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Z Axis Drop during Power failure or emergency stop condition in Fanuc controller OiT and OiM Series

A robust step-by-step procedure to prevent Z axis drop during an emergency stop should combine safety category selection, brake/counterbalance integrity checks, proper E-stop wiring to Sitop or SMPS (Switching Mode Power supply), and CNC/drive parameter timing for brake control and torque removal sequencing. The end goal is that the gravity axis remains at position with no uncontrolled fall when E‑stop or power-loss occurs.

Define the symptom

• Gravity axis (typically Z in VMC and X axis in Turning machine) moves downward when E‑stop is pressed or when power is lost, indicating brake, counterbalance, or stop-category sequencing issues that remove torque before a holding mechanism is engaged.

Safety concept selection

• Choose appropriate stop category for emergency: Category 0 (STO, uncontrolled coast) or Category 1 (controlled stop then STO), based on a risk assessment; vertical/gravity axes often require controlled stop or mechanical anti-fall measures.
• Before diving into the specifics of STO & SS1, it’s essential to understand the categories of safe stopping defined in standards. The standard PN-EN 60204-1 outlines three stopping scenarios (categories) that correspond to our safety functions, But in Industrial machinery field we should focus on two categories .
• Category 0 (STO) – Emergency stop by immediately cutting off the drive power without controlled braking. This is the fastest way to stop a machine, akin to pressing an emergency stop button. However, it is an uncontrolled stop and may be too harsh for delicate machinery, potentially leading to long restart times.
• Category 1 (SS1) – Controlled stop where the system actively brakes the machine before cutting off the power (transitioning to STO). This minimizes mechanical shocks and allows for a more civilized stop. It requires a moment to brake but reduces the risk of mechanical damage. It is typically used when safety requires slowing down rather than immediate power cut-off, such as in production lines with delicate elements.

This classification allows for selecting the stopping method tailored to the machine’s specifics and hazards. Different scenarios require different stopping methods, and a thorough risk analysis during the design phase should indicate which stopping scenario ensures the safety of people and equipment. Remember, emergency stopping (E-STOP) should be implemented in category 0 or 1, i.e., as STO or SS1

Wiring and safety function

• Ensure the E‑stop is wired to the certified safety inputs that command STO/SS1 in the inverter/drive per Safety Integrated guidelines; STO safely removes motor torque, while SS1 performs a controlled decel before STO.
• For multi-axis machines, a central E‑stop should brake axes rapidly then transition drives into STO; verify the safety PLC/relay logic and the drive’s fail-safe inputs are mapped correctly.

Brake and counterbalance integrity

• Inspect the gravity-axis motor brake: verify power-off engagement, air gap, wear, and response time; a worn or slow brake is a common cause of drops on E‑stop.
• Verify presence and function of counterbalance: Pneumatic, hydraulic, nitrogen, or weight; insufficient compensation can allow head drop when torque is removed.

Drive/CNC parameters for brake sequencing

• Implement proper brake control timing so the brake engages before torque is fully removed; configure brake control validity and delay timers in the CNC/drive (example FANUC: enable brake control, set brake control timer, and delay STO at the power module so the brake has time to set).
• Verify any “emergency stop feed” or anti-sag function that micro-moves against gravity during E‑stop to cancel the slip, where available and permitted by the safety concept.

E‑stop behavior verification

• Confirm that on E‑stop the control issues a controlled stop (SS1) for the gravity axis if required by risk assessment, then commands STO after brake engagement delay; avoid immediate torque cut if it causes drop.
• Ensure the machine’s E‑stop category matches the implemented timing; uncontrolled Category 0 with no brake/counterbalance will likely result in a drop.

Mechanical checks on the Z stack

• Check ballscrew nut preload, thrust bearings, and guideway friction; excessive free movement increases drop distance when torque is removed.
• Inspect Z-axis limit/reference switches for damage from previous drops and correct any misalignment to prevent compounding faults after an event.

Controller/drive diagnostics

• Review alarm/event logs around E‑stop to see the sequence: brake command, torque inhibit, and STO; a mis-sequenced enable/ready chain can disable servo holding torque too early.
• Confirm the control interface signals used to acknowledge/reset E‑stop and the drive interlocks are correct for the platform (e.g., SINUMERIK interface bits and Safety Integrated configuration).

Test plan after adjustments

• Perform no-load E‑stop tests with the head near the top of travel; measure any residual motion and confirm it is within safe microns-level slip, not a noticeable drop.
• Repeat with typical spindle/tool weight and with air/pneumatic counterbalance active; verify no drop in both E‑stop and sudden mains power-off simulations if permissible and safe.

Platform-specific notes

• FANUC: Use gravity-axis brake control parameters and emergency-stop feed function to cancel slide; key examples include enabling brake control and setting brake control timer and E‑stop delay at the power module. Validate on Alpha i/αi‑PS systems.
• Before Changing any parameter we recommend to take Sram Back up of machine.In case of any problem you can put SRAM Again in machine.
• You can first compare below parameter with your exiistiing machine parameters.

Please set the above parameters and check for Gravity axis Drop Retraction during Power fail and emergency stop condition.

When hardware changes are required

• If the head still drops with correct timing, add or service a spring-applied, power-off brake sized for the axis load, or repair/replace the existing motor brake.
• If the machine lacks counterbalance and the load is heavy, retrofit pneumatic/hydraulic/nitrogen counterbalance or a weight system per machine builder guidance.

Documentation and sign-off

• Record the risk assessment, stop-category choice, wiring diagrams, parameter sets, and validation test results; include measured drop distance and response times.
• Train operators on proper E‑stop usage and distinguish between emergency stop and normal stop/power-off procedures to avoid unnecessary uncontrolled torque removal.

Quick troubleshooting checklist

• Does the Z-axis brake hold with power removed during static tests? If not, service/replace brake.
• Is E‑stop configured as SS1→STO with adequate brake engagement delay? If not, revise safety function timing and parameters.
• Are mechanical elements (ballscrew, guides, thrust bearings) tight and undamaged from prior crashes? If not, repair before validation.

Disclaimer: The blogs shared on CNC machines are created purely for *educational purposes*. Their intent is to help readers understand CNC controls, alarms, diagnostics, and general troubleshooting methods. We strictly avoid any copyright violations, and all explanations are written only for learning and knowledge-sharing.  

These blogs should not be considered as official repair or service manuals. For detailed instructions, critical repairs, or advanced troubleshooting, it is always necessary to contact and work under the guidance of the respective *machine manufacturer* or *CNC controller support team*.  

The content provided is focused only on *diagnosis and awareness*. We do not take responsibility for any kind of damage, error, or malfunction that may occur if someone directly applies the information shared here without proper technical supervision.#