Stacker Cranes from A-Z: Limitations and Remaining Relevance

In the dynamic realm of warehousing, technology is ever-evolving to meet the sector's shifting needs. Stacker cranes once stood tall as pioneers, automating vast warehouses and transforming logistical challenges into streamlined processes. However, as the industry progresses, they are being eclipsed by a newer generation of innovations. This article explores the rise and evolution of stacker cranes, their inherent limitations, where they are still applicable, and how newer emerging technologies are reshaping the warehousing landscape.

What is a stacker crane? A retrospective view

Deep within the beginnings of automated warehousing, the stacker crane emerges as an example of early automation. Functioning primarily within automated storage and retrieval systems (AS/RS), these computer-driven giants were, for their time, efficient at navigating the expansive domains of warehouses, efficiently solving storage and retrieval conundrums.

In practice, a stacker crane is a machine designed for storing and retrieving goods in large warehouses or distribution centers. Imagine a tall, vertical structure that moves horizontally on rails and vertically along its mast. Attached to this mast is a carriage that slides up and down, equipped with mechanisms, often forks, to pick up or set down pallets or containers. The entire crane fits neatly within narrowly spaced storage aisles, allowing it to reach any shelf or storage point in its operational area. Think of it as a large robotic arm that can maneuver in both horizontal and vertical directions to access items throughout a storage facility.

Example of how a stacker crane operates

Consider a large-scale e-commerce warehouse. It's the holiday season, and orders are pouring in by the minute. Here's a step-by-step scenario showcasing the role of the stacker crane:

1. Order Initiation: A customer places an order online for a toy, a book, and a pair of shoes. The order is processed in the warehouse's inventory management system.

2. Task Allocation: The warehouse's computer system receives the order and determines where each item is located. It sends instructions to the stacker crane to retrieve the items.

3. Navigation: Leveraging its in-built navigation system, the stacker crane swiftly moves along the designated path in the warehouse. Unlike human-driven forklifts, it can quickly calculate the most efficient route.

4. Height and Depth Adaptability: The toy is located on a high shelf, the book is midway, and the shoes are in a bin on the ground level. The stacker crane's telescopic mechanism allows it to adjust height with precision. It first extends upwards to grasp the toy, descends slightly to grab the book, and then goes further down to collect the shoes.

5. Item Retrieval: Employing its mechanical arms or magnetic systems, the stacker crane retrieves each item and places it in a designated tray or container attached to it.

6. Delivery to Packing Station: Once all items are collected, the crane transports them to a packing station. An operator or another automated system then takes over, packing the items for shipment.

7. Restocking: Once a shelf's inventory dips below a certain point, the warehouse management system can also instruct the stacker crane to restock items. The crane picks up goods from a receiving area and places them in the designated storage spots.

This might sound like a neat and efficient process. However, as is the case with all pioneering technologies, stacker cranes, despite their innovations, faced challenges as the industry evolved.

From pioneering technology to evolutionary shifts

In warehouses back in the days, stacker cranes operated with remarkable precision compared to other systems, thanks to advanced computing systems guiding their movements. Gliding effortlessly along tracks, their dual-axis movement—vertical and horizontal—enabled them to access items regardless of their height. This meant the height of the warehouse was fully utilized, leading to improved storage densities.

The introduction of these cranes brought about a slew of benefits. Warehouses could now store more in less space, operations were expedited, and there was a significant reduction in errors commonly attributed to human oversight.

But, like all technologies, stacker cranes were not without flaws. Their reliance on intricate software made them vulnerable to system glitches, potentially stalling operations. Their track-bound movements sometimes led to logistical challenges in busier settings. Additionally, the hefty investment required for installation deterred smaller operations from adopting them.

These limitations suggested that while stacker cranes used to be a leap forward in warehousing, the industry was still on the brink of further evolutionary shifts to meet the limitations of stacker cranes.

Evolution and Ongoing Limitations of Stacker Cranes

Over the years, stacker cranes have undergone significant evolution to overcome their initial limitations and enhance their efficiency in warehousing and logistics operations. However, these advancements have been accompanied by persisting challenges, which have, in turn, driven the development of newer technologies.

Evolution of Stacker Cranes

1. Multiple Cranes in One Aisle

Initially, stacker cranes were limited to operating one at a time within a single aisle. This constraint resulted in slower throughput and reduced the overall efficiency of the storage system. To address this challenge, modern stacker crane systems have evolved to allow for the deployment of multiple cranes within the same aisle. This technological advancement, often referred to as "twin" or "multishuttle" technology, enables several cranes to work concurrently in the same aisle.

2. Increased Storage Depth

Older stacker crane designs faced limitations in terms of storage depth, restricting access to only a limited number of storage positions within a single aisle. In response, modern stacker cranes have been developed with enhanced reach capabilities, facilitating access to deeper storage locations.

3. Lower Approach Dimensions

Historically, stacker cranes required relatively wide aisle dimensions to operate effectively, which reduced the overall storage density within a facility. Over time, innovations in stacker crane design have allowed for narrower aisle widths, known as "very narrow aisle" (VNA) configurations. These narrower aisles, often facilitated by guidance systems such as wire guidance or laser navigation, enable warehouses to make more efficient use of available space. However, even with this improvement, they still require four times more space compared to cube storage automated storage and retrieval.

4. Higher Construction Height

Early stacker crane designs were limited in how high they could stack goods within a storage system, constraining the vertical utilization of warehouse space. To overcome this issue, modern stacker cranes have been engineered to reach greater heights, accommodating taller storage configurations. This increased construction height is pivotal for warehouses seeking to optimize vertical space utilization while maintaining accessibility to stored items.

Ongoing Limitations and the Emergence of New Technologies

While stacker cranes have evolved significantly to address their limitations, several challenges persist:

• High Capital Costs: The substantial upfront investment required for stacker crane installation continues to limit their adoption compared to other systems, particularly among smaller businesses with budget constraints.
• Space Limitation: Stacker cranes, due to their design and aisle requirements, still require wide aisle dimensions, which can reduce overall storage density within a warehouse. This limits their ability to fully utilize available space, making them less efficient. This space limitation can be a significant drawback, particularly in facilities with limited square footage seeking to optimize storage efficiency.
• Maintenance Complexity: Stacker cranes, due to their complexity, demand meticulous maintenance. Even infrequent breakdowns or malfunctions can disrupt operations, leading to potential delays and increased operational costs.
• Single Point of Failure: Stacker cranes remain susceptible to single points of failure. If a single crane experiences a breakdown, it can render the entire aisle's inventory inaccessible, disrupting order fulfillment and halting warehouse operations.

These persisting limitations have driven the warehousing industry to explore and develop alternative solutions that offer greater flexibility, higher efficiency, and improved cost-effectiveness. Newer technologies have emerged to address these challenges more comprehensively, ultimately taking over and reshaping the landscape of warehouse automation. While stacker cranes continue to play a role in certain contexts, the evolution of warehousing technology has been driven by the need to overcome these persistent limitations.

Modern technologies replacing stacker cranes

The recognized limitations of stacker cranes prompted the industry to pursue innovative solutions aligned with the multifaceted demands of contemporary warehousing.

Cube Storage AS/RS

The next generation of advanced warehouse technology is cube storage AS/RS (automated storage and retrieval system) pioneered by AutoStore. This has been another groundbreaking advancement in the history of warehouse automation technology. Due to the cube design, it delivers the most space-efficient solution in the market, which is cost-saving on many levels. Owing to the speed at which the robots can retrieve inventory Bins and deliver them to a workstation operated by a human, it is currently the fastest AS/RS per square foot in the market. This system also has a global uptime of 99,7%, which is a statistic that is extremely hard to beat for stacker cranes and other systems.

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The cube storage AS/RS system also offers increased flexibility compared to traditional systems such as stacker cranes. Although referred to as a cube design, it doesn’t have to look like a 1x1 rubric cube. In fact, the storage Grid containing the inventory Bins, can easily fit around pillars, oddly shaped corners, and even on different floor levels. As long as the Robots have a path they can drive on top, and as long as the floor is flat, there is almost no limit as to how the AutoStore can fill every corner of the warehousing space. This is a huge advantage compared to traditional stacker cranes who need tall warehouses and at least 4x more space (thereby real estate costs) between shelves to allow the cranes to pass.

Moreover, since the products or SKUs are placed in several inventory Bins, even if a Robot was to break down, the product can simply accessed by another robot from another inventory Bin who would then take over. This means that there are no costly disruptions to operations and order fulfillment as could be found with stacker cranes and more traditional technologies.

To sum it up, it's evident that the traditional stacker cranes are not so suitable for supporting modern online order fulfillment, as that requires significantly higher speed, flexibility and system uptime.

Stacker cranes: Where they (still) shine

1. Heavy Load Mastery

One of the undebatable strengths of stacker cranes is their proficiency in handling weighty items. Their robust build allows them to lift and transport loads that many contemporary technologies might struggle with.

2. When vertical real estate is more important than speed

If leveraging a tall vertical space is more important than speed, especially if the goods are large and heavy, stacker cranes are still suitable.

Who Stands to Benefit?

• Steel and Metal Manufacturing: In a sector that regularly grapples with hefty metal bars, plates, and coils, stacker cranes can be invaluable.
• Heavy Machinery Production: For businesses that deal with sizable components or entire machinery units, these cranes offer a reliable handling solution.
• Timber and Lumber Industries: Managing and transporting bulky logs or stacks of wood is a breeze with stacker cranes.
• Automotive Sector: Especially in storage areas or assembly lines where large automotive parts need to be moved or stored efficiently.
• Dense Warehousing Operations: Facilities that emphasize storage density, or those with pre-existing crane infrastructure, can particularly benefit.

In conclusion, while newer technologies are taking over with their innovative features, stacker cranes still carve out a niche for themselves in the modern industrial landscape. Their capability to handle heavy loads, makes them a viable choice for specific sectors within the industrial vertical. Businesses dealing with bulky items or keen on optimizing vertical storage might find stacker cranes not just relevant, but indispensable.

Conclusion

Warehousing has historically been a domain characterized by constant progress and adaptation. In earlier phases, the introduction of stacker cranes marked a significant milestone, pioneering the transition to automation and establishing foundational benchmarks for operational efficiency within the industry.

However, as global commerce evolves and supply chain complexities grow, the warehousing sector has been compelled to innovate further. The industry's dedication to optimization and efficiency has led to the development of a new suite of technologies. Building upon the lessons and insights gleaned from earlier systems like stacker cranes, these modern tools are intricately designed to cater to the multifaceted and ever-shifting needs of the global supply chain.

From software solutions offering real-time analytics to hardware systems enabling faster and more accurate product retrieval, these technologies signify the industry's commitment to staying ahead of logistical challenges. As global demands continue to evolve, the warehousing sector is well-positioned to adapt, leveraging these advancements to ensure a responsive and efficient infrastructure.

FAQ

What is a stacker crane?

A stacker crane is an automated machine used for material handling in warehouses, primarily for the storage and retrieval of goods in racking systems.

What are the advantages of stacker cranes?

Stacker cranes offer increased vertical storage efficiency, can lift heavy products, enhance safety compared to a manual warehouse, and minimize damage to goods.

What are the different types of stacker cranes?

Stacker cranes can be categorized based on their structure and functionality, including single-mast, double-mast, telescopic fork, and rotating fork variants.

What is a stacker crane in ASRS?

In Automated Storage and Retrieval Systems (ASRS), a stacker crane is a component that automatically places and retrieves loads from specific storage locations, optimizing warehouse operations and vertical space utilization.