Joints & constraints
Pin bodies together, hinge them, weld them, or pull them toward a goal.
A door that swings on its frame, a chain whose links stay linked, a ragdoll whose limbs hang from its joints. A small set of nodes covers these, and they all feed the same solver, so they cooperate inside one Rigid Body Sim or particle setup.
Articulation joints
The Spring node (Physics Joint) connects two rigid bodies and holds a relationship between them. Pick the kind on the node.
Ball. The bodies share a pivot point and rotate freely around it. Chain links, pendulums, ragdoll shoulders and hips.
Hinge. Rotation is allowed around a single axis, like a door or an elbow. Angle limits stop the swing, and a position-servo motor drives it toward a target angle.
Fixed / Weld. The two bodies are locked together at the relative orientation they had when the simulation started. Use it to glue parts into a single rigid assembly that can still break apart under enough force.
Joints are animatable. Anchor points, stiffness, damping and limits are ordinary parameters, so a hinge motor can be keyframed to open a door on cue, or a joint's stiffness driven from audio. See Keyframing.
A weld captures its hold-angle at the moment the sim begins cooking. If a welded assembly looks wrong, check that the bodies are already in their intended relative pose on frame 0. The weld preserves whatever it sees first.
Goal-pull (shape matching)
Not every constraint is a joint between two bodies. The solver can also pull a cluster of points back toward a target shape, a soft and springy version of rigidity. This is how Soft Body Sim keeps its form while still wobbling, and it is the basis for approximately rigid looks that recover after being squashed.
There is no separate node to wire. Goal-pull comes from the soft body and shape-matching behaviour built into the sims themselves: remember where you started, and try to get back there.
How they all relate
Ball, hinge and weld joints, distance links inside cloth and hair, and the goal-pull of soft bodies are all resolved together in a single pass. There is no execution order to choose and no constraint that wins; they are balanced against each other every frame.
Mix them freely. A hinged Rigid Body Sim panel can rest on a Cloth Sim sheet, joined parts collide with everything else, and the lot settles in one solve. See Forces & collisions for how contact fits into the same picture.
Choosing a constraint
Two solid objects that must stay connected: Physics Joint (Spring), set to Ball, Hinge or Weld.
A door, lid or limb that swings: Hinge, with limits and an optional motor.
A rigid assembly that can shatter: Weld between the parts.
A blob that holds its shape but jiggles: Soft Body Sim goal-pull, not a joint.
A strand or sheet that must not stretch: built into simulation.hair and Cloth Sim, with no separate constraint node.
There is no CPU physics engine. Every joint and constraint runs on the GPU as part of the same simulation step.