Abstract: In an accumulation conveyor comprising load-carrying rollers, a roller drive is so devised that all rollers under one and the same load at a time are either being positively driven by, or are disengaged from, the said drive. Mechanism controlling engagement of the drive comprises a combination of a clutch-actuating linkage and a priming linkage which prepares the actuating linkage for operation. The mechanism may be positively actuable by a return spring of a sensor device, for example a roller depressable by a load. An assembly of load-carrying rollers may have sprockets drivable by a standard-width chain movable in a constant line. The mechanism may include a transversely movable dogclutch part which forms part of a continuously rotating sprocket assembly engageable with and disengageable from a load-carrying roller, the said clutch part passing through an opening in the sprocket so that the positive drive is picked-up and positively transmitted to the roller.