MechanismsandThingsThatMove

My Gila

Class9

Filed in: Main.Class9 · Modified on : Sun, 10 Apr 11

Attendance

Statics vs. Dynamics

The study of rigid bodies (not rubber bands) that are in equilibrium
Equilibrium = “a state of rest or balance due to the equal action of opposing forces” (dictionary.com)
Why study things that don’t move in a class about how things move?
- A static problem is a snapshot of a dynamic one
- Most of the time we can isolate a statics problem that represents the worst case scenario in a dynamic system

Free Body Diagrams (FBD)

A FBD is a simplified graphical depiction of all the forces and moments happening on a body
- Applied, normal, friction, weight, gravity, spring, drag, etc.
Examples
- Book on a table (Newton’s 3rd law - Every action has an equal and opposite reaction)
- Hanging from two ropes - Graffbot
- Weight on a high crane

Three steps to draw a FBD

Isolate the body from all supports
Identify all external forces acting on the body
Sketch the body, showing all forces

Three steps to solving a Statics problem:

Draw a FBD
Choose a convenient point to calculate moments around (if necessary)
Sum forces and moments in all directions until unknowns are found

Combining Simple Machines

All complicated machines, mechanisms, and robots, are made up of combinations of simple machines. They help us convert rotary motion - the most common input motion - into linear, up and down, or intermittent output motion. You can often use the mechanisms we'll talk about to achieve more than one output with just one input motion.

First things first - what types of motion are there?

Ben Hopson: 3 Boxes

Cranks

A crank can be thought of as a lever attached to a rotating shaft or a wheel and axle machine

Traditional - crank to raise bucket in well
Backwards - drive the shaft, use the crank to do work, like this

Cams and followers

Cam videos
A cam is basically any eccentric or non-circular shape that converts rotary motion into some other type of motion
Types of cams:

Edge/disk/peripheral cams (most common) - circles with lobes: snail cam

The part that the cam moves is called a follower. It can be

flat
roller
curved

Rob Ives cams

Linkages

A linkage is a connection that transfers motion from one mechanical component to another

Linkages in the real world of design: frog can crusher
Eric's heart linkage
Doug Repetto's foals

Why use a linkage?:

Change direction, altering the motion
Alter speed and/or acceleration
Change timing of moving parts
Apply a mechanical advantage
Elegant and efficient motion (don't throw technology at a problem unless you have to) - KMODDL

Four bar linkages are the simplest closed loop kinematic linkage
Consist of 4 segments:

Crank
Ground/Frame
Follower
Lever/Coupler

linkage examples
examples of different configurations
Designing a 4 bar linkage
Grashof's law:
The sum of the shortest and longest link of a planar four bar linkage cannot be greater than the sum of remaining two links if there is to be continuous relative motion between the links

Interactive 4-bar

Uses

windshield wiper
umbrella

Ratchet & Pawl

Gives a motion that is not continuous (stepped, intermittent)
Ratchet: basically a wheel with notches cut into it
Pawl: pushes against the notches and drives the wheel around in steps. A ratchet/pawl system can also be used as a clutch
Ex: ratcheting screwdriver
Motion conversion options (see page 248 in book)

Automatons

Early kinetic sculptures that explored movement through gears, springs, linkages, etc
Powered by winding, water, gravity
Music boxes and mechanical (wind up and pendulum) clocks were precursors to more toy-like machines

Leonardo's robot 1495

Jacques de Vaucanson