In relation to storage space, shuttle systems cost at least twice as much as a small parts warehouse with a conventional ASRS. Nevertheless, shuttles are becoming more and more popular. Why is that? And why would anybody need such a high throughput and accept high costs in return?
The majority of shuttles supplies goods-to-man workplaces. When the goods travel to the man – well, to be correct it should be called goods-to-person – the efficiency of the order picking process jumps up. That’s because the operator doesn’t have to walk from one bin location to the next, or from and to the place where he assembles the orders. In conventional warehousing, these walking times typically take up between 40 % and 70 % of the entire workday.
This is something to think about very clearly: more than half of the workday in conventional warehouses is wasted on a completely unproductive activity. By the way, just because the goods are transported to a stationary order picking workplace, doesn’t mean that the process is optimal. The efficiency and the ergonomic design of the workplace also matter, a lot actually. We already talked about what this means in detail in one of our earlier blog posts.
Why it makes sense to speed up the handling of slow moving items
The goods-to-man principle is particularly profitable when handling products with low and medium rates of access. This is understandable considering that the 80-20 rule holds true in most distribution centres. A small number of fast moving items (20%) can be grouped in optimal storage positions (= less travel time), which improves performance by very simple means.
But then we could be tempted to believe that some low-budget technology should be sufficient for the remaining articles. So why invest in high-performance equipment, such as a shuttle?
Well, in most cases, medium and slow moving articles are only requested in small amounts per order. For example, a pharmacy is usually going to order Aspirin, a fast-mover, in large quantities, but rarely needs a specific drug against metabolic disorders, and only in small quantities.
As a result, each order picking for medium and slow moving items only takes a short amount of time. After all, the amount to be picked is very small. To get the order picker to work efficiently, it is important to have the next article ready without delay. A goods-to-man workstation is essentially like a vacuum cleaner, sucking in items from all over the warehouse.
It usually “sucks“ between 500 and 1000 different articles each hour. Because these articles are stored separately in storage bins (boxes or cartons), the warehouse needs to provide 500 to 1000 double cycles per hour for each workstation. If we just add up as little as 3, 4 or 5 of these workstations, we end up with an enormous thirst for performance.
This explains why, in particular, medium and slow moving items in a goods-to-man concept need a fast-operating storage device. In fact, in many cases even a shuttle system wouldn’t be fast enough.
How to compare the speed of storage and retrival machines
But how can we compare different storage devices with regard to speed? As a reader of this blog, you’re most likely familiar with the term double cycles (DC). However, in order to make a comparison of systems, we need a reference value. A single shuttle can roughly achieve 30-50 double cycles per hour. This would make it a lame duck, even compared to a standard ASRS.
The best way to compare them is to look at the number of double cycles per hour, but also per 1000 storage positions. This is an indicator that is completely independent from the design of the warehouse. Something like “performance per aisle” on the other hand cannot work in systems that don’t have aisles, but instead use lifting bars or carousels.
Shuttle systems typically perform at 100 to 150 DC/h per 1000 storage positions. A conventional ASRS can only do 20 to 40 DC/h per 1000 storage positions. But the following number is much more striking: the Schaefer Carrousel System (SCS) achieves up to 250 DC/h per 1000 storage spaces. This means, it’s about 60% faster than a shuttle system!
It’s therefore no surprise that the SCS has been beating its own sales records time and again over the last couple of years. The increasing requirements coming from highly efficient and ergonomic goods-to-man workstations can hardly be met with shuttle systems, at least not without unjustifiably high costs. Even when we consider cost per storage position, the SCS doesn’t fall behind a shuttle system.
In case you need more performance than a shuttle system is capable of delivering, you should take a look at the SSI Schaefer website for more details on the Schaefer Carousel System.