Impact of Ergonomics on Cycle Times: From Variable Fatigue to Maximized OEE (Horizon 2025)

In modern methods engineering, ergonomics has ceased to be a peripheral discipline of prevention (OHS) to become a critical mathematical vector of productivity. If we analyze cost structure of any industrial plant, biomechanical load acts as a “silent thief” of Takt Time, forcing engineering departments to inflate Rest Coefficients (K) and destroying theoretical operator saturation.

In this Cronometras technical analysis, we dissect direct correlation between TMU (Time Measurement Units) reduction, variable fatigue management, and OEE stability. We do not speak of occupational health; we speak of shielding Standard Time.

The Hidden Equation: How “Muri” (Overburden) Alters Standard Time ($T_t$)

For a time engineer, Standard Time formula is sacred. According to ILO regulations and industrial engineering best practices, it is defined as:

$T_{t} = T_{b} \times (1 + \%Allowances)$

Where:

• $T_{t}$ = Standard Time.
• $T_{b}$ = Basic Time (Normalized to 100 Bedaux/Centesimal activity).
• $\%Allowances$ = Personal needs + Base Fatigue + Variable Fatigue.

The “Class 1” Error in Predetermined Systems

Systemic error in most plants is calculating basic times ($T_b$) using predetermined time systems (like MTM-1, MTM-UAS, or MOST) assuming ideal or “Class 1” conditions. Analyst models motion on paper, but plant reality introduces “Muri” (Overburden).

When an operator works with awkward postures (e.g., trunk flexion >20º) or under static loads, physiology imposes a toll. This forces applying Variable Fatigue allowances oscillating between 12% and 20% over basic time to comply with agreements and ILO regulations.

Micro-pauses and Variance Phenomenon

Even more critical for OEE is what is not measured. A workstation with poor ergonomics causes operator to introduce non-standardized motions (micro-pauses, stretching, grip changes) to recover blood flow. These deviations are not detected in punctual timing but generate variance ($S^2$) destabilizing line balancing and making maintaining constant 100 Pace impossible for 8 hours.

Technical Synergy: MTM and EAWS Integration to Eliminate Wastes

At Cronometras we hold a technical maxim: A work method is not valid if it is not physiologically sustainable. A fast cycle exhausting operator in 4 hours is a false cycle.

High-level methodology demands auditing processes with EAWS (Ergonomic Assessment Worksheet) simultaneously with MTM-UAS analysis.

Case Study: Line Assembly (Metal-mechanic)

Let’s analyze a screwdriving station in low zone (height < 50 cm):

• Situation A (Current): Operator performs MTM AB (Bend) and AS (Arise) motions.
  • MTM Cost: ~60 TMU per cycle.
  • Necessary Fatigue Allowance: 15% (due to severe postural load).
• Situation B (Ergonomic Redesign): Implementation of lifting platform or rotator.
  • Impact 1: Physical elimination of AB/AS motions (-60 TMU direct).
  • Impact 2 (Hidden Multiplier): By eliminating postural load, Fatigue Coefficient drops from 15% to 5%.

Result: Not only do we reduce Basic Time ($T_b$), but we reduce formula multiplier. Net technical productivity gain exceeds 20%, validating engineering investment.

Regulatory Framework 2025: Cost of Ignoring ISO 11228 and Demographic Factor

Plant Managers must prepare for restrictive regulatory and demographic scenario in Spain and Europe.

1. Ageing Workforce

Average industrial operator age will exceed 48 years in 2025. Functional capacity decays, and predetermined times (designed under “young average man” standards) will cease to be realistic without adjustments. If technical ergonomics is not improved, unions will demand upward revisions of fatigue allowances, increasing operator-minute cost.

2. ISO Audits and Frequency Penalty

Reference standards for time calculation must be crossed with:

• UNE-EN ISO 11228 (Manual Handling): It is no longer enough to evaluate weight (kg). The standard exponentially penalizes frequency (cycles/minute). A high-frequency cycle with moderate weight can be declared “Not Suitable”, forcing line speed reduction or staff rotation (inefficiency).
• ISO 11226 (Static Postures): Impact of Holding Time (time maintaining posture) directly affects assembly quality and is a blind spot in traditional timing.

High Performance Engineering Solutions (“Cronometras” Portfolio)

We do not sell comfort; we sell process stability and Takt Time assurance. Solutions must be hard engineering:

1. Passive Exoskeletons:
   • For stations with reach motions (Reach/Move) above shoulder level.
   • Technical Objective: Does not seek to reduce motion TMU, but reduce metabolic consumption to maintain stable 100 Bedaux Pace until last hour of shift.
2. Cobotics for “Pick & Place”:
   • Strict transfer of heavy loads to robot (ISO 11228).
   • Operator is freed for fine skill and value-added operations, reducing cycle variability.
3. Video-Analytics & AI:
   • Use of advanced software overlaying ergonomic heatmaps (REBA/RULA in real-time) over timing. Allows identifying exact second where cycle breaks due to biomechanical overload.

Impact on OEE and Ergonomic Intervention ROI

For Operations Management, financial data is conclusive. Applied ergonomics is a profitability tool:

• Variance Reduction ($S^2$): Ergonomically optimized stations (RULA score < 3) show cycle time standard deviation 40% lower. This allows MRP reliability and production planning without safety “buffers”.
• Quality Rate: 30% of defects in manual processes occur in last 90 minutes of shift. Root cause is localized muscle fatigue preventing motor precision. Ergonomics = Quality at shift end.
• Return on Investment (ROI): Our average recovery calculations sit at 4.5 months, based solely on time savings (allowance reduction) and technical absenteeism reduction.

Technical FAQ on Times and Ergonomics

How is variable fatigue allowance % calculated according to ILO?

It is calculated via valuation tables scoring factors such as force exerted, posture, vibration, and environmental conditions. These points convert to a percentage added to base rest coefficient (generally 4-5%) and personal needs.

Difference between MTM-UAS and MTM-2 in high physical load stations?

MTM-UAS is preferable for serial production of large batches with medium-long cycles and allows better description of compound body motions. In high load stations, MTM-UAS facilitates identification of “Auxiliary” motions symptomatic of poor ergonomics, while MTM-2 might mask them due to lower granularity.

When is introducing an exoskeleton in assembly line profitable?

Profitable when operation implies keeping arms above shoulders more than 30% of cycle or repetitive lumbar manipulations, and activity drop greater than 10% is detected in last hours of shift.

How does RULA score affect OEE calculation?

High RULA score (>6) indicates high probability of micro-stops and speed reduction (Performance Loss), as well as increased errors (Quality Loss). Improving score directly impacts two of three OEE factors.

Do Your Rest Coefficients Reflect Your Plant Reality?

Standard Time is money. If your operators do not reach Takt Time without unauthorized fatigue, or if your OEE fluctuates inherently at shift end, your methods engineering has a profitability leak.

At Cronometras we don’t “guess” times, we shield them scientifically integrating work measurement with advanced biomechanics.

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