Sealing and IP ratings for robot joints

By Tolerance Labs engineering team · Updated 18 October 2026

A joint that works on the bench can fail in rain, dust or a wash-down. Seals keep dirt and water out, but every contact seal adds friction, and an IP code says less than it seems. This guide covers what the codes mean and what they leave out.

What an IP code means

IP codes come from IEC 60529. The first digit rates protection against solid objects and dust, the second against water. An X in either place means that part was not rated.

Typical meanings for a robot joint:

• IP54: dust-protected and splash-proof. Suits indoor robots, workshops and brief exposure to light rain.
• IP65: dust-tight and protected against water jets. Suits outdoor robots and light hose cleaning.
• IP67: dust-tight and survives brief immersion. Suits legged robots that walk through puddles and wet grass.

A higher digit is not always a superset. Up to 6, each water level covers the ones below it. Immersion (7 and 8) is a different test from jets, and a product that passes immersion may fail the jet test. Products that pass both carry both codes, such as IP66/IP67.

IP tests are usually done on a new unit, at room temperature, and often not moving. A robot joint rotates, heats and cools, ages and takes impacts. A lip seal that is tight on day one wears. A housing that is sealed when cold breathes as it warms. Treat an IP code as the starting point for a new joint, and ask how the maker tested it.

How joints are sealed

• Rotary lip seals: an elastomer lip, often loaded by a garter spring, presses on a turning shaft. The usual seal on joint outputs and motor shafts. NBR and FKM are common lip materials; PTFE lips run with lower friction but need a good shaft finish.
• O-rings: on static joints such as covers, housing halves and connector flanges. They seal well and add no running friction.
• Labyrinths: narrow, winding gaps with no contact. No friction and no wear. They keep out debris and splash, but not water under pressure or immersion.
• V-rings and axial face seals: a lip on a rotating ring that bears on a flat face, often used as a dust excluder in front of a lip seal.
• Potting and coating: potted or conformally coated electronics, as a second defense against condensation.

The friction cost of seals

Contact seals add drag, which grows with shaft diameter, lip load and cold, and is higher when the seal is new or dry. In a backdrivable joint, seal drag on the motor shaft appears at the output multiplied by the ratio N, because the motor side sits behind the reducer.

Worked example, with seal values assumed for illustration. A quasi-direct-drive joint with a 9:1 reducer has a lip seal on the output that adds 0.2 N·m of drag and one on the motor shaft that adds 0.03 N·m.

• Motor seal, seen at the output: 0.03 × 9 = 0.27 N·m.
• Total added backdrive torque: 0.2 + 0.27 = 0.47 N·m.
• On a 100:1 joint, the same motor seal would add 0.03 × 100 = 3 N·m.
• At a motor speed of 3,000 rpm, which is 3,000 × 2π ÷ 60 ≈ 314 rad/s, the motor seal turns 0.03 × 314 ≈ 9.4 W into heat at the lip.

So in low-ratio joints, seal the output and avoid contact seals on the fast motor shaft where the layout allows. Seal drag also changes with temperature and wear, which disturbs current-based torque estimates; see joint torque sensors. Ask for no-load starting torque with seals fitted, warm and cold.

Venting and condensation

A sealed housing changes pressure as it heats and cools. If no air could get in, air inside a joint cooling from 60 °C to 20 °C would drop in pressure by the ratio of absolute temperatures: 101.3 kPa × 293 ÷ 333 ≈ 89.1 kPa, about 12 kPa below the outside. That difference pulls air, and any water on the seals, into the housing. Lip seals are not designed to hold a pressure difference, so they let it in.

Inside, moist air condenses on surfaces that cool below its dew point, and the water has no way out. A pressure-equalizing vent, a small fitting with a membrane that passes air but blocks liquid water, keeps the pressure balanced. Place vents where spray does not hit them directly, and check that the vent’s own rating matches the joint’s.

Outdoor and wash-down use

• Grit: sand wears lip seals and shafts. Put a dust excluder or labyrinth in front of the lip seal.
• Cold: elastomers stiffen and grease thickens, so drag rises. Check the seal and grease temperature ranges, and the backdrive torque at the lowest temperature you expect.
• Sun: UV and ozone age some elastomers and cable jackets. Ask which materials are used.
• Wash-down: detergents and disinfectants attack some seal materials, and high-pressure spray can force water past a lip. Ask for chemical compatibility, and do not aim jets at shaft seals.
• Salt: fasteners, connector shells and bare aluminum corrode. Ask about coatings and stainless fasteners.
• Heat: a sealed joint sheds heat less well. See thermal management of robot joint actuators.

Connectors are the weak point

A joint’s IP code often applies only with its connectors mated, or with caps fitted. An unmated connector is an open hole. M8 and M12 circular connectors are commonly rated IP67 when mated and screwed down, while many board-style and latching connectors carry no rating at all.

• Check whether the rating applies mated, unmated or capped.
• Match cable glands to the cable’s outer diameter. A gland on an undersized cable does not seal.
• Water travels along the strands inside a cable. A cable with a wet, open end can carry water into a sealed housing, so use overmolded cordsets or sealed cable entries.
• Route cables so water runs away from connectors, with a drip loop below each entry.
• Cables through a hollow-shaft joint need a seal at the bore as well.

When the rating is missing

Many joint modules list no IP rating, and some list one without a test behind it. Ask the maker:

• Which IP code, and was it tested or only designed for? Who ran the test?
• Was the joint rotating during the test, and how many hours had it run?
• Which seal is on the output, and which on the motor shaft?
• What is the no-load starting torque with seals fitted, warm and cold?
• Is there a vent, and what is it rated for?
• Do the connectors keep the rating mated, unmated or capped?
• Can the seals be replaced, and does opening the housing void the rating?

Checklist

• Match the IP code to the real exposure: IP54 for splash, IP65 for jets, IP67 for immersion.
• Ask for dual marking if the joint must survive both jets and immersion.
• Prefer output-side seals in low-ratio joints, and multiply motor-side seal drag by the ratio.
• Check backdrive torque with seals at the lowest operating temperature.
• Plan a pressure-equalizing vent for sealed joints that heat and cool.
• Treat connectors and cable entries as part of the seal.
• Get answers in writing when no tested rating is given.

Compare robot joint modules and humanoid and legged robot joints, and ask for seal and IP details when you request a quote.