With so many different automated storage and retrieval systems (AS/RS) now available, it can be difficult to determine which one is optimal for your warehouse, distribution, or manufacturing environment. Vertical Lift Modules VLMs have been shown to significantly increase the throughput, efficiency, and accuracy of the picking process, but is a VLM the best solution for you?
In this guide, you’ll find everything you need to know to get started with VLMs, including:
- What a VLM is and how it differs from a vertical or horizontal carousel
- The different types of VLMs and how they work
- The VLMs that offer the fastest throughput and the most storage density
- The most common VLM options
- Features to review when evaluating a VLM
Our goal is to help you understand the basic features and functionalities of VLMs so you can choose the solution that best suits your needs.
What is a Vertical Lift Module (VLM)?
A vertical lift module (VLM) is an enclosed automated storage and retrieval system (AS/RS). It consists of two racks of shelves stacked upright with an elevator or extractor running vertically between them. The elevator responds to commands and automatically retrieves and delivers trays to an operator at a workstation.
Because VLMs can significantly increase the throughput, efficiency, and accuracy of the picking process, they are now commonly used in warehouse, distribution, and manufacturing environments.
How a Vertical Lift Module (VLM) is different than a vertical carousel
A vertical carousel has shelves (and trays on those shelves) that rotate vertically on an elongated oval path. The shelves are driven by a mechanism that is similar to the mechanism that runs a Ferris wheel. By contrast, with a VLM, only the desired tray is extracted and moved. The difference between a VLM and a vertical carousel comes down to this: With a vertical carousel, the entire payload moves, while with a VLM, only one tray at a time moves.
How a Vertical Lift Module (VLM) is different than a horizontal carousel
A horizontal carousel has shelves (and trays on those shelves) that rotate horizontally, powered by a mechanism similar to the one that drives a merry-go-round. By contrast, with a VLM, only the desired tray is extracted and moved. Put another way, with a horizontal carousel, the entire payload moves. With a VLM, only one tray at a time moves.
The different types of Vertical Lift Modules
Vertical lift modules (VLMs) have evolved considerably over the past few years, and today’s VLMs have a wide range of capabilities, specifications, and features.
For example, a VLM can be a single column, single elevator VLM; a single column, dual elevator VLM; a single column, double deep, dual elevator VLM; a multi-column, single elevator VLM; a single column, high speed tote based VLM; or a pallet based VLM. Each one of these VLMs has advantages and disadvantages, and which type is optimal depends on the particular business challenge that needs to be solved.
- Single column, single elevator VLM
The single column, single elevator VLM is the most common VLM in use today. It consists of two racks of shelving stacked vertically, one-shelf wide, with an elevator or extractor system running vertically between them. The elevator extracts one tray at a time and delivers it to an operator at their workstation. - Single column, dual elevator VLM
Like the single column, single elevator VLM, a single column, dual elevator VLM has two sets of shelves stacked vertically, with an elevator or extractor running vertically between them. However, with the single column, dual elevator VLM, the extractor is capable of holding two trays at the same time. That means the extractor can pick up a tray and then, simultaneously, as it is presenting the first tray to an operator, it can proceed on to the next order and pick up a second tray. Once the operator is finished with the first tray, the extractor returns it to its proper storage shelf and presents the second tray to the operator. This dual elevator operation happens continuously, enabling multiple lines of an order to be fulfilled quickly and efficiently. - Single column, double deep, dual elevator VLM
The single column, double deep, dual elevator VLM is similar to the single column, single elevator VLM, except that in the double deep, dual elevator variety, the extractor can retrieve a tray that is stored on a shelf behind another tray. Because the extractor can reach behind the first tray, it is possible to have two racks of shelving on the frontside of the VLM and two racks of shelving on the backside of the VLM. A single column, double deep, dual elevator VLM provides high levels of storage density and can be quite valuable for a business that needs to prioritize space savings. - Multi-column, single elevator VLM
While standard VLMs have extractors or elevators that run only vertically, a multi-column, single elevator VLM has a sled on its extractor that enables it to move not only vertically, but horizontally, as well. A multi-column, single elevator VLM can be useful for a business that needs to store and retrieve parts or products of varying heights. - Single column, high speed tote based VLM
A single column, high speed tote based VLM has an extractor with multiple picking totes. The extractor can elevate, retrieve a tote, slide that tote to the left, elevate, grab another tote, and then slide that second tote to the right. This multiple picking works in conjunction with each order so that for example, if the operator needs five different picks, all five are presented at the workstation at the same time. A single column, high speed tote based VLM accelerates throughput and the results is that the operator is never waiting on the machine; the machine is waiting on the operator. - Pallet based VLM
Pallet based VLM. A pallet based VLM is a VLM that has been modified so that it can handle full pallets weighing up to 3,000 pounds. Usually, the modification involves adding a forklift to the front of the VLM.
How does a Vertical Lift Module work?
A VLM has an elevator that runs vertically between two stacks of shelves. One of these stacks of shelves is located at the front of the VLM, and the other is located at the back. The elevator can respond to commands and it automatically retrieves and delivers trays to an operator as instructed. Typically, the trays are delivered to a workstation at waist-high level (i.e., at the ergonomic “golden zone” of 37-40” high).
The Vertical Lift Modules that offer the fastest throughput
When it comes to throughput, the single column, high speed tote based VLM is by far the fastest among all of the different types of VLMs. That is one of the main reasons high speed tote based VLMs are becoming increasingly common in distribution environments.
One operator working a high speed tote based VLM can pick up to 800 lines an hour (versus 280 lines per hour with a standard single column VLM). Keep in mind though, that as VLM speed increases, storage density decreases. A high speed tote based VLM may be five totes wide, and that can mean the loss of storage density.
It is also worth noting that VLM speed can be accelerated by software. For instance, software can control how a VLM is zoned and/or optimized for slotting. In some cases, VLMs are optimized overnight (a process to similar to defragmenting a hard drive).
The Vertical Lift Modules that offer the most storage density
The VLM that provides the most storage density is a single column, double deep, dual elevator VLM. With a single column, double deep, dual elevator VLM, it is possible to have two racks of shelving on the frontside of the VLM and two racks of shelving on the backside of the VLM. This essentially doubles the storage capacity of the VLM while only increasing its footprint on the warehouse floor by about one-third.
A multi-column VLM can also provide storage density—if offers more than a standard single column VLM, but less than a double deep, dual elevator VLM.
The VLM with the least storage density is the high speed tote based VLM; however, with a high speed tote based VLM, the loss in storage density is offset by an increase in picking speed.
The most common Vertical Lift Module options
To complement their basic functionality, VLMs can be customized with additional options. For example, VLMS can have product identification capabilities, such as pick-to-light systems that use light-directed technology to identify product locations or voice automation systems that use verbal commands to drive the operator to a specific cell.
VLMs can also have different access levels—on a mezzanine, for instance, or at the front, or the back, or both the front and back. Even the access opening itself can be specially configured for stock of different heights.
Lastly, there are also options for fire suppression, either with water or inert gas, and for climate control, including refrigeration.
Features to review when evaluating a VLM
When evaluating a VLM, be sure to review all of its features, from construction details and the quality of structural components to drive systems and software.
- VLM construction details. Assessing the quality of construction of a VLM involves examining its various structural components. For instance, be sure to look at the tray supports. Some VLMs have tray supports that are fabricated externally and then placed within the machines; others have tray supports that are welded and built into the machine itself. In addition, be sure to inspect the VLM trays to see how they are constructed. Some VLM trays are built from multiple pieces of steel riveted together; others are bent steel.
- VLM drive systems. A VLM can be driven by a belt system, a rack and pinion or gear system, or a chain system. Which type of VLM drive system is optimal depends on the specific business application and horsepower required. In the past, nearly all VLMs were driven by chains; however, VLM chains can stretch over time, necessitating frequent maintenance for optimal functioning. Today, more and more VLMs are being built with rack and pinion or gear drive systems. These rack and pinion systems do not stretch and generally require less maintenance than chain drive systems. Plus, VLMs driven by rack and pinion systems are also much faster. X
- VLM software. Software is a critical feature of any automated storage and retrieval system—without it, a VLM is nothing more than a storage unit. The VLM software is what enables the various VLM capabilities, such as batch picking, storage optimization, and slotting optimization. The VLM software must also be able to seamlessly integrate with the warehouse management system (WMS) or ERP system already in place.
Get started with Vertical Lift Modules (VLMs) today
Adding the right VLM can dramatically improve the throughput, efficiency, and accuracy of your warehouse, distribution, or manufacturing environment. This blog post covered several of the primary issues you need to consider when determining which VLM is best for you. For more information and to discuss the details of your particular application