Line balancing and bottleneck capacity: work from the constraint

A line's output is constrained by the operation that can deliver the least effective capacity under the actual mix and operating conditions. Adding the nameplate rates of serial stations does not describe line capacity. The useful first question is which operation repeatedly prevents the next good unit from leaving the line.

FIG. 03At the inspection gate

Start with a simple serial example

Three operations take 30, 45 and 35 seconds per part. With independent staffing, no losses and adequate transfer, the slowest station limits the serial line to one part every 45 seconds: 80 parts/hour. The total work content is 110 seconds, but dividing an hour by 110 estimates a single resource doing all three jobs sequentially, not this three-station line.

If the 45-second operation has 90% availability and 98% good yield, its effective rate becomes 80 × 0.90 × 0.98 = 70.56 good units/hour. This is a screening estimate. Stops that starve or block other stations, shared operators and variable cycles require time observations and, sometimes, a simulation.

Parallel is a physical claim

Two 45-second machines do not necessarily double output. A shared loader might only serve 100 parts/hour; a common inspection bench might accept 90. Record shared resources and measure their occupied time. The production capacity calculator treats entered lanes as independent, so do not use it to hide a common constraint.

Balance work against demand

Calculate takt from the same shift and demand boundary. Then split each station's cycle into work elements: handling, processing, walking, checking and waiting. Identify tasks that can move safely to another station without creating a new constraint or making operator work inconsistent.

As an illustrative lower bound, 110 seconds of work content divided by 45-second takt is 2.444, suggesting at least three fully usable parallel work resources. It does not prove three people will suffice: task precedence, machine occupation, reach, ergonomic constraints and variation can make that allocation impossible.

Verify an intervention

Compare good units at the line exit, WIP and operator workload before and after the change. A faster station accompanied by larger queues may only have moved the bottleneck. Keep product mix and planned time comparable, and distinguish a short demonstration from a sustainable shift result.

Use the downtime log to explain interruptions, the SMED observation sheet for changeovers, and the automation business-case guide when capacity evidence supports an investment.

Reference

Lean Enterprise Institute: cycle time distinguishes machine, operator and effective cycles. The capacities above are Shopfloor's illustrative arithmetic under the stated assumptions.