Storage and Retrieval Methods and Equipment

Storage and retrieval equipment is distinguished from material transport equipment by its primary function, which is :

 To house material for staging

 To house material for building inventory

 To retrieve material for use

In some cases, the material retrieval system can be a part of the material transport system. In other cases a new equipment type may be introduced.

Unit Load Storage and Retrieval:

Unit load storage and retrieval systems are subdivided into:

 Unit load storage systems which house the unit loads

 Unit load retrieval systems which allow access to unit loads for retrieval.

Unit Load Storage Systems:

 Block Stacking

 Stacking Frame

 Single-Deep Selective Rack

 Double-Deep Rack

 Drive-In Rack

 Drive-Thru Rack

 Pallet Flow Rack

 Push-back Rack

 Mobile Rack

 Cantilever Rack

Block Stacking:

Unit loads are stacked on top of each other. It is a very efficient method when multiple pallets are received or withdrawn at one time. (Figure 1 and 1-a)

Disadvantages:

Advantages:

Figure 1. Pallets and unit load position in block stacking

Figure 1-a. In block stacking, lane depth determines the effect of honeycombing.

 

Stacking Frame:

A stacking frame is portable and enables the user to stack materials on top of each other. These frames are used with unit loads that are not conveniently stackable. Honeycombing can be an issue, so the lane depth must be determined carefully to avoid honeycombing. Portable racks can either be frames that are attached to wooden pallets or self contained steel units made up of decks and posts. When not in use, the racks can be disassembled and stored in a minimum space. (Figure 2)

Figure 2. Stacking frames and portable racks.

Figure 3. Single-deep selective rack.

 

Single-Deep Selective Rack:

A single-deep rack (Figure 3) is a construction of metal uprights and cross-members and provides immediate access to each load stored.

Advantages:

Disadvantages:

Double-Deep Rack:

A double -deep rack is merely single-deep selective rack that is two unit load positions deep (Figure 4). In most cases more than 50 % aisle space savings can be achieved compared to single-deep rack. Since unit loads are stored two load depth, a double-reach fork lift is required for storage and retrieval.

Advantages:

Figure 4. Double-deep rack.

Figure 5. Drive-In Rack

 

Drive-In Rack:

Drive-in racks provide for storage lanes from 5 to 10 unit loads deep. They reduce the aisle space to a minimum (one aisle), with the most efficient use of floor space. (Figure 5). Drive-in racks allow a lift truck to drive in to the rack several positions and store and retrieve a unit load.

Advantages:

Disadvantages:

Drive-Thru Rack:

A drive-thru rack is merely a drive-in rack that is accessible from both sides of the rack (Figure 6). Drive -thru rack provides one of the most efficient utilization of floor space (two aisles). One aisle is used for storage and the other aisle is used for retrieval of the load. Just -like the drive-in rack, the only disadvantage of drive-thru rack is that the lift truck must travel at a reduced speed for safe navigation through the rack.

Figure 6. Drive -thru rack.

Figure 7. Pallet flow rack.

 

Pallet Flow Rack:

Load is moved on wheels, rollers, or air cushions from one end of a storage lane to the other (Figure 7). As the load is removed from the front of the storage lane, the next load is moved to the front. The purpose of the pallet flow rack is to simultaneously provide high throughput and storage density, which are required for the items with high turnover.

Push-Back Rack:

Push-back rack provides last-in-first-out deep lane storage (Figure 8). As the load is placed into the storage, its weight and the force applied by the storage vehicle (forklift, etc) pushes the other loads in the lane to create room for the additional load. As a load is removed from the front of a storage lane, the weight of the remaining load automatically advances remaining loads to the front of the rack.

Figure 8. Push-back rack

Figure 9. Mobile rack

 

Mobile Rack:

A mobile-rack is a single-deep selective rack on wheels or tracks(Figure 9). This design permits an entire row of racks to move away from adjacent rows for retrieval or storage purposes. The rest of the time, single-deep racks are pushed together without leaving any aisle space. Access to a particular storage row is achieved by moving the adjacent row and creating a temporary aisle in front of the desired row. Mobile racks are useful when the floor space is limited and the inventory turnover is low.

Cantilever Rack:

The load bearing arms of a cantilever rack are supported at one end. The rack consists of a row of single upright columns with arms extending from one or both sides of the uprights to form supports for storage (Figure 10). The advantage of cantilever racks is that they provide long unobstructed storage shelves with no uprights to restrict the use of horizontal space. These racks are applicable for long items such as pipes, bars, sheet metal, wood, rugs and sofas.

Figure 10. Cantilever rack

Unit Load Retrieval Equipment:

Unit load retrieval equipment types vary by their degree of automation, lift height capacity and aisle width requirements. The popular devices for retrieving unit loads are walkie stackers, riders and counterbalance lift trucks.

Walkie Stackers and Riders:

The walkie stacker extends the lifting capability of the pallet jack to allow unit loads to be stacked or placed in storage racks. The walkie stacker can either have a straddle or reach design (Figures 11 and 12)

Figure 11. Straddle stackers are typically used in narrow-aisle storeroom and warehouse areas to handle uniform load sizes.

Figure 12. Reach stackers

Walkie stackers are divided into low-lift and high-lift types. Low-lift models handle floor level pallets or skids. High-lift equipment has stacking capability and is offered in straddle, reach, and counterbalanced versions.

Low-lift walkies :

Low lift walkies are often called pallet trucks. Forks are inserted between the top and bottom boards of pallets to raise the load a few inches off the ground for transport. These products are not used for stacking. (Fig.13)

Figure 13. Low lift walkie trucks lift the pallet 6 in. off the floor to allow free travel over dock boards, ramps, and obstacles

Low-Lift Riders:

These vehicles use the same lifting mechanism and perform the same handling tasks as the walkie. This type is used when longer travel distances or quicker handling is required. (Fig. 14)

Figure 14. Walkie/rider pallet trucks travel about 8 mph when empty, which is two or three times faster than walking but slower than a traditional lift truck.

High-Lift Walkies:

Straddle stackers (Figure 11) and reach stackers (Figure 12) are examples for high-lift walkies. These equipment have the capability of handling between 4,000 and 8,000 lbs of loads for up to a height of 16 ft. As the height capacity increases, the load capacity goes down .

Counterbalanced Lift Truck:

A variety of the materials handling requirements are handled by counterbalanced lift trucks. Although it is usually referred to as a fork lift, not all designs use forks for lifting. It is common to see design that have platforms or ram attachments. Counterbalanced lift trucks can be either battery powered or powered by an internal combustion engine (gasoline, propane or diesel fuel). The tires can be cushion for indoor use or pneumatic for outdoor use. The load carrying capability of the counterbalance lift trucks may range from 1,000 lbs to over 100,000 lbs. Lift height capacities of 25 to 30 ft can be achieved.

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Last Update: July 26, 1999

Prepared by : Serdar Z. Elgun