Mechanics in Exercise: Other Machines

Wheels and Axles | Pulleys | Cams | Incline Plane | Gears, Belts, Chains

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Wheels and Axles

  • Examples
    • Can opener, volleyball net crank, tricycle, old bicycle.
  • Example of wheel and axle-like arrangements in body
    • shoulder internal rotation with elbow bent during overhand throw or tennis serve
    • forearm pronation in badminton
  • Mechanical advantage
    • Radius of wheel/Radius of axle
    • Increased range of motion advantage during throwing movement.

Pulleys

  • Function
    • Main function of pulley system is to change effective direction of applied motive force
    • Compound pulleys allow a greater load to be hoisted through a shorter distance
    • Split pulleys allow a single load to be lifted with two independently acting motive forces
  • Example
    • cable pulley weight training machines

Simple pulley: No force/distance trade- off, only directional change. Cables may travel through multiple stationary pulleys and will still offer no force/distance trade-off.

Compound pulley: Load (R) moves half distance, cable is pulled so the effort (F) is halved. Opposite end of cable is stationary so compound pulley rotates and travels upward with load.
Split pulley: When both cables are pulled simultaneously, load is distributed equally between each side (lower pulley does not rotate and load is moved full height). When only one cable is pulled while the opposite cable is anchored, the split pulley acts as a compound pulley.

  • Pulley-like arrangements in body
    • Patella at knee
      • pulled between the condyles of femur
      • patellar tendon - patella - quadriceps arrangement
      • quadriceps muscle pull proximally toward their bellies
        • yet knee extension (from flexed position) cause leg to move in opposite direction
    • Lateral malleolus of ankle

Cams

  • Characteristics of first class lever and simple pulley.
  • Typically implemented with lever, pulley, gear and chain mechanisms.
  • Examples
    • Used in weight training machines to vary resistance throughout movement
      • in attempt to match natural strength curve (e.g.: Nautilus, Cybex, etc.)
      • or develop strength curve throughout a targeted portion of the movement

Requires half the effort to raise load as compared to simple pulley. In this position, load also moves through half the distance as compared to side with motive force.

Requires at least the same motive force as resistive force to lift load. Both sides of cam move through same distance.
 
Requires twice the effort to raise load as compared to simple pulley. In position, load moves through two times the distance as compared to side with motive force.

Incline Plane

Pushing a load up a 30° ramp is twice the distance as raising the load vertically, but requires half the effort (plus the effort to overcome friction).

Pushing a load up a 45° ramp requires approximately 71% the effort as raising the load vertically.

Gears, belts, and chains

  • Characteristics
    • Can change direction, speed, or force
    • Speed of rotation inversely affects force transmitted
      • Bigger wheel rotates more slowly but with more force at the axle
    • gears are comparable to first class lever
    • belts and chains are similar to simple pulley

  • Types of Gears
    • Spur:
      • two gears on same plane (parallel axles)
    • Rack and pinion:
      • one wheel and one rack (flat toothed bar)
      • converts rotary motion into linear motion or vice versa
    • Bevel
      • AKA: pinion and crown or pinion and ring gears
      • two wheels intermesh at an angle changing the direction of rotation (the axles are not parallel)
    • Worm
      • one wheel gear (a pinion) and one shaft with surrounding screw thread

  • Belts and chains
    • work like spur gears, but do not change direction of motion
    • Examples
    • Some weight training equipments
    • Cycling


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