What causes FANUC alarm 422?

The allowable speed parameter for the PMC axis torque control is set too low for the actual machine operation requirements. An external load or mechanical interference is causing the axis to accelerate beyond the programmed speed limit during torque control mode. Incorrect or missing PMC ladder logic is sending improper torque control commands that allow runaway speed conditions. A faulty or miscalibrated servo encoder is reporting incorrect speed feedback, triggering a false overspeed condition. The torque control speed limit parameter (typically set via PMC R or D registers) has been accidentally changed or reset.

How do I fix FANUC alarm 422?

1. Identify which axis (n) triggered the alarm and check the corresponding PMC axis control parameters, specifically the allowable speed limit register being sent from the PMC ladder to the CNC. 2. Review the PMC ladder logic for the torque control sequence on the affected axis and verify the speed limit data being written to the control register is appropriate for the application. 3. Check Fanuc servo parameters related to the axis speed limit in torque control mode and compare current values against the original machine setup documentation or backup parameters. 4. Inspect the axis for any mechanical binding, loose couplings, or unexpected load changes that could cause the axis to overspeed when torque control is active. 5. Verify the servo encoder feedback by monitoring actual axis speed in the servo diagnostic screen during a controlled torque control test to confirm whether the overspeed is real or a feedback error. 6. If parameters were recently changed or a battery replacement occurred, restore parameters from a known good backup and retest the torque control operation.

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FANUC Controller

Servo/Axis · Advanced · 1-3 hours depending on root cause

Alarm 422

Fanuc Alarm 422 - SERVO ALARM: n AXIS SPEED EXCEEDED IN TORQUE CONTROL

Alarm 422 occurs when a PMC-controlled axis operating in torque control mode exceeds its programmed speed limit. This is a protective function to prevent runaway conditions when the axis is being controlled by torque commands rather than position/velocity commands. The alarm indicates the actual axis velocity has surpassed the allowable speed threshold set in the PMC axis control parameters.

FANUCServo/AxisAdvanced1-3 hours depending on root cause

422

Fanuc Alarm 422 - SERVO ALARM: n AXIS SPEED EXCEEDED IN TORQUE CONTROL

SERVO ALARM: n AXIS EXCESS ERROR IN TORQUE CONTROL

Root Cause Summary

Alarm 422 occurs when a PMC-controlled axis operating in torque control mode exceeds its programmed speed limit. This is a protective function to prevent runaway conditions when the axis is being controlled by torque commands rather than position/velocity commands. The alarm indicates the actual axis velocity has surpassed the allowable speed threshold set in the PMC axis control parameters.

Safety

Position lost
Disconnect power to the CNC machine before working on any electrical components. Lockout/Tagout procedures must be followed.
Be careful when working near moving parts. Ensure the axis is properly supported before disconnecting any components.
Electrostatic discharge (ESD) can damage sensitive electronic components. Use proper ESD precautions when handling encoders and servo amplifiers.
Refer to the machine's electrical schematics and service manuals for specific voltage and current ratings.
Do not attempt to repair the encoder itself. Replace it with a new or refurbished unit.
High voltages may be present in the servo amplifier. Only qualified personnel should work on the amplifier.
Risk of position loss, which could lead to inaccurate movements or collisions.
Electrical hazard when handling cables; ensure power is off before proceeding.

Causes and Fixes

1
Navigate to SYSTEM > PMC > PMC AXIS CONTROL > PARAMETER screen. Check the speed limit parameter for the affected axis (typically parameter D300-D399 series depending on axis number). Verify the speed limit value is appropriate for the application.
2
Check PMC ladder logic controlling the torque command (G address outputs to axis control). Verify the torque command value being sent is not excessive for the load conditions. Monitor real-time G addresses in PMC > TRACE or I/O DIAG screen.
3
Review parameter 2192 (PMC axis control speed limit) and axis-specific speed clamp settings in the D-parameter range. Increase the allowable speed limit if operational requirements demand higher speeds in torque mode.
Ensure mechanical system and safety circuits can handle increased speed limits before modifying parameters.
4
Reduce the torque command gain or ramp rate in the PMC ladder program if the axis is accelerating too rapidly. Implement torque command ramping or limiting logic in the ladder to prevent sudden speed excursions.

DIY Feasiblehigh confidence

Call a technician if:

·Servo amplifier replacement required
·Encoder replacement on difficult-to-access motors
·Oscilloscope signal analysis needed

Prevention

Regular encoder cable inspection during PM
Maintain proper cable separation from power lines
Keep encoder connections clean and properly torqued
Monitor servo diagnostic screens for early warning signs

Common Mistakes

Working on encoder connections with power on
Using incorrect encoder replacement part numbers
Ignoring cable routing requirements
Failing to perform reference position return after encoder replacement

Related Alarms

401 414 415 421 423 424

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Always verify with your machine manual or a certified technician before performing service procedures.