Robot gearbox oil leaks: causes and what to do

An oil stain under axis 2 or a drip at the robot base is one of the most underrated warnings on the shop floor. It rarely stops the machine right away, so it gets postponed, and weeks later an overtemperature or excessive-torque alarm shows up on a gearbox that has run dry. This article explains how to identify where the oil comes from, tell a real leak apart from a poorly cleaned oil change, and decide whether you can keep producing or need to stop.

Where do leaks come from in a robot gearbox?

The gearboxes of an industrial robot (usually cycloidal or harmonic depending on the axis) are sealed and share an oil or grease bath. The leak almost always appears from one of these points, ordered by frequency:

• Output shaft seal. The most common cause. The seal lip hardens with hours and temperature, loses elasticity and lets oil through toward the arm flange. It shows as a film running down the casting.
• Flange gasket between gearbox and structure. If the gasket or sealant has degraded, oil weeps along the joint line. Typical after a poorly done re-torque or an impact in the area.
• Fill, level or drain plug loose or with a dried gasket. Very common after an oil change. Often fixed with a new gasket and the correct torque.
• Clogged breather. If the breather is blocked, internal pressure rises as the oil heats up and pushes it out through the weakest point (usually the seal). Here the leak is a symptom, not the cause.
• Cracked housing. Rare, but it happens after hard collisions. If you suspect this, first review the post-collision checklist.

Real leak or maintenance residue: how to tell

Before overreacting, rule out the obvious. Many «leaks» are actually oil left on the casting after a change that keeps running off with vibration and heat over days. To tell them apart:

1. Clean the area thoroughly. Degrease the whole gearbox surface and nearby structure with a cloth and cleaner. Leave it dry.
2. Mark a reference point. Note the date and the oil level if the gearbox has a sight glass or level plug.
3. Let it run 24-48 h under normal conditions.
4. Inspect again. If fresh, wet oil reappears from a specific point, it's an active leak. If there's only a thin layer of stuck dust with no new moisture, it was residue.

A classic shop-floor mistake is replacing the seal over a stain that was residue, with the downtime and disassembly that involves. Another is the opposite: cleaning the stain and never checking again, letting a real leak progress.

How to assess severity

Not all leaks are equally urgent. What matters is how much lubricant you're losing relative to the gearbox volume:

• Light weeping (stain that doesn't drip). You can keep producing while monitoring the oil level each shift or weekly. Schedule the repair for the next planned shutdown.
• Slow but steady drip. The level drops measurably between inspections. Shorten the monitoring interval and plan the intervention in days, not weeks.
• Heavy drip or rapid level loss. Real risk of running with insufficient lubrication. A gearbox running low on oil heats up, torque rises and it can seize. Here you must stop and act.

The warning sign you must never ignore is the combination of leak + new noise, vibration or torque/temperature alarm on that axis. That's no longer a leak: it's a gearbox in distress. If you detect abnormal vibration, cross-check with the causes of robot vibrations.

Step-by-step inspection checklist

1. Identify the axis. Locate the source gearbox by tracing the oil trail upward: gravity makes the stain appear below the leak point.
2. Check the oil level of the suspect gearbox per the manufacturer's procedure (robot posture, level plug).
3. Check the breather. If it's dirty or clogged, clean or replace it before touching the seal; sometimes the leak disappears.
4. Inspect the plugs for fill, level and drain: torque and gasket condition.
5. Examine the flange line between gearbox and structure for weeping at the joint.
6. Document with photo and date to compare the evolution at the next inspection.
7. Record oil consumption: note how much you have to top up and how often. It's the best severity indicator.

When you can fix it yourself and when to call a specialist

Topping up oil to level, cleaning a breather or re-torquing a plug with a new gasket are tasks you can handle on-site with the manufacturer's procedure in hand. If you've never worked with these gearboxes, first review the lubrication and oil change guide so you don't get the lubricant type or level wrong.

It's worth calling a specialist service when:

• The leak comes from the output seal or the flange gasket: it means opening the gearbox, controlling cleanliness and respecting specific torque values.
• There's rapid level loss and you can't guarantee safe lubrication until repair.
• Associated symptoms appear (noise, vibration, overtemperature, high torque): the damage may already be done and you should assess the gearbox replacement.
• You suspect a cracked housing after an impact.

In any case, don't mix oil types or fill above the maximum level: overfilling raises internal pressure and causes exactly the leak you're trying to avoid. Our corrective maintenance team can diagnose the source and replace seals or the complete gearbox with original or exchange parts.