Storage Strategies – Stacking Options

Warehouse worker checking the inventoryIf you are in production, you have material. You probably have too much, except for the one thing that’s missing. In this blog post I will give an overview of the options for storing material. Hopefully this will give some inspiration. Please note that this is not on how to manage the material, merely on how to store it. This is the start of a small series on how to store material.

Floor Storage

Probably the easiest approach is floor storage. You just stack your boxes, pallets, and pallet cages on the floor. This can make the best use of your space but requires stackable packaging. It also requires the lowest upfront investment as you do not need any shelves or similar stuff. However, it can be a pain to organize, it is difficult to find your things again, and depending on which item you require, you may need quite a bit of time to dig it out. It is only used if space is at a premium and there is not much shifting around. A prime example is a container ship. Usually you need at least some empty space in storage to shift things around in order to get out material from farther back. Please keep in mind that no matter how good your system is, sometimes you will spend a lot of time searching for material that you believe you have.

Container Ship

A variant of this leaves at least some rows free. This makes access somewhat easier, but you may still have to shift material around to access your parts. This is frequently used at container terminals, although they do have some special equipment to move their containers around. Both approaches are usually not used in lean manufacturing, since lean generally needs easy and frequent access to the material.

Container Terminal

 Shelf Storage

Shelf StorageAnother option is shelf storage. Now you need to invest in shelves, hence your up-front cost is higher. You also need to keep access lanes for forklifts, people, or automated retrieval systems. However, now you can efficiently store material that is not stackable, and you can access every item right away (unless you run out of space and your forklift drivers place their items in the corridors anyway … but that of course has never happened in your place …).

This type of storage is common in industry. Please keep in mind, however, that you have a management overhead, and your system needs to remember which part is where. If you have multiple identical items, you also need to remember which one should be used first.

Naturally it is possible to combine shelves with a dedicated automated storage and retrieval system. The robot shown here stores and removes trays with data tapes for the US military.

Mobile Shelving

Mobile shelving is shelves that can be shifted sideways. Often they are found in libraries, but there are also industrial applications. This can utilize space better than a normal shelf, as you do not need space for access between every shelf. On the other hand, you need to move the shelves around in order to access certain locations. Hence it is better suited for storage with less-frequently-needed access. This shelving can be powered by hand or motorized. The more slots between shelves you have for access, the less you have to move the shelves around to get to what you need.

Rotating Shelf Storage

A variant of shelf storage is storage shelves that can rotate or revolve. Most often this is for small-part storage, and is used quite frequently with electronics and other smaller products. The set-up is a bit more expensive than a normal shelf, but it gives easier access. Some of these towers are quite high, but the smallest such systems can be put on your desk for your stationery. Still, a system would need to track which part is where.

Rolling Rack

The rolling rack system uses … well … rolling racks. The material is added on one side, and (usually gravity) has the items roll to the other side for consumption. The out side usually has a stopper of some sort. Hence this system has a clearly defined in and out side. Most importantly, this system maintains FIFO. This is very commonly used in lean manufacturing. If you have reusable containers, then this system often includes return lanes for these containers.

Warehouse rolling rack

There is also a cheaper variant of the rolling rack for pallets. Instead of actual expensive rollers, they merely attach some rails to the floor. The forklift puts pallets or pallet cages into one side, and removes them from the other side. Since on an even floor there is no gravity moving the pallets along, the forklift simply pushes the whole row forward until the end of the slide. The forklift driver must take care that he doesn’t overshoot the target; otherwise the pallets will stick out too far on the other end. Sometimes there is a stopper on the other end, although in this case the receiving end needs to lift the pallet over this stopper, which may be a problem for small hand-operated pallet carriers.

Amazon Kiva Storage System

Of course, there are also more-exotic options to store your goods. Amazon uses small Kiva robots to move material from storage to picking (see my series on The Inner Workings of Amazon Fulfillment Centers). This small robot lifts a shelf and transports it to picking. The image below shows such a shelf storage. You can clearly see that the shelves are packed in 4×4 or 4×7 groups, with one-way access ways in between. While it takes a bit more time to maneuver a shelf from the inside of a group, it allows for a much higher storage density. I believe that this system has quite a bit of potential, both for high storage density and low labor cost retrieval, and I know of at least two Chinese knock-offs using such a method.

Amazon Kiva Floor Overview

This has been a brief overview of the different ways to store inventory.  If I missed one, please let me know. For bulk material you also have other options like making a pile or storing it in silos or tanks. Depending on which storage you have, there are also different ways to manage such inventories (see for example my post series on milk runs). I also have a whole series with twelve ways to create space around your assembly. In my next post I will look at where exactly to store your items. Now go out, set up your storage, and organize your industry!

6 thoughts on “Storage Strategies – Stacking Options

  1. Hi,

    The article was awesome. I was drawn to this article because I already knew what amazon is using in terms of the storage systems. But I had a question here. As you said amazon is using robots to lift up particular shelves and that is transported to a respected area. I assume the speed of robots does create injuries to humans because alongside robots there are humans working too. Do you believe that not only Amazon warehouses but other warehouses do consider human safety and take that into serious consideration? If not, I think the best alternative stacking option would be mobile storage.

    Akshay

  2. There is another combined option for purchased materials supermarkets in a high rise racking – the bottom part consists of gravity fed roller racks with parts in re-usable containers ready for delivery to PoU (both local and overseas). The top part consist of standard shelving racks, where the overflow of parts is stored (for overseas, imported parts on pallets). The supermarket below is then replenished with parts from the racks above. the lower part of the supermarkets looks like this – https://planet-lean.com/wp-content/uploads/2019/04/Making-materials-flow-with-a-supermarket.jpeg

  3. I found this article to be very informative! There are so many different types of storage systems to be used for different materials. I’ve always been intrigued by Amazon’s Kiva robots. The only disadvantage to that would be if they have a mechanical problem how much would it set them back? I am curious how many defects the robot has and how it can impact the distribution center. But overall it seems really cool and an efficient way of moving product!

  4. Hi Rachel, mechanical problems with Kiva’s will be worked around by other Kiva’s. A bigger issue are software failures, which will stop the entire system. However, I have been told that these are rare and short.

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