Four-barcrank–rocker.

Four links. Four pins. The crank spins all the way around. The rocker only swings. That is the shop use of a crank–rocker.

A–B–C–D A and D are fixed. B is the crank pin. C joins the coupler to the rocker.

Setting the linkage on the bench…

Lesson Shop / engineering: four-bar linkage (crank–rocker)

A four-bar turns spin into a swing. Drag to look around the pins.

02 Bench Shop / engineering: four-bar linkage (crank–rocker)

Turn the crank.

The crank is the shortest link, so it can spin all the way around. The rocker is pinned to the ground, so it can only swing.

Crank rpm 8

What a four-bar is for

Spin becomes a swing

A four-bar is four links pinned in a loop. Ground link A–D does not move. Crank A–B is driven. Coupler B–C floats. Rocker C–D swings.

In shop class this changes a spinning shaft into a back-and-forth motion. A wiper arm, a pump jack, and a small press ram use the same idea.

The brass dot is fixed on the coupler, not on a pin. Its path is the coupler curve. Turn the path off to see only the four links.

Try this

Watch one full turn of the crank. Count how the rocker angle rises, stops, and falls. It never goes all the way around. Then hide the coupler path and name the four links out loud: ground, crank, coupler, rocker.

03 Shop floor Ten minutes at the bench

Name thefour links.

Use this page next to a cardboard model or a wiper linkage. The classroom use is the crank–rocker, not a rendering study.

03.1

Learning objectives

Name ground, crank, coupler, and rocker on a four-bar.
Point to joints A, B, C, and D.
State that the crank rotates fully and the rocker oscillates.
Say what the coupler curve is: the path of a point on the floating link.

03.2

Classroom loop

Cover the readout. Students predict: does the long right link spin or swing?
Run the crank. Record the rocker angle at the two stops.
Turn the coupler path on. Trace the brass dot with a finger for one crank turn.
Name one shop machine that needs a swing, not a second spin.

03.3

Shop examples

Windshield wipers, a beam pump, a treadle, a folding bench latch. This model is a planar crank–rocker. It leaves out friction, joint play, and links that lock.