Material Handling Equipment

IV. Storage Equipment

Storage equipment used for holding or buffering materials over a period of time. The major types of storage equipment are:

  1. Block stacking (no equipment)
  2. Selective pallet rack
  3. Drive-through rack
  4. Drive-in rack
  5. Flow-through rack
  6. Push-back rack
  7. Sliding rack
  8. Cantilever rack
  9. Stacking frame
  1. Shelves/bins/drawers
  2. Storage carousel
  3. Automatic storage/retrieval systems (AS/RS)
    1. Unit load AS/RS
    2. Miniload AS/RS
    3. Man-on-board AS/RS
    4. Deep-lane AS/RS
  4. Split case order picking system
  5. Mezzanine

The most common reason for storing a product allows the other elements of production to operate more efficiently on a per-unit basis because the fixed costs associated with utilizing the element can be spread over more products; e.g., storing up to a truckload of product in a facility reduces the per-unit costs of shipping; and buffering or storage of WIP enables batch production which reduces the per-unit setup costs.

Other potential reasons for storage include: time bridging—allows product to be available when it is needed (e.g., storing spare machine parts at the facility); processing—for some products (e.g., wine), storage can be considered as a processing operation because the product undergoes a required change during storage; and securing—e.g., nuclear waste storage.

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1. Block Stacking (No Equipment)

Bulk storage using block stacking can result in the minimum cost of storage since cube utilization is high and no storage medium is required, but material accessibility is low since only the top of the front stack is accessible and loads at bottom of a stack must not require support

Storage racks are used when support and/or material accessibility is required

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2. Selective Pallet Rack

Most popular type of storage rack

Pallets are supported between load-supporting beams

Special attachments and decking can be used to make the racks capable of supporting other types of unit loads besides pallets (e.g., coils, drums, skids)

Selective racks can be used for the following types of storage:

Standard—single-deep storage using a counterbalanced lift truck

Narrow-Aisle—storage using a narrow-aisle lift truck

Deep-Reach—greater than single-deep storage (typically double-deep storage)

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3. Drive-Through Rack

Loads are supported by rails attached to the upright beams

Lift trucks are driven between the uprights beams

Requires similar-width loads

Open at both ends, allowing access from both ends (FIFO)

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4. Drive-In Rack

Same as drive-through rack, except closed at one end, allowing entry from only one end (LIFO)

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5. Flow-Through Rack

Loads are supported on an incline to enable gravity-based movement of the loads within the rack (via, e.g., a gravity roller conveyor)

Loaded at the higher end and unloaded at the lower end (FIFO)

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6. Push-Back Rack

Same as push-back rack, except loaded and unloaded at the lower end and closed at the higher end (LIFO)

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7. Sliding Rack

Only one mobile aisle is used to access several rows of racks

Location of the aisle is changed by sliding the rows of racks along guide rails in the floor

Typically found in library stacks

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8. Cantilever Rack

Loads are supported by cantilever "arms"

Used to store long loads (e.g., bar stock, pipes, lumber)

Similar to pallet racks, except the front upright beams and the front supporting beams are eliminated

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9. Stacking Frame

Interlocking units that enable stacking of a load so that crushing does not occur

Can be disassembled and stored compactly when not in used

Pallet frames can be used to enable multilevel block stacking

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10. Shelves/Bins/Drawers

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11. Storage Carousel

Carousel consists of a set of vertically or horizontally revolving storage baskets or bins

Materials (and the storage medium) move to the operator, "part-to-man," for end-of-aisle picking

Each level of the carousel can rotate independently in a clockwise or counter-clockwise direction

Control ranges from manually activated push buttons to automated computer controlled systems

Provides an alternative to typical "man-to-part" AS/RS, where the S/R machine moves to the part

Similar to a trolley conveyor with storage baskets

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12. Automatic Storage/Retrieval Systems (AS/RS)

Consists of an integrated computer-controlled system that combines the storage medium, transport mechanism, and controls with various levels of automation for fast and accurate random storage of products and materials

Storage/retrieval (S/R) machine in an AS/RS operates in narrow aisle, serving rack slots on both sides of aisle; can travel in horizontal (along the aisle) and vertical (up and down a rack) directions at same time

Advantages: fewer material handlers, better material control (including security), and more efficient use of storage space

Disadvantages: high capital and maintenance costs, and difficult to modify

12(a) Unit Load AS/RS

Used to store/retrieve loads that are palletized or unitized and weigh over 500 lbs.

Stacking heights up to 130 ft. high, with most ranging from 60 to 85 ft. high; 5 to 6 ft. wide aisles; single- or double-deep storage racks

12(b) Miniload AS/RS

Used to store/retrieve small parts and tools that can be stored in a storage bin or drawer

End-of-aisle order picking and replenishment

Stacking heights range from 12 to 20 ft.; bin capacities range from 200 to 750 lbs.

Termed a "microload AS/RS" when used in assembly, kitting, and testing operations to deliver small containers of parts to individual workstations, where workstations are typically located on the sides of a pair of racks and the S/R machine operates between the racks to move containers to openings in the racks (storage lanes) located next to each station

12(c) Man-On-Board AS/RS

Used for in-aisle picking; operator picks from shelves, bins, or drawers within the storage structure

Manual or automatic control

S/R machine is similar to an order picker or turret truck and can sometimes operate as an industrial truck when outside an aisle, except the S/R is guided along a rail when operating in an aisle

12(d) Deep-Lane AS/RS

Similar to unit load AS/RS, except loads can be stored to greater depths in the storage rack

A rack-entry vehicle is used to carry loads into the racks from the S/R machine, and is controlled by the S/R machine

Termed an "automated item retrieval system" when used to automatically retrieve individual items or cases, with replenishment (storage) taking place manually from the rear of a flow-through storage lane and items are pushed forward with a rear-mounted pusher bar for automatic picking from the front of the storage lane

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13. Split-Case Order Picking System

Unlike an AS/RS, a split-case order picking system enables fully automated picking of individual items

Two general categories of split-case order picking system are robotic based systems and magazine/dispenser based systems

Robotic based systems are similar in construction to robotic pick and place palletizers

Magazine/dispenser based systems are similar to vending machines, but larger in scale

"A-Frame" dispenser system (pictured) is popular within pharmaceutical distribution centers; items are dispensed onto a belt conveyor that carries them into a container

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14. Mezzanine

Inexpensive means of providing additional storage or office space

Makes use of clear space over activities not requiring much headroom (e.g., restrooms, block storage, etc.)

At least 14 ft. of clear space is needed for a mezzanine

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Last modified: September 30, 1999