It is well known that order pickers in a conventional warehouse spend almost
half their time walking from one
picking location to another. Just think of the report two years ago
by the BBC journalist who went undercover to work in a warehouse at
Amazon.com. He complained of sore feet as a result of walking distances that
could amount to as much as 17 kilometres during a single shift.
Many warehouses try to limit walking distances by allocating items to picking locations as efficiently as possible, a process known as slotting. In practice, this means placing fast-moving items at the front and slow-moving items at the back. The warehouse management system (WMS) often lends a hand by using past picking orders to calculate
which items are picked most frequently or least frequently. These items are then allocated to locations that are either closest to or furthest from the start or end point. This often results in a significant reduction in walking distances, but with a few handy tips, further improvements are possible:
1. Look to the future
As mentioned, the WMS bases its calculations of fast-moving and slow-moving items on past picking orders. However, what was a fast-moving item last month may well become a slow-moving item next month. Demand forecasting is therefore far more interesting than historical data: how many units of each item will be sold next month? So consult with sales and marketing to find out their expectations and adjust the storage locations of items accordingly. Sales promotions and marketing campaigns, in particular, can significantly influence demand for certain items. Vanderlande, Ortec, Centric and Slimstock have
explained previously in a joint white paper.
2. Identify which items belong together
In virtually every warehouse, there are examples of items
that frequently appear together in a single order. Think of plastic coffee cups and
stirrers, matching scarves and gloves, or smartphones and matching
cases. Anyone wishing to reduce walking distances would be well advised to
items close to one another. Don’t just rely on common sense
common sense, but also carry out a thorough analysis of all orders. There’s a
high that this will reveal combinations of items that nobody had
thought of. Retailers have been doing this for some time with their receipts. For example, a supermarket chain
once discovered that beer and nappies were often bought together in the evenings. It turned out
that many young fathers were sent out around that time to top up the supply
nappy supplies and, whilst they were at it, pick up a refreshing drink for themselves.
3. Consider the next link in the chain
When planning slot assignments, consider not only efficiency within your own warehouse, but also the efficiency of the next link in the chain. Supermarket chains already take into account the order in which items are stocked on the shop floor when picking orders. Product groups that are
are placed next to one another in the shop are also grouped together in the warehouses. As a result, there is a good chance that these product groups will end up on a single roll container, which prevents shelf-stockers from having to weave their way through the entire shop with roll containers amongst all the customers during opening hours. What applies to supermarket chains may well apply to other warehouses too. Consider, for example, warehouses that supply production lines.
4. Consider the walking route
What makes slotting complex is that an order usually involves more than one item. Placing fast-moving items at the front and slow-moving items at the back effectively ignores the fact that an order picker typically makes a round trip past several picking locations. If the fast-moving items are spread across several aisles, there is therefore a high chance that the order picker will still have to visit several aisles and is therefore not actually working efficiently. Ronald Mantel and Peter Schuur of the University of Twente have therefore devised a method aimed at creating the most efficient possible route past all the pick locations to be visited. With ‘order-oriented slotting’, items are placed in such a way that the total distance covered by all routes is kept as short as possible. According to both researchers, this can yield a further time saving of twelve to fifteen per cent.
5. Take the picking height into account
Usually, when slotting, only
the distance to the picking location, not the height of the picking location. The
Peruvian scientist José Larco concluded in his PhD research at Erasmus
University of Rotterdam that the picking height does
does indeed have an impact on productivity. This is particularly true when dealing with bulky
items or large quantities, picking from very high or very low
locations takes a relatively long time. According to Larco, we should
should therefore be placed in the ‘golden zone’ between hip and shoulder height. This
is also preferable from an ergonomic point of view. Anyone who has to bend over or
stretching all day are more likely to tire and may develop physical complaints. This, too,
leads to reduced performance and lower efficiency in the long run.
Marcel te Lindert - Journalist logistics and supply chain
