Why can two Technic-style cranks turn together but reach their peaks apart?

Two cranks can turn at the same angular speed while reaching a chosen coordinate peak at different input angles. Their starting positions differ by a fixed angle, called phase. If both are locked to one shaft, each makes one complete revolution per shaft revolution. A quarter-turn starting offset does not make either arm a quarter as fast or add a quarter turn on every cycle.

Give phase a model you can hold

Our new Quarter-Phase Crank Bench puts two equal five-hole arms on one retained shaft at separate heights. Each has a marker two hole spacings from the shaft centre, a radius of sixteen millimetres. The lower yellow arm points across the frame. The upper red arm starts perpendicular to it, toward the frame’s short end. Two separated pins attach each arm to its own keyed mounting hub.

The model uses thirty-one compatible parts and thirteen illustrated steps, with an estimated twenty-minute assembly. Its two 24-tooth gears are mounting hubs. They do not mesh with each other, so there is no gear reduction between the arms. The axle explanation shows how cross holes transmit rotation while a round bearing allows it. Look at the shaft before guessing a ratio from the tooth counts.

Brick Technology’s credited air-engine demonstration combines different cylinder and crank arrangements. A visible reader in its accompanying discussion wonders how firing order affects the result. That broad question contains several different jobs: arranging cranks, moving pistons, timing valves and supplying air. Our original bench isolates only the first idea, a fixed starting offset. It does not recreate the creator’s engine, modified cylinders, pneumatic circuit or measured performance.

Predict changes rather than starting angles

Think of two runners fixed to different positions on one rotating platform. They stay separated, but the platform carries both through the same angle. Counting their initial positions as distance travelled would confuse setup with movement. The same mistake happens when you add a phase offset to accumulated crank turns.

In the lab, choose the prediction before touching the input angle. Set the shared input to 360 degrees, then 720. Both arms accumulate one turn, then two turns. The upper marker still has its starting offset. Next choose a virtual zero-degree offset: the coordinates agree. At 180 degrees, the equal-radius markers have opposite coordinates while still accumulating the same turns.

The two circles in the diagram are separated to keep them readable. The physical arms share one shaft, one above the other. The graph plots projected X coordinates against a full input revolution; it is not a trace of elapsed time or a measurement of your hand speed. Other phase choices are mathematical comparisons. Only the authored quarter-turn arrangement is reviewed as this physical guide.

A peak needs a reference direction

Choose a top view with positive X to the right. In our convention, increasing input angle moves the lower marker from the right toward the top of the drawing. The upper marker begins at the top, ninety degrees ahead in that direction. At zero input, lower X is sixteen millimetres and upper X is zero. At ninety degrees, lower X is zero and upper X is minus sixteen millimetres.

This also explains why words such as ahead and later can be confusing. The upper marker’s rightmost peak occurs at an input angle of 270 degrees in the next forward cycle, while the lower marker peaks at zero and 360. You can describe the upper peak as a quarter turn before the next lower peak, or three quarters after the preceding lower peak. State the direction and reference event instead of treating a cyclic offset as a stopwatch reading.

The ideal coordinates are sixteen times the cosine of the input angle for the lower marker, and sixteen times the cosine of input plus ninety degrees for the upper marker. This is a projection of a circle. Neither pin slides in a straight guide, and reaching an X extreme does not stop its movement around the circle. The Crank Orbit Bench explores that single-marker distinction first.

Try a fair marked-angle comparison

Build the supported model, then hold only its stationary frame on a level table. Look straight down and identify the centres of both marker pins. Keep a separate sketch beside the model; paper and rulers must stay outside its swept circles. Turn gently to the same four angle landmarks in each trial. Count a complete return to each marker’s own starting position as one revolution.

Repeat three times with the same viewing side and reference axis. Then gently reverse the input and compare the order of the landmarks. We have not physically tested the model; the observed column is deliberately blank. Perspective, hand-set angles and pin width limit precision. Record uncertainty rather than inventing exact measurements. No chosen animation speed predicts how fast you will turn it.

Support and separation come before timing

The frame’s pinned bridge is a real bearing. Half bushes immediately above and below it retain the whole shaft in both directions. Each hub and arm has seated support below and a cap above. The arms occupy different levels so their circular sweeps remain clear. The bush guide, frame guide and pin guide help identify those jobs.

If the arms do not keep their relative angle, stop and check the mounting pins and hub seating. If the shaft lifts, inspect the underside retainer; if the upper assembly rises, inspect its cap. A loose bridge cannot serve as a reliable reference. Stop for rubbing or binding instead of forcing a hand-turn or holding a moving arm in place. The guide’s animation and finite geometry tests remain model checks, not a load rating or physical prototype certificate.

When this isolated idea is clear, compare the Twin-Piston Engine. Connecting rods, piston guides and real valve or combustion events add further constraints. Equal crank speed alone does not establish equal instantaneous piston speed, a smooth engine, a particular firing order or improved power. First ask which parts share an input, which start at different angles and what holds each unwanted motion.