Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 1
What is Drum Buffer Rope?
Drum Buffer Rope (DBR) is a planning and scheduling solution derived from the Theory of
Constraints (ToC).
The fundamental assumption of DBR is
that within any plant there is one or a
limited number of scarce resources which
control the overall output of that plant.
This is the “drum”, which sets the pace
for all other resources.
In order to maximize the output of the system, planning and execution behaviors are focused
on exploiting the drum, protecting it against disruption through the use of “time buffers”,
and synchronizing or subordinating all other resources and decisions to the activity of the
drum through a mechanism that is akin to a “rope”.
Theory of Constraints
ToC (Theory of Constraints), also called Constraint Management, is a philosophy and set of
techniques used to manage an organization. Most widely implemented in manufacturing
operations, it teaches management how to identify and direct their focus on the few critical
drivers that matter to the bottom line performance.
Eliyahu Goldratt originated the idea in his book The Goal as a way of managing the business
to increase profits. ToC is a proven method that can be used by existing personnel to
increase throughput (sales), reliability, and quality while decreasing inventory, WIP, late
deliveries, and overtime. Successful organizations also adopt ToC to help make tactical &
strategic decisions for continuous improvement.
The crucial insight of ToC is that only a few elements (constraints) in a business control the
financial results of the entire company. ToC tools identify these constraints, and focus the
entire organization on simple, effective solutions to problems that seemed insurmountably
complex and unsolvable.
The Scheduling Problem
When one looks at the load versus capacity, one must look at each resource individually. The
aggregate view of, for example, 1000 hours available in the factory versus 880 hours of
demand doesn’t adequately describe the situation. In figure 1, we see that most work
centers have extra capacity, while work center 3 is fully loaded and cannot accept more work.
The true state of this plant is that it is full and cannot accept more work that involves WC3.
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 2
In addition, we must consider the time frame in which the demand occurs. A monthly or
weekly aggregate view of demand may not be sufficient to take action and deliver work on
time.
To solve this problem, most systems will offset by some standard fixed lead time, but all that
does is move the peak over. Forward scheduling algorithms will not “see” the peak until it’s
too late
The peak demand must be moved to open capacity.
If you ignore peak demands, you will have expediting, overtime, additional WIP, late
deliveries because capacity may not be available when needed. This will have negative
effect on system throughput, due date performance, and lead times.
ToC in Production
The Theory of Constraints is an integrated management philosophy and set of techniques
which serve to manage & optimize the activity of the business.
ToC begins with one underlying assumption; the performance of the system's constraint will
determine the performance of the entire system. To help you understand explain, we use a
chain as an analogy. The strength of the chain is determined by its weakest link. What
determines the strength of the chain? Its weakest link.
Capacity
0
500
1000
1500
2000
2500
WC1
WC2
WC3
WC4
Demand
Capacity: What is available?
Demand: What is needed?
0
100
200
300
400
500
600
700
800
900
D1 D2 D3 D4 D5
Minutes
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 3
The process of delivering a product or service is very much like a chain; each resource
and function are linked. It only takes one element in the system to fail, to cause the
entire system to fail.
In order to improve the system, we must optimize the weakest link; the constraint or
drum. All other resources are subordinated to that. In scheduling terms, we
1. Develop a detailed schedule for the drum resource
2. Add buffers to protect the performance of that resource
3. Synchronize the schedule of all other resources to the drum schedule
The Drum Buffer Rope Solution
Identify the system’s constraint
The first step is to identify the drum. The drum is typically the most heavily loaded
resource (or workcenter) in the plant.
Capacity
0
500
1000
1500
2000
2500
WC1
WC2
WC3
WC4
Demand
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 4
Exploit the constraint
Once the drum has been identified, a detailed schedule is prepared to satisfy the
customer requirements, resolving the peak loads.
The impact on the non-constraints is to smooth
out the load, because their processes are
connected to the constraint resource.
The Drum Schedule
DAY PART QTY MIN
1 P2 10 240
1 P2 10 240
2 P2 10 240
2 P1 20 240
3 P1 30 360
3 P2 5 120
4 P2 15 360
5 P1 40 480
6 P1 11 132
P2
P1 P2P2
P2
P1
P2
P2
P1
P1
Capacity: What is available?
Demand: What is needed?
0
100
200
300
400
500
600
700
800
900
D1 D2 D3 D4 D5
Minutes
40 hrs / week
P1 demand
51 on day 5
50 on day 3
P2 demand
10 on days 1-5
0
100
200
300
400
500
600
D1 D2 D3 D4 D5
WC1
WC3
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 5
The Buffer
The buffer is a period of time to protect the drum
resource from problems that occur upstream from
the drum operation. Its effect to provide a
resynchronization of the work as it flows through
the plant.
The buffer compensates for process variation, and
makes DBR schedules very stable, immune to most
problems. It has the additional effect of eliminating the need for 100% accurate data for
scheduling. It allows the user to produce a “good enough” schedule that will generate
superior results over almost every other scheduling method.
Since the buffer aggregates variation, it also allows to operate the plant with much lower
levels of work in process, producing dramatic reductions in production lead times and
generating a lot of cash that was tied up on inventory.
The “extra” capacity at the non-constraints helps, too. Since the plant is not overloaded with
work it cannot do, the resources can “catch up” when problems strike, without affecting the
drum or global throughput.
Synchronize to the Drum - Subordination
After the drum has been scheduled, material release and shipping are connected to it, using
the buffer offset. Material is released at the same rate as the drum can consume it. Orders
are shipped at the rate of drum production.
DBR Scheduling Algorithm
The process of scheduling the factory first focuses on the primary objective of the facility, to
ship to committed delivery date. Thus we first find the due date of the order, and add a
shipping buffer to create an “ideal” finish date with confidence.
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 6
From this planned finish date, the order is backward scheduled to identify an “ideal” time to
work on the drum resource, a “latest due by” (LBD) date.
All orders are scheduled to fit on the drum using two passes; first, by assigning all batches an
ideal placement on the drum schedule.
When the batch does not fit, i.e., there is another occupying its space, the batch is scheduled
earlier in time so the order due date is not violated. This may result in some jobs starting
before today, and not all jobs may be ready to start at the drum resource.
The drum is then forward scheduled to resolve these conflicts, and potentially late jobs are
identified (the red batch).
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 7
After the drum is schedule, the operations after the drum are scheduled forward in time from
the drum completion date.
Then, the jobs feeding the drum are backward scheduled from the start of the resource
buffer.
Introduction to Drum Buffer Rope (DBR)
© 2004-2008 Mark Woeppel http://pinnacle-strategies.com Page 8