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2. INTRODUCTION TO KINEMATICS OF MECHANISMS


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Consider a pair of adjustable vice grips.



Some definitions,

Machine - a collection of components that will do work.
Mechanism - a collection of components to transform motion
Kinematics - consider positions/velocities/accelerations in mechanical systems
Structure - a collection of components to make larger static structures
Statics - estimate forces in mechanisms that are in equilibrium
Dynamics - determine motion that results when forces are out of balance
Link - rigid body between joints
Binary Link - has two joints only
Ternary Link - has three joints
Quaternary Link - has four joints
Pair or Joint - a connection between two links
Driver / Follower - the driver link will be driving the follower
Kinematic Chain - a sequence of links making up a mechanism
Open Loop - a snake like set of connected links
Closed Loop - a kinematic chain has one or more links that go back in the chain
Frame - a grounded or fixed link in a mechanism
Spatial - in 3 dimensions
Relative/Absolute - a position, velocity, etc. is measured based on a fixed (absolute) or moving (relative) point.

A Degree Of Freedom (DOF) is an independently controllable variable. As an example, a machine that has two degrees of freedom might need two motors to control it.

Lower Pairs, - constrained position/orientation of both sides of the joints are identical

Turning / Revolute - basically a pin joint (R)
Prismatic - a slider (P)
Screw/Helix - a nut and screw pair (H)
Cylindric - a shaft in a collar (C)
Globular/Spherical - a ball joint (S)

Higher pairs include, - typically other equations are needed to constrain the joints, such as gear ratios (if the joint has more than a single degree of freedom)

- flat/planar - constrained to move over a plane
- belt on pulley
- meshing gears
- sliding wheel on a surface
- etc.

The definition of higher/lower pairs given in Shigley [1995] is, "the lower pairs, such as the pin joint, have surface contact between the pair elements, while higher pairs, such at the connection between a cam and its follower, have line or point contact between the surface elements." They go on to point out that the definition is not exact, which is somewhat disappointing.

A better definition of a higher pair is - A higher pair is not a lower pair, where a lower pair permits the following relative motions between links; circular, linear, helical, cylindrical, spherical, planar.

If a link has one joint, it is a unary link. A link with two joints is binary, with three it is ternary, with four it is quaternary, etc.

Planar linkages use lower order pairs, and are constrained to a single plane of motion.

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