Attendance
Observation Assignment
Pulley and Sprockets, Belts and Chains
Power screws
Springs
Kinds of Springs...
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 Hooke's Law... Force = stiffness (k) * distance (x)
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
- Torque = stiffness (k)* angle (in radians)
- degrees x (pi/180) = radians
- How to buy...
- Spring listings will only give you torque. This is the torque at maximum deflection (closed). To find the torque at an intermediate location, use a direct proportion: if listing says 1 in-lb at 90 degrees, then it will have .5 in-lbs of torque at 45 degrees
Disc/spring washers
Use these to preload screws as in the Diagram
Leaf spring
Uses: vehicle suspensions, limit switches
Spiral springs: clock, constant force, etc.
Uses: wind up toys, vending machines
CAD Exploration
There are a variety of programs out there you can use for design. Anything that is 3D, parametric, and can include several parts in an assembly will work. Bonus points if it can export a part to a .stl file - this is what 3D printers use.
Worth mentioning...
- blender Not the best for simple 3D part design but can export .stl files (Windows and Mac)
- Rhino has a demo version for Windows that will save 25 times and version for Mac OSX that is free during development. You'll also need the Rhino Assembly plugin to make assemblies of multiple parts.
- Google Sketchup Good for 2D and some 3D, gear plugin. Check here for tutorial on accurate design. This plugin makes adding dimensions easier. Use the plugin here or the su2stl.rb plugin here to convert the models to .stl for 3D printing. (Windows or Mac)
- OhYeahCAD Free sketchup downloads, gear plugin
- Art of Illusion Free, open source 3D modelling and rendering studio (Windows and Mac)
- CB model pro Free demo version (Windows and Mac).
- eMachineShop Free software, new export to .stl function (Windows only)
