Inside a humanoid joint: motors, gearboxes and why they matter

A humanoid robot moves because each joint contains an actuator: an electric motor, usually paired with a gearbox that trades speed for force. The choice of motor and gearbox shapes most of what an event audience notices, including how the robot sounds, how hot it gets, how gently it reacts to a bump, how long it lasts and what it costs. This explainer covers the main designs in plain language.

What an actuator actually is

An actuator is the part that turns electrical energy into movement. In almost every humanoid built today, this means a compact electric motor, a gear reduction, a position sensor (an encoder) and a small motor driver, all packed into one module at the joint.

Unitree, for example, lists 23 degrees of freedom for the standard version of its G1 humanoid: six in each leg, five in each arm and one in the waist. On its G1 product page the company describes the joint motors as low-inertia, high-speed internal-rotor permanent magnet synchronous motors (PMSM), fitted with crossed roller bearings and two encoders per joint. Each degree of freedom needs its own actuator, so the actuator is repeated many times in one robot.

Why a gearbox is needed at all

Electric motors spin quickly but produce limited torque (turning force) for their size. A knee carrying the weight of a robot needs the opposite: a lot of torque at fairly low speed. A gearbox bridges the gap. With a 10:1 ratio, the output turns ten times slower than the motor but with roughly ten times the torque, minus friction losses.

The ratio is a trade-off. A high ratio lets a small, light motor hold a heavy load, but it also adds friction, multiplies the motor's own inertia as felt at the joint, and makes the joint stiff to push from the outside. A low ratio keeps the joint light and responsive but asks more of the motor. Engineers call a joint that can easily be turned by an outside force backdrivable, and that property matters a great deal for robots that hit the ground with every step or brush against people.

Quasi-direct drive: big motor, small gearbox

Many dynamic legged robots now use what researchers call quasi-direct-drive (QDD) actuators. The idea is to use a wide, high-torque motor and only a modest gear reduction, often a single planetary stage. A 2025 paper on single-stage planetary gearbox actuators for legged robots by Singh and colleagues summarises the appeal: low inertia, high efficiency and transparency, meaning the motor can sense and respond to forces at the foot without a separate force sensor. The same paper studies ratios in the range of about 5:1 to 11:1, far lower than in a classic industrial robot arm.

The benefits for a walking robot are practical. When a foot lands, the impact passes back through the gearbox to the motor. A low-ratio joint gives way slightly and lets software measure the force from the motor current, which helps with balance (see our explainer on how humanoid robots balance).

QDD is also linked to cost. In a 2019 ICRA paper on quasi-direct drive for low-cost manipulation, a University of California, Berkeley team argued that robots in human environments need to be both cheaper and force-controlled, and described a seven-joint arm built on this approach that they said could cost less than $5,000.

The drawback is heat. Because the gearbox multiplies torque only a little, the motor must work hard just to hold a pose. A 2026 study using the Unitree G1 notes in its abstract that humanoids with quasi-direct-drive actuators keep generating joint torque while standing, unlike a person who can lock their knees or sit down. Holding torque means current, and current means heat in the motor windings and drain on the battery, even when the robot appears to be doing nothing.

Harmonic drives: precise, compact, high ratio

The other common gearbox in robotics is the strain wave gear, widely known by the trade name Harmonic Drive. Its manufacturer explains the operating principle as three parts: an elliptical wave generator with a thin bearing, a flexible steel cup with external teeth called the flexspline, and a rigid circular spline with two more teeth than the flexspline. As the wave generator turns, it flexes the cup so that its teeth mesh at two opposite points and creep forward one tooth every half-turn.

Harmonic Drive states that this gives a very high reduction ratio in a small, light package with zero backlash, because the teeth are always engaged along the major axis. Backlash is the small play between gear teeth that makes a joint wobble or clunk when it changes direction.

The trade-offs mirror QDD. A high reduction ratio makes the joint harder to backdrive and less able to feel external forces through the motor alone. The flexspline is a precision-machined part that flexes on every rotation, so its load limits and service life must be respected, and sudden impacts such as a fall are a known concern for designers. In practice, many humanoids mix approaches: low-ratio planetary joints in the legs, and higher-ratio, more precise gearing in the arms and wrists.

Other gear types you will hear about

Cycloidal drives are a third option. They use a lobed disc that rolls around a ring of pins, and designers choose them mainly for their tolerance of shock loads. Like any gearbox, they add their own friction and dynamics, which software has to model if the robot is to control force accurately.

The lesson is that no gearbox is free. Each design moves the compromise between torque, precision, robustness, weight and how easily software can predict what the joint will do.

Linear actuators and linkages

Not every joint is driven by a rotating motor sitting on the joint axis. Some humanoids use linear actuators, where a motor turns a screw that pushes a rod in and out, working rather like a muscle pulling on a bone. Others place motors higher up the leg and connect them to the knee or ankle through rods and linkages.

Ma and colleagues described one such design in a 2025 paper on a variable-ratio knee joint. A linear actuator drives a guide-rod mechanism whose effective gear ratio falls as the knee straightens, giving high torque at the start of a jump and more speed at the end. The authors report a 0.5 m vertical jump and a 1.1 m long jump with the design fitted to a humanoid.

Linear actuators can deliver large forces in a slim package and allow heavy motors to sit closer to the hips, which lightens the lower leg. The cost is mechanical complexity: more parts, more joints in the linkage, and joint behaviour that changes with angle and is harder to control.

How actuators show up in noise, heat, durability and price

For anyone putting a humanoid in front of an audience, the engineering above shows up in four practical ways.

• Noise. Motors and their drivers produce a whine that rises with speed, and gear teeth make sound as they mesh. Quieter designs exist, but a walking humanoid is never silent, and foot strikes add their own thud, which matters for speech and filming (see speech, languages and noisy venues).
• Heat. Joints warm up under sustained load, especially when holding poses. Manufacturers build in thermal limits, and a robot may slow down or need a rest when actuators reach them. Hot rooms, stage lights and direct sun reduce the margin, which is covered in our guide to outdoor events, weather and heat.
• Durability. Falls, repeated jumps and dance routines load gears and bearings hard, and actuators are wear parts.
• Price and capability. Torque scales with motor size and cost. Unitree lists a maximum knee torque of 90 N·m for the standard G1 and 120 N·m for the EDU version, with a standard list price of US$13,500 excluding tax and shipping. On its H1 page, the larger H1 is listed at about 360 N·m at the knee and roughly 47 kg, with a peak actuator torque density of 189 N·m/kg. Bigger, stronger joints mean heavier and more expensive robots.

The shift towards electric actuators is visible across the industry. When Boston Dynamics retired its hydraulic Atlas and showed an all-electric successor in April 2024, IEEE Spectrum's report on the electric Atlas noted that the new robot runs on batteries and electric actuators instead of hydraulics, and that the company said it would be stronger with a broader range of motion than previous generations. Battery-electric actuation also links directly to runtime, which we explain in why humanoids run for only a few hours.

What this means for event organisers

• Ask about duty cycles, not just features. A routine with long poses, repeated jumps or extended dancing works the actuators harder than walking and waving. Ask the supplier how long the robot can sustain your planned routine before it needs a pause.
• Plan for heat. Schedule breaks in long shows, keep the robot out of direct sun and away from hot stage lights where possible, and build rest periods into the running order.
• Test the sound in your space. If the robot will speak or be filmed close up, include a sound check with the robot moving, not only standing still.
• Treat falls as a mechanical event. A fall can damage joints even if the robot stands up again. Agree with the supplier how a fall during the event is handled and whether a backup unit is available.
• Compare like with like. Two robots that look similar can have very different joint torque and robustness. When reviewing offers, use the same brief and questions for each, as described in how to compare robot hire quotes.

Frequently asked questions

What is a quasi-direct-drive actuator in a robot?

It is a joint module that combines a relatively large, high-torque electric motor with a small gear reduction, often a single planetary stage. The low ratio keeps the joint light and easy to move from outside, so the motor can sense and absorb forces such as foot impacts. The trade-off is that the motor works harder to hold a pose, which produces more heat and uses more battery.

Why do humanoid robots make a whining noise?

The sound mostly comes from the electric motors and their drive electronics, which whine at a pitch that changes with speed, and from gear teeth meshing inside the joints. Foot strikes add a thud when the robot walks. The level varies by model and movement, so it is worth hearing the robot move in the actual venue before deciding where it will perform or speak.

What is a harmonic drive and why is it used in robots?

A harmonic drive, or strain wave gear, uses an elliptical wave generator to flex a thin toothed steel cup inside a rigid toothed ring. This gives a high reduction ratio in a small, light package, and its manufacturer states it has zero backlash. That makes it well suited to precise joints such as arms and wrists, though high-ratio joints are harder to backdrive.

Why do humanoid robots get hot during events?

Electric motors turn current into torque, and current produces heat in the windings. Humanoids with low-ratio actuators keep producing torque even while standing still, so their joints warm up during long poses, repeated jumps or dancing. Warm rooms, stage lights and sunshine reduce the cooling margin, and a robot may slow down or need a break when it reaches its thermal limits.

How this article was made: written by the humanoid4rent editorial team with AI assistance; every factual statement was checked against the sources listed below on 2026-11-04. Spotted an error? Write to info@humanoid4rent.com.