Sorting out conveyor sortation systems

The following article from Dematic illustrates some of the key issues that need to be addressed when determining the most appropriate sortation system for different applications and throughput requirements, from as little as 5-20 items per minute to several hundred per minute.

An increasing number of companies are looking to enhance the speed and efficiency of their material handling, and the implementation of high performance conveyor sortation can be critical to success, particularly in applications with a high level of cross docking activity.

Recent years have certainly seen a rapid improvement in both the speed and reliability of sorters but, at the same time, a host of new systems have emerged that have left many users with a potentially confusing array of options to choose from.

Advances in mechanical and electronic controls, tracking systems and computer and IT techniques have achieved high throughput and virtually error-free operation across a wide range of applications including order fulfillment, palletising in manufacturing, and parcel, freight and baggage handling.

Tracking Products Through the System
The advent of barcoding and sophisticated control software has also played a major part in providing real-time information on the status of each individual item within the handling process.

30 years ago, sorters employed manual coding – product was placed at the start of the sorter and the destination was keyed in from a handwritten label.
Today a barcode can be read using either laser scanners or a vision system. Other forms of package identification: eg: alphanumeric symbols, can be read by using a camera-based vision system and RFID systems can also be integrated within conveyor sortation systems.

To keep track of where products are at any given time on a conveyor sortation system, most sorters use window tracking, where a window or space on the belt is generated for each item being sorted.

Product information is contained within that window and its progress is closely monitored. Any problems such as missing products can be identified and subsequent items remain unaffected.

System performance can be further enhanced by the use of variable window lengths, where the size of the window reflects the size of the product within it. A set distance is still maintained between each product.

Fixed windows, where the distance is pre-set, are usually less efficient, but may be appropriate for relatively low throughputs, or where consistently sized items, such as tote boxes, are sorted.

Although a number of new sorter types have been launched in recent years, they still fall into two broad categories – straight line or continuous loop.

Straight Line Sorters

Straight line conveyor sortation systems are most cost-effective where the number of sortation points required is relatively small, for instance, 10-40 delivery points.

A number of different types of diverts can be used on straight line sorters with selection based on a number of factors including product characteristics and the required throughput. The range of in-line sorters include devices to sort from 5-20 cases per minute, up to high speed units operating at greater than 200 cartons per minute.

Sliding shoe-type sorters, where the product is supported on tubes or slats, and a positive divert is performed to either side of the sorter by shoes which slide across the tubes or slats, are suitable for throughput rates of between 9,000 and 12,000 items per hour.

As well as providing quiet and reliable operation, sliding shoe sorters can also handle a wide product range from packages as small as 150 mm long and weighing as little as 50 grams up to packages over 1 m long and weighing up to 50 kg.

Tracking is by variable window, but goods cannot be recirculated within the sorter unless conveyors are linked specifically for this reason.

A second straight line sorting option is the steerable roller.

Located within the conveyor bed frame, the steerable roller divert is a pop-up mechanism, with the rollers lifting and twisting left or right to gently drive products off the sorter. Throughput rates of around 7500 items per hour are possible, as is variable window tracking.

However, the majority of typical conveyor applications require much simpler solutions for medium throughput rates of from 40-80 parcels, cartons or totes per minute (2400-4800 per hour).

Drawing on its extensive conveyor sortation systems design experience, Dematic has developed a totally new style of conveyor sorter for these type of applications.

Called the Pivot Arm Sorter, it is a remarkably compact and versatile system.

Because it has been designed to adapt to a variety of conveyor types and sortation requirements, the Pivot Arm Sorter can not only be installed on new systems, it can also be retrofitted to existing conveyors, or removed and relocated to a new location at any time.

Designed to suit just about all types of conveyor systems including roller and belt, the Pivot Arm Sorter is a totally modular, self-contained system, which can be bolted onto any conveyor frame with beds ranging in width from 550-900mm (24-36”).

The device provides positive sortation through a combination of the pivoting arm diverter working in conjunction with a powered “Vertibelt”.

The Pivot Arm Sorters can be configured in a number of ways to provide single sided, two sided or bi-directional sorting, and are suitable for handling products weighing up to 25kg at the rate of 40-80 per minute depending on carton length and operational parameters.

Applications include shipping sortation, task routing, zone routing and replenishment.

Continuous Loop Sorters

Continuous loop sorters are typically more expensive than straight line systems, but they provide a much greater degree of functionality and flexibility than straight line sorters. They can be designed to handle much higher throughput rates of up to 80,000 items per hour and can sort to several hundred delivery points.

It is also easier to induct goods onto a continuous loop sorter as the machine not only acts as a sorting device, it also acts as a merge for multiple inputs, which makes them particularly suitable for cross docking applications.

In addition to sorting goods for despatch, continuous loop sorters can also be used to fulfil a number of other functions, such as transferring goods from one part of a facility to another, or from one process to another. They can also be used to consolidate items from various areas and, by their very nature, provide a level of on-line accumulation. They also can be used more than once every circuit, increasing the capacity, potentially by as much as 400%.

Crossbelt Sorter

The most recent development in continuous loop sorters is the crossbelt sorter, a high performance, bi-directional sortation conveyor, which transports goods safely and gently to their destinations, irrespective of their shape, size and surface characteristics.

Positive package control at all times is a key feature of the crossbelt sorter. Individual carriages, each with their own belt conveyor, allow the crossbelt sorter to deliver product to a particular destination with pin-point accuracy. Because of this, divert points can be concentrated at close intervals making the crossbelt sorter very space efficient.

The design of the crossbelt sorter also enables a very wide range of products to be handled, including many items previously considered non-conveyable, making it suitable for sorting even split-case items such as small bottles or packages.

Sortation rates in excess of 40,000 items per hour are achievable with crossbelt sorting systems by increasing the number of induct points and through the use of double-deck carriages. Throughput rates over 80,000 items an hour can even be achieved through the latest advance in crossbelt sorting technology, the QUAD Sorter, which comprises four individual crossbelt cells on a single carrier. The four cells can be used singly, in pairs or as a single large cell to handle very large products.

A major benefit of crossbelt sorters is their reliability. The use of multiple linear motors means the system has a high degree of built-in redundancy: the failure of one motor does not stop the whole system. Similarly, the failure of a single carriage on the loop has a minimal effect, as the rest will continue operating.

Tilt-Tray Sorter

There have also been a number of developments in tilt-tray design in recent years.

Tilt-tray conveyor systems, which carry goods in individual trays that tip and discharge their contents at appropriate outfeed points, are also suitable for handling a wide range of products.

Because they use gravity to discharge their contents, tilt-tray conveyors are best suited to handling non-fragile, irregular products such as loose items of clothing and many other products normally considered non-conveyable.

Throughputs of around 12,000 items per hour can be achieved and this figure can be increased further still by using multiple inducts and a variety of discharge chute configurations.

Both crossbelt and tilt-tray sorters allow recirculation of goods, providing some on-line storage and enabling goods to be staged or sorted in particular sequences.

Summary

Realising the expected levels of throughput demands a full analysis of the complete material flow, taking into account factors such as the types of products to be handled, fluctuations in throughput levels, how the products will be inducted, whether the handling systems at the input/exit points can match the speed of the sorter and whether product grouping can avoid unnecessary delays in delivery procedures and simplify point-of-sale stocking operations.

With all the relevant data collected and evaluated, simulation can be used to test various alternatives and identify potential bottlenecks. Eventually a schematic layout can be produced, showing information such as flow rates. Only then, should equipment be selected to provide the most effective solution.

While this article has tended to focus on the top-end of the performance scale, it is equally important to stress that it is possible to justify the introduction of a new sorter system for throughputs of just a few hundred items per hour, using simple pneumatic pushers/pullers.

Whatever the scale of the application, the best approach is always to identify the requirements of the system, then select the right sorter for the job.

For more information, please contact John Briscoe:

john.briscoe@dematic.com
Ph. 61 (0) 408 443 358