How cams work: make a Technic-style machine play a drum beat

A cam is a bump on a turning shaft. As the shaft turns, the bump lifts whatever rests on it, called the follower, and then lets it drop. Put bumps in the right places and a turning shaft becomes a rhythm: every bump is a beat, and one turn of the shaft is one bar of music. The Technic-style Cam Drummer plays a two-stick beat this way, and you can work out its rhythm before you turn the handle.

What a cam is

Most machines turn things round and round. A cam turns that into a lift and a drop. The shape decides the movement: a round wheel with its axle off-centre gives a smooth rise and fall; a wheel with knobs gives quick taps; a single peg gives one big lift and a sudden drop.

Something has to bring the follower back down. In engines a spring does it. In both cam builds here it is gravity: the follower hangs on a pin and falls back under its own weight. The Cams idea page has the short version.

Cams are old. The engineer al-Jazari used cams and camshafts in the automata he described in 1206, and water-driven automata with cams go back more than two thousand years.

Inside the Cam Drummer

The Cam Drummer (level 3, about 30 minutes) has one cam shaft and two 15-hole drumsticks hinged at one end, their far ends resting on a drum.

• The yellow stick rests on a knob wheel. Its 4 knobs lift it 4 times a turn for a light "tak".
• The blue stick is lifted by two red axle pins pushed into a 24-tooth gear on the same shaft. Two lobes give 2 bigger lifts a turn, each followed by a drop: "BOOM".
• The crank goes straight onto the cam shaft, with no gears between. As the build puts it, one turn of the handle is one bar of music.

So in each turn you get 4 taks and 2 booms. The build's animation lands the blue stick a moment after a yellow tap, so they never hit together and you hear 6 separate sounds a turn. The build's own notes are careful to say the sound, the bounce and the real timing haven't been measured, so treat the animation as a plan and your ears as the test.

Work out the beat

Two numbers set the rhythm:

1. Beats a minute = crank turns a minute × bumps on that cam. Crank at 20 turns a minute and the yellow stick taps 20 × 4 = 80 times.
2. Where the hits fall depends on where each bump sits around the shaft. Turn the 24-tooth gear on its axle and every BOOM moves earlier or later against the taks. Line them up and two of the taks become a tak and a BOOM together, so you hear 4 sounds a turn instead of 6.

Try both in the lab. The clock face is one turn of the shaft, and the hand shows where the sticks are in the bar.

Why fast isn't always better

Every lift has to be followed by a fall, and falling takes time. At 30 turns a minute, a turn takes 2 seconds, and the BOOM comes only about 0.13 seconds after a tak. Crank much faster and a stick may not have landed before the next bump picks it up again, so it skips beats.

Car engines meet the same limit. Springs close their valves behind the cams, and at very high engine speeds the valves can stop following the cam properly. Engineers call it valve float, and one fix is a stiffer spring. On the Cam Drummer, gravity is the spring, so crank at a steady, moderate speed.

A simpler cam to start with

The Woodpecker Automaton (level 2, about 20 minutes) uses the same knob wheel as its cam. A heavy tail keeps the bird's tail resting on the knobs, so each knob lifts the tail and the beak pecks: four pecks a turn. Its play notes suggest a cam challenge: swap the knob wheel for a 24-tooth gear with one red axle pin sticking out and count the pecks again. It is also part of a short tour of levers and cams in six simple machines you can build.

If gears are new, the 8-minute Gear Friends is a gentle warm-up, and the Cam Drummer's build page lists it as one to try first.

A rhythm activity for a class or club

1. Predict. Before cranking, write the bar: four taks and two booms. Where do the booms fall?
2. Count. Crank slowly for one minute against a timer and count the yellow taps. Does the count match crank turns × 4?
3. Clap along. One person cranks, the group claps the taks and stamps the booms.
4. Re-program. Turn the 24-tooth gear a little on its shaft. Does the rhythm change? Try moving one red pin so the two lobes are no longer opposite each other.
5. Push it. Crank faster until a stick starts missing beats, and write down the speed.

Check yourself