In general, accumulation conveyors have been developed to regulate the flow of articles on conveyors. More specifically, accumulation conveyors are employed to allow a line of articles to be stopped on a powered conveyor without damage to the articles to allow processing or handling of the articles at a rate which is different from the rate of arrival of the articles on the section of the conveyor which has been stopped. For example, such conveyor techniques are often employed where the articles are offloaded from the conveyor in groups whereas the articles arrive at regular intervals of time. In practice, the line of articles is stopped by placing a barrier in the path of the lead article, and power is decoupled from the conveyor bed in the region where the articles are stopped. The great majority of accumulation conveyors devised heretofore have employed rollers driven by arrangements of either belts or chains engaged with a motor. Belts and chains are not as compact as might be desired since not only must the active (usually the top) run be accommodated but also the return run which in most cases does not engage any operating mechanisms of the conveyor. This requires space on the conveyor structure in excess of what is actually needed for operating the rollers. In addition, the belts are subject to stretching and wear while the chains although stretching less than belts are generally noisy.
Line shafts, that is, longitudinally extending conveyor drive shafts, have been employed on accumulation conveyors to address some of the problems associated with belts and chains. However, in most cases, the conveyor rollers are drivingly connected with a line shaft by individual endless belts, sometimes referred to as "O-rings". The result is a great multiplicity of such belts which have to be maintained. The replacement of such belts is very laborious because when a belt breaks, it is necessary to shift all the remaining belts toward the empty space before a belt is added. In the case of accumulation conveyors, an intermediate mechanism is required for control of the rollers of a zone, such as an intermediate shaft and a decoupling mechanism.
Accumulation conveyors have not always performed as would be expected. Normally, a sensor is positioned in each accumulation zone and is connected to a drive coupling mechanism in the previous or immediately upstream zone to deactivate the rollers in the previous zone when the first zone is occupied by an article to prevent a collision therewith by an oncoming article. Depending upon the position of the sensor in a zone, the weight of an article on the conveyor, the freewheeling ability of the rollers, and whether a brake is employed; an article entering a zone may or may not engage the sensor in the zone. The usual result is that unnecessary gaps are left in the line of accumulated articles. The gaps could be eliminated by connecting the article sensor in each zone to the decoupling mechanism in that zone. However, there would be no provision for starting up a line of articles in such an arrangement once the articles had been halted.
In order to more efficiently utilize accumulation conveyors by eliminating gaps while providing a start-up mechanism therefor, two-stage accumulation control was devised. On a two-stage accumulation conveyor, the drive to the rollers of a zone is decoupled when an article engages the article sensor of the zone, and the preceding zone is conditioned to decouple the drive therein when an article engages the sensor therein. The conditioning mechanism also serves as a start-up sensor such that when an article in a zone is disengaged from the sensor therein, the drive to the rollers in the next upstream zone is recoupled; and the process is repeated in an upstream direction for all the article occupied zones such that the line of articles is placed in motion.