Joint module size vs torque

Two questions come up more than any other when a joint is being specified: “how much torque can this diameter make?” and “how small a frame will reach the torque I need?” They are the same dataset read from opposite ends. Read down the first table to see what each frame size is capable of; read down the second to find the smallest frame that clears a torque target.

By frame size: what each diameter can deliver

Each row covers one frame size across every manufacturer we carry. “Highest rated” is the strongest build at that diameter in the catalogue, “highest peak” the strongest start–stop figure, and “lightest” the build that adds least mass at that frame — usually the short-body variant. The last column counts how many builds exist at that diameter, which is a rough measure of how much choice the size gives you.

Figures are the catalogue values normalised onto one scale; brand names are shown so the outlier at each size is visible. Full per-model tables: joint modules & actuators.

By torque: the smallest frame that reaches it

Start from the torque the axis actually has to hold, then take the smallest diameter that clears it. “Other frames that reach it” shows how much room there is above the minimum — if the answer is a single diameter, that torque level is a hard step in the catalogue and the frame choice is effectively fixed.

Torque targets are rated output torque, not peak. If the duty cycle is intermittent — a lift that happens twice a minute rather than continuously — quote against peak and the frame usually drops one size.

Why the same diameter gives different torque

Torque at a fixed frame is set by two things: the reduction ratio, and how much of the envelope the ring gear is allowed to use. A short-body build trades ratio for length; a through-bore build trades wall thickness for a hollow shaft. Φ52 is the diameter with the widest cross-brand choice in the catalogue, so it shows the effect most clearly — same outer diameter, same mounting pattern, torque spread across the range below.

The higher-ratio builds reach more torque but spin the output slower — at 101:1 the output turns at roughly 20 rpm from a 2000 rpm input, against 40 rpm at 51:1. If the axis needs speed rather than torque, the low-ratio build at the same diameter is the right answer.

How the numbers are compared

Manufacturers quote torque on at least four different bases: rated at 2000 rpm input, allowable average load over a duty cycle, start–stop peak, and instantaneous maximum. Two quotes that look 30% apart often differ only in which of those is on the page. Everything here is normalised: rated is continuous output torque at 2000 rpm input, peak start–stop is what the gearbox survives briefly during acceleration and braking, and neither is the instantaneous maximum. Where a catalogue only publishes an average-load figure we say so on the brand page rather than converting it silently.

Weight is the bare module without a brake, torque sensor or harness. Adding a holding brake and a torque sensor always adds mass at the same frame — 1.15 kg against 0.83 kg at Φ68 in the RBV3 line — which is why brake-equipped builds look heavy for their diameter.

FAQ