Hinge (New for 5.1)

The Hinge constraint is one of the most commonly used constraint types. Whenever you need to have physics objects to rotate on a certain axis, then you can use this type of constraint to ensure the direction of rotation.

1. Apply a prop (in this case, a gear). Transform it to a desired position, size and angle.
   
2. Duplicate it and relocate the new gear.
   
3. Set one of them as Dynamic and the other as Kinematic so that you can control the kinematic one and push the other gear.
   
4. Because the Dynamic rigid body will fall by default, you need to constraint it to where it is. In the Object Physics Settings panel, click the button to open the Constraint Settings panel (Shortcut: Shift + F10).
   
5. Click the button and select Hinge constraint type from the list.
   
6. In the Connection section, you can connect the constraint to the world or another target physics object by clicking the Pick Target or World buttons. Please refer to the Setting Connection Relationship section for more information.
7. In the Limit section, you are able to decide the rotation axis by selecting the desired radio button.
   
8. To decide the rotating axis, turn on the Gizmo mode and switch to the Local Move or Local Rotation tools.

   In this case, the dynamic gear shall freely rotate by the Z axis (R = X, G = Y, B = Z).

9. Select the radio button of the correct axis and click the Free button; which ensures the dynamic object can freely rotate along the Local Z axis.
10. Set transform keys or puppet the kinematic object to animate. The dynamic one will then be triggered to move along with it instead of dropping down to the floor.