Joints

Use the Joints tool to create and edit joints.

Joints create lower pair constraints. A constraint is defined as the removal of a degree of freedom (DOF) from a body. Each body has a total of six DOF, three translational and three rotational.

A lower pair constraint is an idealized form of constraint in which the constraint can be represented by two reference frames (coordinate systems), belonging to two different bodies.

For information on how to add or edit an entity like a joint, see Entity Creation.

With combinations of constraints in different directions, various types of joints are possible.

Table 1.
Joint Type	Description	Removes Translational DOF	Removes Rotational DOF	Removes Total Number of DOF	Add Friction
Revolute	A one DOF constraint, also known as a pin or hinge joint, enabling rotation around a single axis. It is commonly used in mechanisms such as door hinges and folding structures.	3	2	5	Yes
Ball and Socket	A three DOF constraint, also known as a spherical joint, allowing rotation around three axes. It is commonly used in mechanisms such as steering racks connected to knuckles via tie rods and knuckle-to-control arm joints.	3	0	3	Yes
Translational	A one DOF constraint, enabling linear motion along a single axis. It is commonly used in mechanisms such as splined shafts and slider assemblies.	2	3	5	Yes
Universal	A two DOF constraint, enabling rotation about two axes. While functionally identical to a Hooke joint, the distinction between the two lies in how they are defined. Universal joints are commonly used in applications such as propeller shafts, drive shafts, and steering columns.	3	1	4	Yes
Inline	A four DOF primitive constraint that restricts motion such that the origin of a reference marker on one part (Part 1) translates along the Z-axis of a reference marker on another part (Part 2). It allows three rotational and one translational DOF between the two parts. *	2	0	2	No
Fixed	A zero DOF constraint that creates a rigid connection between two parts, forcing them to move as a single unit. It is used to simulate connections where relative displacement is idealized as zero, such as bolted or welded joints and components that remain fixed in both motion and orientation.	3	3	6	No
Inplane	A five DOF primitive constraint that restricts one part (Part 1) to remain within the XY plane defined by the connected part (Part 2). It allows three rotational and two translational DOF while preventing motion perpendicular to the plane. *	1	0	1	No
Cylindrical	A two DOF constraint, similar to a translational joint, but it adds an additional rotational DOF. It is commonly found in mechanisms such as shock absorber tubes, rods, and hydraulic cylinder-rod pairs.	2	2	4	Yes
Planar	A three DOF constraint that restricts a plane on one part to remain within a plane defined on another part. The two planes can only move and rotate parallel to each other.	1	2	3	No
Constant Velocity	A two DOF constraint that restricts the rotation of a part about a specified axis to be equal to the rotation of the other part. Constant velocity joints are widely used in drive shafts of vehicles with independent suspension.	3	1	4	No
AtPoint	A three DOF constraint that is identical to the ball and socket joint.	3	0	3	No
Orientation	A three DOF constraint that restricts all rotational motions between two parts, while allowing translational motions to remain free. *	0	3	3	No
ParallelAxes	A four DOF primitive constraint, restricting two parts to rotate only about a common axis. All translational motion is free. *	0	2	2	No
PerpendicularAxes	A five DOF primitive constraint, restricting an axis on one part to remain perpendicular to an axis on another part. All translational motion is free. *	0	1	1	No
Screw	A five DOF constraint, imposing a relationship between the rotation of one part (Part 1) about an axis to the translation of the other part (Part 2) along another axis. The relationship is defined by the pitch of the joint, where one full rotation of Part 1 translates Part 2 by a distance equal to the pitch. Screw joints are commonly used in applications such as bolt and nut assemblies and rack-and-pinion steering mechanisms.	1	0	1	No

Inspire joints can be set as compliant or non-compliant. This feature helps in toggling the representation of any connection either as an idealized constraint (joint) or as a bushing with stiffness and damping.

Non-compliant Joints: Non-compliant joints act as pure constraints and allow relative motion only between the DOF of that particular joint type.
Compliant Joints: Compliant joints are identical to bushings and allow relative motion in all six DOF. The relative motion is dependent on the stiffness and damping of the compliant joint. In order for a joint to be compliant, the Compliant option must be enabled.

Note: When the Compliant option is enabled, the compliance state of joints in a system can be controlled at the system level.

Note: Unlike regular joints, primitive constraints may not have a physical form but are useful for imposing specific constraints where conventional joints are not feasible.

Joint Properties

Descriptions of joint properties in the Property Editor.

Table 2.
Property	Description
General
Label	Descriptive label for the entity
Variable Name	Variable name of the entity
ID	Integer identifier
Part 1	First part of the joint
Part 2	Second part of the joint
Symmetry	To make the entity symmetric about the x-z plane, set Symmetry to LEFT or RIGHT. The side on the positive y-axis is considered the RIGHT side and will act as the leader if selected. The opposite side will follow. To make the entity asymmetric, set Symmetry to NONE. For more information, see Pairs and Symmetric Modeling in Inspire Motion.
Origin	Origin of the joint
Type	Type of joint
Use Virtual	Set the joint as virtual
Compliant	Compliance state of joint. True or False. Joint is compliant if True and represents a bushing. This property is visible when the joint is created with Allow Compliance option.
Friction
Use Friction	Turn on friction for the joint. Refer to Friction Properties section.
Orientation	Use this section to orient the non-compliant joint. Types and methods of orientation depend on Joint Type.
Alignment Type (Type 1 \| Type 2)	Applicable for Universal, Inline, Inplane, Planar, and ParallelAxes only. Select the type of alignment desired. Refer to the comment below.
Method (Method 1 \| Method 2 )	Method of orienting the axis or axes of the joint. Available choices are Point and Vector.
Point\|Vector	Select a Point or a Vector based on the Method selected. The joint will be oriented along the direction of the point or the vector from the Origin.
Initial Conditions	Apply initial velocity conditions to the joint. Applicable for Revolute, Translational and Cylindrical joints.
Use rotational	Apply initial rotational velocity for revolute or cylindrical joint
Rotational Velocity	Rotational velocity value in rad/sec
Use translational	Apply initial rotational velocity for translational or cylindrical joint
Translation Velocity	Translational velocity value in model velocity units
Notes and Tags
Attachment Candidate	Add an attachment candidate tag. See Attachments.
FlexBody Connection Radius
Override	Override automatic connection radius. Applicable when the joint is applied on a flexbody.
Connection radius	Overriding search distance to connect the entity with rigid elements when the joint is applied on a flexbody Refer to Connection Radius for more details.

Note: Properties of compliant joints are the same as Bushing properties.

Note on Non-compliant Joint Orientation

By defining a joint between two parts , the constraint between the parts is imposed using two markers or coordinate systems (I Marker and J Marker) belonging to each part. By orientating a joint, these markers are oriented in certain directions along or about which the joint has degrees of freedom (usually Z axis of the markers).

Figure 1. Z Axes of the revolute joint oriented along a point

Revolute | Translation | Cylindrical – The orientation causes the Z-axis of these joints (direction of DOF) to be aligned in the given direction.

Alignment types

Certain joints have different ways of alignment based on the joint type.

Universal joints can be aligned either by aligning the Shaft direction or the direction of the Crosspin (which is perpendicular to the shaft and part of the yoke) on either side.
Inline joints can be aligned by either aligning the axis along a point or a vector or providing another origin Origin 2.
Inplane, Planar, and Parallel axis joints can be aligned by either specifying the normal to the plane (a point or a vector) or defining the plane with two inputs using points or vector.

For information on virtual joints, see Constraint: Joint, Comment 17 in the MotionSolve documentation.

Adding Friction

Friction can be added to the following non-compliant joints - Ball, Revolute, Translation, Cylindrical, and Universal. Refer to Edit Joints, Joint Properties table in the HyperWorks documentation for more information.