How to Use Rigid Body Constraints in Blender

Blender's rigid body physics engine allows you to link simulated objects together to create realistic mechanical assemblies like doors, pistons, and suspension systems. By using constraints such as Hinge and Slider, you can limit the movement of connected bodies to specific rotational or translational axes. This guide covers the essential steps to prepare your objects, apply rigid body physics, and configure both Hinge and Slider constraints effectively.

Step 1: Prepare Objects for Rigid Body Physics

Before adding constraints, your target objects must be part of the Rigid Body simulation.

  1. Select your base object (e.g., a wall or anchor) and go to the Physics Properties tab.
  2. Click Rigid Body and set the Type to Passive (if it should not move) or Active (if it should react to forces).
  3. Select the moving object (e.g., a door or piston head), add a Rigid Body, and set its Type to Active.
  4. Apply the scale of all involved objects by selecting them and pressing Ctrl + A > Apply All Transforms.

Step 2: Setting Up a Hinge Constraint

A Hinge constraint restricts movement to rotation around a single axis (by default, the local Z-axis of the constraint).

  1. Press Shift + A and choose Empty > Plain Axes to create a constraint controller.
  2. Place the Empty at the exact pivot point where rotation should occur.
  3. Align the Empty's local Z-axis with the intended axis of rotation.
  4. With the Empty selected, navigate to the Physics Properties tab and click Rigid Body Constraint.
  5. Set the Type to Hinge.
  6. Under the Objects section:
    • Set First to your base/anchor object.
    • Set Second to the rotating active object.
  7. (Optional) Enable Angular Limits in the settings below to restrict the rotation angle within a specific minimum and maximum range (e.g., 0° to 90° for a standard door).

Step 3: Setting Up a Slider Constraint

A Slider constraint restricts movement to translation along a single straight line (by default, the local X-axis of the constraint).

  1. Press Shift + A and add a new Empty > Plain Axes.
  2. Position the Empty at the starting point of the linear motion.
  3. Rotate the Empty so that its local X-axis points in the direction the object should slide.
  4. Go to the Physics Properties tab and click Rigid Body Constraint.
  5. Change the Type to Slider.
  6. Assign the two interacting objects in the First and Second fields.
  7. Under Linear Limits, check the X box to define the minimum and maximum distance the object can slide along that axis.

Step 4: Testing and Troubleshooting

  1. Press Spacebar in the viewport to play the timeline and test the simulation.
  2. Jittering or Breaking: If constraints snap or shake uncontrollably, go to the Scene Properties tab, open the Rigid Body World panel, and increase the Substeps Per Frame and Solver Iterations (values between 20 and 50 often resolve stability issues).
  3. Incorrect Direction: If the object moves or rotates in the wrong direction, select the constraint Empty, enter local transformation mode (G then X/Z twice), and rotate the Empty so its local coordinate axes match the required motion path.