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Forma

Lab · Mechanical engineering

An arm turns. A block slides.

How can those movements be connected? Meet three parts, follow one turn, then use the idea to explain a machine.

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Lab · Mechanical engineering

How a turning arm moves a sliding block

Start with a question

How can a turning handle make a block slide back and forth?

Machines often need to change one kind of movement into another. A turning handle can drive a pump that pushes and pulls in a straight line.

Before you start: No mechanics knowledge is needed. First meet three parts, then follow one movement. The model shows motion; it does not simulate a running engine.

Your goal: Explain how the turning arm, connecting rod and straight guide make the piston move out and back.

  1. A turn and a slide are different movements

    A point on a turning arm travels around its fixed center. A sliding block moves along a straight path. Joining them needs a link that can change its angle while staying the same length.

  2. Meet the three working parts

    The crank is the turning arm. A pin near its end joins it to the connecting rod. The rod’s other end joins the piston, the sliding block. Both joints let the rod pivot, like a hinge, instead of forcing the piston to follow a circle.

  3. The guide keeps the piston on one path

    The cylinder is the fixed guide around the piston. It keeps the piston moving along one straight line. Because the rod does not stretch, the changing position of the crank pin makes the piston move closer to the crank and then farther away.

Words you’ll see

Crank
The arm that turns about a fixed center; its outer pin travels around that center.
Connecting rod
A rigid link that joins the turning pin to the piston. Its joints let it tilt.
Piston
The block that slides inside the cylinder. One trip from one end of its path to the other is called a stroke.

Now match the three numbered parts to their names. Follow the guided turn, make a prediction, then explore the controls if you want to try a different movement.

Watch the connection

Find the turning arm, the link and the sliding block

Match the numbers on the model to the names below it: 1 is the crank, 2 is the connecting rod, and 3 is the piston. The short crank turns about its fixed center. Start by watching its outer pin.

Guided first turn · 1 of 4

Follow the numbered parts
The slider–crank at 0 degrees with a 30 mm crank radius. The rod joins the crank pin to the piston in a cutaway cylinder.
Piston at its far endPiston at its far end

Saved front-section drawings show the same quarter-turn positions. Use Step to follow the mechanism.

The exposed joints show how rotation becomes reciprocation. The dark connecting rod remains rigid; the cutaway cylinder guides the piston. Loading the 3D view. The saved drawing is available while it loads.

How far does the piston move?

A stroke is one trip between the piston’s two end positions. The crank’s radius is the distance from its center to its outer pin. We measure distance in millimetres, written mm.

At one end, the crank pin reaches one radius toward the piston. Half a turn later, it points one radius in the other direction. Those positions are two radii apart. The rod has the same length at both ends, so the piston’s stroke is twice the crank radius. A larger radius makes a longer stroke in this straight, in-line arrangement.

Worked example: a 30 mm crank radius gives a 30 + 30 = 60 mm stroke. A full turn gives one 60 mm inward trip and one 60 mm return trip: two strokes, with 120 mm of total travel.

The 30 mm crank at 0 degrees: piston farthest from the shaft.

0° · Farthest from the crank

The 30 mm crank at 180 degrees: piston closest to the shaft.

180° · Closest to the crank

These front views put the crank on the left and the piston to its right. Compare the piston’s two end positions: they are 60 mm apart. The rod keeps the same length in both views. Original illustrations by Forma.
Optional: the geometry and model assumptions

You do not need this equation for the lesson. L is the rod length (110 mm), r is the crank radius, θ is the crank’s turn angle, and x is the distance along the piston’s path from the shaft center to the piston’s joint.

The rod’s horizontal reach is √(L² − r² sin² θ). Adding the crank pin’s horizontal position gives x = r cos θ + √(L² − r² sin² θ). At the two ends, x is L + r and L − r, so the difference is 2r. The drawing and readout use this geometry.

An ideal in-line slider–crank with a rigid 110 mm connecting rod. Playback is deliberately slow. The cutaway exposes the joints; this lesson models motion, not combustion, friction, forces, inertia, or manufacturing tolerances.

Reference: MIT OpenCourseWare · Machine Elements, slider–crank kinematics (PDF).

A free mechanics lab. No account needed. Also explore the computer memory lab.