Resolving SINUMERIK 840D SL Brake Test Alarm 20096 Fault Code 4 on DMG MORI Eco800V

On SINUMERIK 840D sl-controlled machine tools that implement Safety Integrated, the brake test is an obligatory power-on check verifying each axis holding brake can hold the rated static load with drive torque removed. On DMG MORI Eco800V vertical-spindle milling machines built around an NCU 710.3B PN, a SINAMICS S120 drive line-up, and ET200S distributed I/O, two alarms commonly surface together when the Z axis brake test cannot complete:

• Alarm 20096 (NCK, red) — brake test aborted, fault code 4: monitoring window exited.
• Alarm 700545 (DMG MORI machine builder warning) — "Brake test Z axis: axis does not stop — move axis to lowest position before opening the door."

The combination of the two is the diagnostic fingerprint of a degraded Z axis holding brake, not a parameter error. This reference walks through system identification, alarm decoding, the relevant machine data 36960–36969, SINAMICS S120 drive-side checks, mechanical inspection, environmental considerations, and a verification sequence before returning the machine to two-shift production.

System Identification

Before any diagnosis, confirm the hardware and software stack on the machine. Each component influences how the brake test is invoked, what alarms appear, and which machine data is active.

Open HMI > Commissioning > Version and record NCK, PLC, and SINAMICS firmware versions. NCU 710.3B PN is typically paired with SINUMERIK Operate and SINAMICS S120 firmware V4.7 SPx or V5.x; the machine data index, the SINAMICS parameter set, and the brake-test variant (SINUMERIK-only, drive-integrated, or Safety Integrated) all depend on the active version. Verify against the current Siemens Industry Online Support entry for your NCU build.

Functional Description of the Brake Test

The brake test is a Safety Integrated function that runs in three phases per axis. The NCK orchestrates the sequence; the drive executes the torque; the NCK observes the response.

1. Phase 1 — Approach. The axis is moved to a defined safe position. For the Z axis on a vertical-spindle machine, this is typically the lowest mechanical position (Z0 reference or a parameterized test position) so that gravity acts in a known direction and any drop is detected as downward motion.
2. Phase 2 — Torque application. The drive applies a defined holding torque T_test against the closed brake. The torque is ramped up over MD36964 (or the equivalent parameter in your SW release) so the brake is not shock-loaded.
3. Phase 3 — Monitoring window. The NCK observes the actual position through the drive telegram. If the axis moves beyond ±MD36962, or if the sequence exceeds MD36963, the test fails and 20096 is raised with the relevant fault code.

A passing test confirms the brake can hold the rated static load with drive torque removed. A failing test means one of three things: the brake is mechanically degraded, the drive cannot produce the test torque, or the monitoring window is set too tight for the application.

Alarm 20096 — NCK Side Analysis

Alarm 20096 is the SINUMERIK NCK alarm raised when the brake test sequence aborts before completion. The accompanying fault code identifies the abort reason. Decoding the fault code is the single most important step in the diagnosis because it tells you whether the failure is mechanical, electrical, or sequencing.

How to confirm the fault code at the HMI

1. Acknowledge the alarm list.
2. Select alarm 20096 and press Further information (the info icon at the bottom of the Operate alarm line, or the info hotkey on older HMI Advanced).
3. The lower info area shows the fault code as a sub-code: e.g. 20096 4. On older SINUMERIK Operate versions, navigate to Diagnostics > Alarm log and inspect the time-stamped entry; the sub-code is in the alarm properties dialog.
4. Record NCK time stamp and ambient temperature for trend analysis.

Alarm 700545 — DMG MORI Machine-Builder Alarm

Alarm 700545 is not a Siemens alarm. It is generated by the DMG MORI PLC program and wraps around the NCK brake test to enforce a specific operator sequence: the Z axis must be moved to its lowest safe position before the operator can open the machine door. This wrapper is safety-relevant because, if the brake fails after the door is open, a vertical-axis drop can injure the operator.

700545 as a stand-alone warning means the test sequence cannot begin because the Z axis is not in the expected pre-test position. When 700545 appears with 20096 FC 4, the test started, the brake failed, and the wrapper alarm was raised on the way out. Treat 700545 as a symptom of 20096 FC 4, not as an independent fault.

Machine Data 36960–36969 — Brake Test Reference

The brake test is parameterized in axis-specific machine data within the 36960–36969 range. The MD names and bit assignments below are typical for current SINUMERIK 840D sl Safety Integrated; confirm them against the active Lists Manual for your SW version before editing.

Should you change MD36960–36969 to bypass the alarm?

No. Do not loosen MD36962 (position tolerance) or stretch MD36963 (time tolerance) to mask a brake failure. The brake test exists to prevent vertical-axis drop, which is a personnel hazard. Loosening these values to make the machine pass removes the only automatic safeguard protecting the operator when the door is open. Treat the alarm as a hard fault: stop, inspect the brake, fix the mechanical cause, return to original MDs, then re-run.

SINAMICS S120 Drive-Side Brake Test

The S120 participates in the brake test through telegram 701 and a coordinated sequence. The drive checks enable state, torque accuracy at low speed, encoder feedback quality, and the state of the motor holding brake control word.

If the drive raises any of the F0790x alarms during the brake test, the NCK-side alarm 20096 follows. Always clear and inspect the drive-side fault log via Starter / Startdrive / AOP30 before chasing the brake mechanically. A clean drive fault buffer with 20096 FC 4 confirms the brake; a populated drive fault buffer shifts the root cause to the drive system.

SINAMICS parameters that affect the brake test

• p2003 reference torque — used as a base for the test torque on many 1FK7/1FT7 motor configurations.
• p1531 power limit, motoring — if too low, the drive cannot reach the test torque and FC 2 results.
• p1226 threshold for standstill detection — interacts with the brake test's monitoring window.
• r0899 state word — reflects whether the drive is in brake-test-active state.

Read the active values with Starter / Startdrive before changing any MD or drive parameter; record them in the maintenance log.

Brake Hardware Verification

For Eco800V, the Z axis servo motor (typically 1FK7 or 1FT7) holds the spindle head via an integrated holding brake. With 20096 FC 4 in the alarm log and a clean drive fault buffer, the brake is mechanically suspect. Verify in the order below.

Step 1 — Brake power supply

1. Power off and lock out / tag out the machine at the main disconnect.
2. Open the Z axis motor terminal box or follow the brake cable to the Motor Module terminal block (X5 on the SINAMICS S120 Motor Module for the integrated brake).
3. Measure the brake coil resistance with a calibrated multimeter. Compare to the motor data sheet — typical 1FK7 / 1FT7 holding brakes read between 30 Ω and 120 Ω depending on frame size. Open circuit or short circuit means the brake must be replaced.
4. Measure the brake supply voltage at the Motor Module output under load (drive enabled, brake commanded closed). It must be within ±10 % of the nominal 24 V DC.

Step 2 — Air gap and friction-plate wear

1. Remove the brake cover on the non-drive end of the motor.
2. Measure the air gap between the armature plate and the rotor with feeler gauges. Compare to the motor data sheet (typically 0.2–0.4 mm). A gap larger than the spec means the friction plate has worn and the magnetic force can no longer develop full rated torque.
3. Inspect the friction surface for glazing, oil contamination, or uneven wear. A brake that has slipped produces a polished mirror-like surface. The plate cannot develop full torque in this condition even if the air gap is correct.
4. Check the splined teeth of the inner plate (drive plate) for chipping. A chipped tooth slips under load without warning.

Step 3 — Mechanical load path

1. With the brake engaged and drive torque removed, attempt to move the Z axis by hand using a service crank (or low-pressure hydraulics where applicable). Any drift indicates the brake cannot hold rated load.
2. Inspect the ball-screw nut for backlash. Excessive backlash looks like a brake fault because the axis drops briefly during the test torque ramp.
3. Check counterbalance (if Eco800V is equipped with hydraulic or pneumatic counterbalance). The brake is only one of two static holding elements; both must be functional.
4. Inspect couplings between motor and ball screw for slipping or wear.

Step 4 — Thermal environment

High ambient temperature affects brake torque in two ways:

• Friction coefficient drop. Organic friction linings lose 10–30 % of rated friction coefficient above 80 °C ambient, and 50 %+ above 120 °C.
• Brake coil resistance rise. A 24 V brake coil pulling 0.5 A at 25 °C ambient pulls only ~0.42 A at 70 °C ambient because copper resistance rises. The magnetic flux drops and so does the holding torque.

Record motor and ambient temperature during the test. If the failure correlates with daytime / afternoon shifts but not with night / morning shifts, the brake is thermally marginal and replacement is the only correct repair. Consider specifying a high-temperature brake variant with the OEM if the application requires it.

Diagnostic Procedure (Field-Proven Order)

1. Confirm the alarms. Read both 20096 and 700545 from the alarm screen. Note the fault code (4) on 20096.
2. Check the S120 drive fault log. Open Starter / Startdrive, connect to the NCU, navigate to the Z axis Motor Module, and read the fault buffer. Address any F07902 or F07903 first.
3. Verify the brake power supply. Measure coil resistance and supply voltage as described above.
4. Inspect the brake mechanically. Air gap, friction plate, drive plate teeth.
5. Inspect the mechanical load path. Ball screw, counterbalance, couplings.
6. Capture motor temperature and ambient. Read the motor thermal model via p0601 / r0035 and confirm against ambient.
7. Decide on repair. Use the decision matrix below.
8. Repair, replace, or escalate to OEM. When the brake has to be replaced, the servo motor must be removed and shipped to a certified repair shop, or replaced with a new unit. The brake alone is not field-serviceable on most 1FK7 / 1FT7 motors — replacement requires re-impressing the rotor magnets and the friction plate assembly.

Repair Decision Matrix

Why the Existing Service Calls Did Not Solve the Problem

The maintenance history in the case shows three service interventions:

1. Reconnect contact — likely addressed a loose brake connector. Correct for an intermittent open circuit but does not address friction-plate wear.
2. Reload program — restored the PLC and NC program. Correct for a corrupted project but irrelevant to a mechanical brake fault.
3. Check brake torque and request motor replacement — correct conclusion but not yet executed.

The pattern indicates the OEM service correctly identified the brake as the root cause on the third visit but has not yet completed the replacement. Until the motor with integrated brake is replaced, the alarm will recur on every cold start. The two-hour lost-time cost per failed start is a direct consequence of running without the corrective replacement in place.

Cost-of-Downtime Analysis

Use the data from the case to size the repair decision. The brake test alarm costs the operation approximately two hours of lost production per failed start. With two shifts per day, this is up to 14 hours per week if the failure is intermittent. At the quoted DMG MORI service rate of $70/h plus transport and waiting time, an in-house or scheduled repair is significantly cheaper than reactive calls. Schedule the motor replacement on a planned weekend stop rather than paying for an emergency call-out during production.

Verification Procedure

After the corrective action, run the following verification sequence before returning the machine to production:

1. Power on the machine cold (cold start, ambient temperature).
2. Wait for NCK and drive to complete initialization; confirm no drive-side faults in Starter / Startdrive.
3. Run the brake test from the HMI: Diagnostics > Brake test (or via the operator-startable test that DMG MORI exposes on the OP).
4. Confirm no alarm 20096 and no alarm 700545.
5. Open the door and visually confirm that the Z axis holds position with drive torque removed. Use a dial indicator on the spindle nose to verify zero motion for at least 30 seconds.
6. Power off and let the machine sit for 4 hours; restart and re-run the brake test. This is the hot-environment verification: the brake must hold with motor and brake at thermal steady state.
7. Run a representative part program with the door open and observe the Z axis. Any drop indicates an incomplete repair.
8. Update the maintenance log with motor serial number, brake part number, brake torque, and ambient temperature at the time of the test.

Only return the machine to operator use after steps 1–8 pass with the door open and the brake tested hot as well as cold.

Documentation Cross-Reference

When validating the repair, cross-reference the following official sources:

• The active Siemens Industry Online Support entry for "SINUMERIK 840D sl Safety Integrated" — current commissioning manual and alarm description.
• The current SINUMERIK 840D sl Lists Manual — for the exact machine-data identifiers in the 36960–36969 range for your SW release.
• The SINAMICS S120 Function Manual — brake-test chapter.
• The SINAMICS S120 List Manual — drive parameter p2003 and brake-test parameters.
• DMG MORI Eco800V Operator Manual and Maintenance Manual — machine-builder PLC logic that generates alarm 700545.
• The 1FK7 / 1FT7 motor data sheet — brake coil resistance, air gap, holding torque.

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