Materials and Stock
Materials
Some material characteristics to consider when selecting candidates:
Density
Thermal stability
Corrosion resistance
UV resistance
Melting temperature
Ductility
Brittleness
Cost
Chemical resistance
Embodied energy
Magnetic properties
Flammability
Food-safe
Electrical conductivity
Hardness
Yield strength ("elastic limit")
Impact toughness
Appearance
Porosity
Compatible manufacturing processes
Nearest stock sizes
Chart of material families, comparing Young's Modulus against density. Adapted from Michael F. Ashby, Hugh Shercliff, David Cebon - Materials_ Engineering, Science, Processing and Design-Butterworth-Heinemann (2018)
Stress-strain curve visual for several material properties.
Image credit to efficientengineer.com
To compare the performance of materials when maximizing one characteristic against another, a material index can be developed to quantify a score for each material. Using software such as Ansys Granta, Ashby plots comparing material properties can easily be made.
Ashby chart for the first material index from the above example. Taking into consideration a minimum elastic modulus of 1 GPa (blue line), the material index slope of 1 (orange line) suggests the Natural Materials family is ideal.
Stock Parts and Mechanisms
Outside of extreme circumstances (think spacecraft or nano machines), it's better to make use of existing stock hardware for designs to keep reliability up and costs down. The more common the component, the better.
From past projects I have experience selecting the following stock parts, balancing some or all of the applicable criteria:
Fasteners
Fasteners
Permanent vs. non-permanent
Ease of removal (permanent rivets, adhesives, threads with Loctite, simple screw, cotter pins, temporary tape, etc.)
Engagement length (three threads minimum)
Head style (socket, button, flat head, etc.)
Drive style (Phillips, flat, hex, Torx, etc.)
Thread type (thread forming, thread cutting)
Available installation space
Clamped material(s) (plastic, aluminum, steel, wood, etc.)
Magnetic/non-magnetic
Electrical conductivity
Corrosion resistance (oxide coating vs zinc plating)
For quick adhesive advice, try thistothat.com!
Source images credit to walmartimages.com, waykenrm.com, jmpwood.com, wikimedia.org, 3erp.com
Image credit to istockphoto.com
Bearings
Engagement length with shaft (ball, needle, etc.)
Rated speed
Sealing type (dual rubber seals, shielding, open)
Thrust loads
Axial misalignment
Type of fit with shaft/pocket (how much interference and installation force)
Magnets
Magnets
Required strength (ferrite, neodymium, samarium-cobalt)
Acceptable losses (eddy currents)
Stock shape availability (bar, ring, arc, etc.)
Compatible pre- and post-processing (electrical conductivity for EDM, brittleness for machining)
Risk of demagnetization loading (localized heat, mechanical shock, etc.)
Supply chain import sensitivity (rare earth metals vs ferrite)
Protective coatings
Gears
In general:
Tradeoff between efficiency and precision (backlash and friction)
Specifying gear size and teeth to achieve desired output torque and speed
Types of teeth and their uses:
Spur (cost-effective option suited for low speeds, loads, and high noise level)
Simple
Common
Can be made using wire EDM
Shorter life due to large instantaneous loading
Imprecise (large backlash from single engaged tooth)
Image credit to medital.com
Helical (budget option for higher speeds, low-medium loads, and low noise)
Moderate
Quieter operation from gradual engagement
Smooth power transmission (multiple teeth engaged)
Unbalanced loading requires thrust bearing to secure in place
Image credit to misumi.com
Herringbone (higher performing option for higher speeds, medium-high loads, and low noise)
Complex to produce
Smooth power transmission (multiple teeth engaged)
Symmetric tooth design yields balanced loading, no thrust bearing needed
Image credit to made-in-china.com
Bevel
Compact
Angular transmission of torque
More difficult to produce
Can be combined with the above tooth patterns
Image credit to savree.com
Worm (suited for compact, high torque applications)
Compact
Angular transmission of torque
Transmits very large torques
High energy losses (due to large contact area)
Image credit to stewmac.com
Special cases and newer designs:
Rack and pinion
Convenient conversion of rotational to linear motion in the same plane
Precise control
By OSHA Directorate of Technical Support and Emergency Management - Point of Contact Between a Rack and Pinion. The original uploader was Brian0918 at English Wikipedia., Public Domain, https://commons.wikimedia.org/w/index.php?curid=186765
Epicyclic gear trains
Planet/sun systems
Compact
Change output by holding different portions stationary
Schematic diagram of gearing system
By Jahobr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=57167542
Stationary ring (red)
By Jahobr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=53918826
Stationary sun (yellow)
By Jahobr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=53918827
Stationary carrier (green)
By Jahobr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=53918824
Direct drive (all)
By Jahobr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=55217840
Harmonic gearing
Flexible gear ring deformed by rotating cam inside static ring
Unequal numbers of teeth to control angular advance per rotation
No backlash
Compact
By Jahobr - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=53789383