Design for Quality and Product Excellence

Design for Quality and Product Excellence (Part 2)

Reliability

• Definition: The probability that a product performs its intended function for a stated time under specific conditions.
  • Expressed as a probability (0 to 1).
  • Time is crucial; longer operation can enhance reliability.
  • Performance linked to intended function.
  • Operating conditions vital (usage, environment).

Types of Reliability

• Inherent Reliability: Predicted reliability based on product or process design.
• Achieved Reliability: Actual performance observed during use.
• Types of failure:
  • Functional Failure: Occurs at the start of the product's life due to manufacturing or material defects.
  • Reliability Failure: Happens after a period of use.

Measures of Reliability

• Reliability measures include failures per unit time (failure rate, λ).

Example: Massive Corporation

• Tested five motors over 900 hours:
  • Failures at 200, 475, and 750 hours, with 2 running full 900 hours.
• Failure Rate Calculation:
  • λ = Total Failures / Total Operating Time = 3 / (200 + 475 + 750 + (2 x 900)) = 0.00093 failures/hour.

Product Life Characteristics Curve

• Three critical phases:
  1. Early Failure Period (Infant Mortality)
  2. Useful Life Period
  3. Wearout Period

Mathematics of Reliability

• Reliability Function (R(T)): Probability item will survive a given time.
  • Relationship: R(T) = 1 - F(T)
  • Failure Probability: F(T) = 1 - e^(-λT)
  • Probability during an interval: F(t2) - F(t1) = e^(-λt1) - e^(-λt2)
  • Reliability: R(T) = e^(-λT)

Example: Flatpanel, Inc.

• Failure Rate = 0.000072 units/hour; Calculate reliability function.
• Probability of failure within 8,000 hours:
  • F(T) = P(x < 8000) = 1 - e^(-0.000072 * 8000)
  • = 0.4379 (43.79% chance monitor survives < 8000 hours).

Hazard Function

• Probability of immediate failure at time t:
  • h(t) = f(t) * [1 - F(t)] = λ
• Mean Time to Failure (MTTF): reciprocal of failure rate;
• Mean Time Between Failures (MTBF) for repairable items.

Example: Spacescope, Inc.

• Failure Rate = 0.0000165/hour.
• MTTF = 60606.06 hours.
• Probability component survives 20,000 hours: R(T) = e^(-20000/60606.06) = 0.71892.

System Reliability

• Reliability data predicts overall system reliability.
• Configurations: Series, parallel, or mixed.
• For Series Systems: R_s = R1 * R2 * … * Rn = e^(-Σ(λi)T).

Example: Bestronics

• Process reliability example for sales where:
  • R(information system) = 0.998
  • R(point-of-sale) = 0.992
  • R(credit card system) = 0.978
• Overall system reliability:
  • R_total = (0.998)(0.992)(0.978) = 0.968.

Redundancy in Systems

• Backup components for reliability in case of failure.
• For Parallel Systems: R_s = 1 - (1 - R1)(1 - R2)…(1 - Rn).

Example: MagnaPlex, Inc.

• Manufacturing process with three operations in series using redundancy:
• Reliability calculation:
  • For single machine: Ra Rb R_c = (0.85)(0.92)(0.90) = 0.704.
  • With redundancy: Raa Rbb R_cc = (0.9775)(0.9936)(0.99) = 0.962.
  • Improvement: from 0.704 to 0.962.

Design for Excellence

• Objectives:
  • Higher functional and physical performance, user-friendliness, reliability, and environmental friendliness.

Activities for Design Excellence

• Focus on improving design and manufacturing processes over merely solving problems.
• Understand and exceed customer expectations rather than just meeting them.
• Optimize desired features while reducing costs without compromising quality.

Design Optimization

• Focus on robust design (insensitive to variations).
• Identify tools for optimization to improve reliability and reduce manufacturing defects.

Design for Manufacturability (DFM)

• Affects costs, quality from early design stages (70-80% impact).
• Guidelines for Quality Assurance:
  • Minimize parts and complexities to improve reliability and reduce assembly errors.
  • Design for robustness and eliminate adjustments.
  • Choose parts that survive processing and design for efficient testing.

Design for Environmental Responsibility

• Consider environmental concerns in product processes to reduce liabilities and enhance end-of-life value.

Design Failure Mode and Effects Analysis (DFMEA)

• Evaluates product safety, potential failures, and corrective actions.
• Scoring rubric to assess severity, occurrence, and detection ratings.

Fault Tree Analysis

• Describes conditions/events leading to failures and complements DFMEA with "and" and "or" gates in the tree structure.

Design Verification

• Design reviews at preliminary, intermediate, and final stages ensure accuracy and prevent costly changes.

Reliability Testing

• Essential for liability protection, assessing warranties, and understanding failure distributions.
• Types include burn-in, life testing, accelerated life testing to uncover latent defects.

FAQ: Reliability vs. Durability

• Reliability focuses on consistent performance; Durability emphasizes lifespan and resistance to wear.