Lubricating robot joint reducers: grease, oil and service

By Tolerance Labs engineering team · Updated 18 October 2026

The lubricant in a reducer sets much of its friction, its behavior in the cold and its life. The wrong grease, or the right grease in the wrong amount, is a common cause of early reducer failure.

Grease or oil

Oil flows back into each contact after it passes, carries heat away and keeps friction low. It needs a well-sealed housing and a fill level that suits the mounting orientation. Grease is oil held in a thickener. It stays in place, needs simpler seals and works in any orientation, which is why almost all compact robot joints use it. The cost is higher churning loss at speed, less heat transfer and a finite life.

Small quasi-direct-drive joints often run their single planetary stage with a light grease, because friction there costs backdrivability; see backdrivability in robot joints. Harmonic vs planetary vs cycloidal reducers covers how the three types work.

Why harmonic drives need the maker's grease

A harmonic reducer has contacts that general gear grease is not made for. The flexspline flexes twice per input revolution, its teeth slide as they engage, and its inner surface rubs against the outer ring of the wave generator bearing with small oscillating motion. That last contact is prone to fretting wear, which makes fine debris that darkens the grease and can start cracks in the flexspline.

Makers select or develop greases for these contacts, with base oil, thickener and additives matched to them, and they publish ratings and rated life for operation with that grease. A substitute can run well for weeks and then fail early, and a maker may not support its ratings with another lubricant. When you buy a harmonic joint, ask which grease it uses, the fill quantity, and whether the maker supplies the grease. Do not replace it with a general-purpose grease because it looks similar.

Base oil viscosity and temperature

A grease’s behavior comes mostly from its base oil. Viscosity is given in mm²/s at 40 °C and 100 °C. Higher viscosity builds a thicker film at slow speed and high load, which protects heavily loaded teeth, but it costs more friction, most of all in the cold. The NLGI grade describes how stiff the grease is, from fluid grades such as 000 to firm grades such as 3. Softer grades are common in reducers because they flow back into the mesh.

Viscosity falls steeply with temperature. For a typical mineral base oil it roughly halves for every 15 to 20 °C rise in the working range. Synthetic base oils such as PAO change less with temperature, so greases for wide temperature ranges often use them. Check both ends of your range: thin oil in a hot joint can let metal touch metal, and thick oil in a cold joint raises starting torque.

Starting torque in the cold: worked example

Harmonic reducer datasheets often plot no-load running torque against temperature, measured at the input. Assume a 100:1 joint whose reducer needs 0.05 N·m at the input at 25 °C and three times that, 0.15 N·m, at −10 °C. The motor has a continuous torque rating of 0.6 N·m. These values are assumed for illustration; read the real curve for your reducer.

• At 25 °C the reducer takes 0.05 ÷ 0.6 ≈ 8% of the motor’s continuous torque.
• At −10 °C it takes 0.15 ÷ 0.6 = 25%.
• Multiplied by the ratio, the drag equals 0.05 × 100 = 5 N·m warm and 0.15 × 100 = 15 N·m cold at the output. A torque estimate from motor current cannot tell that drag apart from load.
• At an input speed of 2,000 rpm, which is 2,000 × 2π ÷ 60 ≈ 209 rad/s, the loss grows from 0.05 × 209 ≈ 10 W to 0.15 × 209 ≈ 31 W.

The extra 21 W heats the grease, so friction falls as the joint runs. A slow warm-up motion through the range for a few minutes before full-speed work brings the joint near its rated performance and reduces false collision trips caused by cold friction. For outdoor robots, check the low limit of the grease and measure current at the coldest temperature you expect.

Regreasing intervals and quantity

Most joint modules are lubricated for life, which means for the rated life of the reducer under rated conditions, not forever. Component reducers and some joints have a stated regreasing or replacement interval. Makers commonly give it in hours or input revolutions at a reference temperature, and shorten it when the grease runs hotter. Ask whether the joint can be regreased in the field or must go back to the maker. Robot joint lifetime and maintenance puts this into a schedule.

Quantity matters in both directions. Too little grease starves the contacts. Too much raises churning loss and temperature, and the excess is pushed toward the seals. Use the fill weight the maker gives for each size, and weigh it rather than estimating it. For rolling bearings, a fill of about 30 to 50% of the free space is a common guideline. Remove the old grease before refilling, so that the new fill does not carry old debris.

Mixing greases and contamination

Greases with different thickeners, such as lithium, lithium complex, calcium sulfonate, polyurea and clay, can be incompatible. A mix can soften and leak, or harden and starve the contacts. Base oils can also clash: silicone oil does not mix with mineral or PAO oils, and some esters attack seal materials that a mineral oil does not. If you do not know the old grease, clean it out fully before you fill with the specified grease.

• Particles. Dust enters past worn seals. Hard particles act as a lapping compound on teeth and races.
• Wear debris. Steel debris turns grease dark grey or black. Some darkening is normal in a harmonic reducer, but metal flakes or a gritty feel are not.
• Water. Condensation and wash-down water emulsify some greases and cause rust.
• Samples. At each service, keep a small sample in a labeled bag with the running hours. A series of samples shows a trend that one sample cannot.

Leakage and seals

Grease does not stay fully in place. When hot, the base oil separates from the thickener, which is called bleed, and thin oil migrates along shafts and through gaps between housing parts. Some bleed is how grease lubricates, but it should not reach the outside. Common leak paths are the input and output lip seals, the faces between housing parts, bolt holes that open into the grease space, and the bore of a hollow shaft.

• Seal housing faces with an O-ring or a sealant the maker approves, and bolt holes with sealing washers or thread sealant.
• Follow the maker’s instructions for vertical mounting. Grease moves under gravity and centrifugal force, and some makers specify extra grease or other measures for it.
• Fit a vent where the maker allows one, so that warm-up does not pump grease past the seals. Sealing and IP ratings for robot joints covers vents and seal materials.
• A thin oil film at a seal is often normal. Drips, or a ring of grease that has collected dirt around the seal, are not.

Checklist

• Ask which lubricant each reducer uses, and in what quantity.
• For harmonic reducers, use only the grease the maker specifies.
• Check the grease temperature range against your coldest and hottest conditions.
• Measure running current cold and warm, and plan a warm-up motion if the difference is large.
• Get the regreasing interval and whether service can be done in the field.
• Never mix greases of unknown type; clean out the old grease fully.
• Keep grease samples, and inspect seals, housing faces and vents for leaks.

Compare harmonic joint modules and high-ratio and cycloidal joints, and ask which grease and service interval each uses when you request a quote.