SINUMERIK 840D Alarm 300608: Vertical Axis Drop Diagnosis

Problem Overview

A vertical (gravity-loaded) axis on a loader machine controlled by a SINUMERIK 840D drops uncontrollably to the floor when the axis is mechanically blocked or momentarily stalled. The event is followed by alarm 300608 "Speed controller output limited" (sometimes written "Speed controller at limit"). The axis is driven by a Siemens 1FK6083-6AF71-1EH0 synchronous servo motor with an integrated absolute encoder and a built-in holding brake, and there is no hydraulic or mechanical counterbalance. A secondary anomaly appears when the motor power and encoder cables are reconnected while the controller is still on, causing a fast uncontrolled up/down excursion on the next control-ON.

This article consolidates the field diagnosis path: confirm whether the drop is mechanical (brake failure) or control-side (speed-controller saturation, drive bus, LT module, or referencing state), then apply targeted machine data (MD) corrections and verification steps. The methodology applies to any 840D sl/sl plus HMI solution line controller driving an un-counterbalanced vertical axis with a brake-equipped servo.

Alarm 300608 - Technical Definition

Alarm 300608 is an axis-specific alarm. The format is:

• Class: 3 axis alarm (NCK), display via HMI as axis-name + alarm text.
• Numbering: 3006xx block corresponds to "follow-up / monitoring" alarms. 300608 indicates the controller has been at the speed-controller output limit for longer than the configured monitoring time on a specific drive.
• Reaction: NCK sets the axis to "follow-up" by default; on a vertical axis without counterbalance this is the moment of free-fall unless the holding brake engages first.
• Clear: Reset by NC Reset/Cancel after the speed-controller output returns below the threshold.

Per the official SINUMERIK 840D Diagnostics Manual, alarm 300608 is raised when:

The speed controller output has reached its limit for longer than the time set in MD36020 with a higher axis torque than allowed in MD36000.

Cross-reference related alarms before assuming a single root cause:

Root Cause Analysis - Why the Axis Drops

On an un-counterbalanced vertical axis, gravity is a continuous force. Holding the position depends on either (a) the closed-loop speed/torque controller generating a counter-torque continuously, or (b) the holding brake applying when the controller is disabled. If the speed controller saturates (alarm 300608) and the brake does not engage in time, the only thing keeping the load up is torque production - and the moment it saturates, the load drops.

Three failure modes are possible:

1. Mechanical stall with brake intact: a blockage forces a torque demand above the drive limit, the controller saturates, but the brake is meant to clamp once pulses are removed. The 300608 should be momentary; the load should not drop. If it does, the brake is the suspect.
2. Mechanical stall with brake degraded: worn friction plates reduce holding torque below the load weight, so the moment the controller saturates, the load slips through the weakened brake.
3. Drive / control-side enable loss: the drive pulses are removed (drive fault, bus fault, LT module drop, encoder plug reseat). The controller has no torque output, and the brake must clamp within the configured ramp time. If MD timing or brake voltage is wrong, the load falls during the ramp.

Brake System Investigation

The 1FK6083-6AF71-1EH0 is fitted with a permanent-magnet holding brake (24 VDC, integrated in the motor, released by excitation). The brake is opened by the drive module's brake output; the closing time is typically 30-60 ms and the opening time 60-100 ms depending on age and temperature.

Step-by-step brake check

1. Static current test: command a low feed (e.g. 10% rapid) UP and DOWN on the vertical axis only and capture the motor current on the HMI service screen. The UP current should be slightly higher than DOWN (gravity-assist), but the difference must be repeatable. A difference greater than ~10-15% indicates a dragging or partially-released brake.
2. Brake voltage check: measure the brake supply at the drive module output (terminals X7 on SINAMICS S120). Voltage must be 24 VDC +/-10% with the brake released and < 2 VDC with the brake applied. A low release voltage (below ~22 V) prolongs opening time and can cause a 1-2 mm initial drop after enable.
3. Mechanical brake inspection: with the axis safely supported and machine isolated, measure the rotor's free rotation. There must be zero rotation with the brake applied. A click-free, immediate stop with no audible creep is normal; any rotor drift indicates air-gap growth or worn plates.
4. Air-gap measurement: on 1FK6 motors, the brake air-gap is checked between the armature plate and the rotor plate through the side inspection hole. Spec: 0.25 - 0.45 mm typical. Gap > 0.6 mm means the brake must be re-shimmed or replaced - the holding torque is no longer rated.

Field signature of a degraded brake

A typical failing brake shows a slow drift under static load (no command), 1-2 mm of immediate fall the moment the controller is disabled (closing time exceeded), and a small jog back up the moment the next enable occurs. If the operator reports "the axis settles a little every time I power off," replace the brake or the motor with a verified MLFB.

Servo Motor and Encoder Diagnostics

Motor: 1FK6083-6AF71-1EH0 - 80 mm frame, 6-pole synchronous, integrated absolute encoder (likely DRIVE-CLiQ via 1FK6 converter or SSI absolute on the older flange). The "1EH0" code = holding brake (1) + absolute encoder (E) with singleturn (H) resolution + shaft with keyway (0). To inspect the actual motor MLFB in the controller, read:

MD1105 = motor type index (compare to S120 configuration)
MD1106 = motor code (for 1FK6 this is the Siemens-commissioned value)
MD1107 = encoder code

If the controller was hot-swapped, encoder cables were disconnected, or the drive module was replaced, the absolute position reference can be lost. On 840D this is normally recovered by referencing (homing) the axis, but if the referencing was never executed after the cable reseat, the drive will use the last valid absolute position, which on a vertical axis with a quick retract can be off by a full revolution - causing the unexpected fast movement observed on the next control-ON.

LT Module and Drive Bus Check

The Line Module (LT, "Line Transducer" - actually Smart Line Module / Active Line Module / Basic Line Module) supplies the DC bus to all drive modules. A drop in DC bus voltage forces every drive on that bus into pulse-disable, and the brakes apply by default. If the LT module is weak or the bus is overloaded, the bus voltage can dip below the threshold during a sudden motor current spike (e.g. the 60% stall current observed) and the drives will pulse-disable momentarily. When the bus recovers, drives re-enable - and on a vertical axis this is observed as a brief drop, brake clamp, then bounce back as the controller re-engages.

Bus voltage diagnostics

1. Read DC bus voltage via HMI service screen or Drive Monitor: r0070 on each drive module.
2. Capture during a stall event using Trace: Drive + Line Module r0070, r0026, r0030 and the vertical-axis r0078 (current actual) and r0060 (speed setpoint).
3. Verify the line filter, line reactor, and pre-charge circuit on the LT module. Capacitors age and ESR rises; pre-charge resistors drift. Siemens publishes a 10-year replacement guideline for the line filter capacitors and the fan.
4. Check the bus bar and screw connections at the LT module output for thermal discoloration or torque loss.

The contributor who suggested "the LT module was faulty" was on the right track: a weak LT module will cause bus sag under transient load and produce exactly the symptoms described (a vertical drop, then an alarm, then a fast excursion on the next enable as the controller fights a charged bus). Replace the LT module with the same MLFB (e.g. 6SL3130-6AE21-0AB1 for a 16 kW Smart Line Module) and re-commission.

Machine Data Parameters for Speed Controller Monitoring

The 300608 alarm thresholds live in the following MD block. Tune them for the load profile of the loader:

Brake Control Timing and Configuration

SINUMERIK 840D manages the holding brake through the drive's internal brake control. The sequence is:

1. NC issues pulse enable -> drive closes internal relay, applies 24 V to brake -> brake releases.
2. Drive enables torque production only after the brake release time has elapsed (MD36050).
3. On pulse disable, drive removes 24 V from the brake, the brake clamps by spring force, torque production is removed after MD36060 elapses.

For an un-counterbalanced vertical axis, MD36060 should be as short as the brake's actual closing time allows. If the closing time is 40 ms and MD36060 is set to 200 ms, the load will drop for 160 ms with no torque, no brake. Field rule: set MD36060 to the brake's published closing time + 20 ms margin. For a 1FK6 with 24 VDC holding brake, that is 40 + 20 = 60 ms.

Drive-side brake parameters (SINAMICS S120)

p1215 = 1     // brake configuration - internal holding brake
p1216 = 0.06  // brake release time (s) - from motor nameplate
p1217 = 0.04  // brake closing time (s) - from motor nameplate
p1218 = 1     // signal source for brake control
p1220 = 1     // apply brake at speed threshold (rpm) - e.g. 10
p1221 = 0.5   // brake applied if speed < threshold for > this time (s)
p1222 = 1     // pulse suppression with brake (CRITICAL for vertical axis)

Setting p1222 = 1 ensures the drive removes pulses only after the brake has had its full closing time to clamp. Without this, pulses can be removed early under a fault and the load drops before the brake closes.

Commissioning and Verification Procedure

1. Verify brake electrically: with the machine isolated, apply 24 V directly to the brake leads and confirm the rotor is free; remove 24 V and confirm zero rotation under full load. Document the closing time with a stopwatch - should be < 60 ms for a healthy 1FK6083 brake.
2. Verify drive enable sequence: enable axis from HMI in JOG mode, observe on the service screen: pulses enable, then 24 V on brake, then 60-100 ms later the motor produces torque. Reverse: command stop, observe torque removal only after brake has had its closing time.
3. Verify absolute referencing: check axis status in HMI -> Axis -> Reference. The status should be "Referenced." If the cable was disconnected at any point, force a homing pass before continuing.
4. Run a controlled stall test: at low feed (5%) and low RPM, mechanically block the axis (e.g. with a steel plate and a 10 mm spacer) and observe the current rise. It should ramp to 60-80% of rated current within 200-400 ms and the drive should report 300608 and pulse-disable with the brake clamping. The axis must not drop. If it drops, the brake is failing or MD36060 is too long.
5. Capture a trace: record the fault on the HMI trace with channels: r0070 (DC bus), r0026 (DC bus setpoint), r0078 (current actual), r0060 (speed setpoint), r0063 (speed actual), and the brake output status (r1229.1). Use this as a baseline for any future axis event.
6. Run a full unload test: command the axis to mid-travel, disable controller enable, time the drop. A healthy brake on a properly tuned axis drops < 1 mm. A degraded brake drops 5-50 mm. Replace the brake or motor if the drop is measurable.

Troubleshooting Matrix

Recommended Long-Term Fixes

• Add a collision / blockage detection using current or torque monitoring. Configure MD36030 and a message block in the PLC (DB 31-61, FC 18 / FC 19 "axis stop with alarm") to feed the spindle/axis current from r0078 into a comparator and trip a controlled stop before saturation.
• Reduce MD36060 (pulse disable delay) to match the brake closing time + 20 ms.
• Install a second brake on the ballscrew nut (third-party spring-applied safety brake) for an un-counterbalanced vertical axis. Siemens supports this with a separate brake output, but the most common field fix is a Stober / Warner / Kendrion spring-applied brake on the screw, wired to a safety relay.
• Schedule a brake wear inspection at every 12 months or 4000 operating hours, whichever comes first, on any 840D vertical axis without counterbalance.
• Enable Drive-CLiQ diagnostics for the affected drive module and set the bus timeout (p2042) to the recommended 20 ms; a slow drive bus recovery can produce the fast excursion symptom after encoder reseat.

FAQ