Custom planetary gearboxes for robotics

In robot joints the usual arrangement holds the ring gear fixed, drives the sun from the motor and takes the output from the carrier. It gives the highest reduction of the basic single-stage arrangements, keeps input and output on one axis, and splits the load across several planets. A custom stage starts from your motor and envelope, so the ring can sit in your joint housing and the carrier can carry your output flange.

The design study gives 1 + 72 / 18 = 5, a 5:1 reduction. Two checks decide whether a set of tooth counts can be assembled:

• Concentric: for standard gears of one module, Zr = Zs + 2·Zp. Here 18 + 2 × 27 = 72.
• Equal spacing: with N equally spaced planets, (Zs + Zr) / N must be a whole number. Here (18 + 72) / 3 = 30.

Single stages typically run from about 3:1 to 10:1; at high ratios the sun gets small and the planets start to crowd each other. Stages in series multiply, so two 5:1 stages give 25:1. To try other combinations, check tooth counts and ratios in the planetary gear ratio calculator.

Send what exists today. Partial information is fine; we will ask for what is missing.

• Gear set: sun, planets and ring gear, to your drawings or to a design agreed in the review.
• Gear set with carrier: adds the carrier, planet pins and planet bearings, ready to drop into your housing.
• Complete subassembly: adds the housing, output shaft or flange, purchased bearings and seals, assembled and inspected to an agreed plan.

A ring gear cut into the housing is a design option we evaluate in the review. For assembled and kitted deliveries, see gearbox subassemblies and kits; for the parts around a stage, see joint housings, bearing carriers and adapters.

A quiet, low-backlash planetary gearbox comes from many small decisions:

• Gear accuracy: profile, pitch and runout errors add play, noise and uneven loading.
• Profile and lead modification: tip relief and crowning ease mesh entry and tolerate misalignment.
• Center distance and carrier precision: they set the remaining tooth clearance and how evenly the planets share load.
• Load sharing: a floating sun or ring helps the planets share torque.
• Bearing clearance: play in the planet and output bearings adds backlash and tilt.
• Lubrication: grease or oil choice and fill change friction, noise and heat.

Pico publishes no backlash, noise or efficiency figures for its parts. Your requirements are written into the acceptance criteria and confirmed on the pilot build.

• Sun and planets: hobbed or shaped, then ground where a hardened gear needs higher accuracy.
• Ring gears: shaped, broached or power skived; wire EDM can suit some prototypes.
• Heat treatment: through-hardening, carburizing or nitriding where the steel and the load call for it.
• Polymer stages: machined for prototypes and molded at volume; see custom plastic gears.

Specialized steps may come from outside suppliers, and the scope states where each step happens. If US manufacturing is required or preferred, the route is evaluated against it. Spur and helical gears outside a planetary stage are covered under custom gears and gear sets.

1. Requirement: motor, ratio, torque, envelope, quantities and timing.
2. Engineering review: tooth counts, materials, manufacturing route and inspection, with a written proposal before anything is made.
3. Pilot build: a prototype or first batch, accepted against criteria agreed up front, including any backlash or noise test.
4. Repeat supply: the same drawings, inspection plan and point of contact.

Describe your planetary stage and start a project.