An Unbiased Comparison of Captive-Aisle & Aisle-

Changing AS/RS Technologies

Solution Profile

DAMBACH Lagersysteme manufactures Automatic Storage / Retrieval Machines of all shapes, sizes, types, and performance – including “aisle-changing” and “aisle-bound”.

Customer requirements will dictate the best technology and material flow for the application.

Aisle-changing Automated Storage and Retrieval Systems (AS/RS) involve the utilization of fewer Storage and Retrieval Machines (SRMs) than there are aisles of storage.

The following factors are considered when planning any AS/RS, including both aisle-bound and aisle-changing:

1. Throughput2. Inventory management3. Asset utilization4. Reactivity5. Investment6. System sizing7. Technical complexity8. Vulnerability9. Storage capacity

1. Throughput

Throughput is a function of system layout and machine performance. Aisle-changing systems (when changing at the rear) work best when machines can perform multiple transactions before changing aisles (i.e. “wave based” systems). Aisle-changing machines have equal performance to aisle-bound machines in terms of speed, acceleration, and positioning.

Example calculation (loads in/out per machine):

System 80ft tall x 300ft long. Pick-up / drop-off at front-end of system. Aisle change

at rear (1) machine, (2) aisles. Single-deep. Both technologies capable of multiple-

deep storage. For the aisle change machine it changes aisles every 10

double cycles. If aisle-bound then 34.7 combined cycles / hour /

machine. If aisle-changing then 32.7 combined cycles / hour /

machine. This means the aisle-changing machine sacrifices 5.7%

in performance to change aisles.

Aisle-bound systems generally tune machine performance to minimize the number of aisles, with limited ability increase performance in the future without expanding the complete system.

Aisle-changing systems performance can be increased without changing the complete system (adding rack and building) by simply adding additional machines.

2. Inventory Management

Incoming loads are balanced across all aisles in both aisle-changing and aisle-bound systems.

Warehouse control software will dictate frequency of aisle-change to optimize material flow, shipping / receiving requirements, and equipment utilization.

Contrary to aisle-bound systems, any aisle-changing machine can access any aisle. This means that even if a machine is out of service, 100% of storage locations are accessible by the remaining machines.

3. Asset Utilization

Aisle-changing systems can be single-ended or double-ended. It depends on the application.

In many scenarios, the aisle-changing occurs at the front of the system. This has the advantage that only (1) set of input / output points is required for each machine (as opposed to each aisle, for an aisle-bound system).

For example, in a system with (2) machines in (6) aisles – the aisle-changing system can be designed with (2) pairs of inputs/outputs as opposed to (6) pairs. Of course, there are many other design possibilities. It depends on the application.

As with aisle-bound systems, aisle-changing systems usually “velocity zone” the rack.

4. Reactivity

Reactivity of the aisle-changing system depends on system design. Particularly, whether the system uses front or rear aisle-changing.

Using the previous application (80ft tall x 300ft long). In the aisle-bound system, the “average” single-cycle (hot pick) is 62.3 seconds. In an aisle-changing system with rear-aisle change, where the machine is in the “wrong aisle” a single-cycle will take 126.7 seconds.

On the other hand – the system design should not be configured around a very small percentage of “hot picks”. If there are high-percentages of “hot picks” throughout the day, other factors should be considered in the design of your material handling system.

Periodic “hot picks” will not significantly affect total system performance.

5. Investment

Total cost of ownership (and original capital cost) should be evaluated on a case-by-case basis for each customer. In many cases, choice of technology is “operationally based” or “material flow” based. Capital cost is only one part of the equation.

Each system is unique, but generally speaking an aisle-changing system requires fewer SRMs, thereby minimizing maintenance and spare-parts requirements. Track switches are very reliable and only require 15 minutes of preventative maintenance 3x per year each aisle.

Aisle-changing machines by DAMBACH Lagersysteme do not require batteries to change aisles. The machines are in constant contact with 480VAC buss-bar.

6. System Sizing

Both aisle-changing and aisle-bound systems are sized for the application by adjusting design elements such as: machine performance, rack length and height.

Both aisle-changing and aisle-bound machines are capable of the same machine performance (speed / acceleration) in all axis’. Both technologies are capable of single, double, triple-deep and deep-lane storage.

Both technologies allow system heights in excess of 120ft. In fact, DAMBACH Lagersysteme built an aisle-changing machine in 1982 (27 years ago!) that was 117ft tall. We’ve been building aisle-changing machines that height and taller ever since.

7. Technical Complexity

Aisle-changing machines have been used in the industry for over 30 years. They utilize time-proven designs, are reliable and easy to maintain.

DAMBACH aisle-changing machines do not use batteries; no controls or electronics on upper guide rail (all “intelligence” at floor level); “firmware” changes are automatic across all machines simultaneously through the “Master Controller” in the event of changes to the rack or system layout.

Both aisle-changing and aisle-bound machines require similar preventative maintenance and spare parts inventories. Maintenance hours and cost depends on number of machines and aisles.

8. Vulnerability

Both aisle-changing and aisle-bound SRMs by DAMBACH Lagersysteme provide an availability rating of 99%. However, with aisle-changing machines access to inventory is virtually 100%.

In an aisle-bound system, if the machine is out of service – the inventory in that aisle is completely unavailable. With an aisle-changing machine, the out of service machine can be moved to a maintenance area and the remaining machines can service all aisles.

9. Storage Capacity / Building Size

Aisle-changing machines require very compact run-out dimensions. For example:

Aisle-bound machine with GMA pallet, 80ft tall: Front runout: 106” Rear runout: 204” Total runout (front + rear): 310”

Aisle-changing machine with GMA pallet, 80ft tall: Front runout: 106” Rear aisle change (cross-aisle): 193” Total runout (front + rear aisle change): 299”

The required building footprint with both technologies is very similar. All else is equal in terms of clearances, storage depths, heights, etc.

Example System Designs

Aisle Change at Back

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Aisle Change at Front

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Aisle-change at back increases throughput when machines stay in aisle for multiple cycles.

Aisle changing at front minimizes front end conveyor complexity.

Aisle changing at front allows additional storage for “staged” loads and “A” movers.

For more information regarding DAMBACH Lagersysteme, call (616) 334-9969 or visit www.dambach-usa.com.