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Shuttles or Stacker Cranes? "What counts is system performance"

Photo: Witron

Photo: Witron

Photo: Witron

Photo: Witron

The entire intra-logistics industry is currently talking about shuttles. But in the course of those discussions, economic issues and decisive questions seem to be ignored. WITRON founder, Walter Winkler, and Martin Stich, head of logistics design, explain the benefits of stacker cranes from their point of view and why WITRON has not opted to use shuttles instead.

Mr. Winkler, about five years ago, WITRON decided not to invest in shuttle solutions.  What were the main reasons for this decision?

Walter Winkler: We design and build automated logistics systems that handle a large volume of articles and that need to achieve a high level of performance. Based on the low system output of shuttles, such a performance level cannot be achieved as cost efficiently as required. Instead, we use proven stacker cranes with high availability and performance for the inbound and outbound of totes. We have very good experience with that approach. Because one thing we noticed again and again during the past years: Our customers, especially those in the retail business, know very well that it is system performance that counts. The fact that WITRON has increased its employee base by 60% over the past four years in the midst of economic recession is evidence that we have opted for the right technology.

And where specifically do you see concrete disadvantages of the shuttles?

Winkler: The summarized performance of the stacker cranes used in our systems generally corresponds to the entire system output. This is ensured by sophisticated software strategies and structured material flow systems with efficient elements. Compared to the aisle-bound stacker cranes in the automated small parts warehouse (AS/RS), shuttle systems, lose a great deal of system performance. The reason for this is a much smaller accessibility to storage locations or articles per shuttle element and the associated need for an extremely complex sequencing. A large number of articles in combination with high performance cannot be realized as cost-efficiently as possible with shuttles based on their poor system performance - this is the main problem.  

Where do you see concrete benefits of stacker cranes?

Winkler:  As already mentioned, stacker cranes achieve a very high system performance of more than 93 percent. With our Picking Mini Loads (PML), we are able to handle 300 inbounds and 300 outbounds per hour per device without any problems. The double-deep storage which has become a WITRON standard provides a significant increase in performance. Just imagine that you need at least 14 shuttles on 14 levels and two lifting devices to achieve the performance of a single PML.  
This is due to the fact that conventional/classic shuttles - also called captive systems - have a very limited, strictly aisle-bound accessibility; it means that shuttles have access to a limited number of articles only. As a consequence, and due to the higher complexity, there are considerable performance losses.  

Which performance losses do you mean?

Winkler: These performance losses are caused by the high number of interfaces between the lifting device, the shuttle, and the conveyor system, and the synchronization between them all. If you compare the shuttle solution as a whole system, you will determine that in total there are considerably more drives, sensors and interfaces. Due to the high number of elements, and due to the multitude of transfer points of the totes or trays, the shuttle concept is much more complex in my opinion - which inevitably leads to more problems.

Besides the one-level-shuttles, there are also systems with shuttles that change levels. What do you think about this solution?

Winkler: These so-called roaming systems with shuttles that change the levels via vertical conveyors to have access to more storage locations and articles are not an option for me. The reduction of performance due to the need to change levels is a big disadvantage. This level-changing requirement seems to contradict with the primary intent of providing a higher performance with shuttles.
In my opinion, neither single-level nor multi-level shuttles will be able to cover the full range of market requirements. In this context, the current trend is quite interesting for me – shuttle system providers are offering shuttles with access to as many as six storage levels per shuttle.  In those instances, I ask myself, what is the difference to a stacker crane?

Do your customers ask directly for shuttle solutions?

Martin Stich: As already mentioned, our customers - first and foremost - look for the performance of the entire system and for a state-of-the-art logistics philosophy. The components used to realize their project is not decisive for our customers. In the past, we sometimes competed directly against shuttles. So far, we have not lost any order to a shuttle solution.  
There is one thing that we notice again and again: Some people seem fascinated by shuttles based on their portrayal as modern technology described by sweeping statements like "light", "energy-saving", or "battery-powered". But what really counts for us and our customers are real use factors like better store service, increase of competitiveness, cost-efficiency of the entire system in terms of costs per pick,  as well as aspects like system output and availability.  

Where can shuttles still be used?

Winkler: I definitely see potential for customer requirements with a low number of articles that are to be stored and shipped with middle- to high-performance, and for systems with limited payload of 30 or 50 kilograms. Shuttle solutions are strongly tailored to their respective provider. If one has gained good experiences with the respective system supplier, and if the mentioned restrictions are not exclusion criteria, the use of shuttles may of course make sense.   
The question of using a "shuttle" or a "stacker crane" is all about one component in an overall logistics system for inbound and outbound - and both solutions can be justifiable. In such cases, it could make sense to carry out an economic comparison with shuttles.

An aspect which is always highlighted as a great benefit of the shuttle technology is the allegedly higher energy efficiency of the systems. Can you confirm this fact based on your experiences in practical use?

Stich: No. A serious comparison of the energy costs can only be carried out on a system level basis, and stacker cranes are not necessarily worse than shuttles. And the impact of any possible energy-savings for the individual shuttles would be minimal in comparison to the energy costs for the entire system, according to our opinion.

How do you see the future of shuttles and stacker cranes?

Winkler: I think the media is creating too much hype around shuttles. Shuttles or stacker cranes are only used for a certain purpose - to store and dispatch totes or trays. Decisive are the picking systems themselves, their performance, availability, and cost-efficiency. There will be new systems and also new discussions. WITRON is permanently working on innovations and we will continue to release new technologies and components for the market in the future.
During my 40 years of experience in the field of conveyor systems, and especially in the design of complex logistics systems, I have experienced many innovations that meanwhile are only used where they achieve real benefits.

Between theory and practic

With respect to the shuttle technology, WITRON has verified the current state of technology in the course of a diploma thesis, supported by the Technical University of Dresden. Prof. Dr. Thorsten Schmidt, Head of Technical Logistics and Institute Director at the Technical University of Dresden, sets out his opinion regarding the advantages and disadvantages of shuttles compared to stacker cranes.  

"For me, the current market situation was clearly revealed in the course of a round-table session at the "Muenchner Materialflusskongress" (Munich Material Flow Congress) earlier this year.  Intensive discussions were held about the development of stacker cranes and shuttle systems.  Upon agreement of all discussion members - all system providers of stacker cranes and/or shuttle systems - there is the need for both; and not all theoretical benefits of the shuttle systems can be fully attained in practical use.  
According to statements from the experts, the potential performance per storage aisle was the fundamental characteristic of the shuttle. This performance depends not only on the number of used shuttles, but also on the type, number, and position of the vertical conveyors per storage aisle that considerably determine the achievable throughput. Here, there is a particularity: the diversity of the commercially available solutions makes the determination of the system throughput essentially incalculable.

Shuttle systems require sequence buffers

An Interesting issue in the course of the round-table session was the evaluation of how sequence requirements can be handled. The unanimous opinion was that sequencing processes can only be achieved through an increased number of shuttles in the system - experiences show a considerable higher demand of shuttles - which is inefficient in most instances. Due to this reason, shuttle systems require additional sequence buffers that need to be installed in front of the pick stations.
Parallelism is an essential prerequisite for achieving high availability. In principle, shuttle systems that use many shuttles and at least two lifting devices in one aisle should offer significant benefits. Considering availability values for today’s stacker cranes, this benefit will lead to academic discussions. It is difficult to compete against the stacker crane’s average one-year maintenance interval.
Shuttle technology reaches its limits when it comes to higher unit loads and to a high number of articles - in the latter case especially due to cost-efficiency reasons".