MechanismsandThingsThatMove

Addressing some class feedback:
*Toru from Fabricating Information is coming to class for a guest lecture
*Machine shop tour: dates and times?
*Welding: Check out this Class at the Education Alliance (starts April 20th!). The have many other affordable classes as well.

Simple Machines:

1. Gear
2. Lever
3. Block and Tackle (Pulley)
4. Wheel and Axle
5. Inclined Plane and Wedge
6. Screw

Resources:
Physical Computing pages 271-281: Basic Mechanics: Converting Motor Motion to Usable Motion (on blackboard)
Cabaret Mechanical Movement - most of the book

• Levers (pg 17) - on blackboard
• Shafts (pg 33)
• Drives & Gearing (pg 85)

Basic Machines and How They Work - the whole book (Google Books has the chapters on Levers and Pulleys)
Designing Cost-Efficient Mechanisms - examples throughout book, and notes on where to get parts

Levers

Last week we learned about how gear ratios give you a mechanical advantage

• Mechanical advantage, or the difference between the work you get out and the work you put in, can also be achieved with levers

A lever is a rigid object used with a pivot point or fulcrum to multiply the mechanical force on an object

• Three classes of levers: Each type has a fulcrum, load, and effort
  1. Fulcrum is between effort and load
     • Ex: seesaw, crowbar, shoehorn, oars, can opener
     • Double lever: pliers, scissors
  2. The load is located between the effort and the fulcrum
     • Ex: wheelbarrow, nutcracker
  3. The effort is between the load and fulcrum
     • Ex: Mandible, biceps, fishing rod

3 lever types in one

Pulleys (a.k.a. Blocks, Sheaves)

1 fixed pulley does not magnify force or direction
However, 1 un-fixed pulley can: a runner
Next step: gun tackle. Use another pulley to change the direction of pull. Mechanical advantage stays the same.
Next step: higher mechanical advantage using a "luff tackle" - two sheaves at top, one movable on bottom.

Wheel and Axle

A simple wheel and axle gives you mechanical advantage: steering wheel, door knob, valve wheel
Note: We're only talking about wheel-and-axle machines that consist of a wheel or crank rigidly attached to the axle.

Inclined Plane and Wedge

Example: Driver loading barrels into truck
Where is mechanical advantage? Small force over long distance vs. large force over short distance
A wedge is a special application of an inclined plane: 2 of them back to back
Example: axes, chisels
Mechanical advantage? Width vs. Length

Screws

A screw is really just a modification of an inclined plane (it’s an incline plane wrapped around a cylinder)
Generally used to raise things with mechanical advantage: house, car, heavy machinery

• Like any system, the mechanical advantage (MA) is the ratio of what get out to what you put in

Ex: Say a screw jack has a handle length r=2 ft. For one revolution, 2*pi*r, the screw raises a height of p (pitch). If the pitch is ¼”, and MA=(2*pi*R)/P, MA=603

• The actual MA will be lower due to friction
• Note: These type of screws are cut at a “self locking” angle.

Examples: Micrometer, Turnbuckle
Types of Power Screws:

• Acme screw
  • 30-50% loss from friction
  • Will not unwind
  • Low speeds: Wildflower Meadow Glacier, Video download
• Ball screw
  • ~90% efficient

Also used in positioning systems where mechanical advantage isn't the concern:

• Velmex
  

Springs

Resources:
Cabaret Mechanical Movement pg 63

Kinds of Springs:

• Compression
• Extension (Tension)
• Torsion
• Radial
• Disc/spring washers
• Leaf
• Clock

Compression Springs
Characterized by:

• Inner diameter
• Outer diameter
• Wire diameter
• Free length (spring all by itself length)
• Solid height (squished together height)
• Spring rate or stiffness (the k in F=kx)

Uses:

• Shock absorber, return spring for solenoid, projectile launcher, belt tensioner
• Construction tip: mount it on a shaft to prevent buckling

Simple harmonic motion
MIT TechTV spring demo
Slinky Metronome

Tension/Extension Spring

• One more parameter: extended length

Uses: belt tensioner, tension element, motion limiter, shock absorber, garage door mechanisms, trampolines, keep a cam follower against a cam

Torsion Spring
These exert a torque or rotary force Uses: active hinge, energy storage, locking mechanism, hair clips, mousetraps, clothespins, clipboards, doorknobs, SMS latch

Radial Spring

Disc/spring washers
Use these to preload screws as in the Diagram

Leaf spring Uses: vehicle suspensions, limit switches

Clock spring Uses: wind up toys

IDDS/SMS pdf