A deep hole drilling machine's hydraulic system powers critical functions: clamping, steady rests, bushing slides, tailstock positioning, and sometimes spindle speed variation. When a hydraulic problem stops a machine function, production stops completely. Systematic diagnosis gets the system back online faster than random component swapping.
Hydraulic Pump Types
Common Pump Types
| Pump Type | Pressure Range | Flow Range | Application | Failure Mode |
|---|
| Gear pump | 50–200 bar | 5–100 L/min | General hydraulic power units | Wear — internal leakage increases over time |
| Vane pump | 50–150 bar | 10–200 L/min | Machine tool hydraulics | Vane wear, cam ring wear |
| Piston pump (axial) | 100–350 bar | 10–300 L/min | High-pressure clamping, accumulators | Piston slipper wear, valve plate scoring |
| Piston pump (radial) | 200–700 bar | 5–100 L/min | High-force clamping | Piston wear, eccentric wear |
Pump Failure Symptoms
| Symptom | Likely Cause | Diagnostic Check | Corrective Action |
|---|
| No pressure | Pump not priming, drive coupling failed, pump seized | Check shaft rotation, coupling, prime pump | Repair coupling, prime, rebuild pump |
| Low pressure (all functions) | Pump wear (internal leakage), relief valve set low | Measure pump case drain flow, check relief valve setting | Rebuild pump, adjust relief valve |
| Low pressure (one function) | Downstream leak, valve problem | Check pressure at each function | Isolate and repair affected circuit |
| Excessive noise (cavitation) | Suction restriction, low oil level, high viscosity | Check oil level, suction strainer, oil temperature | Clean strainer, add oil, check heater |
| Excessive noise (aeration) | Air leak on suction side | Check suction fittings, shaft seal | Tighten fittings, replace shaft seal |
| Overheating | Relief valve passing, pump worn, low oil level | Check relief valve setting, case drain flow | Adjust relief, rebuild pump, add oil |
| Erratic pressure | Contamination in relief valve, pump wear | Check pressure gauge response, sample oil | Clean valve, check oil cleanliness |
Hydraulic Valve Functions
Valve Types
| Valve Type | Function | Common Failure | Symptom |
|---|
| Directional control valve (DCV) | Directs flow to actuators | Spool sticking, solenoid failure | Cylinder does not move, moves slowly, or drifts |
| Pressure relief valve | Limits system pressure | Stuck open (low pressure), stuck closed (overpressure) | Low system pressure, pressure spikes |
| Pressure reducing valve | Reduces pressure for a branch circuit | Spool wear, spring fatigue | Branch pressure too high or too low |
| Flow control valve | Controls actuator speed | Orifice clogging, compensator wear | Cylinder moves at wrong speed |
| Check valve | Allows flow in one direction only | Seat leakage, poppet sticking | Load drifts, system pressure decays |
| Pilot-operated check valve | Holds load position | Pilot piston seal leak, poppet wear | Load drifts down over time |
Valve Troubleshooting
| Valve Problem | Symptom | Check | Corrective Action |
|---|
| Spool sticking | Cylinder moves intermittently or not at all | Manual override — does spool move freely? | Clean or replace spool, check oil cleanliness |
| Solenoid failure | Valve does not shift | Check coil resistance, voltage | Replace coil or valve |
| Relief valve stuck open | System pressure below setting | Remove and inspect seat and poppet | Clean or replace relief cartridge |
| Relief valve stuck closed | Pressure spikes on start | Check pressure gauge at startup | Clean or replace relief cartridge |
| Check valve leaking | Load drifts | Pressure test — does pressure hold? | Replace check valve cartridge |
| Flow control drifting | Cylinder speed varies with temperature | Check compensator | Replace flow control valve |
Diagnostic Procedures
Pressure Testing
| Step | Action | Detail |
|---|
| 1 | Install pressure gauge at pump outlet | Read system stand-by pressure |
| 2 | Note system pressure at idle | Should match relief valve setting |
| 3 | Actuate each function one at a time | Note pressure drop during operation |
| 4 | Compare readings to machine specifications | Identify circuits with excessive pressure drop |
| 5 | Install pressure gauge at problem function | Read pressure at the actuator |
| 6 | Compare function pressure to system pressure | Significant drop indicates restriction or leak |
Flow Testing
| Test | What It Checks | How To | Interpretation |
|---|
| Pump flow (no load) | Pump displacement | Flow meter at pump outlet, relief valve fully open | Compare to pump displacement × speed |
| Pump flow (at pressure) | Pump internal leakage | Flow meter at pump outlet, load to operating pressure | > 10% drop from no-load = pump wear |
| Case drain flow | Pump wear (piston pumps) | Flow meter in case drain line | > 10% of pump flow = rebuild needed |
| Cylinder bypass test | Cylinder seal leakage | Extend cylinder, apply pressure, measure drift | > 5 mm/min = seal replacement needed |
| Valve leakage test | Valve seat leakage | Isolate valve, pressure test each port | Any leakage = valve repair |
Temperature Monitoring
| Temperature | Condition | Action |
|---|
| 30–45°C | Normal operating range | No action |
| 45–55°C | Elevated — acceptable for continuous duty | Check cooler condition, ambient temperature |
| 55–65°C | High — reduced oil life | Check cooler, reduce load, consider oil cooler maintenance |
| > 65°C | Critical — oil degradation accelerates | Stop machine, investigate cause |
Contamination Control
Contamination Sources and Effects
| Contaminant | Source | Effect | Prevention |
|---|
| Particulate (chips, dust) | Coolant ingress, worn seals, dirty oil | Valve sticking, pump wear, cylinder scoring | Proper filtration, seal maintenance |
| Water | Coolant leak, condensation | Corrosion, oil degradation, bacterial growth | Seal hydraulic system, breather filter |
| Air | Suction leak, low oil level | Cavitation, spongy actuator response | Tighten suction fittings, maintain oil level |
| Thermal degradation products | Overheating | Varnish, sticky valves | Maintain temperature < 55°C |
Oil Cleanliness Targets
| Component | Target ISO Code | Target NAS Class | Filtration Required |
|---|
| Gear pump | 20/18/15 | 8 | 25 µm |
| Vane pump | 18/16/13 | 7 | 15 µm |
| Piston pump | 17/15/12 | 6 | 10 µm |
| Directional valve | 18/16/13 | 7 | 15 µm |
| Servo/proportional valve | 16/14/11 | 5 | 3–5 µm |
Troubleshooting Flowcharts
No Hydraulic Pressure
| Step | Check | If Yes | If No |
|---|
| 1 | Is the pump motor running? | Go to step 2 | Check motor power and starter |
| 2 | Is the pump shaft rotating? | Go to step 3 | Check coupling |
| 3 | Is the oil level adequate? | Go to step 4 | Add oil |
| 4 | Is the suction strainer clean? | Go to step 5 | Clean strainer |
| 5 | Is the relief valve stuck open? | Clean/replace relief valve | Pump may be worn — rebuild |
Cylinder Fails to Hold Position
| Step | Check | If Yes | If No |
|---|
| 1 | Does cylinder drift when valve is centered? | Valve spool leaking | Rebuild or replace valve |
| 2 | Does cylinder drift when valve is locked out? | Cylinder seals leaking | Rebuild cylinder |
| 3 | Is the pilot-operated check valve leaking? | Replace check valve | Check for external leak |
| 4 | Is there an external leak at the cylinder? | Repair leak | Cylinder may be OK — check load |
Maintenance Schedule
| Component | Daily | Weekly | Monthly | Annually |
|---|
| Oil level | Check | — | — | — |
| Oil temperature | Check | — | — | — |
| System pressure | — | Check and record | — | — |
| Filter indicators | — | Check | — | — |
| Oil sample | — | — | — | Send for analysis |
| Relief valve setting | — | — | Verify | — |
| Accumulator pre-charge | — | — | Check | — |
| Pump case drain flow | — | — | Measure | — |
| Cooler/heat exchanger | — | — | Clean | — |
| All hoses | — | Visual | — | — |
| Complete oil change | — | — | — | Per manufacturer |
FAQ
What causes low hydraulic pressure on a deep hole drilling machine?
Low pressure is most commonly caused by: pump wear (internal leakage increases), relief valve stuck partially open (contamination on seat), low oil level (pump cavitation), clogged suction strainer (restricted flow), or a leak in the system (external or internal). Diagnose in order: check oil level, check relief valve, check pump flow — the most likely cause is the simplest to check.
How do I diagnose a sticking hydraulic valve?
A sticking valve causes intermittent or no movement of the actuator. Check: does the valve spool move freely when manually actuated? If not, contamination is likely causing the spool to stick. Check oil cleanliness — if ISO code exceeds 20/18/15, the oil is contaminated. Clean or replace the valve and improve filtration. If the valve moves freely but does not shift electrically, check the solenoid coil resistance and voltage.
Why does my hydraulic cylinder drift when it should hold position?
Cylinder drift is caused by: spool leakage in the directional valve (oil bypasses past the spool lands), pilot-operated check valve leakage (poppet not seating), cylinder seal bypass (oil leaks past piston seals), or an external leak. To isolate: lock out the valve and see if drift continues. If drift stops, the valve is leaking. If drift continues, the cylinder seals or check valve are leaking.
How often should hydraulic oil be changed on a deep hole drilling machine?
Change hydraulic oil per the machine builder's recommendation — typically every 2,000–4,000 operating hours or annually. The best practice is to use oil analysis (sampling every 6–12 months) to determine actual oil condition and change based on results rather than on a fixed schedule. Maintain oil temperature below 55°C to maximize oil life.
What is the most common hydraulic problem on deep hole drilling machines?
Contamination is the most common hydraulic problem. Chips and coolant from the drilling process find their way into the hydraulic system through damaged seals, breather caps, and cylinder wiper seals. Contaminated oil causes valve sticking, pump wear, and cylinder seal damage. Prevention — maintain seals, use breather filters, and check oil cleanliness regularly — is more effective than troubleshooting contamination problems after they occur.
Hydraulic systems on deep hole drilling machines are reliable when maintained and persistent sources of downtime when ignored. Systematic diagnosis — pressure first, flow second, component third — finds the root cause faster than removing parts and inspecting them. When in doubt, check the oil cleanliness first. This article reflects industry practice as of 2026.
