Bidirectional conveyors

Bidirectional conveyors are used in intralogistics to cover two processes with one conveyor line. It is not uncommon for the installation of parallel lines to be dispensed with and replaced by bidirectional operation of one line because the throughput calculation shows that the conveyor lines are utilized less than 50% on average. In this case, bidirectional means, for example, that only one line is available for storage and retrieval, which changes the conveying direction at intervals or when required.

As the system itself cannot handle transports in both directions on one conveyor at the same time, it must not be instructed to do so by the material flow control system. Time slices are often introduced in which one direction is transported at a time.

The problem

The problems arise during the changeover, as not only the route itself, but also the upstream and downstream transport areas and systems have to change operating status. As long as transports are underway in one direction, no transports may be started from the other direction. This means that the order from the currently valid direction must be stopped in good time so that the entire route can be switched at all. As a result, this part of the system is completely emptied each time it is switched, which drastically reduces the overall performance of the system.

Bidirectional conveyors are only useful in rare cases

If the average load only requires 10 – 20% of the conveying capacity of the line, the transports are not subject to any time restrictions and there is never a need to switch over at short notice, this savings potential can be utilized. However, a closer analysis often shows that these conditions do not apply in reality.
One way to increase the efficiency of a bidirectional line is to use buffer storage on the upstream and downstream lines. This solves the problem of the long idle phase and makes it possible to occupy the reversible route in one direction with many transports. In return, however, this also results in double set-down and pick-up processes and additional costs for the batch buffers to be installed, so that ultimately the cost saving by omitting the second route must be critically scrutinized.

The frequently proposed solution of decoupling the upstream and downstream routes from the bidirectional route by placing buffer stores directly at their ends solves the problem of the long idle phase and then makes it possible to occupy the reversible route in one direction with many transports. Due to the resulting doubling of set-down and pick-up processes and the costs of the installed buffers, the cost savings by omitting the second route must be critically scrutinized.

You can find more information on conveyor technology under Multi-station conveyors.

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