Mechanical Devices
Mechanical Devices
Mechanical devices are used in products to create movement, change the direction of force or multiply force. Understanding how these mechanisms work is essential for designing products that move. This lesson covers the four types of movement, levers, linkages and rotary systems.
Types of Movement
There are four types of movement (motion) that mechanical devices produce:
Linear motion — movement in a straight line in one direction (e.g. a drawer sliding open). Rotary motion — movement in a circle around a central point (e.g. a wheel turning). Reciprocating motion — backwards-and-forwards movement in a straight line (e.g. a sewing machine needle). Oscillating motion — swinging movement back and forth along an arc (e.g. a pendulum in a clock).
The four types of motion are linear (straight line), rotary (circular), reciprocating (back-and-forth in a line) and oscillating (swinging arc). Many mechanisms convert one type of motion into another.
Levers
A lever is a rigid bar that rotates around a fixed point called the fulcrum. Levers multiply force (making it easier to lift heavy objects) or increase the distance or speed of movement. There are three classes:
First-order lever — the fulcrum is between the effort and the load. Examples: seesaw, crowbar, scissors. These can multiply force or distance depending on where the fulcrum is placed.
Second-order lever — the load is between the fulcrum and the effort. Examples: wheelbarrow, nutcracker, bottle opener. These always multiply force (the effort moves further than the load).
Third-order lever — the effort is between the fulcrum and the load. Examples: tweezers, fishing rod, human forearm. These always multiply distance/speed (the load moves further than the effort), but require more force.
Linkages
Linkages are systems of bars connected by pivot points that transmit and change the direction of movement.
A bell crank changes the direction of movement by 90 degrees. When you pull a bicycle brake lever (input), the bell crank converts the pulling motion into a pushing motion that presses the brake pad against the wheel rim. A push/pull linkage (also called a reverse-motion linkage) reverses the direction of input motion — push in one end and the other end pulls out.
Rotary Systems
Rotary systems transmit rotary motion and can change speed, direction or type of movement.
Cams and followers convert rotary motion into reciprocating motion. The cam is a shaped disc on a rotating shaft. As it turns, a follower rides along its edge, moving up and down. The shape of the cam determines the pattern of movement. Used in engines (opening valves) and toys.
Simple gear trains use toothed wheels that mesh together. When gears of different sizes are used, the smaller gear (pinion) turns faster than the larger gear, but with less force (torque). The gear ratio determines the speed and torque relationship. Meshing gears also reverse the direction of rotation.
Pulleys and belts transmit rotary motion between shafts using a belt wrapped around wheels. Like gears, different-sized pulleys change speed and torque. Unlike gears, pulleys connected by a flat belt rotate in the same direction. A crossed belt reverses the direction.
Cams convert rotary to reciprocating motion. Gears transmit rotary motion and can change speed, torque and direction. Pulleys transmit rotary motion over distance using belts. All three are common in exam questions.