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Mates and Joints

Mates and Mate Entities

In a SolidWorks® assembly, you specify geometric relationships between parts using mates. Each mate applies a kinematic constraint between geometric entities of different parts. Geometric entities include points, lines, and surfaces.

A nut and bolt system provides an example of a mated system. To center the two parts on a common axis, you might add a concentric mate between the cylindrical surfaces of the bolt and the nut hole. This mate restricts motion to translation along the common axis as well as rotation about that same axis.

During CAD Import, SimMechanics™ converts mates between parts into joints between rigid bodies. In the nut-and-bolt example, SimMechanics might translate the concentric mate between the two cylindrical surfaces into a revolute joint between frames on the nut and bolt rigid bodies.

SimMechanics supports most mates and mate entities in SolidWorks. The supported entities are:

  • Circle/arc

  • Cone

  • Cylinder

  • Line

  • Plane

  • Point

The supported mates are:

  • Angle

  • Coincident

  • Concentric

  • Distance

  • Parallel

  • Perpendicular

Supported mates are valid only for certain entity pairs. The table shows the entity pairs compatible with the supported mates. This table is symmetric with respect to the diagonal row. For conciseness, the redundant half of the table is omitted.

CAD assemblies with other mate-entity combinations may present issues during CAD import into SimMechanics. By default, SimMechanics replaces each unsupported mate with a rigid connection. Parts joined by a rigid connection have zero degrees of freedom with respect to each other. You can replace these rigid connections with the appropriate joints once CAD Import is complete.

SimMechanics Joints

SimMechanics joint and constraint blocks are the functional equivalent of SolidWorks mates. They apply between frames the kinematic relationships that determine how they can move. For example, a revolute joint aligns the Z axes of two frames while keeping their origins coincident. In this configuration, the two frames have one rotational degree of freedom (DoF) about the common Z axis.

Joints are combinations of joint primitives, the building blocks that provide the joint its degrees of freedom. These primitives vary according to the number and type of DoFs that they provide. SimMechanics includes three joint-primitive types:

  • Prismatic primitive — Joint primitive with one translational DoF

  • Revolute primitive — Joint primitive with one rotational DoF

  • Spherical primitive — Joint primitive with three concurrent rotational DoFs

The spherical primitive differs from a set of three revolute primitives in the way it represents 3-D rotation:

  • Spherical primitive — Represents rotation in three dimensions as one rotation about an arbitrary 3-D axis.

  • Three revolute primitives — Represents rotation in three dimensions as a sequence of three rotations about different axes. During rotation, two axes can become aligned, causing the joint to lose one rotational degree of freedom. This phenomenon, named after the gimbal devices in which they often occur,

The table summarizes the joint primitives present in each SimMechanics joint. In this table, P, R, and S denote prismatic, revolute, and spherical joint primitives. Note that the weld joint, which provides zero degrees of freedom, has no joint primitives.

Joint BlockJoint Primitives
6-DOF JointP-P-P-S
Bearing JointP-R-R-R
Bushing JointP-P-P-R-R-R
Cartesian JointP-P-P
Cylindrical JointP-R
Gimbal JointP-P-R
Pin Slot JointP-R
Planar JointP-P-R
Prismatic JointP
Rectangular JointP-P
Revolute JointR
Spherical JointS
Telescoping JointP-S
Universal JointR-R
Weld JointN/A

During CAD Import, SimMechanics translates the mates in a SolidWorks assembly into an equivalent set of joints. For example, a coincident mate between a point-point entity pair maps onto a spherical joint in SimMechanics.

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