pressure accumulator in the percussion circuit stores hydraulic energy and releases it at the moment of piston reversal, filling the gap between pump supply and instantaneous circuit demand. When the nitrogen pre-charge pressure drops—through diaphragm degradation or gradual gas permeation—the accumulator can no longer buffer the pressure spike at reversal. The result is that the piston undergoes a secondary impact: it switches direction prematurely, the return stroke is too short, and impact energy per blow drops measurably below rated values.

The diagnostic indicator is distinctive: a dull, irregular percussion sound with visible pressure gauge pointer oscillation. The COP1838 manual describes this as the sound changing from crisp to hoarse—an accurate description of what secondary impact timing distortion sounds like. Impact pressure readings around 14 MPa with pointer vibration and violent oil pipe movement are characteristic of accumulator failure on that model. Checking and correcting nitrogen pre-charge is a 15-minute task with the correct charging tool; replacing the diaphragm takes around two hours.

Never operate a drifter with a suspected accumulator fault. Running with no air or insufficient pre-charge pressure concentrates maximum hydraulic oil pressure on the accumulator shell, which can cause housing fracture—a significantly more expensive repair than a diaphragm replacement.

Fault 2: Percussion Circuit Pressure Set Below Rated Value

Every drifter has a rated percussion pressure—the hydraulic pressure at which the impact piston generates its specified blow energy. The relief valve in the percussion circuit limits maximum pressure; if this valve is set too low, or if it drifts due to spring fatigue or contamination, the piston never reaches the pressure required to generate rated impact energy.

This fault produces a gradual, symmetric reduction in impact power rather than the erratic pattern of an accumulator fault. Penetration drops consistently across all hole positions, not just occasionally. The fix is straightforward: measure actual percussion pressure at the test port (most drifter models have one), compare against the rated value in the service documentation, and adjust or replace the relief valve. Contaminated relief valves that are stuck partially open are common after hydraulic oil change intervals have been extended—particle contamination seats on the poppet and prevents full closing.

Fault 3: Percussion Piston Seal Bypass Leakage

A worn percussion piston seal allows hydraulic oil to bypass the piston face during the power stroke. The oil that passes the seal contributes to pressure in the return circuit rather than accelerating the piston toward the shank—effective force on the piston drops in proportion to the bypass volume. Unlike the previous two faults, this one typically develops gradually over hundreds of operating hours and produces slow performance degradation rather than an acute event.

The diagnostic sign is elevated hydraulic oil temperature in the return line combined with reduced penetration rate. Bypass oil is converting the pressure differential into heat rather than mechanical work—the return oil temperature rises 10–15°C above the normal value for the circuit before any visible external leak appears. A drain-line flow test—measuring the actual flow rate from the percussion cylinder drain port against the manufacturer specification—confirms bypass leakage without disassembling the drifter.

The fix is a percussion seal kit replacement. HOVOO supplies percussion circuit seal kits for the major drifter models with PU or HNBR compounds appropriate to the operating temperature. Full model references at hovooseal.com.

Fault 4: Control Valve Flow Restriction

The directional control valve that sequences the piston through its impact cycle must pass the full rated flow at minimal pressure drop. A valve with worn spools, scoring on the bore, or particle contamination from degraded hydraulic oil reduces the flow rate available to the percussion cylinder—effectively the same symptom as an undersized pump, but localized to the percussion circuit rather than affecting all hydraulic functions simultaneously.

The distinction between a control valve fault and a pump fault: with a pump problem, all hydraulic functions on the carrier show reduced performance simultaneously. With a percussion valve fault, only the percussion circuit is affected—rotation, feed, and boom functions continue normally. Measure percussion circuit flow at the test port and compare to specification. If flow is low but carrier hydraulic pressure is normal, the fault is in the percussion circuit downstream of the main supply.

Diagnostic Sequence: Low Impact Fault Tree

After the Fix: Preventing Recurrence

The best single predictor of recurring low-impact faults is hydraulic oil cleanliness. Particle contamination in the 10–50 micron range is invisible to the eye but is the primary cause of relief valve drift, control valve scoring, and premature seal abrasion. A used oil sample analyzed at 200 and 500 hours provides early warning of contamination levels that will produce these faults. ISO cleanliness code 16/14/11 is the target for most percussion circuit applications—most sites run dirtier than that without realizing it.

Record operating parameters every shift: impact pressure, rotation pressure, push pressure, and buffer pressure. The COP1838 service protocol specifically recommends this practice and identifies the early warning pattern—when these four parameters go out of their interrelated balance, a percussion fault is developing before the obvious symptom of low impact appears. Addressing it at the parameter-drift stage costs a filter change and an oil analysis; addressing it after the symptom appears costs a seal kit, a valve, or a diaphragm.