Short answer: choose a servo motor for a robot joint by working from the load backwards: calculate the load inertia and the torque the motion profile demands, match the motor's continuous torque (not its peak) to the sustained duty, keep the load-to-rotor inertia ratio in a range the controller can handle, and then pick the encoder the control loop needs. Power ratings in a catalogue are the last thing to check, not the first.
Continuous torque vs peak torque
Every servo motor has two torque figures that matter. Continuous (rated) torque is what it can deliver indefinitely without overheating; peak torque — often around three times higher — is available for seconds during acceleration or a push against a load. Robot joints live on both: gravity and process forces draw on the continuous budget, while fast moves draw on the peak. The classic sizing error is checking only the peak figure against the worst-case move, then discovering the motor thermally limits during a sustained hold. Work through the full motion cycle, compute the RMS torque across it, and size so the RMS sits comfortably inside the continuous rating.
Inertia: the ratio that decides how the joint feels
A servo accelerates its own rotor plus the reflected load inertia. The inertia ratio — load inertia at the motor shaft divided by rotor inertia — governs how crisply the controller can command the joint. High ratios make tuning harder and motion spongier; gearing changes the arithmetic in your favour, because a reducer divides the reflected load inertia by the square of its ratio. This is why the motor and the reducer are chosen as a pair: the ratio that fixes your torque may also fix — or break — your inertia ratio. It is also why frameless motors integrated into joint modules are specified so differently from stand-alone servos with their own housings and bearings.
Encoder choices: resolution and memory
The encoder is the joint's sense of where it is. Two decisions matter. Resolution — bits per revolution — sets the finest position the feedback can report; modern absolute encoders offer resolutions far beyond what mechanics can exploit, so choose for the control loop, not the brochure. Multi-turn counting decides whether the joint knows its absolute position after power loss: battery-backed and geared multi-turn encoders remember turns while the machine is off, which is the difference between a robot that resumes work and one that needs re-homing every Monday. Our deeper dive into encoders as a component category is planned separately; here it is enough to say encoder questions belong in the first supplier conversation, not the last.
Brakes, heat and the bus
Three practicalities close the spec. A holding brake keeps a vertical axis from falling at power-off — a safety decision, not an accessory. Heat is the hidden sizing constraint in compact joints: a motor that fits the envelope may not shed the RMS losses inside a sealed arm casting, so ask how the rating was measured. And the drive interface — EtherCAT is the common choice in modern robot controllers, with other fieldbuses and proprietary links still widespread — decides whether the motor drops into your control architecture or fights it.
Stand-alone servo, or an integrated joint module?
A final fork in the spec. The sizing logic above applies to stand-alone servo motors — the right choice for custom machinery and for buyers who want to mix motor, reducer and drive suppliers. Much of the newer robot industry instead buys integrated joint modules, in which a frameless motor, reducer, encoder and drive arrive as one tested unit; our article on frameless torque motors and integrated joints covers that architecture. Modules trade flexibility for speed of development and a single throat to choke on warranty; stand-alone servos trade integration effort for control over every interface and spare part.
The supplier landscape spans giants and specialists. Yaskawa built much of its reputation on servo motors and drives alongside its robot arms, and Switzerland's maxon is a byword for precision motors in compact joints. Both sit in the directory's core components category (160 companies as of 10 November 2026) — a useful checklist of candidates before the sizing conversation starts.
Frequently asked questions
How do I size a servo motor for a robot joint?
Work from the load backwards. Calculate the load inertia and the torque demanded across the full motion cycle, take the RMS torque, and choose a motor whose continuous rating covers it with margin; use peak torque only for short accelerations. Then check the load-to-rotor inertia ratio after gearing, and finally select the encoder, brake and drive interface the application needs.
What is the difference between continuous and peak torque?
Continuous torque is what the motor can deliver indefinitely without overheating; peak torque is a short-duration figure, often around three times higher, for acceleration and brief overloads. Sizing against the peak figure is a common error: a joint that holds a load or works a sustained cycle lives on its continuous rating, and a motor sized on peak numbers will thermally limit in service.
Why does the inertia ratio matter?
The inertia ratio — reflected load inertia divided by the motor rotor inertia — decides how responsively and stably the control loop can command the joint. High ratios make tuning difficult and motion less crisp. Because a gearbox divides reflected inertia by the square of its ratio, motor and reducer selection are linked: the pair must be sized together.
Does a robot servo need an absolute encoder?
For most robot joints, yes in practice. An absolute multi-turn encoder reports joint position immediately at power-up, so the robot can resume without a homing routine. Incremental encoders are cheaper but require the joint to find a reference after every power cycle — an acceptable trade in some machinery, a daily nuisance in a robot cell.