Linkages and mechanisms
What you’ll learn
- The three classes of lever
- How cranks, cams and linkages convert rotation into other motions
- Which mechanism to pick for grippers, legs and flaps
Levers
A lever is a bar that pivots on a fulcrum. Effort on one part moves a load on another.
| Class | Order | Example | Gives |
|---|---|---|---|
| 1st | Effort – fulcrum – load | Seesaw, scissors | Force or distance, depending on lengths |
| 2nd | Fulcrum – load – effort | Wheelbarrow, nutcracker | More force |
| 3rd | Fulcrum – effort – load | Tweezers, your forearm | More distance and speed |
The law of the lever: effort × its distance from the fulcrum = load × its distance from the fulcrum.
Turning rotation into other motion
| Mechanism | Input → output | Robot use |
|---|---|---|
| Crank and slider | Rotation → back-and-forth | Pushing, pumping, walking toys |
| Cam and follower | Rotation → up-and-down in a pattern set by the cam shape | Timed movements, tapping |
| Four-bar linkage | Rotation → swinging or complex paths | Legs, wipers, grippers |
| Rack and pinion | Rotation → straight line | Linear grippers, lifts |
| Lead screw | Rotation → slow, strong straight line | 3D printers, precise lifts |
Choosing a mechanism
- Need force? Use a long effort arm, a lead screw or a worm gear.
- Need speed or range? Use a 3rd-class lever.
- Need a repeating pattern? Use a cam or crank.
Example: a servo-powered gripper
A servo turns a small gear that meshes with a second gear. Each gear holds one jaw, so the jaws close towards each other. The jaws are 3rd-class levers: small servo movement, big jaw movement.
Example: lever maths
A 30 cm lever with the fulcrum 5 cm from a 600 g load. Effort at the far end (25 cm from the fulcrum): 600 × 5 = effort × 25, so effort = 120 g. Five times easier.
Example: a walking leg
A motor turns a crank. A four-bar linkage attached to the crank traces an oval path at the “foot”: down and back on the ground, then up and forward in the air, which is a walking step.
Name the leverEasy
Which class of lever is a bottle opener, a fishing rod and a pair of pliers? Bottle opener: 2nd class. Fishing rod: 3rd class. Pliers: 1st class.
Lift itMedium
You need to lift a 1 kg weight with a servo that can only push 250 g at the end of its arm. How would a lever help? Put the fulcrum so the servo’s arm is 4 times longer than the load’s arm (e.g. load 2 cm from the fulcrum, servo 8 cm away): 1000 × 2 = 250 × 8.
Pick a mechanismHard
A robot must open and close a small door once every 5 seconds forever. Choose a mechanism and explain why. A crank (or cam) on a slow continuously turning motor: one turn every 5 s opens and closes the door with no code needed. A servo would also work but needs code and wears out faster when moved constantly.
Quick check
Finished this lesson?