Storage Equipment


Storage equipment is used for holding or buffering materials over a period of time. Some storage equipment may include the transport of materials (e.g., the S/R machines of an AS/RS, or storage carousels). If materials are block stacked directly on the floor, then no storage equipment is required. Storage racks are used to provide support to a load and/or to make the load accessible.

1. Block Stacking (no equipment)

  • Block stacking is the storage of loads on top of each other in stacks placed in lanes on the floor (a.k.a. floor storage)
  • Advantage: Easy to implement and very flexible. Low investment cost since no storage medium is required
  • Disadvantage: Honeycomb loss can be significant since, on average, half of one lane will be empty for each item. In most cases, only LIFO retrieval is possible in each lane unless there is an aisle at the back. Damage to loads might cause instable stacks
  • Two to ten rows of storage typically used
  • Width of each lane limited by vehicle width, not load width
  • Storage racks are used when support and/or material accessibility is required

2. Selective Pallet Rack

  • Pallets are supported between load-supporting beams. Most popular type of storage rack. 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)
  • Load-on-beam racks are used to provide clearance for straddles; load-on-floor racks can be used when it is not necessary to use straddles

Single-deep Rack
  • Single position (slot) per position
  • Advantage: Provides complete and fast accessibility to all loads with no honeycomb loss
  • Disadvantage: Can result in low cube utilization because of aisle space requirements, which can be influenced by the lift truck used (e.g., a turret truck would increase and a standard counterbalanced would decrease utilization)
  • Double-deep Rack
  • Two pallets stored per position
  • Advantage: Provides greater cube utilization than single-deep racks because more loads can be accessed from the same side of the rack
  • Disadvantage: In order to access rear load in rack, an extended reaching mechanism is required on the lift truck
  • Typically used when the inventory level for an item is at least five or when loads are stored and picked in multiples of two pallets16

  • 3. Drive-in Rack

    • Loads are supported by rails attached to the upright beams. Lift trucks are driven between the uprights beams
    • Advantage: Provides high density pallet storage
    • Disadvantage: Requires uniform-size loads. Lengthy storage and retrieval times due to care required by driver inside of the rack
    • Closed at one end, allowing entry from one end (LIFO)

    4. Drive-through Rack

    • Similar to drive-in rack, except open at both ends, allowing access from both ends (FIFO)
    • Used for staging loads in a flow-thru fashion [Frazelle, WC WH and MH]

    5. Push-back Rack

    • Loads are supported on an incline to enable gravity-based movement of the loads within the rack via roller conveyor. Used to provide highly accessible pallet storage
    • Provides LIFO storage in each lane: Loaded and unloaded at the lower end and closed at the higher end
    • Advantage: Can be used to enable deep-reach storage without the need for extended reach mechanisms for loading/unloading
    • Disadvantage: Rack investment costs are greater than for double-deep racks
    • Maximum depth is 5 loads

    6. Flow-through Rack

    • Similar to push-back rack in terms of storage density, except greater storage depth is possible. Rack is loaded at higher end and unloaded at lower end, providing FIFO storage in each lane
    • Termed pallet-flow rack and carton-flow rack (pictured) when pallets and cartons used, respectively

    Carton-flow Rack
    • Advantage: Allows a large cubic volume of product to be accessible from a small pick face area, supporting relatively high pick rates. Replenishment does not interfere with picking.
    • Disadvantage: More expensive than bin shelving
    • Can have LED displays attached to shelf beam for “pick-to-light” operations

    Pallet-flow Rack
    • Advantage: Replenishment does not interfere with picking
    • Disadvantage: Requires twice as much aisle space as push-back racks, but overall storage density could be higher because of greater storage depth
    • Storage depth of 40 to 50 possible for pallet-flow racks

    7. Sliding Rack

    • Location of the aisle is changed by sliding rows of racks along guide rails in floor (a.k.a. mobile rack)
    • Used when only single-deep storage is possible and space is very limited or expensive
    • Advantage: High cube utilization and complete accessibility to all loads
    • Advantage: More expensive compared to other storage racks. Lengthy storage and retrieval times because one can only pick in one lane at a time. Relies on having a reliable power source available.
    • Provides increased security for items compared to other racks
    • Typically found in library stacks, vaults, and climate-controlled (e.g., refrigerated) storage rooms

    8. Cantilever Rack

    • Loads are supported by two or more cantilevered “arms” (i.e., horizontal beams supported at only one end)
    • Similar to pallet racks, except the front upright and front shelf beams are eliminated
    • Used when there is a need for a full clear shelf that can be loaded from the front without obstruction from rack support uprights
    • Typically used to store long loads (e.g., bar stock, pipes, lumber)

    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 use
    • Pallet frames can be used to enable multilevel block stacking
    • Cost per frame: $100–$300 (can be leased for short-term increases in inventory)

    10. Bin Shelving

    • Alternative to racks to store small, loose, nonpalletized items. Pieces placed either directly on shelves or in bins or cartons
    • Advantage: Low cost
    • Disadvantage:Can result in excessive travel for picker. Difficult to pick from top shelf depending on the height of the picker and the weight of the unit. Replenishment can interfere with picking
    • Several levels of shelves (and storage drawers) on a mezzanine can be used to allow multi-level picking (max 4 levels). A lift truck or vertical reciprocating conveyor, e.g., can be used to help with removing units from the top level

    11. Storage Drawers

    • Drawers provide an alternative to bin shelving to store small, loose items
    • Advantage: Drawers can provide increased security compared to bin shelving and is most important when the demand for a specific item is low and infrequent. Easy to install at point of use
    • Disadvantage: Space is frequently underutilized unless there is an ongoing disciplined approach to managing the location and usage of the units being stored. Replenishment can interfere with picking. Cannot see inside the drawers, making labeling an important issue

    12. Storage Carousel

    • Carousel consists of a set of horizontally (pictured) or vertically revolving storage baskets or bins
    • Advantage: Allows a large number of items to be picked at a high rate
    • Disadvantage: Replenishment cannot occur during picking operations (typically, replenishment takes place during a separate shift or is interleaved between peak picking periods)
    • One operator picks from 2 to 4 carousels (termed a “pod”) in order to minimize waiting time while other carousels are moving
    • 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

    13. Vertical Lift Module

    • Pieces stored on trays inside a multi-bay enclosure that are delivered to the opening of a bay for picking by a servo-driven lift carriage
    • Advantage: Provides high-bay storage, and dense storage since height of trays can vary. All picking occurs at a user-adjustable waist height. Can provide even greater security compared to bin shelving when the operation of the module is often under computerized control, which can increase cost
    • Disadvantage: High cost. Requires reliable power source

    14. A-frame

    • Units are dispensed from parallel arrays of vertical angled channels onto a belt conveyor that carries them into a container.
    • Advantage: Very high pick rate
    • Disadvantage: Only feasible for small, rigid items of uniform shape that are not fragile. Requires manual replenishment
    • In stretch-wrapping, a film is wound around the load while the film is stretched; as compared to shrink-wrapping, stretch-wrapping has lower material, labor, and energy costs
    • Enables fully automated piece picking, with manual replenishment. Popular within pharmaceutical distribution centers
    • Inverted A-frame used for flat items
    • Other types of fully automated piece picking systems include robotic based systems that are similar in construction to robotic pick and place palletizers

    15. Automatic Storage / Retrieval System (AS/RS)

    • Consists of integrated computer-controlled system that combines 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
    • Adv: Fewer material handlers, better material control (including security), and more efficient use of storage space
    • Disadv: Typically, high capital and maintenance costs, and more difficult to modify
    • Although AS/RS were originally developed for warehousing and distribution operations, they are now also being used for in-process storage as part of an automated job shop. In an automated job-shop, an AS/RS can be combined with an automatic identification system and an automatic transportation system (e.g., automatic conveyors and/or an AGV system) to provide real-time material control capabilities. The material stored in the AS/RS can include both finished goods and work in process and even production tools and jigs

    • Racks: A typical AS/RS utilizes high-rise storage racks, ranging in height between 40 and 80 feet or higher, for random storage. High-rise racks require tight rack tolerances and level floors, all of which increase the cost of the racks as compared to a basic storage rack. The racks in an AS/RS can be freestanding or uses to support the building (RSS—rack-supported structure)
    • S/R Machine: An S/R machine in an AS/RS operates in a narrow aisle, serving rack slots on both sides of the aisle. The machine can travel in the horizontal (along the aisle) and vertical (up and down a rack) directions at the same time. Often the machine is captive to one aisle, although, if throughput requirements do not justify dedicating a machine to each aisle, a transfer car can be provided to move the machine from the end of one aisle to another, thus enabling the machine to operate in more than one aisle. The machine is a structural single- or multiple-mast frame that rides on one or two floor-mounted wheel rails. A carriage carrying a load-supporting mechanism (or shuttle) operates within the frame. The shuttle is used to store/retrieve loads at the racks and, at the end of the aisle, to transfer loads onto or away from conveyors, vehicles, or pick-up and delivery (P/D) stations or transfer stations. Deep-reach “mole” S/R machines can detach and run into a lane
    • Control: The operation of an AS/RS can be controlled by an operator working from a console, but in many cases, the control system is under complete computer control. Typically, distributed control, where each S/R machine is controlled by a dedicated computer with interfaces with a central computer, is used to increase system reliability.

    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

    Miniload AS/RS
    • Used to store/retrieve small parts and tools that can be stored in a storage bin or drawer
    • End-of-aisle picking and replenishment
    • Stacking heights range from 12 to 20 ft.; bin capacities range from 100 to 750 lbs
    • Not typically used for order picking because of long cycle times and high cost
    • Termed a “microload AS/RS” when capacity is less than 100 lbs (used in assembly, kitting, and testing operations to deliver small containers of parts to individual workstations)

    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

    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