There are so many types, variations, and factors involved in the decision-making process, and now more than ever before the choice of racking style for storing your products is becoming less and less obvious as warehousing and operations evolve. AGV's (Automated Guided Vehicles) are an example of the latest elements that have been introduced into the mix. The most common types of warehouse racking available are:
1. Teardrop racking
2. Structural racking
3. Single-deep selective rack
4. Double-deep selective rack
5. Pushback
6. Drive in/Drive-thru
7. Portable racking or "Stack Rack"
8. Gravity flow rack
9. Cantilever
Many other options for product storage exist, including shelving, mobile shelving, and mezzanines, but for the sake of this "101" style course, we will stick to the basics. (For additional information on other product storage options, see my previous article on Saving Warehouse Space - Seven Proven Strategies.) To start the process of elimination for your facility, we first must ask the right questions.
The First Step: Discovery
1. What is my product?
The type or number of products or services that a company offers can vary greatly. In a dedicated manufacturing environment for example, perhaps no more than 3-5 SKU's (Stock Keeping Units) exist. However, a 3PL (a Third Party Logistics, or contract warehousing, provider) can have thousands of SKU's depending on any number of factors. For example, how many customers are handled at that one facility? Or is this location dedicated to one customer? How many products of each customer's are at this location? Does the 3PL specialize in only one type of product storage and handling?
Product type is a key factor. Is the product stored in barrels, containers, bags, pieces or cartons? Is it palletized? What are the pallet dimensions when loaded? How much does it weight? Does the product hang over the pallet?
With so many questions, we should start by saying that, if possible, floor-stacking your product, the act of stacking on palletized product on top of another, is the single best method to use with regards to both cost and storage density. Floor-stacking does not require the purchase of warehouse racking that takes up valuable warehouse space, making this the preferred method of 3PL's everywhere (much to the chagrin of pallet racking manufacturers and distributors!) The ability to floor-stack a product is based on the stability of the loaded pallet combined with the product's capacity to handle the weight without buckling on itself. But even if it can handle the weight, floor-stacked product should never exceed four pallets high for stability purposes.
If your business is not a 3PL, or if floor-stacking your product is not possible, then you need to evaluate exactly what it is you are looking to store. How many different SKU's do you have? How much of each SKU is there at any one time? What are the sizes of each SKU? Does 80% of your business draw from 20% of your total SKU's? All of these answers play a role in determining your racking preference.
Also, the smart designer will plan not just for the total maximum amount, but will also add 20%-30% to that figure in order to account for future company growth. While pallet positions sitting empty for a couple of years waiting for the company to grow are a waste of space, using equipment that is designed to expand with you wherever possible is always the intelligent option.
2. What is my throughput and velocity?
Basically, how much product (raw, finished, and mid-process) goes through your facility at any one time, where does your product go within your facility and how fast is your product moving from receiving to shipping? Does some of it sit for a while before it ships out? Do you have a dozen or so forklifts transporting product back and forth at any one given moment in time or does your company have only one forklift driver? Are you a manufacturing plant using "Just In Time" processes? Does your product expire? Do you use FIFO (first in, first out) or LIFO (last in, first out,) or neither?
3. What kind of material handling equipment do I have?
The single best question to ask at this point is the following: Are my forklifts going to determine the racking I use, or is the racking I use going to determine the forklifts I get? Where are you in the process? If you already own (and are locked into) the lift trucks you have (or are acquiring,) then you simply have to work within the confines and limitations of that equipment.
If, however, you have the luxury of being in the enviable position of having a choice of lift truck and storage method to use, then the only limitation you have is your existing budget. If you are not in a position to purchase new (or even used) forklifts, then ask yourself if you wait until you are.
Understand that the material handling equipment you use is vital in this decision. Believe it or not, I once heard of a situation where the racking sales rep didn't ask about nor look at the type of forklifts that his customer was using, and when double-deep racking was proposed, the fresh-out-of-college warehouse manager didn't think about that either. Thank God that the project never made it past the "Letter of Intent" stage, because those four-wheel sit-down forklifts would have had a hard time picking product out of the second (two-deep) pallet position without a reach mechanism!
4. How much room do I have to work with?
The dimensions needed go beyond (length) x (width) x (height). The forklift limitations described above in Question #3 have to be taken into account, which includes the right angle stack aisle clearance width necessary for safe rack interaction. Also, take into account the placement of your other equipment (machine tools, battery charging station, packaging area,) as well as your shipping and receiving areas.
Do you need to create more room? See my previous article on Saving Warehouse Space - Seven Proven Strategies here.
5. How much flexibility do I need?
The savvy warehouse layout planner will understand that the only constant in business (besides taxes) is change. That includes product sizes, volume, throughput and velocity, or even facility moves or consolidations. You need to take into account that today's product may be tomorrow's trash. Flexibility is an absolute must when considering storage alternatives.
Questions to ask are: How quickly can I modify this rack? What modification limitations do I have with this? If I need to change storage equipment, how will that effect my handling equipment (lift trucks, etc...)? How much time do I have to change equipment over if my product changes? What is the lead time when I order new rack?
An additional hint: Whenever you are looking to order anything new, regardless if it is rack, lift trucks, or office supplies, get the lead time. Always, always, always get the lead time. When you get that time frame, you then can work backwards from the longest lead time possible to get the last day you can make a decision on a project. For example, if today is July 1st, you need new warehouse rack delivered and installed by no later than September 1st, delivery lead times are 4-6 weeks out and the project will take one full week to install, then the last day for you to make a decision will be 7 weeks before September 1st, or July 14th.
If you want to factor in Murphy's Law, as the savvy warehouse layout planner will do, you will make your decision by July 7th.
The Next Step: Solutions
Now that we have our questions answered, we can now move on to which type of storage solution may work best. Again, there are other options available, and all options should be explored, but we will stick to the most common for simplicity's sake.
1. Teardrop racking
Teardrop and Structural Racking (#2 below) are not methods of storage, they are different designs of rack, but the two varying styles do need to be covered as there are differences in capacities and cost. The teardrop design is a hole punched into roll-formed racking in the shape of an upside-down tear. The beams have a locking device that slides into the large hole in the frame and locks down into place. Holes are typically spaced 2" apart offering placement flexibility, and the method of installation will save money on the labor needed to assemble the racking.
Most teardrop style racking manufacturers also make interchangeable racking, meaning one manufacturer's racking can be used with another's. This means you're not "locked in" to any one given rack producer, which offers you flexibility later on down the line. This is also the most sought-after racking between the two, meaning should you wish to offer your rack on the used market later on down the line, you can be assured that, more than likely, there will be a demand for it if it is in good shape.
2. Structural racking
The Structural design of racking is designed for heavy-duty applications. A stronger steel design is used and beams are bolted on to the uprights or frames instead of being slid into place. While structural steel does offer a stronger rack module with a higher capacity, it is more expensive when compared to teardrop both in the equipment purchase and in the installation. Structural racking has its place, and in some cases may be mandatory. Ask a qualified consultant for assistance in determining your needs.
3. Single-deep selective rack
This is, by far, the most common storage solution I see, and for good reason. Single deep selective rack interacts with most if not all types of forklifts, can fit into almost any environment, and offers the most flexibility. While it is not the most efficient use of floor space available, it rates highest in selectivity. You can always pick every pallet at any time anywhere in single-deep selective rack.
4. Double-deep selective rack
Offering more storage density (up to 40% more than single deep selective rack in some applications,) the only true drawback to double-deep storage racking is the necessity for a special type of forklift, the reach truck. The reach truck is a specialty type of forklift that is designed primarily for rack interaction and product staging to shipping or pallet pick-up from receiving. High throughput and velocity operations will benefit the most from this storage method as this environment usually requires dedicated forklifts such as the four-wheel sit-down lift trucks and the walkie-riders, which can handle most of the other pallet transport functions throughout the facility.
If you have a high ceiling, this racking also offers a larger module footprint than single-deep racking, thus making for a more stable system, and some reach trucks can now lift pallets to 37' high! Overall available square footage must be weighed against the total SKU count however since selectivity decreases.
5. Pushback
A LIFO application, pushback racking systems can typically go anywhere from 2-5 pallets deep. Pallets are placed by forklift on a gravity-controlled cart within the rack. If there is product already on a cart and space is available behind it, the forklift (pallet) pushes the cart back and the new pallet is placed on the cart in front of the original pallet. Unloading of the pallets simply works in reverse, with the full cart behind the picked pallet moving forward after the picked pallet is unloaded from the rack.
Selectivity decreases, but pallet storage density increases. Low to mid-range SKU counts work best here.
6. Drive in/Drive-thru
A low SKU, high density solution that can be utilized for both LIFO (Drive-In) and FIFO (Drive-thru) storage methods. Drive in/thru modules utilize rails that act as pallet supports, and run as deep as the system provides, eliminating aisles and thus increasing density and cube utilization. One problem with this storage method is called "Honeycombing," whereas single slots can become un-utilized until pallets in front (or behind) of the slots are removed.
Where pushback racking offers more versatile storage than Drive in/thru racking because each lane flows independently, Drive In/Thru may be have the most storage density between the two solutions. There is a trade-off, and a qualified consultant can offer answers.
7. Portable racking or "Stack Rack"
Applications without fixed racking design requirements, changing available racking space, volatile storage needs, granular product stored in large bulk bags, or other special products may require portable or "stack rack." This is a great application for low to mid-range velocity and throughput applications as you are no longer confined to a specific rack design. When not in use (and depending on the design,) these racks typically stack and nest within themselves for easy storage.
8. Gravity flow rack
Flow racking is a specialty type of racking most commonly found in pick modules in a FIFO system, but is also found in bulk racking systems as well.
In a typical bulk rack module, pallets are loaded onto the module onto the back end, and the pallets "flow" via flow-rails to the front picking end, automatically rotating stock. This system works best with a low to mid-range number of SKU's (depending on available space) and is one of the densest storage methods available. Usually the number of forklifts and manpower can be reduced using this technique as well.
In a typical pick module system, flow racking is the bottom row of racking and the top levels are for "replenishment." Full pallets are brought down from the replenishment levels and are loaded into the rear of the system. They then flow via gravity into the front or face of the system, which is the pick area. Pickers are instructed via pick ticket (or a pick to light or other similar system) to pick off of the pallet either cartons or pieces to fulfill an order as it proceeds down the system via conveyor.
9. Cantilever
Cantilever racking is another specialty type of racking. It is found in manufacturing or lumber yards primarily, but can be used anywhere longer product is stored. This can include drywall, sheets of plywood or metal, steel bars used in processes, tubular products, or any other elongated material.
An additional hint: When planning your product placement in the warehouse, use the 80/20 rule mentioned before. Usually 80% of your business comes from 20% of your product. Account for that by placing the 20% of the product closest to the shipping, processing, or picking areas. Placing the least used product in the pallet positions that are the furthest away, then working your way in, will help you in your planning.
This information should serve as a guide and a great starting point for anyone looking to lay out a warehouse floor plan. In the upcoming weeks, I will go into each type of rack with greater detail in order to assist with the intricacies of each style.
