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Warehouse Storage Solutions and Systems Design
Warehouse Storage Solutions & Design Considerations; How Do I Know What I Need?
By A. Wynn Watson III
Although every company is different and the situations are unique, many of the challenges that businesses face are fairly similar. For example, whether a distributor is expanding an existing location, opening a facility in a new market, adding more products and services, offering new sales channels such as e-commerce or direct-to-consumer, or any other major shift in operations, the same challenge exists; implement the physical and operational changes without affecting the customer. Operational and physical changes often go hand in hand or at least significantly impact one another. Generally speaking, in a warehouse or distribution environment, physical facility changes boil down to different types of storage media for product handling, manipulation or picking. The most common types of storage come from four categories: Bulk Storage, Pallet Rack, Shelving, and Carton Flow. With the focus on different types of manual storage systems, below you will find how each type is used, some pitfalls to avoid, and ways to keep the equipment like new.
Also known as “floor stacked”, this is the most basic form of warehouse storage and consists of no physical storage medium other than building square footage. Products are stored directly on the warehouse floor typically arranged several pallets deep and possibly stacked depending on product type. This method offers low cost and is ideal when pallets are stackable, ceiling height prevents vertical racks, products are not sensitive to (FIFO) First-In First-Out sequencing and storage density is not a concern. Challenges include low space utilization (60-70%) since the vertical space is not effectively used, increased possibility for product damage due to stacking, and inefficient product locating and picking due to cumbersome product access and poor product location identification.
Pallet racking consists of a set of upright frames connected in various ways by horizontal beams to provide storage for palletized or non-palletized product depending on the rack configuration and beam supports used. There are five basic types of pallet rack:
Drive-In / Drive-Thru: This storage medium consists of a multi-level system of rails forming several levels of continuous pallet storage. It can be installed as either Drive-In, which is accessed for product delivery and retrieval from only one side (LIFO) Last-In First-Out, or Drive-Thru, which typically delivers product to one side and retrieves it from the opposite thereby preserving FIFO sequencing. This type of storage is used when storage density desired is high (potentially 85%) and SKU base is low. It is well suited for refrigerator or freezer environments where square footage costs are significantly higher which is why this type of pallet storage is often used in the grocery industry. Challenges include the following:
- Requires highly skilled fork lift drivers since they enter the rack system every time they deliver or retrieve product
- Quality pallets since they reside on rails that suspend them on each end
- Trapped product, no easy access to internal pallets
- System is designed for specific size pallets and can not be easily modified to accommodate larger or smaller width pallets
- Increased potential for injury and damage due to fork lift impact
Selective – Single or Double Deep: Single deep selective rack is the most common type of pallet racking in use and is one of the least costly types of basic storage. Think Home Depot, Lowes and Costco warehouses. Single deep offers access to every location giving users flexibility in product storage and retrieval, but it comes at a cost. Space utilization suffers and can only be regained through the use of higher lift technology to reduce aisle space. It is best used when 100% access is required or desired, large numbers of SKU’s are handled relative to total storage, and access trumps cube utilization. Double deep selective rack allows for better space utilization and is no more costly per pallet location than single deep rack, but should only be used if storage density is more important than 100% access, since product can become trapped and require additional labor to maneuver. This type of racking also requires specific lift equipment that can retrieve pallets from double deep locations.
Pallet Flow: This system consists of a rail or conveyor system installed in pallet rack that can be 20+ pallets deep. There are many versions of the flow mechanism, but the basic premise is to feed pallets in one end and pick them up on the other. Typically some sort of speed restrictor is installed in deeper systems to prevent impact damage. These systems provide the density and FIFO storage possible with Drive-Thru without the requirements to drive into the rack system. It is used when SKU count is low, FIFO is desired and square footage is tight. It is significantly more costly than Single Deep or Drive-Thru racking and requires use of quality pallets to prevent product from hanging up in deep locations.
Pushback: This is a system of carts installed into pallet rack. The carts are stacked and allow for storage of up to 6 pallets deep, although typically not used more than 4 pallets deep. Product is placed on a cart and then pushed back when the next pallet is placed in front of it. This is done until the final cart is pushed back and the last pallet is stored onto the rack beam. When a pallet is removed, the successive pallets move forward because the carts are installed on a slight decline toward the front beam. This system provides high storage density and is used when deep storage is desired near a wall or for medium turnover products with multiple pallets per SKU. Also, this storage medium is used when LIFO is an acceptable means to rotate inventory. Pushback racking is typically the most costly type of manual pallet storage.
Cantilever: A very specific type of racking used extensively in the furniture and lumber industry for long products that are better supported by arms instead of beam levels. Cantilever must be designed specifically for the products to be supported and may require special considerations based on the product size, weight, and rigidity.
Wide Span Shelving: Typically a lighter duty version of standard pallet rack, this shelving provides the same versatility, access and sizing as selective pallet rack when the overall capacity stored does not rise to light duty pallet rack standards. Not typically designed for pallet storage, this shelving is ideal for cartons, hand stacked storage, bin or tote storage, etc. Wide span shelving is common in loose pick, totes and small item storage. It can also be stacked to form shelf supported mezzanine structures that can maximize vertical storage.
Industrial Shelving: This type of storage offers multiple levels of storage or picking in a relatively small footprint. Shelves typically store anywhere from 300 – 1,000 lbs. per shelf level. Shelves can incorporate dividers, drawers, and many other accessories that augment the product stored. Used extensively in the automotive and electronics industries for small parts picking, industrial shelving is versatile and can be stacked to form multi-level mezzanine supports. It is less durable than pallet rack or widespan shelving, but is significantly less costly per storage location.
The decision to use carton flow depends on many factors such as overall footprint available, consistency of product sizes and weights, SKU quantity, SKU velocity, and the list goes on. After weighing the pros and cons associated with using valuable space to set up a carton flow picking area, the next decision is to determine what type of carton flow should be used.The size, weight, and consistency of these variables will determine the best type of carton flow for the application. There is a great deal of difference in carton flow requirements for a warehouse that picks product from consistently sized and weighted boxes or containers and a warehouse where size and shape vary greatly. Even warehouse environmental conditions can play a deciding factor in what product to choose. For example, imagine the effects of high humidity on cardboard box’s integrity. By placing a 40 lbs. cardboard box of widgets on 2 narrow rails of wheel track and letting it sit for a few days in the same spot in a humid warehouse, a set of nicely formed wheel ruts in the box appears. Consequently,the pickers will need to use that lovely sky hook to retrieve the stuck packages. Happens every day and there are ways to avoid this and other mistakes. There are a few basic types of carton flow and manufacturers typically provide different versions of them to set themselves apart, but for simplicity we will discuss the following basic categories: wheel track, full width roller, and skatewheel. Any of these types can be configured as either standalone units or rows, integrated into a new pick module or retrofit into an existing storage media..
Wheel Track: This type of carton flow typically consists of a frame or shelf assembly, wheel tracks or rails and dividers. The frame is either welded, bolted or bracketed, the wheel tracks are typically about 1” wide roll formed channels with integrated plastic wheels on roughly 2” centers. The wheels may or may not have axles. Dividers are most likely full length and adjustable.
- Wheel tracks and dividers are adjustable so as your product mix changes you can modify the carton flow
- Tracks and dividers can be replaced as they degrade or are damaged as they are not part of a composite unit
- Shelves can integrate many types of accessories for product presentation, such as knuckle over trays
- Shelves are not bound by the depth of the storage media allowing for ergonomic shelf positioning or excess depth where applicable
- Additional tracks may be added to allow easier product travel
- Wheels can bind or eventually cause friction that reduces product flow
- Boxes may conform to wheels in high humidity environments depending on the product type, resulting in hang ups
- Tracks can be difficult and time consuming to adjust once installed, especially when some presentation accessories are also present
- Adding additional tracks to allow smoother product conveyance quickly adds incremental cost to each pick lane
- This media is somewhat width limited since the entire assembly must be supported by the shelf or frame the tracks are attached to. 96” wide shelves are typical, but sizes approaching 144” can be a challenge especially with heavier products.
- A small amount of pickable space is lost both in both depth and width due to the use of a shelf structure for primary support
Full Width Roller: This type of carton flow typically consists of gravity roller conveyor tracks of varying widths, roller centers and capacities. As with wheel track, it relies on pallet rack or standalone supports to provide the structural support, but typically uses standard pallet rack beams as the primary means for installation. Tracks can either be confined to the depth of the racking using rack beams as end stops or mounting areas for accessories, or attach on top of the beams allowing for more depth, ergo and accessory options. Some require brackets that allow the track to be attached to the beam and others simply lay in place and are secured as is. Deep lane applications may require additional beams for support above and beyond the infeed and discharge zones, although this is typically accomplished through the use of intermediated beams within the normal pallet rack structure. Some manufacturers may also provide a shelf or frame that allows accessories to be installed and depth adjustment beyond beam supports.
- Full width rollers provide more surface area contact and may provide better conveyance for products with flimsy bottoms or in high humidity environments
- Width is only confined by the total bay width and beam capacities if no support shelf or frame is used. No loss of pickable space
- Roller centers and track widths can be modified to strike a balance between cost and functionality. Some product overhang is allowable.
- Tracks are easy to install as new product or in a retrofit. Typically involves infeed and discharge brackets and a lay in section of conveyor for rack beam installation.
- Specific widths mean you are limited in the size product you can accommodate if business changes. They may be too large or too small as things change.
- Cost is typically higher per bay than wheel track applications.
- No ability for width adjustment once installed. Tracks are fixed width so new product is required if changes are desired.
Skatewheel: This type of carton flow blends the concepts of both full width roller and wheel track into a fixed width bed of skatewheel. Beds can be inserted to cover the entire bay width so the no gaps are present. Lane entry guides or full length dividers are used to separate lanes. Wheels can be aligned or staggered depending on the manufacturer and are typically much larger than those on wheel track rails. Wheels ride on an axle and are held in place in various ways.
- Since the entire bay is one big carton flow section, there is no loss of pickable space.
- Lane entry guides are adjustable giving a breadth of adjustments for product size or changes.
- Number of wheels per foot can typically be adjusted by changing axle centers to mitigate cost if possible.
- Larger wheels provide a low coefficient of friction and are more aligned with full width roller surface area contact, mitigating the possibility of boxes forming around the wheel in high humidity.
- Easy to install. More aligned with full width roller install.
- Typically higher cost due to full bay coverage, number of wheels and larger axle sizes than other options.
- Entry guides may allow product to skew during transit if they are not full length.
- Depth is usually limited to beam to beam depth of pallet rack unless standalone units or shelf frames are used.
Pitfalls to Avoid When Planning Your System
It’s important to make sure that whatever rack storage medium you choose will fit well in the footprint of the facility. Too often building internals are overlooked in lieu of overall square footage and that can lead to a great deal of underutilized space. In an ideal scenario, the storage medium should be identified and designed prior to site selection or building design, although this isn’t always possible. Consider your building with the list of characteristics below in mind to avoid purchasing or leasing a building that doesn’t fit the bill:
- Building column layout as it relates to aisles and major equipment design
- Building clear height (useable vertical space below the roof or ceiling)
- Overall square footage (useable floor space)
- Permanent and semi-permanent obstructions (HVAC, offices, electrical, sprinklers, lighting, etc.)
- Simplicity of building layout (a rectangular building is typically more conducive to efficient storage design than one that has large square footage through multiple build-outs)
- Dock door placement
- Environmental Factors (airflow, humidity, etc.)
Aside from the factors listed above, try to plan for future growth, not just what you need today. An efficient storage system never operates at near 100% capacity, so if at Day 1 you are at 85% and expect growth you probably need to rethink your business strategy. Many companies in growth mode choose to move into a facility that can accommodate growth through vertical space utilization, but phase in the storage medium as business dictates. That way they mitigate the initial cost, but allow for future expansion.
Keeping it Fresh: Rack Safety and Guarding Products
One of the most overlooked areas of warehouse layout is safety guarding for people and for the storage products themselves. The question isn’t if you are going to incur damage to your rack system, it’s when. Almost every warehouse will experience some level of damage based on the level of complexity of the building and storage layout, experience of lift drivers, aisle width, type of lift, type of product etc. There are many products out there purporting to provide protection against damage, but here are a couple of pointers to keep in mind.
- The incremental cost of using pallet rack guarding will typically pay for itself fairly quickly by mitigating more costly damage to the structural components of the system.
- Unless you are using self guided lifts that are highly unlikely to impact your rack system, use floor mounted column protection for every pallet rack upright. Floor mounted column protectors with internal anchors are preferred because they do not transfer impacts directly to the rack structure as any rack mounted protector would do. DC managers who become upset when they see their protection devices getting damaged through the course of normal operations should think of it this way: if the guarding is absorbing the damage, the racking is not. Guarding is easily replaced and pallet rack is not. The use of internal anchors are somewhat more labor intensive to install, but less time will be spent maintaining wheels on your lifts from running over exposed anchors, so they are worth it.
- Use full length end-of-aisle guarding in any high traffic areas to provide a visual and physical barrier to lift equipment. These are also highly attractive impact zones so guarding is well worth the cost. Tunnels should be guarded for the same reason.
- Use double highway rail to separate operational areas wherever personnel and powered lift equipment operate in close proximity. Handrail may provide a visual deterrent in these areas, but highway rail is more likely to prevent a major accident.
- Make sure all transition points for lift equipment are well lit, visually clear and marked appropriately so that even inexperienced operators can clearly understand traffic patterns.
- Maintain a rigorous training program and ensure all operators are qualified to operate their respective lift equipment and use its associated safety equipment.
In summary, there are many storage options available for consideration when addressing the business challenges associated with change in your warehouse or distribution operation. By utilizing a trusted partner to help specify and implement the proper storage, picking and replenishment strategy, the probability of project success is increased dramatically. When determining whom to trust, consider the level of experience in design & project management, ask references, find out if they represent multiple lines of storage equipment manufacturers and understand if they have conflicting interests such as lift equipment sales or rental.
A. Wynn Watson is a Product Sales Engineer at TriFactor, LLC, a material handling systems integrator based in Lakeland, Fla. He can be contacted at 904/493-2173 or firstname.lastname@example.org. For more information visit http://www.trifactor.com/