A speed chain conveyor — also written double-speed chain or free-flow chain — uses a special roller chain whose rollers are carried on a larger-diameter pin. The chain itself never stops; the pallet on top can.
How the 2.5–3× ratio appears
The chain runs at, say, 9 m/min. Because each roller is a small wheel on a tall pin, the pallet riding on those rollers advances at roughly 2.5–3× the chain speed — about 24 m/min — as long as nothing holds it back. A pop-up stop blade lifts into the pallet's path and the pallet halts, while the rollers keep spinning underneath it.
Why that matters on an assembly line
Operators need the workpiece still; the line needs to keep moving. With a conventional chain you stop the whole drive, and every restart costs positioning accuracy and drive wear. With a speed chain the drive runs continuously, so stations stop and release independently and the line can absorb uneven manual times.
Where it is the right answer
Palletised assembly, test and repair loops; battery module and pack lines; appliance final assembly; and any line with manual stations of unequal duration. It is usually the wrong answer for loose cartons and for products without a carrier.
Specify the pallet size and load, the number of stop positions and the required indexing accuracy first; the chain type and drive power follow from those three numbers.