Basics in Logistics Automated small parts storage — function, advantages & strategies

Automated small parts storage systems (AKL) offer the possibility to store items in a space-saving and efficient way in containers such as plastic crates or trays. This article explains how AKLs are structured, how much they cost and which access strategies are used to differentiate them!

The starting point for the first automated small parts storage (AKL) in the 1960s were pallet storage systems. These were later followed by specific storages for coils and rods, containers and small parts containers. Today, automated small parts storages are used in almost all sectors of trade and industry.

How are automated small parts storages structured?

In principle, automated small parts storages are similar in design to automated pallet high-rack systems. High-rack systems are economical and efficient, but differ in terms of load weight (usually 50 kg per individual load) and thus in terms of component dimensioning.

The Aberle GmbH presents a 4-aisle AKL for the Schaeffler Group. 44.800 storage spaces with up to 4-fold deep storage enable efficient material flow.

Automated small parts storage systems usually consist of a shelving system comprising one or more aisles, a small parts storage device, a commissioning area, in which items can be picked according to specific commissioning procedures, and a large number of small parts containers. If a shelf compartment contains more than one container, this is referred to as double-deep storage. If the stored goods are comparatively small parts or there is only a small turnover, there is the option of storing different items together in one small parts container. The prerequisite for this is a suitable warehouse management system.

What types of shelves are available in the AKL?

Automated small parts storages can be used as both single and multi-bay storages with single-deep or double-deep storage. Single-deep automated small parts storages allow direct access to all stored goods, while the double-deep version stores several items behind the other. Since the goods at the rear can hardly be accessed in an economic way, such automatic small parts storages are used especially, when high stocks per item have to be handled. Within the storage channels, storage is then carried out by type.

Advantages of double deep storage:

• higher volume efficiency

• enables optimum use of existing storage space

• costs for warehouses, shelving and storage technology are reduced

Although the time required for access to remote storage units is increased by the necessary relocations, the travel time is reduced by the automated small parts storages because they involve shorter travel distances with the same number of storage spaces.

Automated small parts storages are either free-standing or load-bearing. In the latter variant, also known as storage silo, the steel structure of the warehouse supports the side walls and the roof.

How is an automated small parts storage operated?

The storage and retrieval in automatic small parts warehouses can be carried out in different degrees of automation:

• by manually operated narrow-aisle trucks,

• semi-automated storage and retrieval machines (S/R machine) with operator cabins for mobile order pickers and

• by fully automated storage and retrieval machines.

As the height of the automated small parts storage increases, so does the load on the storage and retrieval machine. Therefore, two-mast devices are often used for high shelves. The different variants of the S/R machine can be combined with any of the various storage types in conjunction with the appropriate load handling attachments.

Which technology is used in automatic small parts storages?

Option 1: small parts storage devices and shuttle systems

The small parts storage devices used in the automatic small parts warehouses have been specially developed for use in confined spaces. They consist of one storage and retrieval machine per aisle, one rail and the load handling attachment (LHA). The storage devices work at heights of up to 30 m and can handle loads of up to 300 kg. On average, they reach horizontal moving speeds of six meters per second. The acceleration is between three and four meters per second. These speeds are important factors, especially in motion strategies for storage and retrieval processes. The aim is to achieve maximum acceleration and maximum deceleration within the shortest possible time — resulting in a huge loss of energy. Sustainable systems for automated small parts storage are therefore in great demand.

The storage and retrieval machines have different designs to hold the small parts containers:

• Forklift technology: Telescopic

• Pulling system: Cassette pusher

• Gripping technology: lateral removal gripper

Automated small parts warehouses mainly work with pulling technology and forklift or telescopic technology in different variants. Suction grippers are also used for cardboard boxes. Multiple load handling attachments are often used in warehouses that can transport several load units at the same time. The units are arranged either side by side or in line.

An example of a shuttle system in the warehouse: In this video Viastore shows how the Viaflex shuttle system works.

Innovative shuttle systems are increasingly being used in automatic small parts warehouses for the storage of small parts. These are storage devices that are guided along a shelf front and can be placed on rails integrated into the shelf. The storage, transfer and retrieval takes place laterally to the path of the shuttle.

On the one hand, the flexibility of the shuttle system makes performance scalable, which saves costs. On the other hand, in the event of a breakdown of a single vehicle, there are only minor restrictions in warehouse operation. In addition, the shuttle system enables items to be transported from the warehouse directly to their destination.

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However, such a system also has disadvantages. Special rails and safety devices are therefore required. In principle, rack construction is therefore much more complex than for automatic small parts storage with stacker cranes.

Option 2: Warehouse management systems

The core of every efficient automatic small parts storage (AKL) is the warehouse management system. This system is provided with a list of the required parts by the warehouse management system or the ERP-system. Based on this information, it informs the control software which small parts containers have to be transported to the picking station. On the basis of the specifications, the control software then ensures that the respective storage containers are forwarded to the station where they're needed. An order picker then removes the corresponding parts and places them in the target container.

Option 3: small parts container

In most cases, standardized containers are used for the transport and storage of goods in automated small parts storages. These containers form logistic units, which are the basic prerequisite for automation and mechanization of the material flow.

The AKL containers are usually made of sturdy and resistant plastic. However, there are also variants made of metal (steel, stainless steel or aluminum) or cardboard, which usually have a smaller filling volume. Many small parts containers have compartments or separators which ensure that even the smallest - sometimes even different parts - can be safely stored and removed in the automated small parts warehouse. Depending on the goods to be stored, various formats of AKL containers are available.

Option 4: Trays

Trays are stable conveyor trays on which the storage units are stored together in the automated small parts warehouse. Upward bended edges ensure that nothing can slip. Each tray is provided with its own identification number in the logistics system which allows the stacker crane to access it.

Trays in the automated small parts storage increase flexibility and performance and are well suited for internal material flow because the trays are stable, always have the same size, and can be moved automatically. The standardized dimensions are ideal for automatic storage and allow optimum utilization of the storage area.

How does commissioning work in the AKL?

Automated small parts storages are a components of the commissioning systems and are therefore subject to different requirements. Depending on the intended use, there are two system variants which differ in particular in the degree of automation and the respective order picking procedure: the "goods to man" principle and the “man to goods" principle.

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The principle "goods to man" is used when the storage containers in the automated small parts warehouse move to the order picker. The required parts are then removed at local picking stations in the individual aisles. Since it is unlikely that all items of an order will be assembled in a single aisle of multi-aisle storage systems, all aisles of the automated small parts warehouse are connected by conveyor technology in larger order picking systems. Depending on the picking performance required, picking stations are set up along the resulting conveyor line.

With the "man to goods" principle, the articles are at a fixed location in the automated small parts warehouse to which the order picker moves. If the automated small parts storage contains a very large assortment of parts, aisle-bound order picking devices are used which can move vertically and are automatically controlled to the respective picking stations - sometimes by a mobile order picker.

Logistics and distribution centers are often equipped with combined order picking systems. In these cases, goods with large turnover volumes are provided statically, while items with low turnover volumes are picked from dynamically provided storage containers.

Which operating strategies are used in the AKL?

Automated small parts storages only work economically if the warehouse organization and the associated operating strategies are adapted to the stored goods. The following strategies are particularly relevant:

Access strategy

The access strategy determines the order in which the S/R machine accesses the stored units of the same product. The two most popular procedures in this strategy are:

• First in — First out (FiFo) and

• Last in — First out (LiFo)

With the FiFo principle, the loading unit with the longest residence time is always removed first, whereas with the LiFo principle, the unit with the shortest residence time has priority. The FiFo strategy ensures uniform handling of the stored items in the automated small parts storage and is mainly used in food logistics.

Utilization strategy: The utilization strategy specifies which articles are placed into storage in which storage zones or in which storage spaces in order to keep the storage and retrieval paths as short as possible. Examples for utilization are:

Chaotic storage space assignment: Irrespective of the item in question, it is stored at the closest available location.

Fixed storage space assignment: For each item, storage spaces are reserved for the maximum expected stock level in the automated small parts warehouse. These spaces are blocked for other goods.

Fast seller concentration: Items with a high turnover rate are stored close to the storage and retrieval point in order to keep the average routes in the warehouse short and to enable quick access. This utilization strategy requires precise knowledge of the turnover rate of the various items.

Routing strategy: The routing strategies used in automated small parts storage determine the sequence in which storage and retrieval operations are carried out by the storage and retrieval machine and the associated load handling devices. Among others, the following strategies are used:

Single-cycle strategy: The stacker crane picks up a storage unit from the automated small parts warehouse, moves it to the storage space provided for it and then returns to pick up the next unit. Items are either stored or retrieved. The advantage of this strategy is a higher performance during storage and retrieval. The associated longer empty runs of the storage and retrieval machines are unfavorable.

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Double-cycle strategy: The double cycle combines storage and retrieval. The storage of an item in the automated small parts warehouse on the outward journey is followed by the retrieval of an item stored nearby on the return journey. The advantage of this variant is the optimization of the route. This avoids empty runs and the processes in the warehouse run more efficiently. The individual motion sequences take longer to complete. In addition, this variant requires sophisticated warehouse management software.

Path strategy: Storage and retrieval machines, which are able to hold several storage units, move to different storage and retrieval spaces in a row, choosing the shortest possible distances. The aim is to minimize the distances traveled as far as possible.

What are the advantages and disadvantages of automated small parts storage?

One of the biggest advantages of automated small parts storages is the time savings it achieves compared to manual order picking. In addition, it offers the following advantages:

• maximum degree of automation possible

• can be implemented as a closed system (protection against unauthorized access)

• optimal use of the existing storage volume

• well suited for picking tasks

• continuous work flow

• high handling throughput possible

On the other hand, there are these disadvantages of automated small parts storages:

• container dimensions limited, therefore restrictions on stored goods

• limited technical availability

• high investment costs

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When do automated small parts storages make sense?

Whether an AKL is worthwhile for a company depends on the following factors:

• the usable area

• the item structures and quantities

• the ergonomic requirements

• the expected personnel costs

• the security of data and goods as well as

• the required speed of access to the goods

The greater the demands in these aspects are, the more advisable it is to use automated small parts storage. In principle, an economic use is conceivable if a warehouse aisle with approx. 3,000 to 5,000 storage spaces can be fully utilized. If an integration into already existing rooms is possible, solutions with less than 1,000 places can make sense. If, on the other hand, a new building is required, automated small parts storage usually only pays off with higher numbers of containers.

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