Cell Redesign Under Mixed Demand and Fatigue Constraints
A fabricator runs a U-shaped cell assembling three harness variants. Current standard time is 4.2 min/unit at 95% performance rating, with a 12% allowance. Observed cycle times across 6 operators: 4.6, 4.1, 4.4, 4.9, 4.2, 4.0 min. The cell averages 52 units/shift (480 min) but demand is 60. Management proposes eliminating the 10-minute mid-shift break and adding a fourth operation to each operator's loop. The safety committee notes two operators report wrist discomfort, and the cell's walking distance would rise from 18 m to 27 m per cycle. A time study log shows the 4.9-min observation occurred during a tool change.
1. Compute a defensible standard time and decide whether removing the break is a valid way to close the 8-unit gap.
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Standard time. Discard the 4.9-min observation as a non-routine tool change (foreign element). Average the remaining five: (4.6+4.1+4.4+4.2+4.0)/5 = 4.26 min. Normal time = 4.26 × 0.95 = 4.05 min. Standard time = 4.05 × 1.12 = 4.53 min/unit.
Capacity check. At 4.53 min/unit, 480 min yields ~106 units theoretically, but the cell only makes 52 - a large gap between standard and actual output signals lost time (walking, tool changes, imbalance), not a loose standard. Removing a 10-min break recovers 10/4.53 ≈ 2.2 units, nowhere near 8.
Judgement. Reject break removal: it addresses ~25% of the gap while adding fatigue risk to already-symptomatic operators. The real losses are tool-change time and walking, which will worsen with the longer loop. Target the foreign elements and balance instead.
2. Evaluate the proposed fourth operation and longer walking loop against ergonomic criteria.
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Adding a fourth operation plus 9 m extra walking per cycle increases both cycle time and physical load. Walking at ~1.4 m/s adds roughly 6.4 s per cycle; over 60 units that is ~6.4 min/shift of pure travel - real but modest. The dominant concern is cumulative wrist load: two operators already report discomfort, so adding tasks to their loop violates the principle of reducing repetition and static load for at-risk workers.
A defensible redesign: keep the three-operation loop, use a shared tool cart to cut walking, standardize tool changes via quick-change fixtures (attacking the 4.9-min outlier root cause), and rotate the two symptomatic operators to lower-repetition tasks while a job analysis is done. Re-measure after changes rather than assuming the standard holds.
3. State what data you would collect before committing to a new standard time.
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Collect: (1) a continuous time study with at least 10-15 cycles per element, separating regular from foreign elements; (2) a performance rating by a second analyst for calibration; (3) an allowance study (fatigue, personal, delay) rather than a blanket 12%; (4) walking-distance and frequency data per cycle; (5) tool-change frequency and duration; (6) output and downtime records to reconcile standard vs actual. Without these, any new standard is a guess that will be disputed at the next negotiation.