23.6. Spring Joint

The Spring Joint (SPRING_JOINT) is a linear spring which can be used to connect a spring between two rigid bodies, or between the world and one rigid body. It has four important properties:

  • Elasticity - Spring coefficient, the stiffness of the spring

  • Damping - The damping coefficient for the spring

  • Rest length - The length where the spring will not apply any force on the attached rigid bodies.

  • Current length - The actual length of the spring. If the current length differs from the rest length, a force will be applied on the attached rigid bodies.

A Spring-Joint has two attachment points which can be individually moved by selecting and using the Move tool.

As with all the joints, there are two ways of attaching the Spring Joint:

  1. Attach it directly to an edge or an axis of a solid belonging to a Rigid Body, which attaches the joint between the Rigid Body and the world.

  2. Attach it between two separate solids (each belonging to a separate Rigid Body).

23.6.1. Usage

In the first case you simply select the spring joint tool from the ribbon menu and left-click on an edge or an axis of the Rigid Body you wish to add a joint to, a yellow Spring Joint icon will appear on the edge or axis. See Joint status.

In the second case where you wish to join two Rigid Bodies with a spring joint you simply select the Spring joint from the ribbon menu. Then, Ctrl-click somewhere on one of the Rigid Bodies, and then click the edge or axis on the other Rigid Body that the spring joint should be attached to. A purple Spring Joint icon will appear on the edge or axis. See Joint status.

23.6.2. Moving the joint

With the Move tool activated you can select one or both of the attachment points of the Spring-Joint and move them. The rest length will be updated to reflect the new distance between the two attachment points.

../_images/Spring-JointExample.png ../_images/Spring-JointProperties.png

  • Attachment X,Y,Z: Specifies the position in the world of the currently selected Spring-Joint attachment.

23.6.2.1. Joint Properties

  • Enable: Specifies whether the joint is enabled or not.

  • Collision between Rigid Bodies: Specifies if there can be contacts generated between the two attached rigid bodies. Default is off. If collision is enabled, there might be jamming effects if the two bodies have overlapping geometry.

  • Solve type: Specifies which solver that will solve this joint.

    • Direct And Iterative: (Default), both the direct AND the iterative solver will see this joint. To get stable friction with the Split solver type for Material Pairs, both the iterative and the direct solver must see the joint.

    • Iterative - Only the iterative solver will see this joint. Will lead to instabilities for large mass ratios.

    • Direct - Only the direct solver will see this joint. This might lead to artifacts when involving a Material Pair with Split solve type.

  • Component1: Name of the first component associated to this joint.

  • Component2: Name of the second component associated to this joint.

  • Rigid Body1: Name of the first rigid body associated to this joint.

  • Rigid Body2: Name of the second rigid body associated to this joint.

  • Reverse direction: Visible only if joint is attached to two bodies. Will swap the two attached rigid bodies, effectively flip the direction of the joint.

  • Is Attached: A property that is set to true when the joint is directly attached to an edge/axis on a solid. When the joint is created, it is always directly attached to an edge/axis on a solid. If the solid is moved, the joint will follow the solid. However, if the joint is moved individually from the solid, this property is changed to false. This means that the joint will no longer follow the solid when the solid is moved, but will remain at its current world location. To reset the joint’s world position to the original solid position, set this flag to true.

23.6.2.2. Spring properties

  • Elasticity: Elasticity specifies the “stiffness” (spring constant).

  • Damping: Specifies the damping coefficient of the spring.

  • Current Length: The current (prolonged or shortened) length of the spring during simulation.

  • Resting Length: The resting length of the spring.

  • Current force: The force the spring is currently applying to its attached bodies.