﻿ General Design Information > Basic Kinematics > Kinematics - General > Kinematic Definitions and Elements

# Kinematic Definitions and Elements

## Basic Kinematic Elements

You assemble mechanisms (called kinematic-chains, or linkages) with the Parts and Joints - the basic kinematic elements.

### Some Basic Definitions

Kinematics and Kinematic-Chains

Kinematics

The study of the relative motion of rigid-bodies that are connected with joints. Kinematics does not consider the forces that you need to move a kinematic-chain.

Kinematic-Chain

A series of rigid-bodies and joints such that all move in a predictable way given the motion of a minimum of one rigid-body. One rigid-body in a kinematic-chain is the Reference Frame - called the Base-Part in MechDesigner.

Rigid Body

A rigid-body is not a real body. It is an ideal-body.

A rigid-body:

does not stretch

does not bend

does not twist

does not vibrate

does not expand with heat

has an exact and specified length

Joint

A Joint allows the relative motion between Rigid-Bodies. It is an ideal-joint.

An ideal-joint:

does not have backlash

does not resist the motion with friction or other deleterious force.

does not have a dimension - other than the location of an axis in an ideal-body.

Machines versus Mechanisms

Machines are those kinematic-chains that do useful work, that transmit forces to transmit power from an input to an output

Mechanisms are those kinematic-chains that transmit motion, or information.

In MechDesigner, there is no distinction. However, there is often a semantic debate as to when a mechanism becomes a machine, and vice versa. The distinction is usually made as the design intent

Machines transmit force. But they also transmit motion from part to part.

Mechanisms transmit motion. But they also transmit force from part to part.

### Joints

Joints constrain Parts to move relative to each other in a predefined way. Different types of joint give different relative motion.

A Joint is also known as a 'Kinematic-Pair'. There are three types of Kinematic-Pairs; each gives a different relative motion.

### Motion-Parts and Motion-Dimensions

 There are two elements for which we specify a motion. Motion-Parts: You specify the motion of a Part with motion-values at the input to a Motion-Dimension FB. Motion-Points: You specify the motion of a Point with motion-values at the input to a Motion-Path FB. Rocker icon Slider icon The parameter that the Motion-Dimension FB  specifies is: •Angle between two Parts when we join them with a Pin-Joint. We call the Motion-Part a Rocker- or - •Distance between two Parts when we join them with a Slide-Joint. We call the Motion-Part a Slider Motion-Path icon The parameter the Motion-Path FB specifies is: •Distance relative to the start-Point of a sketch-element