Materials

Drawing on knowledge of existing materials, stock parts, and proven mechanisms is a key ingredient to a successfully mechanical project.

Content List

Material Selection

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 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.

The above example shows how to create material indices to minimize mass and cost of a beam with variable cross section.

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) is lowered from the top and first makes contact with the Natural Materials family, suggesting it is ideal.

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Stock Hardware

Outside of extreme circumstances (think spacecraft or nano machines), it’s better to make use of existing mechanisms and stock hardware in 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

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 adhesives advice, try thistothat.com!

Source images credit to walmartimages.com, waykenrm.com, jmpwood.com, wikimedia.org, 3erp.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

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 (friction and backlash)
Specifying gear size and teeth to achieve desired output torque and speed
Power transmission angle (parallel, perpendicular, coplanar or not)

Spur

Cost-effective option suited for low speeds, and loads. Shorter life and high noise level.

Simple
Common and readily available
Custom profiles possible using wire EDM
Shorter service life due to large, instantaneous loading
Imprecise (large backlash due to single engaged tooth at a time)

Helical

Budget option for higher speeds, low-medium loads, and low noise. Transverse loads out of plane increase design effort.

Moderate complexity
Quieter operation from gradual tooth engagement
Smooth power transmission (multiple teeth engaged)
Unbalanced loading requires thrust bearing to secure in place

Herringbone

Higher performing option suited for higher speeds, medium-high loads, and low noise. More expensive.

Complex to produce
Smooth power transmission (multiple teeth engaged)
Symmetric tooth design yields balanced loading, no thrust bearing needed

Bevel

Best option for angular power transmission in tight space. Difficult to install and intolerant of misalignment.

Compact
Angular transmission of torque within plane
More difficult to produce
Can be combined with the above tooth patterns
High axial loads require thrust bearings, limiting service life

Worm

Suited for compact, high torque applications. Often less efficient and lower speeds.

Compact
Angular transmission of torque out of plane
Reliably transmits very large torques due to load distribution across large contact area
High energy losses due to friction across large contact area
Potential for self-locking property (worm cannot be driven in reverse by manipulating output shaft)