Why does a Technic-style crank move unevenly along one axis?

A crank pin follows a circle, but its position along a fixed straight axis changes unevenly. Near either left or right extreme, a small turn moves it mainly across the other axis. Near the centre crossing, the same angle change moves it much farther along your measurement axis. Our new Crank Orbit Bench makes that distinction visible before you add a rod or sliding output.

Start with the circle you can actually build

redshoebox’s credited demonstration shows several ways to produce oscillating and reciprocating motion. Its explanation around 1:18 introduces an offset connection, and around 1:35 relates the amount of oscillation to distance from the rotating axis. It explains the mechanisms rather than giving a complete part-by-part assembly tutorial. We use that engineering question as inspiration for an original measurement bench, not a replica of the creator’s design.

Open the free Crank Orbit Bench instructions. The level 2 model has 25 parts and ten steps, with an estimated 15-minute assembly time. A four-foot frame supports a pinned bearing bridge. A retained shaft carries a 24-tooth mounting hub and a flat five-hole arm. Two separated pins lock that arm to the hub. An upright marker pin sits at the arm’s outer end, two hole spacings from the shaft.

One hole spacing is eight millimetres, so the outer pin’s centre has a 16 mm radius. Count centre to centre, not from a beam’s rounded end. The gear acts as a mounting hub here: no second gear meshes with it, and there is no gear reduction. Turning the shaft turns the arm once for each shaft revolution. The axles guide explains why the cross hole keys the hub while the bridge’s round hole allows rotation.

The pin moves around a circle above the bridge. The bench has no physical sliding follower. A ruler or a diagram can record one coordinate, but neither magically supplies a guide. This distinction matters when you later connect a piston: its unwanted sideways motion, lift and tilt need real support.

Predict the excursion and compare equal turns

Choose a fixed horizontal X axis on a top-view sketch. At the rightmost position the pin is 16 mm to the right of the shaft. Half a revolution later it is 16 mm to the left. The difference between those extremes is 32 mm. That full excursion is twice the radius; it is not the distance travelled around the circle.

Enter 32 in the lab, check the answer, then move the angle slider through 0, 90, 180, 270 and 360 degrees. Watch the dark marker remain on its circle while the red projected dot moves along the straight axis. At 90 degrees the X coordinate is zero, even though the pin itself is a full radius away from the shaft in the other direction.

Now compare two equal thirty-degree intervals. From 0 to 30 degrees, the 16 mm-radius example changes X by about 2.14 mm. From 60 to 90 degrees it changes by 8 mm. Equal angular changes give unequal coordinate changes. If angular speed were constant, the projected speed would also vary. The physical pin still follows the circular path; it never becomes a slider because you have drawn a projection line.

Double the modeled radius and predict the new excursion before checking. It doubles, while the selected angle stays unchanged. The authored bench’s outer marker is fixed at two spacings. Other radius settings are hypothetical comparisons, not an instruction to move a mounting pin without reviewing the new geometry. Our calculation uses an ideal rigid circle. A real connecting rod changes the detailed piston-position formula, and a rocker linkage has its own dimensions and limits.

Make a fair observation sheet

Place the bench on a level table and hold its stationary frame. Look directly down at the marker to reduce perspective error. Use a separate top-view sketch with the shaft centre and fixed X direction marked. Keep paper, pencils and rulers outside the swept circle. Turn the shaft gently and record estimates at the same angle marks each time. Never force a pin against a ruler as a substitute for a guide.

Repeat your observation three times with the same reference axis and viewing direction. We have not performed physical trials, so the observed column is deliberately blank. Hand-set angles, perspective and the width of a pin all limit precision. Record a range when you cannot distinguish a small difference. Changing the camera, reference axis and radius together would make a fair comparison difficult.

Fix support problems before connecting an output

If the hub turns but the arm lags or swivels, check both separated mounting pins. A single hinge line would let the arm rotate relative to the hub. If the shaft lifts, check the underside retainer; if it drops or rubs, check the upper spacer and seating. The bearing should rotate freely while its bridge remains rigid. Stop if the arm, marker or frame is loose. Holding a moving arm in place by hand would change the mechanism you are trying to observe.

The complete instructions show insertion order and retention, and the animated loop checks the full crank circle. It does not measure friction, loads or real operating speed. Our finite geometry probes check unwanted escape motions as well as the intended rotation; they remain model checks rather than a physical prototype certificate.

When the circular input is clear, compare the Twin-Piston Engine, where rods connect cranks to reciprocating outputs, and the Oil Pump Jack, which couples a crank to a rocking beam. The crank and slider explanation and linkage guide help you name the extra joints. Predict which parts can move and what holds everything else before pressing play. That question is more useful than assuming every smooth-looking animation will work in real parts.