Fanuc SV0400 - Servo alarm: n axis hardware disconnection
The FANUC SV0400 Servo Alarm for n-th Axis Overload is typically caused by mechanical binding, improper servo tuning, or motor/electrical issues. The most likely fix involves inspecting for mechanical obstructions or friction on the affected axis, followed by verifying servo parameters against OEM specifications. If unresolved, electrical testing of motor and amplifier components may be necessary.
FANUCServo/AxisIntermediate30-90 minutes
SV0400
Fanuc SV0400 - Servo alarm: n axis hardware disconnection
SERVO ALARM: n AXIS HARDWARE DISCONNECTION
The servo amplifier has detected that the motor current for the specified axis has exceeded the continuous rated current limit for a sustained period, triggering thermal protection. This alarm prevents motor and amplifier damage from mechanical overload, binding, improper servo tuning, or electrical faults. The axis number 'n' in the alarm message identifies which servo axis is affected.
Root Cause Summary
The FANUC SV0400 Servo Alarm for n-th Axis Overload is typically caused by mechanical binding, improper servo tuning, or motor/electrical issues. The most likely fix involves inspecting for mechanical obstructions or friction on the affected axis, followed by verifying servo parameters against OEM specifications. If unresolved, electrical testing of motor and amplifier components may be necessary.
Safety
Disable servo power (Emergency Stop active) and apply axis locks before manually inspecting ballscrews or couplings to prevent unexpected axis movement
Disconnect servo amplifier power before megger testing motor windings - high voltage test current will damage amplifier output stage if connected
Be aware that reducing servo gains as temporary fix will degrade machining accuracy and corner performance - identify and resolve root cause rather than masking with detuned parameters
Causes and Fixes
1
Visual Inspection for Obstructions
Set machine to Emergency Stop and visually inspect the affected axis for chip accumulation, damaged way covers, or tooling debris. Remove any visible obstructions using compressed air or manual cleaning tools.
Check under way covers and around ballscrew areas—chips often hide in less accessible spots.
Ensure servo power is disabled before reaching into machine areas to avoid unexpected movement.
2
Check Lubrication System
Verify way oil reservoir level and confirm lubrication pump operation via machine diagnostics or manual activation. Inspect ballscrew and linear guides for adequate grease coverage; apply lubricant if dry per OEM specs.
Use a flashlight to inspect ballscrew threads—uneven grease distribution often indicates a clogged lube line.
Do not over-lubricate as excess oil can attract chips and worsen binding.
3
Manual Jog Test for Resistance
Release Emergency Stop, set to JOG mode, and slowly move the axis through its full travel range. Note any grinding, binding, or uneven resistance. If resistance is detected, mark the position for detailed mechanical inspection of ballscrew or guides.
Use a dial indicator to measure axis deviation at suspected binding points for precise diagnosis.
Keep hands clear of moving parts during jogging to prevent injury.
1
Access Servo Parameters
Navigate to MAINTENANCE > PARAM > SERVO on the control panel. Locate Parameters 2020 (position gain), 2021 (velocity gain), and 2023 (load inertia ratio) for the affected axis and record current values.
Take a photo of the parameter screen for reference before making any changes.
Do not alter parameters without documenting originals—incorrect settings can worsen performance.
2
Compare to OEM Baseline
Refer to the machine’s maintenance manual or OEM parameter backup to compare current values. If Parameter 2020 or 2021 is significantly higher than recommended (e.g., >30% deviation), or if 2023 exceeds 100, suspect tuning issues.
Contact the machine builder if OEM values are unavailable—default FANUC settings may not match your machine.
Incorrect parameter values can cause instability; proceed cautiously.
3
Adjust Gains Temporarily
Reduce Parameter 2020 (position gain) by 20% and test axis movement in JOG mode. Monitor load meter (DIAGNOSIS > SERVO) for improvement. If alarm persists, reduce further in 10% increments, but do not go below 50% of original value without mechanical checks.
After adjustment, perform a light cutting test to ensure accuracy isn’t compromised before full production.
Reduced gains can mask mechanical issues—use as a temporary diagnostic, not a permanent fix.
1
Listen for Abnormal Noise
In JOG mode, move the axis slowly and listen for grinding, clicking, or whining noises from the ballscrew or guide areas. Note specific positions where noise occurs for targeted inspection.
Use a mechanic’s stethoscope to pinpoint noise sources on larger machines.
Keep clear of moving parts while listening to avoid entanglement.
2
Check for Mechanical Play
With servo power off and axis locked, manually check for excessive play or looseness in couplings, ballscrew mounts, and linear guides using a wrench or by hand. Any noticeable movement beyond 0.001 inch suggests wear.
Use a dial indicator to quantify play—small deviations can cause significant load increases.
Ensure axis locks are engaged before applying force to prevent unexpected drops.
3
Inspect Brake Operation
Verify axis brake releases properly by checking Parameter 2000 bit 1 (brake enable) and monitoring brake release signal in PMC ladder. If brake does not release, inspect wiring or solenoid for faults.
A stuck brake often causes intermittent overload—check for consistent load spikes at movement start.
Do not bypass brake circuits as they are critical for safety during power loss.
1
Monitor Current Feedback
Access DIAGNOSIS > SERVO > MONITOR and observe current feedback for the affected axis during slow JOG movement. Look for spikes or asymmetry in U/V/W phases indicating amplifier or feedback issues.
Compare with a healthy axis if available—differences >30% suggest electrical faults.
Do not operate machine under load if current readings are erratic; risk of further damage.
2
Inspect Cable Connections
With power off, inspect motor power (U/V/W) and encoder cables at both motor and amplifier ends for loose connections, fraying, or contamination. Tighten or clean as needed using electrical contact cleaner.
Look for signs of arcing or burn marks on connectors—often a sign of intermittent faults.
Ensure power is fully disconnected before handling cables to avoid shock.
3
Perform Motor Insulation Test
Disconnect motor from amplifier and use a megger to test windings phase-to-phase and phase-to-ground. Readings should be >10 megohms to ground and balanced (<5% variance) between phases. Low readings indicate motor failure.
Test at 500V DC for accurate results—lower voltages may miss insulation breakdown.
Never megger test with amplifier connected—high voltage will damage electronics.
1
Review Load Meter During Operation
Monitor the load meter (DIAGNOSIS > SERVO) during typical cutting and rapid traverse moves. Sustained readings above 60% indicate the motor is undersized for the application or parameters are too aggressive.
Log load values over a full cycle—intermittent spikes may point to specific program sections.
Avoid prolonged high-load operation during testing to prevent thermal damage.
2
Check Programmed Feedrates
Review NC program for feedrates and rapid traverse settings (G00 speeds). Compare against machine specifications in the operator’s manual. Reduce feedrates by 25% temporarily to test if alarm clears.
Look for heavy roughing cuts or deep drilling cycles—these often push motor limits.
Ensure temporary reductions don’t affect critical part dimensions before testing.
3
Evaluate Table or Tooling Mass
Check if recent changes to table load, fixtures, or tooling exceed OEM weight limits for the axis. If overloaded, reduce mass or consult machine builder for motor upgrade options.
Use a crane scale to verify fixture weight if documentation is unavailable.
Do not exceed specified axis load limits—risk of permanent mechanical damage.
DIY Feasiblemedium confidence
Call a technician if:
·If mechanical binding persists after cleaning and lubrication, indicating internal damage to ballscrew or guides.
·If electrical tests show motor insulation failure or amplifier faults beyond basic cable repairs.
Prevention
Maintain regular lubrication schedules and inspect way oil levels weekly to prevent binding.
Perform periodic chip cleanup around ballscrews and under way covers to avoid obstructions.
Review and update cutting parameters after tooling or fixture changes to match motor capacity.
Common Mistakes
Ignoring high load meter readings and continuing operation, leading to motor or amplifier burnout.
Reducing servo gains as a permanent fix without addressing underlying mechanical or electrical issues, resulting in poor accuracy.
