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Increase Warehouse Storage Capacity - Racking System Options Simplified

Ways to Increase Warehouse Storage Capacity - Racking System Options Simplified

Supply Chain Digest Blog Entry: May 22, 2013 (here)

When the net working storage capacity of a warehouse or distribution center surpasses the 85% utilization level, there is typically a corresponding reduction in warehouse productivity.  This is caused by symptoms such as over-congested receiving and shipping docks, product being staged in operating aisles, commingling of multiple SKUs and/or pallets in storage locations causing extra work, etc.  If this describes your operating environment, then it may be time to consider warehouse rack and aisle layout changes  to increase storage capacity.  In many cases, the lifespan of a warehouse can be extended by simply changing the type of storage systems in use.  In this blog entry, we describe (at a high level) several different options for racking systems and the considerations associated with each option.  A much more thorough and detailed review can be found here

The storage options we compare include:

  • Block storage
  • Single deep selective racking
  • Double deep selective racking
  • Push-back racking
  • Flow trough racking
  • Drive-in racking
  • Pallet shuttle systems
  • Flow rail systems

Block Storage

  • Storage of pallets on the floor such that there is no need for a racking system.
  • Pallets stacked 2 - 4 high in designated storage lanes
  • Viable option for stackable pallets that do not incur crushing or damage
  • Benefits:
    • "Free System" in the sense that fixed equipment need not be purchased which is a definite cost benefit. 
  • Downsides:
    • Loss of building capacity in buildings > 21' stacking height
    • LIFO rotation
    • Honeycombing effect causes reduced net capacity utilization
    • Potential for product damage and crushing

Single Deep Selective Racking

After block storage, single deep racking is the most common pallet storage system used in warehouses around the world because of its flexibility.

  • Benefits
    • Each storage location can be used to store a different SKU
    • Multiple types of forklifts can be used to store and retrieve pallets
    • Aisle widths can be very narrow, narrow or wide aisle depending on the type of mobile equipment being used.
    • Single deep post and beam racks also provide the lowest cost per pallet position of any racking system option available.
    • The storage and retrieval of pallets fast and efficient
    • Pallets can have varying shapes and sizes if wire mesh decks or safety bars are used
    • FIFO inventory rotation
  • Downsides
    • Relatively low storage density

Double Deep Selective Racking

Double deep selective racks basically consist of two single deep racks positioned together such that pallets are stored 2-deep instead of 1-deep.  A specialized electric reach truck with a scissor extension mast is required.

  • Benefits
    • Can be used for selective or storage purposes
    • Provides a good balance between increased capacity and low cost per pallet position
    • The storage and retrieval of pallets fast and efficient
  • Downsides
    • Forklift operator needs to reach into the 2nd deep location with no visibility of pallet
    • Requires specialized forklift with scissor-extension mast

Push-Back Racking

Push-back racks are specialized engineered racks that can store between 2 and 6 pallets in depth.  At the time of putaway, a pallet is pushed into a storage location (i.e. storage lane) such that pallets already stored in the lane are pushed backwards up a slight incline.  At the time of retrieval, the lead pallet is pulled from the storage lane and gravity causes the unit loads behind the lead pallet to slide slowly forward to the front of the rack system.  A nested system of rails is installed at each storage lane to enable pallets to move backward and forward within the racking system. 

  • Benefits
    • Forklift always stays in the operating aisle therefore the driver is more productive
    • High storage density
  • Downsides
    • Beams are welded in place to ensure precise angle of incline - cannot reset rack elevations
    • Used for full pallet storage and retrieval only
    • Relatively high cost per pallet position
    • Loss of vertical storage capacity due to the use of an angled rail system
    • May not be suited for pallets that have overhang or are leaning or unstable
    • LIFO rotation

Flow Through Racking

Pallet flow through racks store multiple pallets in depth such that FIFO inventory rotation is supported  because pallets are always loaded from the rear and retrieved from the front of the racking system.  These racks are specially engineered with angled rails such that pallets flow forward towards the front of the rack as soon as they are loaded into a storage lane.  A series of brake drums are built into the storage lane rails to create friction such that the pallet glides forward slowly and smoothly to the first open pallet position within the lane.  This racking system is generally used for full pallet storage and not less than pallet order picking purposes.

  • Benefits
    • Forklift always stays in the operating aisle therefore the driver is more productive 
    • High storage density
    • FIFO rotation
  • Downsides
    • Beams are welded in place to ensure precise angle of incline - cannot reset rack elevations
    • Used for full pallet storage and retrieval only
    • High cost per pallet position
    • Loss of vertical storage capacity due to the use of angled rail system

Drive-In Racking Systems

With drive-in racks, the forklift drives into the rack to store and retrieve pallets.  This implies using a vehicle that has a suitable outer width dimension that can fit between the uprights with adequate clearance on either side of the forklift. Pallets are placed into the rack such that the outer edges of the pallets rest on an L-shaped rail bracket installed for the full depth of the racking system.  This means that there are no horizontal beams used to support the entire width of the pallet.   Lastly, a new type of drive-in is now available for the beverage and FMCG industries to support the storage and retrieval of pallets in pairs.

  • Benefits
    • Relatively low cost per pallet position to obtain high density storage
    • High storage density
  • Downsides
    • Only 1 SKU per rack bay hence honeycombing effect reduces net capacity utilization to 70 - 80% which declines as rack depth increases
    • Forklift vehicle drives into rack which implies higher levels of rack damage throughout racking system over time
    • Pallets only supported on outer edges hence uniform pallet size needed and only suited for good quality pallets
    • Vehicle drives into rack which is less productive than accessing pallets from the operating aisle
    • Not suitable for environments where multiple pallet widths may be in use
    • LIFO rotation

Pallet Shuttle Systems

A pallet shuttle system uses a mobile robotic shuttle carrier to transport pallets into available storage locations within a rack storage lane.  The storage system can be as many pallet positions deep as needed since there is no constraint as far as that goes.   Forklift operators need to  insert and remove the carrier as a first step before storing or retrieving pallets.

  • Benefits
    • The elimination of storage aisles provides the highest density of storage system available on the market.
    • FIFO rotation
  • Downsides
    • Significant loss of vertical storage capacity up to 18" per storage level
    • Robotic carriers are battery powered and are expensive to replace
    • Extra time required to put shuttle carrier into and out of the racking system
    • Difficulty to resolve incidents when pallet breaks in the middle of a storage lane
    • High system investment requirement which increases as throughput rises

Flow Rail System

This is a relatively new entrant to the market so it is not widely deployed in North America but nevertheless provides a very interesting high-density storage solution that can be deployed to retrofit existing post and beam or drive-in racking systems.

A pair of rails is installed for each pallet storage lane for the full depth of the racking system up to 10-deep.   The box-shaped rails have chains within them that rotate clockwise and counter-clockwise.  Similar to push-back racks, pallets in the storage lane are pushed back and pulled forward with the storage and retrieval of unit loads.  The difference is that the chains enable this to take place rather than angled rails or drive motors.

  • Benefits
    • System can be used to retrofit existing post and beam or drive-in racks
    • Very high storage density
    • Low loss of storage capacity (about 4" per level)
    • Moderate cost per pallet position
  • Downsides
    • Pallets need to be a minimum weight of about 400 pounds to be stored in this type of environment
    • LIFO rotation

In closing, racking storage systems vary significantly and each option comes with its own pros and cons.  There is no such thing as a best system so it is important to be informed about why some systems work better in certain environments versus others that may not be as suitable.  An unbiased material handling consultant can advise if a system is suitable for a specific distribution or storage operation.

Marc Wulfraat is the President of MWPVL International Inc.  He can be reached by clicking hereMWPVL International provides supply chain / logistics network strategy consulting services. Our services include: supply chain network strategy; distribution center design; material handling and automation design;  supply chain technology consulting; product sourcing; 3PL Outsourcing; and purchasing; transportation consulting; and operational assessments.

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