Toothed Timing Belt

Set up a V-belt drive with equal pulleys. Mark both with a single line, aligned.

Run it under load for 200 turns of the driver, then stop and compare the marks.

Expect them to be out of alignment — the driven pulley has fallen behind by several degrees or more.

Record the error. This is the failure that makes the whole subject necessary.

Swap to a toothed belt on matching toothed pulleys, equal tooth counts. Mark and align as before.

Run the same 200 turns under the same load.

Expect the marks to line up exactly — not approximately. Zero accumulated error, because the belt cannot move relative to the pulley by less than one whole tooth.

Measure the distance across 10 teeth with a caliper and divide by 10. That is the pitch.

Do it on the belt lying flat, and understand that the true pitch is defined at the cord line inside the belt, not at the tooth tips — which is why a belt bent around a small pulley still meshes correctly.

Check the pulley pitch the same way. A belt and pulley of different pitch will appear to fit and will destroy themselves within minutes as the error accumulates across the arc of contact.

Cut a short offcut of belt across its width and look at the cut face.

Expect a row of fine cords running lengthwise near the belt's back.

Those cords carry essentially all the tension. The rubber's job is to hold tooth shape and grip. This is why a timing belt must never be kinked or levered over a pulley — breaking cords does no visible damage and removes the belt's strength.

Slacken the belt progressively and load the output until it jumps.

Watch what happens: the belt climbs out of the grooves and skips whole teeth. It does not gradually slip like a V-belt — it holds exactly, then loses a discrete number of teeth at once.

That is why a slack cam belt is dangerous in a way a slack fan belt is not. A V-belt warns you by squealing. A toothed belt gives no warning and then jumps, and on an interference engine the pistons meet the valves.

Where it came from. Toothed belts were developed in the United States in the 1940s for industrial synchronising duty and sold as Positive Drive belts — the name states the selling point against the V-belt exactly. Sewing machines and machine tools took them before engines did.

The application that mattered. Overhead-camshaft engines need the crankshaft and camshaft locked in phase. That had meant a chain in an oil-filled case, with a tensioner and guides. A toothed belt runs dry, outside the oil, quieter and lighter, and — critically — it was cheap enough to make overhead cams normal on ordinary cars rather than a sporting feature. The 1962 Glas 1004 is generally cited as the first production car with a toothed cam belt.

The trade-off nobody chose deliberately. A chain is a lifetime component that stretches slowly and warns you. A belt is a service item with a mileage interval and it fails suddenly. On an interference engine that failure is catastrophic. The industry accepted scheduled replacement in exchange for cost, weight and noise — and a good many engines have since moved back to chains, which is the same trade-off being re-judged rather than a mistake being corrected.

Where you already rely on one. Almost every 3D printer moves its head on toothed belts, for exactly the reason in step 2: a positioning system cannot tolerate the accumulated error a friction belt gives.