Choosing the right locator height is an important consideration when designing and building automated storage and retrieval systems (AS/RS). The locator height has implications for storage density, ergonomics, capacity, and throughput. This article will examine the key factors to consider when selecting locator heights and provide guidance on best practices.
What is locator height?
Locator height refers to the vertical distance between each level of storage in an AS/RS system. The locators are the locations within the racks where inventory is stored. The height between each locator level is optimized based on the type of system, inventory being stored, and requirements for efficiency and ergonomics.
Maximum Utilization of Vertical Space
A key driver in determining locator height is maximizing storage density within the height constraints of the warehouse. By optimizing locator height, more levels of racking can be installed which allows more inventory storage within the same footprint. Some key factors to consider for storage density include:
- Height of inventory – The locator height should be just taller than the tallest item to be stored to maximize vertical space utilization. Allowance should be made for some clearance between levels.
- Building constraints – Locator heights are physically constrained by building ceiling height, sprinklers, lighting, and other obstructions.
- Number of pallet loads – The target pallet positions per rack level will influence the locator height.
- Pallet load heights – Standard pallet load heights are around 4 feet but can vary by industry and inventory type.
In general, locator heights can range dramatically from around 25 inches for picking systems up to 12 feet or more for very dense pallet storage warehousing. Most standard AS/RS solutions target locator heights between 4 to 8 feet.
The vertical distance for item picking and inventory management should allow for ergonomic working heights. A few key considerations for ergonomics include:
- Worker heights – Accommodating 5th percentile females to 95th percentile males (roughly 4’10” to 6’2″).
- Reaching and bending – Levels should minimize extended reaching or excessive bending.
- Openness – There should not be a closed-in feeling if workers are in picking aisles.
- Pallet access – If pallets are placed and removed manually, the height should accommodate safe pallet handling.
Ergonomic picker reach studies often recommend locator heights between 24″ to 72″ for case picking and 48″ to 96″ for full pallet loads. Other considerations like conveyors and light wells may also impact these ranges.
Locator height also impacts the throughput capacity of AS/RS systems. Some considerations for optimizing throughput include:
- Rack columns – More rack columns and levels accommodate more inventory for higher throughput.
- Picking time – Picker reaching time will increase at higher levels as will lift and lower time for robotic order picking.
- Lift speed – Crane lift speeds may need to increase for facilities with taller racks to maintain throughput.
- Acceleration – Slowing down and speeding up at each level consumes time, favoring fewer locator levels.
In very high throughput facilities with heavily automated picking, lower locator heights around 25-40 inches are common. Manual picking systems need to balance ergonomics and throughput which tends to favor heights in the 5-7 foot range depending on order profiles.
The capabilities of the storage and retrieval equipment can also impact locator height selection. A few key considerations include:
- Crane height – The crane or mast needs to extend higher than the topmost rack level.
- Crane speed – Faster cranes accommodate more locator levels without impacting throughput.
- Interleaving – Narrower aisle for interleaved racking limits level heights.
- Robot reach – Articulated arm robots require lower locator heights for picking.
In general, newer equipment with higher mast heights, faster lift speeds, and smarter warehousing software can support efficient operations with increased locator heights compared to older generations of AS/RS solutions.
Some other factors to weigh when selecting locator heights include:
- Future flexibility – Higher locator heights build in flexibility for future changes in inventory sizes.
- Fire safety – Local fire codes may restrict storage above a certain height without fire suppression.
- Lighting – More vertical space may require higher wattage warehouse lighting.
- Maintenance access – Service technicians will need safe access to all locator levels.
Size Class Method
A structured methodology called the Size Class Method helps optimize locator height based on the spectrum of inventory sizes stored. The basic steps include:
- Categorize all inventory into different size classes based on height.
- Determine the percentage of total inventory for each size class.
- Establish preferred locator heights and allowable height ranges for each size class.
- Calculate the weighted average optimal locator height based on the mix percentages and target heights per size class.
This ensures the locator height works well for the majority of inventory while accommodating exceptions. Large distribution centers may define a dozen or more size classes for very fine tuning.
Locator Height Examples
Here are a few examples of common locator height configurations:
|Case picking AS/RS
|25 to 40 inches
|Lower height for ergonomic picking of individual cases. Very high throughput.
|Full pallet AS/RS
|72 to 120 inches
|Heights optimized for pallet loads with lift trucks. High density.
|Split-case picker module
|48 to 60 inches
|Compromise height to support case and pallet picking in same system.
|Cold storage warehouse
|132 inches (11 feet)
|Very tall racks to maximize density in limited footprint cold storage.
Locator Height Best Practices
Some best practices for determining optimal locator heights include:
- Analyze actual inventory size data using the size class method or 80/20 rule to find heights for the majority of SKUs.
- Involve industrial engineers and ergonomic experts early in the layout design process.
- Model different heights using CAD and digital twin simulations to quantify throughput tradeoffs.
- Validate proposed heights and configurations through pilot testing before full implementation.
- Document racking design standards so locator rationale is captured for future reference.
- Design in flexibility to adjust 5-10% up or down from nominal locator heights where feasible.
The ideal locator height balances storage density, ergonomics for pickers, throughput capacity, equipment capabilities, and practical considerations like cost. While no single locator height is perfect for every distribution center, following structured design methodologies like the size class approach and cross-functional collaboration leads to heights optimized for each facility’s unique inventory mix and operating requirements.
Careful attention during the early phases of AS/RS design to selecting and validating proposed locator heights through simulations and piloting helps ensure the system operates efficiently across dimensions of space, labor, throughput, and flexibility relative to evolving business needs.