Because the teeth engage progressively instead of a roller striking the sprocket.

In a roller chain, each roller approaches the sprocket and seats into a tooth space with an impact. That impact happens once per roller, so the frequency rises with speed and so does the noise, which is why roller chain gets loud fast.

In an inverted tooth chain, the flanks of the link plate teeth make contact with the sprocket tooth flanks and slide into engagement gradually. The contact builds rather than arriving, so the impulse that generates noise is largely absent.

The same progressive engagement reduces vibration and the dynamic loading on the chain and sprockets, which contributes to the chain's high-speed capability and long life.

The name is relative rather than absolute - the drive is not silent - but at speeds where a roller chain would be objectionable, a silent chain is markedly quieter.

Either by guide links running in a groove in the sprocket, or by guides on the outside of the chain - and the arrangement must be specified.

A silent chain is wide and flat and has no flanges of its own, so something must keep it laterally located.

Centre-guide chains carry a row of guide links in the middle of the chain width that run in a corresponding groove machined around the sprocket. This is the most common arrangement and keeps the chain positively centred.

Side-guide chains have guide plates on the outer edges that straddle the sprocket. This suits narrower chains and avoids machining a groove in the sprocket.

The chain and sprocket must match - a centre-guide chain needs a grooved sprocket and will not run on a plain one. When replacing a chain, confirm which arrangement the existing sprockets are cut for, because it is not always obvious from a quick look and the chain will not fit if it is wrong.

It elongates like any chain, but it tolerates elongation far better because of how it meshes.

All chains wear at the pin and bush interfaces, and that wear shows as pitch elongation. In a roller chain, elongation means the rollers no longer sit at the correct pitch relative to the sprocket teeth, so they ride progressively higher up the tooth flanks until eventually the chain climbs the sprocket and jumps - a sudden and damaging failure.

A silent chain engages on tooth flanks, and as it elongates it simply seats slightly further out on the sprocket teeth while continuing to mesh correctly. The drive keeps working, quietly, well past the elongation that would condemn a roller chain.

That gives longer service life and a more forgiving failure approach, but it also means elongation is less obvious. Measure it against the manufacturer's limit rather than waiting for symptoms, since the chain will not warn you the way a roller chain does.

In high-speed, high-power drives where noise, smoothness or power density matter.

Machine tool main drives, pump and compressor drives, printing and paper machinery, and industrial gearbox input drives are typical. They also appear in oilfield and marine drives where a compact high-power transmission is wanted.

Automotive engines have used them extensively for camshaft and balance shaft drives and in transfer cases, precisely for the combination of quiet running and high power density in limited space.

What those share is a requirement the roller chain cannot meet - either the speed is too high for acceptable noise, the power too great for the space available, or the smoothness requirement too tight.

For ordinary industrial drives at moderate speeds, roller chain is cheaper, more widely stocked and entirely adequate, which is why it remains far more common overall.

By selecting a pitch from the speed, then a width from the power - which is a different process from roller chain selection.

The chain's pitch is chosen mainly from the operating speed: a smaller pitch runs more smoothly and quietly at high speed, while a larger pitch carries more load per unit width at lower speed.

Power capacity is then set by width. Because the chain is an assembly of parallel plates, it can be made as wide as needed, and manufacturers publish power ratings per unit of width for each pitch at each speed. That makes selection a two-step process: pitch from speed, then width from power.

Apply a service factor for the driven machine's characteristics and the number of starts, as with any chain drive.

Check sprocket tooth counts too - a small sprocket increases the articulation angle and the chordal action, which raises noise and wear, so a minimum tooth count applies particularly at high speed.

Continuous and reliable - more so than roller chain, because the speeds and power densities are higher.

The wearing surfaces are the pin and plate interfaces, and at high speed those articulate very frequently under load. Manual or drip lubrication is generally inadequate; silent chain drives are usually oil bath or forced feed, with the chain running in an enclosed case.

That enclosure is not optional at speed. Beyond a certain chain speed, oil must be delivered to the chain positively and the case must handle the oil the chain throws off. The manufacturer's data specifies which lubrication method applies at which speed, and using a lower method than specified is a reliable way to destroy an expensive chain.

Cleanliness matters equally. Abrasive contamination reaching the pin joints accelerates elongation dramatically, so the enclosure serves as much to keep dirt out as oil in.

Not directly - the sprockets are completely different and must be changed with it.

A roller chain sprocket has tooth spaces shaped to receive rollers. A silent chain sprocket has straight-flanked teeth shaped to mesh with the link plate flanks, and it may also need a guide groove. The two are not interchangeable in any way.

So an upgrade means new sprockets, and the sprockets must match the chain's pitch, width and guide arrangement exactly.

Space is the other consideration. A silent chain of equivalent power capacity is usually wider than the roller chain it replaces, so there must be room for it and for the wider sprockets - and if the existing drive is enclosed, the case may need changing.

Given that, the conversion is usually made when a drive is being redesigned for higher speed or lower noise rather than as a straight swap on an existing machine.