Mechanics

Introduction to mechanics

---

Concept of degrees of freedom

If DOF = 1 -> Deterministic mechanism
If DOF = 0 -> Structure
If DOF > 1 -> Non-deterministic mechanism

Things can get more difficult to analyse:

And structures can get complex:

Mechanisms

• Linear movement

  • Linear bearings / guides
  • Frictive mechanisms (piston)
  • Screw
    

• Rotational movement:

When things need to have a coupled rotation, we can use gears, belts or chains to transfer the movement to one another. In any of them, we define transfer ratio as the ratio between the rotational speeds. If we consider the number of teeth of a gear, or pinion as

$r$:

$$i={\omega }_2/{\omega }_1=r_2/r_1$$

Gears

When things are close to each other: we use gears:

• Straight gear: simple, but noisy

• Helix gear: more complex, but silent:

• Straight Bevel gear: when we have angles

• Helix Bevel gear: all of the above

• Pinion-rack: from linear to rotation

• Planetary gears!

Belts and chains

When distances are longer, we don't use gears due to it's size, we use either belts or chains. Chains can transmit more power, but they are more noisy and they don't absorbe as much sudden vibrations as belts:

• Chain

• Belt

An continuosly variable transmission (CVT):

Cams

Cams can come in different flavours:

Keeping things in place

• Bearings

  • Linear
    
  • Ball
    
  • Flat/needle
    
  • Conical
    

• Rings

Mechanisms examples

• Bars mechanisms:

  • 4-bar mechanism:
    

    • Locking plyers
      
    • Crane
      
    • Excavator
      

  • Ratchet
    

  • Geneva wheel
    

  • Universal joint
    

  • Core X/Y
    

Some links:

Mechanism Examples
CoreXY
Reprap CoreXY
Machines that make