Robot joint gearboxes and reducers
A robot joint gearbox is the speed reducer between a joint's motor and its output: it multiplies motor torque, lowers output speed, and sets much of the joint's stiffness, backlash and backdrivability.
Pico supplies custom gear sets, planetary gearbox stages for robot joints and the hardware around them from your design, starting with a prototype or pilot batch.
At a glance
| Topic | In short |
|---|---|
| What it does | Raises torque, lowers speed, shapes stiffness, backlash and backdrivability |
| Common types | Planetary, cycloidal, strain wave; polymer or hybrid gears inside planetary or spur stages |
| Choosing a ratio | Load case, motor limits, and reflected inertia, which grows with the ratio squared |
| What Pico supplies | Custom gear sets, planetary stages, subassemblies and joint hardware |
| Not supplied | Stock gearboxes, complete actuators with motors and drives, standard cycloidal or strain wave units. Bearings are purchased, not made, and can be fitted in a kit or subassembly |
What is a robot joint gearbox?
A compact electric motor makes its power at high speed and low torque, and a robot joint usually needs the opposite. The reducer trades one for the other, and it is one layer of the joint stack. From input to output, a typical revolute joint stacks:
- the motor and its mount
- the reducer: planetary stages, a cycloidal or strain wave unit, or a polymer stage
- an output bearing that carries the link's moment, radial and axial loads, so the gears mainly see torque
- the housing, which sets much of the stiffness
- encoders on the motor side, the output side or both
- the output flange that bolts to the next link
Which gearbox types are used in robot joints?
Three reducer families cover most rotary robot joints: planetary, cycloidal and strain wave. Polymer or hybrid gears are a material choice inside a spur or planetary stage. One robot can mix them. Each trades ratio, size, backlash, backdrivability and cost differently.
| Type | How it reduces speed | Typical role | Main trade-offs | What Pico can supply |
|---|---|---|---|---|
| Single-stage planetary | Sun drives planets inside a fixed ring gear; the carrier is the output | Low-ratio, backdrivable joints, quasi direct drive legs | Compact and efficient; modest ratio per stage | Custom planetary stages |
| Multi-stage planetary | Stages in series; their ratios multiply | Higher-ratio planetary joints | More torque density; longer, less efficient, harder to backdrive | Gear sets and stages built from your design |
| Cycloidal | An eccentric drives a lobed disc around a ring of pins | High-ratio joints that see shock loads | High ratio in one stage, shock tolerant; eccentric mass to balance, many precision parts | Hardware around a unit you buy |
| Strain wave | A wave generator flexes a thin flexspline inside a rigid ring with slightly more teeth | Compact high-ratio joints such as wrists and cobot arms | Very little backlash, short and light; torsionally compliant, limited shock capacity, hard to backdrive | Hardware around a unit you buy |
| Polymer or hybrid stage | Spur or planetary meshes with polymer teeth, or a polymer rim on a metal hub | Grippers, lightly loaded or low-noise stages | Light, often quieter, little or no lubricant; lower load and temperature limits | Polymer and hybrid gears |
A single planetary stage typically gives about 3:1 to 10:1, while cycloidal and strain wave reducers typically reach ratios from the tens to the low hundreds in one stage. Pico supplies custom gears and gear sets and planetary stages built from your design, and can scope the hardware around a cycloidal or strain wave unit you buy.
How do you choose a gear ratio for a robot joint?
- Load case: peak and continuous output torque, top output speed and the duty cycle.
- Motor limits: the required torque and speed must fit the motor's continuous and peak limits at your supply voltage.
- Reflected inertia: rotor inertia seen at the output grows with the square of the ratio, so a high ratio makes the joint feel heavy to the controller and to anything it hits.
- Backdrivability: motor-side friction is multiplied by the ratio too.
T_out = T_motor × N × ηω_out = ω_motor / NN is the reduction ratio and η the gearbox efficiency.
J_out = J_motor × N²Rotor inertia as felt at the joint output.
Low ratios, as in quasi direct drive legs, favor backdrivability, torque sensing through motor current and tolerance of impacts. High ratios favor torque density and holding a pose with a smaller motor. Try tooth counts in the planetary gear ratio calculator.
What else goes into a joint besides the gears?
- An output bearing: crossed roller, thin-section or four-point contact
- A bearing carrier and housing that locate the bearings and set the stiffness
- Motor mounts, shaft couplings and output flanges
- Seals, retaining hardware and an encoder mount, often around a hollow cable bore
Pico designs around purchased bearings and does not make bearings. See robot joint housings, bearing carriers and adapters, or have the stack delivered assembled and inspected as one of our gearbox subassemblies and kits.
What causes backlash and compliance in a joint gearbox?
Backlash is the free play at the output when the input is held and the load reverses. Compliance is the elastic wind-up under torque.
- Play comes from tooth clearance, center distance tolerance, gear accuracy and bearing clearance.
- Wind-up comes from tooth, planet pin, carrier and housing stiffness, and the bolted joints between housing parts.
Specify backlash at the output, in arcminutes, under a stated reversing load, and torsional stiffness over a stated torque range. Pico publishes no backlash figures for its parts: the requirement and the measurement method are agreed in the acceptance criteria and checked on the pilot build.
How does Pico supply joint gearboxes and gear sets?
Pico supplies custom gear sets, planetary stages, subassemblies and joint hardware made to your design. There is no catalog and no stock gearbox. Every program follows the same four steps from requirement to repeat supply:
- Requirement: application, design data, quantities and timing.
- Engineering review: a written proposal before anything is made.
- Pilot build: a prototype or first batch, accepted against criteria agreed up front.
- Repeat supply: the same drawings and inspection plan.
Gear cutting, heat treatment or molding may come from outside suppliers; the scope states where each step happens. US manufacturing can be marked required or preferred and is evaluated as part of the proposed scope. Start a project with the design data you have today.
Questions
Do you sell off-the-shelf robot gearboxes?
No. Pico has no catalog of stock gearboxes. We supply custom gear sets, planetary stages and joint hardware made to your design, starting with a prototype or pilot batch.
Can you supply parts around a strain wave or cycloidal reducer we already buy?
That can be scoped as a joint hardware project: housings, bearing carriers and adapters designed around the reducer maker's interface drawings. Name the unit in your inquiry and we review the interfaces with you.
What backlash can a custom planetary stage reach?
It depends on gear accuracy, tolerances, bearing choice and assembly, so we do not quote a figure before the engineering review. The requirement and how it will be measured are agreed in the acceptance criteria.
Which gearbox type is best for a humanoid joint?
There is no single answer. Teams choose joint by joint on torque, speed, space, mass, backdrivability, shock loads and cost. Many legged and humanoid designs use low-ratio planetary stages where backdrivability matters, and higher-ratio reducers where torque density and holding matter more.
What do you need to quote a joint gearbox?
The motor or its interface, target ratio, output torque and speed, duty cycle, space envelope, any backlash or stiffness requirement, quantities for the first build and per year, timing, and any country-of-origin requirement.
Planning a joint transmission?
Share the motor, target ratio, output torque, envelope and quantities. Partial information is fine; we will ask for what is missing.
Start a project