# Shaft coupling (rigid)

> Connection kind `shaft-coupling` (category `mech`, DSL name `couple`, API v1)

Rigid 1:1 coupling between two rotational ports. Propagates ω unchanged; typically used to mount a gear on a shaft or to join two collinear shafts.

## Accepted ports

_Any matching rotational ports (symmetric)._

## Parameters

- **`offsetMm`** — Signed offset along the anchor local Z axis where the driven component is placed (mm). Defaults to 0.

## Parameters schema

| Parameter | Type | Default | Constraints |
| --- | --- | --- | --- |
| `offsetMm` | Number | 0 |  |

## Kinematics

ω_to = ω_from. No ratio, no sign change.

## Layout

This connection computes the driven component position from the anchor.

## Example

```
connect S1.axis -> G1.axis with mech.couple {
  offsetMm: 50
}
```