Dynamic storage with shuttle or RBG - a comparison

Discussions about the extent to which the more recent development of shuttle warehouses will replace automated small parts warehouses operated by storage and retrieval machines (SRMs) have largely given way to the realization that both systems have a right to exist.

If the ideal system is to be planned purely in terms of storage capacity with freely dimensionable floor space and height, the decision will be to use miniload. Although the number of storage locations is limited by the upper and lower access dimensions of the SRM in the miniload system, the shuttle warehouse requires maintenance levels over the entire height of the racking. As a guideline, it can be assumed that with a stored goods height of approx. 400 mm, any AS/RS AS/RS with a rack height of over 14 meters is superior to the shuttle warehouse in terms of pure storage capacity. The AS/RS AS/RS is also ahead in a pure investment comparison: lower requirements for the steel structure, the vertical transport provided by the AS/RS and various other factors make the AS/RS AS/RS more cost-effective.

The limits of the AKL

For a mini-load warehouse (four aisles, each with one SRM / two LAMs (load handling attachments) and double-deep storage) with an aisle length of approx. 80 meters, a rack height of approx. 15 meters and a high storage fill level, a guideline value of approx. four times 100 storage and retrieval operations per hour can be assumed. A 40-foot container in the incoming goods area with approx. 700 cartons would fill this small parts warehouse to capacity for two hours. Of course, the goods could be temporarily stored in the incoming goods area - this is an individual cost issue and a question of the speed of item availability. The capacity of the miniload can be scaled by increasing the number of aisles. This makes sense if the storage capacity is to be increased at the same time. If the existing storage capacity is sufficient or there is no more space available, the number of LAMs per AS/RS could be increased or two AS/RS per aisle could be used. However, if an even higher storage capacity is required in the short or long term, the step towards shuttle storage systems must be taken.

The types of shuttle warehouse

Shuttles are independently mobile rack conveyors that move along a rail system attached to the rack. The stored goods are usually fed into and removed from the racking levels via lifts.

With the captive variant, the shuttles do not leave "their" aisle or level. Captive shuttle storage systems have so far been unbeatable in terms of storage performance, but also require the highest investment. They have to be fully equipped from the outset, which limits the subsequent increase in performance. If the storage geometry remains the same, only the vertical transport capacity can be increased with additional lift systems or the pre-zone can be multiplied accordingly as the overall system capacity.

In roaming shuttle warehouses, not every storage location can be reached without moving the shuttle vehicles to other aisles or levels. Vertical roaming means that the shuttles change racking levels via a lift system. With horizontal roaming, a rail system is usually installed in the pre-zone for transverse movement. 3D roaming combines both movement options. With roaming systems, the performance of the storage system can be increased by retrofitting the performance-limiting elements or by adding more shuttles (for vertical and 3D roaming) and lift systems (for horizontal and 3D roaming).

A breath of fresh air in old buildings

An AKL requires a uniform height over the entire length of the aisle; the lowest point determines the shelf height. Neighboring aisles can differ in height and length, but must be "on the same line". Shuttle warehouses, on the other hand, can make more flexible use of the dimensions of an existing building. In principle, the storage levels of an aisle can be of different lengths due to deviations in height. Even very jagged building geometries can be used with roaming systems.

Speed as required

The unique selling point of the shuttle warehouse is its storage capacity. A captive system with a double pre-zone and two lift systems per aisle can achieve a capacity of over 600 storage and retrieval operations with single transportation per lift platform (depending on the number of levels, aisle length and storage depth). The limiting element here is not the shuttles, but the vertical transport. To ensure that the lifts can develop their full capacity, the individual shuttles should only operate at a utilization rate of 55 to 65 %, even in warehouses with a high capacity.

If the storage and retrieval machine fails in a miniload, the entire aisle or the assigned aisle area is affected. This can only be countered by spreading the articles across several aisles in a way that is as fail-safe as possible. In the shuttle warehouse, on the other hand, the availability of goods is guaranteed with two lifts. Although the failure of one lift would mean a 50% loss of performance, all storage locations in the aisle would still be fully accessible.

Modern warehouse planning also takes energy efficiency into account. Why move equipment weighing several tons to transport a payload of just a few kilos? This discrepancy is obvious with stacker cranes and is only partially compensated for by modern energy recovery. Shuttle warehouses are more energy efficient. The energy requirement for vertical transportation of stored goods using a lift is roughly equivalent to the energy requirement of the lift drive on a SRM. However, significantly less energy is required for horizontal transportation in a shuttle warehouse. Although the energy consumption depends on the use and design of the shuttles, their energy balance is significantly more favorable.

The other side of the coin

Performance, energy efficiency and item availability come at a price in the case of shuttle warehouses. Both the initial investment and the running costs (with the exception of energy costs) are higher than for a comparable mini-load warehouse. Maintenance does not affect the availability of goods in shuttle warehouses unless it can be carried out at short notice - qualified personnel and replacement equipment must be available on site. In addition, shuttle warehouses are more time-consuming and labor-intensive during the ramp-up phase.

Conclusion

AKL and shuttle storage systems are not competitors; their use depends on the task at hand. The decision is always made in favor of the shuttle warehouse if the miniload capacity would not be sufficient or if significant development prospects are required in terms of performance. High stock turnover and reliability also suggest the use of a shuttle warehouse. However, the selection of the ideal system alone does not guarantee efficient processing. Fully automated storage must be effectively linked with the other logistics components.

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Author: Christian Berndt, Senior Consultant at Pierau Unternehmensberatung GmbH