Slotting Your Next Move Improving Warehouse Picking Efficiency through Slow Mover Positioning and Selector Strategies
Slotting Your Next Move
Improving Warehouse Picking Efficiency through Slow Mover Positioning and Selector Strategies
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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.
Slotting Your Next Move
Copyright 2011 Optricity Corporation. All rights reserved.
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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