improving warehouse picking efficiency through slow...

Slotting Your Next Move

Improving Warehouse Picking Efficiency through Slow Mover Positioning and Selector Strategies


Copyright 2011 Optricity Corporation. All rights reserved.

Executive Summary

When slotting a warehouse, slow movers are not to be overlooked, as they typically

constitute 70-80% of total items. A slow moving item is often characterized as one with

few hits, or visits to the pick location. In instances where slow moving items take up the

majority of space in a warehouse, it is critical that the items be slotted in a manner that

facilitates intelligent slow mover picking. There are a number of techniques that can be

employed to reduce selector travel and to increase pick density including layout

configurations, selection strategies, and golden zoning considerations.

Through an evaluation of slow mover activity, one can determine the best size for a slow

moving area and the picking techniques best suited for that area. Slow moving areas are

often defined by cross aisles in the pick path that allow selectors to avoid travelling the

entire length of the pick path. Picking techniques in the slow moving areas often differ

from those in the fast moving areas. Batch picking often makes more sense in slow

moving areas to increase pick density. Height or golden zone considerations are crucial

in many pick to cart or pick to belt areas. Often pick to cart or pick to belt areas for slow

movers are completely separate from the main fast moving area where orders are picked

singularly in waves.

Although there is great benefit in focusing on the fastest moving items in the pick line,

one should also examine the potential gain from evaluating slower moving items in the

pick line.



Table of Contents EXECUTIVE SUMMARY ..................................................................................................... 2 1.0 INTRODUCTION ......................................................................................................... 4 2.0 MOVEMENT MEASURES ............................................................................................. 5

2.1 Selling Unit Movement .......................................................................................... 5 2.2 Visits to Pick Slot .................................................................................................. 5

3.0 DETERMINING THE OPTIMAL RACK LAYOUT AND PICK PATH FOR INCREASED

EFFICIENCY ...................................................................................................................... 6 3.1 Reducing Travel Distance Using a Cross Aisle to Separate Fast and Slow

Movers......................................................................................................................... 6 3.2 Separating Pick Operations to Save Selection Travel .......................................... 7 3.3 Item placement for Pick to Cart Selection ............................................................ 9

4.0 ENHANCING PRODUCTIVITY USING GOLDEN ZONING .............................................. 10

4.1 Sequencing to Minimize Congestion ................................................................... 12 4.2 Additional Strategies for Enhancing Selector Productivity ................................ 12

4.3 Using a Hybrid Approach to Shorten the Pick Path ........................................... 13 5.0 CONCLUSION ............................................................................................................ 14



1.0 Introduction

Common wisdom dictates that when evaluating the locations of items in the pick line,

working on the 20-30% fastest movers provides the best return on investment. But that

doesn’t mean that looking at the slowest 70-80% of items is not worthwhile. In fact, one

must continuously evaluate all items in the pick line not only to make sure the fastest

items are in the prime locations, but also to ensure that there are no slow movers taking

up prime space where fast movers should be. One simple process for maintaining an

optimal pick environment is to, on a continual basis, free up the most efficient pick

locations by taking slower movers out of them, search for faster moving items that should

occupy the newly freed up locations, and then move them. Of course, this process is

quickly muddied because of the number of constraints that must be adhered to, including

grouping and sequencing rules, as well as making sure that the slotting unit (e.g., pallet,

case, inner, each) will actually fit in the new location while still maintaining a reasonable

amount of inventory in the pick slot.

Because the warehouse environment is dynamic, there is a need to continuously shift the

placement of items in the pick line. Factors that account for this dynamic environment

include, but are not limited to, the addition and deletion of items from the pick line,

seasonal item demand fluctuations, and promotional or allocation ‘pushes’ of items to

customers. When these events occur, it is likely that the movement of some items will

temporarily increase, and then, after some period of time, normalize. The time period is

very short for an allocation or item ‘push’, longer for a seasonal item, and even longer

over the life cycle of any particular item.

By exploring various slow mover selection strategies, one can see how fixing slow

movers can increase selector productivity by both creating space for fast movers and

cutting down the travel path.



2.0 Movement Measures

When discussing fast and slow moving items, two movement measures must be used:

selling unit movement and visits to the pick slot. These movement measures are directly

related to profiling, sequencing, reducing travel path, and golden zoning.

2.1 Selling Unit Movement

A selling unit might be a pallet, case, inner or each of any item; however, in many

situations, the selling unit is different from the slotting unit. A slotting unit defines how

an item is actually slotted in a location. Typical slotting units are pallet, case, inner, each,

bin, etc. Selling unit movement is required in order to determine the correct profiling of

an item to a location type (e.g. single high pallet rack, 5 level flow rack, 9 level carousel,

etc.). With an understanding of selling unit movement, slotting unit dimensions and

weight, and a targeted number of days of product that is desired to keep in the pick slot,

an item can be profiled to fit in a particular location type, where the goal would be to

keep the targeted days supply of product in the smallest location that can provide that

number of days supply. So, unit movement has much to do with profiling to the best type

of location in the pick line. In many cases, unit movement values are also used to

sequence items down the pick path or within groups down the pick path, with faster

movers slotted earlier on in the pick path or the group. Care must be taken to not cause

congestion during the pick process if sequencing items based on unit movement.

2.2 Visits to Pick Slot

The term ‘visits’ is sometimes referred to as ‘lines’ or ‘hits’, simply indicating the

number of times a selector must travel to a location, regardless of the number of selling

units that are selected. This is a key value when trying to minimize travel distance and

bend and reach during the selection process. The visits value and the unit movement

value may not be in alignment, especially for smaller items. It is possible to visit a

location one time but select many of that item during the visit, resulting in differing

values for unit movement and visits. Where these instances occur frequently, one can see



the importance of sequencing items down the pick path using the visits value rather than

the unit movement value. Additionally, the visits value aids in the effort to golden zone

(i.e., ergonomically slot) items to improve selector productivity, and keep selectors safe

from possible injury.

3.0 Determining the Optimal Rack Layout and Pick Path for Increased Efficiency

The gain from paying attention to both fast and slow moving items is heavily dependent

on the physical rack layout and pick path in use. There’s really not much to gain if every

inch of the pick path must be traveled for every order or batch of orders. By examining

various layouts and pick path options, one can see how the arrangement of fast and slow

moving items and the use of a cross aisle can contribute to minimizing the travel path and

improving selector productivity.

3.1 Reducing Travel Distance Using a Cross Aisle to Separate Fast and Slow Movers

For serpentine pick environments, it is common to see cross aisles, usually about 2/3 of

the way down each aisle (away from the shipping dock). One purpose of the cross aisles

is to enable a selector to skip a portion of the pick path, if there is no need to select

product on the back portion of the aisle (beyond the cross aisle). In this environment it

would make sense to slot faster movers before the cross aisle and slower movers after the

cross aisle. Additionally, in the one-way, serpentine environment, if a selector uses the

cross aisle, he is skipping not only the back portion of the aisle that he is coming from but

also the back portion of the aisle he is entering via the cross aisle. Because of this, the

layout of product in the pick line should clearly delineate faster movers in front and

slower movers in the back as to increase the likelihood that the cross aisles can be used,

reducing travel distance. In addition, as pictured in Figure 1, behind the cross aisle, items

should be slotted in a velocity sequence along the serpentine pick path so as to maximize

the likelihood of walking only one or two aisles behind the cross aisle. Slotting the



fastest 30% as well as some ‘B’ movers before the cross aisle typically increases the

opportunity to utilize the cross aisle.

Figure 1: Eliminating Excess Travel by Using the Cross Aisle

3.2 Separating Pick Operations to Save Selection Travel

A clever approach in a traditional serpentine environment separates the selection

operation before the cross aisle from the selection operation behind it. In the front

section, orders may be picked by order, perhaps within a wave. This operation handles

the fastest movers. Behind the cross aisle, orders are batch picked, gaining significant

pick density, and then married to their specific orders on the shipping dock. The trade-

off, of course, is the saving of selection travel against the double handling of outbound

items. Figures 2 and 3 illustrate this approach.



Figure 2: Varying picking strategy by area can improve selector productivity.

Careful evaluation of fast versus slow movers helps to further refine this approach, where

perhaps only half of the aisles have their slow moving items selected separately in a batch

mode.

Figure 3: Batch Picking Only the Slowest Moving Items



3.3 Item placement for Pick to Cart Selection

Pick-to-cart scenarios must consider unique techniques

to realize savings. Consider a pick-to-cart (or pick-to-

belt) operation where the cart is pulled down a center

aisle with items slotted in short aisles that are

positioned perpendicular to the pick path. The selector

performs two operations. He pulls the cart down the center aisle. And he travels into the

perpendicular aisles to select items for an order (or batch of orders). To minimize his

travel path, one would place the fastest moving items (those with the most visits) closest

to the center aisle, with slower movers away from the center aisle, as illustrated in Figure

4.

Figure 4: Placing Fast Moving Items Closet to the Pick Path

Pick-to-cart scenarios

must consider unique

techniques to realize

savings.



4.0 Enhancing Productivity Using Golden Zoning

The approach illustrated in Figure 4 above can be enhanced by applying golden zone

rules to the items and slots at the same time (in a multi-level pick environment). Golden

zone rules typically have the fastest of the fast items slotted at a height between the

shoulders and the waist. The challenge in the described environment is determining the

trade-off between ‘bend and reach’ ergonomics and productivity versus the opportunity

to reduce horizontal travel down an aisle. The clever solution involves something called

a ‘productivity wedge’, which is pictured in Figure 5.

Figure 5: The productivity wedge reduces bend and reach and horizontal travel.

The productivity wedge slotting rule considers the trade-off between bend and reach and

horizontal travel by a selector. It applies the rule to assign items to slots based on that

trade-off. One extreme trade-off is that bend and reach always costs less (in time) than

horizontal travel. In this situation, one might assign all levels of an entire bay the same

selection productivity value, with the next bay down the aisle having lesser productivity

value, and so forth. The other extreme trade-off is that horizontal travel always costs less

(in time) than bend and reach. In this situation, one might assign an entire level or levels

between the shoulders and waist the highest productivity value, with lower and higher

levels given a lower productivity value, as illustrated in Figure 6.



Figure 6: Golden Zone slotting improves ergonomics.

If the trade-off between the two is considered, then the productivity wedge takes shape.

The pick path described above must be maintained regularly in order to benefit from the

productivity based slotting. It is easy to see how the errant slotting of a fast moving item

at the back of a perpendicular aisle will increase travel time significantly, especially if

that same item could have been slotted immediately next to the center aisle. Consider this

error across multiple perpendicular aisles and the problem becomes enormous, as there

would be a need for multiple trips to the back of the perpendicular aisles. One might

think of the slotting objective in this scenario to be to minimize trips down the

perpendicular aisles, rather than maximize placement of fast moving items along the

center aisle. The reason for this is that the selector will travel the entire center aisle

anyway, meaning the only way to affect travel distance is to minimize travel in the

perpendicular aisles.



4.1 Sequencing to Minimize Congestion

Slotting by velocity can often lead to congestion in the

pick path. In our example the fastest moving items

should be slotted in the center aisle. However, since the

selector will always travel the length of the center aisle,

there is no need to group the fastest items near the

beginning. Rather the fast moving items should be

randomly distributed along the center aisle. This will greatly reduce the likelihood of

selector congestion near the beginning of the pick path.

4.2 Additional Strategies for Enhancing Selector Productivity

Other strategies may also be applied to enhance selection productivity, especially in cases

where a selector can visit more than one pick slot in a single trip down a perpendicular

aisle. For example, one may consider slotting medium movers down the first set of

perpendicular aisles, followed by slower movers, followed by the slowest movers, as

pictured in Figure 7. If it is found that significant trips are being made down the medium

mover perpendicular aisles, then one might consider serpentine picking that set of

perpendicular aisles, as this will eliminate walking up and back the same aisle.

Furthermore, one might consider slotting items that are commonly ordered together near

each other in the pick line. And lastly, if batch order picking, one might consider the

order selection criteria to include the proximity of slower moving items across orders,

since the slower moving items cause the extra travel distance.

Since the selector will

always travel the length of

the center aisle there is no

need to group the fastest

items near the beginning.



Figure 7: Placing Medium Movers Before Slow Movers in the Pick Path

4.3 Using a Hybrid Approach to Shorten the Pick Path

Consider a hybrid rack layout where on one side of the pick path are flow rack and on the

other side are static shelving in aisles perpendicular to the pick path (see Figure 8).

Clearly the fastest movers (and larger items) would be slotted in the flow rack

immediately adjacent to the pick path. The fastest, smaller items (requiring less space to

keep the targeted days of product in the pick slot) would go into the perpendicular aisles

closest to the pick path. And then the same technique as described above would be

employed. The advantage of this hybrid approach is that the fastest movers (and larger

items) would need less facings and/or stackings to maintain the targeted days in the pick

slot; hence, the pick path has an opportunity to be shortened.



Figure 8: Reducing Travel Distance into Perpendicular Aisles

5.0 Conclusion

It makes great sense to focus on the fastest moving items in the pick line when

maintaining a slotting layout. The purpose of this paper is to point out the potential for

increasing selector productivity by also evaluating the slower moving items in the pick

line. Fixing slower moving items not only frees up valuable space for faster moving

items but also allows for increasing the odds that portions of the pick path will not need

to be travelled at all.



Optricity Profile

Based in Research Triangle Park, NC, Optricity provides a progressive approach to solving complex Supply Chain problems through best of breed optimization software. Optricity currently offers a variety of solutions from which to draw depending on the client’s supply chain optimization need including:

slotting optimization tool, OptiSlot DC™,

warehouse profiling tool, OptiProfile™

Juncture Optimization™ tools, which are custom-developed optimization solutions using proprietary RASSQ* technology. These engines optimize least-cost solutions based on competing goals and constraints of multiple functions where data and systems come together at merge points within the supply chain.

The Optricity team has a long history of providing supply chain solutions for clients whose problems require a combination of skills and expertise in order to solve correctly. Optricity solutions optimize operations, provide rapid payback and integrate easily with existing technology. The Optricity team capitalizes on strengths in advanced mathematics, software engineering, and deep supply chain industry expertise to design tools that achieve client’s operational improvement goals.

In addition to the commercial products, Optricity team members have specialized skills in solving complex Supply Chain problems through the application of advanced optimization engines namely in the areas of RASSQ - Routing, Assignment, Sequencing, Scheduling and Queuing. Most supply chain problems can be broken down into a combination of these functions. Using RASSQ technology Optricity develops custom software to rapidly deploy optimization solutions at Supply Chain Junctures – where systems, data, and functions merge. Leveraging the RASSQ engines allows significant reduction in traditional software development cycles and quickly puts an optimization solution into the customer’s hands. Capitalizing on improved processes, advanced computing algorithms and analysis techniques Optricity’s tools integrate with and power forward thinking supply chain solution providers. For more information, visit www.optricity.com.

With Questions Contact:

Sheila Benny [email protected] (919) 280-4418

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