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Logistics Systems Engineering Maintainability/Serviceability/Human Factors

SMU SYS 7340. NTU SY-521-N. Logistics Systems Engineering Maintainability/Serviceability/Human Factors. Dr. Jerrell T. Stracener, SAE Fellow. Maintainability Maintainability is - an engineering and management function spanning the product or service life cycle

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Logistics Systems Engineering Maintainability/Serviceability/Human Factors

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  1. SMU SYS 7340 NTU SY-521-N Logistics Systems Engineering Maintainability/Serviceability/Human Factors Dr. Jerrell T. Stracener, SAE Fellow

  2. Maintainability • Maintainability is • - an engineering and management function • spanning the product or service life cycle • - a characteristic of equipment design and • installation which is expressed in terms of ease and • economy of maintenance, availability of the • equipment, safety and accuracy in the performance • of maintenance actions.

  3. Maintainability • Objective of maintainability • - to design and develop systems and • equipment which can be maintained in the least • time, at the least cost, and with a minimum • expenditure of support resources, without • adversely affecting the item performance or safety • characteristics

  4. Product/Service Support Resources • logistics personnel utilization • spare parts • tools and test equipment • support services • support facilities

  5. What is Maintainability? • Converters for driving factory belts 1. Motor Burn-out 2. Wire replacements 3. Torque Adjustments 4. Lubrication • What are its associated cost? • Down time: Staffing • Production: Product to market • Human factors: Stress, Leaning Curve • Reliability: Service performance and guarantees

  6. What is Maintainability? • Maintainability greatly influences reliability and availability of a system or subsystem. • Maintainability must be addressed early in the design stage to prevent or reduce failure or down times of the system.

  7. Why is Maintainability Required?1 • Infinite Reliability is not achievable • When a system is discarded, it must be discarded or it must be repaired • Cost usually dictates that a faulty system must be repaired • In addition to repair, most systems must be serviced (Consumables replaced - fuel, oil, coolant, etc.) • Incipient failures must be detected

  8. Why is Maintainability Required?1 • To verify that equipment has not deteriorated, its overall capability to perform must be reviewed • Maintenance is the repair, servicing, and inspection of equipment

  9. Maintenance Concept • Maintenance defines all those activities performed on an item to retain it in or to restore it to s specified state.4 • Can be divided into two categories: 1. Preventive Maintenance • Prescribe procedures to reduce the probability of failure or degradation 2. Corrective Maintenance • Initiated after fault recognition • Regain state of system for performing required function

  10. Maintenance Concept

  11. Maintenance Concept Failure Occurs Detection Failure Confirmed Preparation for Maintenance Active Maintenance Commences Location and Isolation Faulty Item Identified Disassembly (Access) Corrective Maintenance Cycle6 Disassembly Complete or Repair of Equipment Removal of Fault Item Installation of Spare/Repair Part Re-assembly Re-assembly Complete Alignment and Adjustment Condition Verification Repair Completed

  12. Maintenance is Conducted:7 • On equipment repair • Remove and replace faulty item • Adjust of align an item that has drifted out of specification • Off equipment repair • In a local shop • In and industrial facility

  13. Achieving Maintainability • Achieving maintainability is done through planning and realizing maintenance concepts: • Fault Detection and isolation • Partitioning equipment or systems into LRUs • User documentation • Training • Logistical Support

  14. Achieving Maintainability • Fault Detection and isolation • Goal is to localize faults down to LRU’s (last repairable unit / line replacement unit) by performing the following: • BIT (Built-in test): 1. Degree of fault 2. Degree of isolation 3. Correctness of the fault isolation 4. Test duration

  15. Achieving Maintainability • BITE (Built-in test equipment): 1. Simplicity 2. Standardization 3. Reliability 4. Maintenance • Equipment and System Partitioning • Partition complex equipment and systems into LRUs: PCB • Accessibility: Ease of LRU • Adjustment: Digital reduces need • Exchange: Careful of obsolescence

  16. Achieving Maintainability • User Documentation • General Description • Operating Manual • Preventive Maintenance • Corrective Maintenance • Illustrated Spare Parts Catalog • Logistical Support • Training of Operating & Maintenance Personnel • Well trained and motivated • Human Errors

  17. Achieving Maintainability • User Logistical Support • Four Levels 1. Operating personnel 2. First line maintenance personnel 3. Maintenance personnel 4. Specialist from arsenal or industry

  18. Achieving Maintainability • Specify • Specifications, Contracts, Warranties • Program Plan • Design • Equipment Arrangement • Equipment Location • Servicing Locations • Weapon Location • Turnaround Arena • Accessibility • Fault and Servicing Cues

  19. Achieving Maintainability • Plan • Predesign Homework • By Analysis • Mock Ups • Demonstrate Supportability • Verify Operation Environment

  20. Achieving Maintainability

  21. Bottoms Up Models • Provide output to monitor design progress vs. requirements • Provide input data for life cycle cost • Provide trade-off capability • Design features vs. maintainability requirements • Performance vs. maintainability requirements • Provide Justification for maintenance improvements perceived as the design progresses

  22. Bottoms Up Models • Provide the basis for maintainability guarantees/demonstration • Provide inputs to warranty requirements • Provide maintenance data for the logistic support analysis record • Support post delivery design changes • Inputs • Task Time (MH) • Task Frequency (MTBM) • Number of Personnel-Elapsed Time (hours) • For each repairable item

  23. Bottoms Up Models • Input Data Sources • Task Frequency • Reliability predictions de-rated to account for non-relevant failures • Because many failures are repaired on equipment, the off equipment task frequency will be less than the task frequency for on equipment

  24. Bottoms Up Models • Input Data Sources (Continued) • Task Time • Touch time vs. total time • That time expended by the technician to effect the repair • Touch time is design controllable • Total Time • Includes the time that the technician expends in “Overhead” functions such as part procurement and paper work • Are developed from industrial engineering data and analyst’s estimates

  25. Task Analysis Model • Task analysis modeling estimates repair time • MIL-HDK-472 method V • Spreadsheet template • Allow parallel and multi-person tasks estimation • Calculates elapsed time and staff hours • Reports each task element and total repair time • Sums staff hours by repairmen type • Estimates impact of hard to reach/see tasks

  26. Why Do Maintainability Modeling? • To identify the important issues • To quantify and prioritize these issues • To build better design and support systems

  27. Design Guidelines for Maintainability9 • General Guidelines • Plan and Implement a concept for automatic fault detection down to the last LRU • Partition the equipment • Aim for standardization of parts, tools, and testing equipment • Conceive operation and maintenance procedures to be as simple as possible • Consider environmental conditions

  28. Design Guidelines for Maintainability9 • Testability • Degrees of failure detection and isolation • The correctness of test results • Test duration • Accessibility and Exchangeability • Provide self-latching access flaps • Plan for accessibility • Use preferably indirect plug connectors • Provide for speedy replaceability • Prevent faulty installation or connection

  29. Design Guidelines for Maintainability9 • Operation and Adjustment • Use high standardization in selecting operational tools • Consider human aspects • Order all steps of a procedure in a logical sequence • Describe system status • Avoid any form of hardware adjustments

  30. Elements & Terminology of Maintainability • MTTR: Mean Time to Repair • T0.5: Median Time to Repair • TMAX: Maximum Time to Repair) usually the 95th percentile • MTTPM: Mean Time to Preventive Maintenance • MTBPM: Mean Time Between Preventive Maintenance • MDT: Mean Down Time • MTBM: Mean Time Between Maintenance

  31. Maintainability Prediction • System Mean Time to Repair, MTTRS System without redundancy E1 E2 En

  32. MTTF = 400 h MTTR = 2.5 h MTTF = 250 h MTTR = 1 h MTTF = 100 h MTTR = 0.5 h MTTF = 500 h MTTR = 2 h Maintainability Prediction • Example 1: Compute the mean time to repair at the system level for the following system. • Solution:

  33. MTTF = 100 h MTTR = 0.5 h MTTF = 500 h MTTR = 2 h MTTF = 400 h MTTR = 2.5 h MTTF = 250 h MTTR = 1 h MTTF = 100 h MTTR = 0.5 h Maintainability Prediction • Example 2: How does the MTTRs of the system in the previous example change if an active redundancy is introduced to the element with MTTF = 100h? • Solution:

  34. MTTF and MTBF • Mean Time to Failure (or Between Failures) MTTF • (or MTBF) is the expected Time to Failure (or • Between Failures) • Remarks: • MTBF provides a reliability figure of merit for expected failure • free operation MTBF provides the basis for estimating the • number of failures in a given period of time Even though an • item may be discarded after failure and its mean life • characterized by MTTF, it may be meaningful to characterize • the system reliability in terms of MTBF if the system is • restored after item failure.

  35. SMU SYS 7340 NTU SY-521-N Logistics Systems Engineering Modeling & Analysis of Time to Repair Dr. Jerrell T. Stracener, SAE Fellow

  36. Definition • Maintainability is an inherent design characteristic of a system or product and it pertains to the ease, accuracy, safety, and economy in the performance of maintenance actions.2 • Maintainability can be created into a four-part definition:3 1. Maintainability is the probability that a failed system 2. will be restored to specified performance 3. within a stated period of time 4. when maintained under specified conditions.

  37. Definition • Maintainability is a characteristic of an item, expressed by the probability that preventive maintenance (serviceability) or repair (repairability) of the item will be performed within a stated time interval by given procedures and resources (number and skill level of the personnel, spare parts, test facilities, etc.).4 • Maintainability is the ability of an item to be retained in, or restored to, a specified condition when maintenance is performed by people having specified skill levels, using prescribed procedures and resources.5

  38. Maintenance and Design8 • The system’s design determines its requirements for maintenance • Reliability (How often maintenance) • Configuration (How much time for access) • Built in Test (Fault Isolation Time) • Subassembly life span (Inspection/forced replacement) • Adjustment/alignment requirements (Inspection) • Capacity/fill rate (Servicing) • Corrosion susceptibility (Inspection/repair)

  39. Normal Distribution: A random variable X is said to have a normal (or Gaussian) distribution with parameters  and , where - < <  and  > 0, with probability density function - < x <  where  = 3.14159… and e = 2.7183... f(x) x

  40. Normal Distribution: • Mean or expected value of X • Mean = E(X) =  • Median value of X • X0.5 =  • Standard deviation

  41. Normal Distribution: Standard Normal Distribution If X ~ N(, ) and if , then Z ~ N(0, 1). A normal distribution with  = 0 and  = 1, is called the standard normal distribution.

  42. Normal Distribution: f(z) f(x) x z  0

  43. Normal Distribution: Standard Normal Distribution Table of Probabilities http://www.smu.edu/~christ/stracener/cse7370/normaltable.html Enter table with and find the value of  f(z)  z 0 z

  44. Normal Distribution - example The following example illustrates every possible case of application of the normal distribution. Let X ~ N(100, 10) Find: a. P(X < 105.3) b. P(X  91.7) c. P(87.1 < X  115.7) d. the value of x for which P(X  x) = 0.05

  45. Normal Distribution - example solution a. P(X < 105.3) = = P(Z < 0.53) = 0.7019 f(x) f(z) x z 100 105.3 0 0.53

  46. Normal Distribution - example solution b. P(X  91.7) = = P(Z > - 0.83) = 1 - P(Z  -0.83) = 1 - 0.2033 = 0.7967 f(x) f(z) x z 91.7 100 -0.83 0

  47. Normal Distribution - example solution c. P(87.1 < X  115.7) = = P(-1.29 < Z < 1.57) = F(1.57) - F(-1.29) = 0.9418 - 0.0985 = 0.8433 f(x) x 87.1 100 115.7

  48. Normal Distribution - example solution d. P(X  x) = 0.05 P(Z  z) = 0.05 implies that z = 1.64 P(X  x) = therefore x - 100 = 16.4 x = 116.4 f(x) x 100 116.4

  49. Normal Distribution - Example: The time it takes a field engineer to restore a function in a logistics system can be modeled with a normal distribution having mean value 1.25 hours and standard deviation 0.46 hours. What is the probability that the time is between 1.00 and 1.75 hours? If we view 2 hours as a critically time, what is the probability that actual time to restore the function will exceed this value?

  50. Normal Distribution - Example Solution:

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