SV0421

Inspect Coupling and Ballscrew

Disable servo power and engage axis brakes. Remove the motor coupling cover and visually inspect for wear, cracks, or loose set screws. Check ballscrew for visible damage or bending.

Use a flashlight and mirror to inspect hard-to-reach areas of the coupling for hairline cracks.

Ensure Emergency Stop is active to prevent unexpected axis movement during inspection.

Measure Backlash

Use a dial indicator mounted on the table or spindle to measure backlash while manually rotating the ballscrew. Backlash exceeding 0.02mm indicates wear or damage requiring repair or replacement.

Mark the ballscrew with a marker to track rotation and ensure accurate measurement.

Secure the axis mechanically if working on a vertical axis to prevent gravity-driven movement.

Test Manual Movement

Disconnect the motor from the ballscrew (if safe) and manually rotate the ballscrew while observing the linear scale position on the ABSOLUTE screen. Inconsistent or no position change indicates mechanical binding or failure.

If manual rotation feels uneven, apply light lubrication to isolate friction from binding.

Counterbalance vertical axes before disconnecting motor to avoid sudden drops.

Inspect Linear Scale

Power down the machine and inspect the linear scale read head for debris, coolant residue, or physical damage. Check the air gap between read head and scale (typically 0.5-1.0mm) using a feeler gauge.

Clean the scale with a lint-free cloth and isopropyl alcohol to remove contaminants without damaging the surface.

Avoid using compressed air near the scale as it can force debris into sensitive areas.

Test Scale Position Reading

Power on the machine and slowly jog the axis while monitoring the ABSOLUTE screen for linear scale position updates. Erratic or no updates indicate scale failure or read head issues.

Compare scale readings with motor encoder readings on the CURRENT screen to pinpoint discrepancies.

Keep hands clear of moving parts during jogging tests.

Check Cable Integrity

Inspect the linear scale feedback cable for cuts, wear, or loose connections at the scale, junction box, and amplifier. Test cable continuity with a multimeter if accessible, and verify shield grounding.

Look for cable drag marks or pinch points near moving axis components that could cause intermittent faults.

Disconnect power before testing cables to avoid electrical shock.

Verify Dual Feedback Enable

Access the parameter screen and check Parameter 1815 bit 5 (DPCx). Ensure it is set to 1 to enable dual position feedback for the affected axis.

Document the original setting before making changes to avoid unintended consequences.

Incorrect parameter changes can cause machine instability; proceed with caution.

Check Excess Error Limit

Verify Parameter 1828 (excess error limit) is set to an appropriate value (typically 500-2000 detection units) for the axis. Compare with machine manual or similar axes on the same machine.

If unsure, temporarily increase the limit by 50% to test if the alarm clears, then restore to original if unrelated.

Do not leave an excessively high limit as it may mask other issues.

Confirm Feedback Selection

Check Parameter 1825 to ensure the dual feedback type matches the installed encoder (e.g., linear scale or rotary). Cross-reference with machine specifications or encoder label.

If recently replaced, ensure the new encoder type matches the parameter setting.

Backup all parameters before editing to allow rollback if needed.

Check Amplifier Diagnostics

Inspect the servo amplifier for LED fault codes or error messages. Refer to the FANUC ALPHA/BETA series manual for code meanings related to feedback processing.

Take a photo of the LED display for reference when consulting manuals or support.

Do not open amplifier covers without proper electrical safety training.

Review Alarm History

Access ALARM > HISTORY on the control to check for related alarms like SV041x or SV0440, which indicate intermittent feedback loss or amplifier issues.

Note the timestamp of related alarms to correlate with machine events like crashes or power surges.

N/A

Swap Amplifier Test

If a spare or identical axis amplifier is available, swap the suspected amplifier with a known good unit after powering down. Retest the axis to see if the alarm follows the amplifier.

Label all cables before swapping to ensure correct reconnection.

Ensure power is off and capacitors are discharged before swapping amplifiers to avoid shock.

Test Axis Movement

Jog the axis slowly in both directions using the control panel while listening for unusual noises (grinding, clicking) and feeling for resistance via handle feedback if available.

Record a video of the jog test to capture subtle noises or hesitations for later review.

Keep clear of moving parts during jogging to avoid injury.

Inspect for Crash Damage

Visually inspect the axis guideways, ballscrew, and table for signs of impact, deformation, or scoring. Check for bent components or misaligned mounting brackets.

Use a straightedge to check for ballscrew straightness if a crash is suspected.

Secure the axis if inspecting vertical components to prevent unexpected drops.

Check Bearing Condition

Manually rotate the ballscrew (motor disconnected if possible) to feel for rough or seized bearings. Excessive resistance or grinding indicates bearing failure requiring replacement.

Compare resistance with other axes on the same machine to establish a baseline for normal feel.

Ensure axis is mechanically secured before disconnecting motor for manual tests.