Drum‑Buffer‑Rope Tuner — Balance flow, not capacity
💡
Goal:
Optimize profit or cost while maintaining service.
Bottleneck (drum) capacity — units/hr
10
Release rate (rope) — units/hr
10
Buffer in front of bottleneck — units
15
Demand rate — units/hr
12
Downstream capacity (C2) — units/hr
12
WIP cap between BN→C2 (Q2 cap) — units
20
Variability (0–1, higher = noisier)
0.20
WIP carrying cost ($/unit·hr)
0.50
Lost‑sales penalty ($/unit)
20
Revenue per unit ($/unit)
40
Balanced to the Drum
Over‑release (chaos)
Starved Bottleneck
Bigger Buffer
Reset
Instructor notes
Shift the objective from “minimize cost” to “maximize profit = Revenue − (WIP + Lost‑sales costs).”
Discuss when raising buffers increases profit (prevents pricey stockouts) vs when it just adds carrying cost.
Ask: should we pace release to **BN** or to **market demand** when price (revenue/unit) is very high?
Bottleneck out
Downstream out (C2)
Buffer level (pre‑BN)
Q2 level (BN→C2)
Release in
Demand
Avg Throughput
–
Avg WIP (buf1+Q2)
–
Service Level
–
Avg Lead Time*
–
Total Cost (H)
–
Cost / Hr
–
Total Revenue (H)
–
Profit / Hr
–
*Approx Little’s Law: Lead ≈ WIP / Throughput.
What’s happening under the hood?
The
rope
releases work at the chosen rate with random noise based on variability.
Work waits in the
buffer
; the
drum
(bottleneck) pulls up to its capacity each tick.
Output from BN flows into a finite
Q2
before the downstream station
C2
. If Q2 is full, BN is blocked.
Demand is compared to shipped units to compute service level; excess demand counts as missed.
Costs & Profit:
each tick adds WIP carrying cost and lost‑sales cost; revenue = shipped × price; profit = revenue − cost.