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The axis has spent too long drawing more current than it can sustain and the thermal protection of the motor-amplifier pair has cut out. It is not a momentary spike, it is accumulation. The machine is telling you that axis has been working hard for a while, and it is worth finding out why before it turns into a mechanical repair.
For reference only: alarms change with the manufacturer, the control and the machine PLC. Check the full code in the official manual before doing anything. Legal notice.
If it returns after removing the load, or there is noise, stiffness or heating.
Reset only after removing the obvious cause and testing a slow move in a safe area.
Choose the manual matching your control series, software version and machine builder. A support portal is a starting point; it does not establish applicability to every model.
The drive does not only watch instantaneous current: it keeps a thermal count, a current-times-time integral that estimates how much heat is building up in the motor windings and in the amplifier power stage. A motor can deliver a lot of current for an instant with no problem, but it cannot sustain it, and the thermal model reflects exactly that. While the load is normal the count rises when moving and falls when stopped, never reaching the threshold. When the axis has to push too hard for a sustained period, it rises faster than it cools and eventually trips SV0400. The practical consequence is that this alarm is an indicator of effort, not an isolated event: if it trips, that axis has been working above normal for some time even if nobody noticed. The reasons are the same ones that make a slide stiff: poor lubrication, ways with chips or scoring, a damaged ball screw or nut, an axis brake that does not fully release, an unbalanced counterweight or balance cylinder on vertical axes, or simply accelerations and feed rates beyond what the machine can give. It can also come from cooling: an amplifier with a dead fan or a cabinet running hot trips sooner with exactly the same mechanical load as always.
In order, from the fastest and cheapest check to the most expensive.
01Note the axis and when it trips
If it trips in the middle of heavy cutting, suspect the process. If it trips moving in air or on positioning rapids, it is almost certainly mechanical: in air an axis should not come anywhere near its thermal limit. If it trips after the machine has been running for hours and not at start-up, look at the cabinet cooling.
02Check lubrication and cleanliness of that axis's ways
Reservoir level, pump cycling, a visible oil film on the ways, bellows intact. It is the cheapest check and the most frequent cause of sustained abnormal effort. A vertical axis with poor lubrication suffers the most, because it works against the weight all the time.
03Jog the axis with no load and compare against another axis
Run the full travel at low feed through a clear area. Look for stiffness, noise or spots where it takes more effort. Then do the same with a healthy axis on the same machine as a reference: comparing by ear and by handwheel between axes finds differences you would never notice on one axis alone.
04Look at the axis load percentage on the servo monitor
The control shows load per axis on its servo diagnostic screen; the location changes between series, so look it up in the maintenance manual as the servo monitor. Move the axis in air at medium speed and note the value; repeat with another axis. A clearly higher no-load figure on the axis that alarms confirms mechanical resistance and there is nothing left to argue about.
05If it is a vertical axis, think about the brake and the counterbalance
A holding brake that does not fully release makes the motor work against it on every move, and this is exactly the picture it produces. The same goes for a counterweight or a balance cylinder with low pressure. Neither can be checked from the panel: pass it to maintenance with the information that the overload is only on the vertical axis.
06Check the cooling of the cabinet and the amplifiers
Clogged filters, stopped fans or a cabinet running hot make the protection trip earlier at the same load. Filters are cleaned with the machine stopped; everything else, including any check inside the cabinet, is electrical maintenance work with the machine locked out.
A dragging brake or insufficient counterbalance pressure. Confirm it by comparing the vertical axis load going up and going down in air: if it is far higher than reasonable in both directions, it is not the weight, something is braking. Maintenance checks it.
Progressive mechanical deterioration: chips getting in through a torn bellows, a ball screw wearing out, or lubrication that stopped reaching one point. Confirm it with the alarm history and by feeling stiffness when jogging. The sooner it is dealt with, the cheaper it is.
It is process: the cutting force is loading the axis above its continuous rating. It is confirmed because the same travel in air gives no trouble and because reducing feed makes it disappear. Adjust it in the program, splitting passes or checking the tool condition.
Central lubrication checked properly, not just the reservoir level: verify that oil actually reaches every point, because a blocked distributor leaves a slide dry without the level dropping. Bellows and wipers replaced as soon as they tear. Cabinet filter cleaning on the periodic plan. Cutting parameters that are realistic for the machine you have, not the ones in the tool catalogue. And use the axis load monitor as tracking data: writing down each axis's no-load figure once a month turns this alarm into something you see coming months in advance.
Fanuc reports an overload on the specified axis or abnormal servo load. It is an alarm from the Servo / ejes area, and in the catalog it is listed as “Axis servo overload”. On the machine you notice it like this: Alarm when moving an axis, The axis may feel stiff or run hot, It may appear in heavy machining.
On the shop floor it almost always comes from one of these causes: High mechanical load, Guides without lubrication, Motor or amplifier overheated, Crash or chips in the guards. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset only after removing the obvious cause and testing a slow move in a safe area. Resetting without fixing the cause first only hides the problem: SV0400 trips again and, in the meantime, you keep working with the machine in bad shape.
With the machine stopped and in a safe state, start with these checks: Note the indicated axis, Check lubrication and visible obstacles, Check axle load in diagnostics if available. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With SV0400 active, avoid this: Do not bypass safety devices, Do not manipulate energized electrical cabinet, Do not continue producing if the alarm reappears. Skipping any of these points is what turns a minor stop into an expensive breakdown.
If it returns after removing the load, or there is noise, stiffness or heating. If you are not sure how far it goes, stop the machine and report it: a repeat alarm always works out cheaper than a crash.
It is critical: there is a real risk of damage to the machine, the tool or a person. Leave the machine stopped, do not keep resetting it to see if it goes away, and call maintenance before producing again. Guidance reference. Always confirm the full text in the exact machine/control manual.
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