Crank and Slider Mechanism Explained: How One Turn Makes One Full Stroke Along a Curved Guide
Motors turn. Many machines need something to move back and forth instead. The crank and slider is one of the oldest and most useful answers: a rotating crank, a connecting rod and a slider combine to turn spinning into a stroke.
The interactive lab from 3D Mechanism shows a version of this mechanism with a twist: the slider runs in a curved guide. Its big idea in one line: turning in, curved sliding out: one turn, one full stroke. The model and its motion come straight from the CAD file.
What Is a Crank and Slider Mechanism?
It has only four parts. The lab lists them in the order the motion travels:
Crank → connecting rod → slider → curved guide
Crank: the input. It turns around a fixed point.
Connecting rod: links the crank to the slider.
Slider: the output. It moves back and forth.
Curved guide: the track the slider runs in. In this model the guide is curved, not straight.
How It Works, Step by Step
The lab has five steps:
Crank. The motion starts at the crank, which makes a full turn.
Connecting rod. The rod links the crank to the slider, and it rocks about 19 degrees as the crank turns.
Slider. So the slider moves back and forth, about 49 mm.
Curved guide. Because the guide is curved, the slider also tilts, about 29 degrees.
All together. Every part moves at once.
Why a Turning Crank Makes the Slider Move Back and Forth
The crank's end moves in a circle. The connecting rod is attached to that end, and its other end is attached to the slider, which is held in the guide and can only travel along it. As the crank pin goes around, it pulls the slider one way, then pushes it back. The rod does not turn with the crank. It swings from side to side, which is why the lab shows it rocking by about 19 degrees.
One full turn of the crank gives one full forward-and-back cycle of the slider. This is the same working principle you find in engines, pumps and presses, only usually with a straight guide.
What Changes With a Curved Guide
In the classic version, the slider moves in a straight line, and it stays parallel to itself. Here the guide is an arc, so the slider follows a curved path and turns slightly as it moves. The lab measures that tilt at about 29 degrees. This is useful whenever the output needs to swing along an arc while it moves, for example when the part that is driven must follow a curved path or pivot as it advances.
The cost is a bit more design work, because the guide's shape now decides both the stroke and the tilt of the slider.
Reading the Numbers in the Lab
The lab shows numbers measured on the 3D model while it moves:
Crank: 358°. That is one full turn of the crank in the model's loop of about 5 seconds.
Live panel: it shows the crank's turn in degrees and the slider's travel in millimeters while the model plays.
Technical view: it draws the crank angle on a diagram that you can switch between front, back, left, right and top.
The lab plays the motion at half speed so that you can follow each part. The 49 mm, 19 degrees and 29 degrees figures are measurements of this CAD model, not ratings for a real product.
What to Try in the Lab
Go through the five steps and watch which part lights up at each one.
Use Auto to play the sequence, or Manual and press Space for the next step.
Watch the connecting rod in the Front view. It rocks while the crank goes around.
Turn on X-ray to see through the parts, and CAD colours to tell them apart.
Follow the crank angle in the technical view while the slider moves.
The motion loops continuously, so one full turn always leads back to the start.
Where Crank and Slider Mechanisms Are Used
The same idea appears wherever rotation has to become a stroke, or the other way round:
Piston engines and compressors, where a piston drives a crank, or a crank drives a piston.
Pumps and presses, where a rotating drive makes a repeated stroke.
Machines with a repeating motion, such as feeders, cutters and sewing or packaging equipment.
The lab does not name a particular product, so treat these as typical uses of the principle, not as claims about this model.
Designing a Crank and Slider in CAD
These are general design points, not specifications from this model:
Start from the stroke you need. The crank radius and the rod length determine how far the slider travels.
Keep the rod long compared with the crank. A longer rod rocks less and gives a smoother motion.
Model the guide first. In a curved-guide design, the arc decides the slider's path and tilt.
Use assembly joints and a motion study. Check that the rod does not hit the guide at either end of the stroke.
Look at the force direction. The rod pushes on the slider at an angle, so the guide has to take side loads.
Designing for 3D Printing
A printed crank and slider is a good teaching model.
Print the crank, rod and slider flat, so layers run along their length.
Use pins at the joints with clearance in the holes, and test the fit.
Add a smooth, slightly loose slot for the slider in the guide, and check that it does not stick.
Print thicker walls near the pin holes, where the load concentrates.
Common Problems
The slider sticks in the guide: too little clearance, or the guide's curve is not what the slider's path needs.
The mechanism jams at the ends of the stroke: the rod and crank line up, or the rod is too short.
The motion is jerky: the pins have play, or the slider tilts and binds.
The slider wobbles: the guide is too wide, or too short to hold the slider.
Frequently Asked Questions
How does a crank and slider mechanism work? A crank turns, and a connecting rod joins its end to a slider. Because the slider can only move along its guide, the turning of the crank becomes a back-and-forth movement.
Why does the connecting rod rock? Its crank end goes around in a circle, while its other end moves along the guide. The rod swings from side to side to connect the two, about 19 degrees in this model.
What is different about the curved guide? The slider follows an arc, so it tilts as well as moving. In the lab, the slider travels about 49 mm and tilts about 29 degrees.
Does one turn give one full stroke? Yes. One full turn of the crank gives one forward-and-back cycle of the slider.
Are the lab numbers real specifications? No. They are measured on the model while it moves and describe this model only.
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