Mechanical Constraints and Materials

Mechanical Constraints

• There are five principal mechanical constraints: flexion, tension, torsion, compression, and shearing
• Flexion: bending stress, where an object is deformed by a force that is applied perpendicularly to its surface
• Tension: stretching stress, where an object is deformed by a force that is applied along its length
• Torsion: twisting stress, where an object is deformed by a force that is applied to rotate it around its axis
• Compression: squeezing stress, where an object is deformed by a force that is applied to reduce its size
• Shearing: sliding stress, where an object is deformed by a force that is applied parallel to its surface

Effects of Mechanical Constraints on Materials

• Elastic deformation: temporary deformation that returns to original shape when force is removed
• Plastic deformation: permanent deformation that does not return to original shape when force is removed
• Fracture: breaking of material due to excessive stress

Mechanical Properties

• Hardness: resistance to scratching or abrasion
• Stiffness: resistance to bending or deformation
• Tensile strength/rigidity: maximum stress that a material can withstand without breaking
• Elasticity: ability of a material to return to its original shape after deformation
• Ductility: ability of a material to be stretched or drawn without breaking
• Malleability: ability of a material to be shaped or molded without breaking

Metals, Alloys, and Plastics

• Pure metal: an element that is not combined with other elements
• Alloy: a combination of two or more elements, at least one of which is a metal
• Corrosion: a chemical reaction that breaks down a material, often due to exposure to air or water
• Metals have high electrical and thermal conductivity
• Examples of alloys: steel, bronze, brass
• Ferrous alloys: contain iron, e.g., steel
• Non-ferrous alloys: do not contain iron, e.g., bronze, brass
• Plastics: materials made from synthetic or natural organic polymers
• Thermoset plastics: cannot be melted and reformed, e.g., bakelite
• Thermoplastics: can be melted and reformed, e.g., polyethylene

Basic Mechanical Functions

• Definition: the role played by a component or group of components inside a technical object
• Guiding: controls the motion of one or more moving parts
  • Rotational: guides rotational motion, e.g., bearings
  • Translational: guides linear motion, e.g., rail guides
  • Helical: guides helical motion, e.g., screw threads
• Linking: connects two or more parts of a technical object
  • Direct: connects parts directly, e.g., welding
  • Indirect: connects parts through an intermediate component, e.g., bolts
  • Removable: can be disconnected, e.g., snap fits
  • Non-removable: cannot be disconnected, e.g., rivets
  • Rigid: does not deform, e.g., rigid frames
  • Elastic: deforms and returns to original shape, e.g., springs
  • Complete: connects all parts of a technical object, e.g., a complete linkage
  • Partial: connects only some parts of a technical object, e.g., a partial linkage

Complex Mechanical Functions

• Comprises multiple components that work together
• Driver: the component that provides the power or motion
• Driven component: the component that receives the power or motion
• Intermediate: the component that transmits the power or motion from the driver to the driven component
• Motion transmission: systems that transmit motion from one component to another
  • Friction gear system: uses friction to transmit motion
  • Belt and pulley system: uses a belt and pulleys to transmit motion
  • Gear trains: uses gears to transmit motion
  • Chain and sprocket: uses a chain and sprockets to transmit motion
  • Worm and worm gears: uses a worm gear to transmit motion
• Motion transformation: systems that change the type of motion
  • Slider-crank: converts rotary motion to linear motion, e.g., pistons
  • Cam and follower: converts rotary motion to linear motion, e.g., camshafts
  • Rack and pinion: converts rotary motion to linear motion, e.g., steering systems
  • Screw gear: converts rotary motion to linear motion, e.g., screw jacks
• System reversibility: when the driven component can become the driver and vice versa