What the two digits actually promise

An IP code comes from IEC 60529. The first digit is solids, the second is water. For robots only the top of each scale matters. A 5 on dust means some fine particles may get in without doing harm; a 6 means none get in at all. Water is where the tests get physical: a 5 is a low-pressure jet, a 6 is a powerful jet from a 12.5 mm nozzle at 100 litres a minute, and a 7 is immersion to one metre for 30 minutes.

Two details trip buyers up. The water digits aren’t cumulative, so an IP67 robot has passed immersion but not necessarily the 100 L/min jet. And a rating belongs to the whole system as tested. Bolt on a gripper, a sensor or an unsealed connector and the weakest interface sets the real rating.

Who publishes a rating

Quadrupeds got here first. Industrial inspection dogs – ANYbotics’ ANYmal, Unitree’s B2, DEEP Robotics’ X30 – ship at IP67, while Boston Dynamics’ Spot sits at IP54. Humanoids are only now catching up. Atlas and DR02 are the two with published ratings; Agility’s Digit 5 launch in September 2026 covered payload, safety and charging but gave no IP code, and Unitree’s G1 doesn’t publish one either.

The gap isn’t an accident. A humanoid has more joints than a quadruped, many of them small, and the arms and hands move through far more of their range. A dog’s twelve leg joints sit in compact, stout housings. A humanoid wrist has to fit its seal into a few millimetres of radial space.

Where water gets into a joint

Every joint module has three kinds of opening. Static joints, where two housing parts bolt together. The dynamic gap, where the output flange turns against the housing. And the cable exit, where power and bus wires leave the module. Each needs a different kind of seal.

The static joints are the easy part: an O-ring in a groove, compressed once and left alone. The cable exit is harder than it looks. Sourcing guides for sealed actuators flag cable exits as a common ingress point and tell buyers to ask exactly how phase wires, power, CAN or EtherCAT and sensor lines leave the housing – gland, moulded connector or M8/M12 plug – and whether the IP test was run with that exit fitted.

The dynamic gap is the real problem. It turns, it has to stay tight for millions of cycles, and whatever seals it rubs against a moving surface.

The seal menu

Industrial robot arms have been sealing joints for decades, and humanoid makers borrow straight from that parts bin. Trelleborg says every industrial robot joint carries mini or nano radial oil seals whose job is “keeping lubrication in and contaminants out”. Its lower-friction options swap the rubber lip for PTFE, or use a U-shaped PTFE ring pushed onto the shaft by a metal finger spring, sold on low breakout force and no stick-slip. Bal Seal pitches a similar idea – spring-energised seals with a canted-coil spring – and markets versions pre-certified to IP67 and IP69 for robot joints, actuators and motors.

What sealing costs: friction and heat

A lip seal works because it touches. That contact costs torque, and in a humanoid joint torque is the currency. For a stiff, high-ratio strain-wave joint the loss is small next to the gear’s own friction. For a quasi-direct-drive joint built to be back-driven, a grabby seal eats straight into the transparency the design paid for, and seal breakout force shows up as a dead band at low speed. That’s why the low-breakout PTFE designs keep getting pitched at robots.

Seal geometry makes a measurable difference. An SKF team comparing a wave-lip design with a plain lip seal under the same conditions found lower friction torque and seal-contact temperatures more than 10 °C cooler, and showed that friction is a mix of viscous shearing in the lubricant film and losses from flexing the rubber. Their work was on industrial gearboxes rather than humanoid joints, but the physics carries over.

Heat is the bigger bill. A sealed motor can’t breathe. Sealing “limits internal convection and can trap heat around the stator”, and continuous torque normally has to be de-rated unless heat is conducted efficiently to the housing or chassis. Potting the stator in a thermally conductive compound claws some of that back, at the price of a module you can’t open in the field.

It also explains the operating ranges vendors quote next to their IP codes. Atlas is rated for −20 to 40 °C (−4 to 104 °F); DR02 claims −20 to 55 °C. Cold stiffens elastomer lips and thickens grease, heat softens them and piles onto the motor’s own losses. The seal sits at the junction of both.

What to ask a vendor

• The IP code for the whole robot as delivered, and which end effectors and accessories were fitted when it was tested.
• The test report: depth, duration, sample condition, cable-exit configuration and how the unit was inspected afterwards.
• Whether the rating covers jets, immersion or both. IP67 and IP66 are different tests, and neither is a pressure-washer rating.
• Continuous joint torque in the sealed configuration, at the top of the rated temperature range.
• Seal replacement interval and whether a joint can be resealed on site or has to go back to the factory.

Indoor logistics doesn’t need any of this, which is why most humanoids skip it. Outdoor yards, cold chains, food plants and anything that gets cleaned with a hose do. Those are also the sites where the first IP-rated humanoids are being pitched.

FAQ

Sources