Attendance
Cool things of the week: Awesome Gears, Long Road Home
Plan: Guest speaker Heather Knight, Liza midterm presentation, learn about mechanisms guts for an hour, take stuff apart for an hour
Bearings and bushings
Bearings permit constrained linear or rotary motion between parts. They keep shafts stable and help machines run smoothly. A bearing could be as simple as drilling a hole in a block of wood, or as complicated as a tapered angular roller bearing...
How Bearings Work from howstuffworks.com
See Chapter 7
Bushing (also called Sliding/journal/sleeve/plain bearing) = bearing with no rolling elements
Bearing construction
- The housing holds the outer race
- The shaft constrains the inner race
- Normally, rotating ring is a tighter fit
- Example of installation
Ordering bearings
Almost any general parts store (McMaster, SDP-SI, etc.) will have them
- Some bearings are ABEC (Annular Bearing Engineers Committee ) rated
- The higher the number, the better the bearing
- Only need to pay attention to in high speed, long life designs
Couplings
What's a coupling? Something that joins two rotating things to transfer torque
Coupling to a motor shaft:
- ALWAYS look for non-round motor shafts (D-shaped, keyed, and 2 flats are popular).
- ANY SHAPE TRANSFERS TORQUE BETTER THAN A CIRCLE!
- Splines are common on servos. Use hubs and adaptors like these shaft couplers that are made to interface with servos.
- Make your circular shaft D shaped with a file
- Rough up shaft (i.e. by sanding) if you must use it circular
Coupling gears to shafts
- Pin
- Match drill/pin
- Capture with washers/bearings/nuts
- Use supplied hub/clamp/set screws
Gears
Spur Gears
Bevel
- Teeth formed on conical surface
- Transfer motion between non-parallel shafts
- Miter gear is special case of bevel gear at 90 deg angle
Worm
- Worm gears resemble screws
- Transmit motion between non-parallel and non-intersecting shafts (most of the time at right angles)
- Used to reduce speed and increase mechanical advantage – each rotation of the worm advances the meshing gear just one tooth
- Non back drive-able: the worm can turn the gear, the gear can’t turn the worm
How Differential Gears work - an excellent ~9min long YouTube video from the 1930's
Gear Anatomy: Boston Gear Design Guide
Making Gears
