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Chapter 8

Chapter 8. Manual Materials Handling Limits. Introduction. Robotics has decreased manual labor repetitive and structured jobs mostly successful industries CATCH 22: capital investment for robots, need to be successful to get investment Unstructured jobs still manual labor

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Chapter 8

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  1. Chapter 8 Manual Materials Handling Limits

  2. Introduction • Robotics has decreased manual labor • repetitive and structured jobs • mostly successful industries • CATCH 22: capital investment for robots, need to be successful to get investment • Unstructured jobs still manual labor • construction, assembly, equipment repair, fire fighting, police, nursing

  3. Results of 1981 NIOSH Study: • Overexertion claimed cause ~ 60% of low back pain. • If significant lost time, less than 1/3 with back pain return to previous work. • Overexertion injuries account for ~1/4 of all reported occupational injuries in the US (some industries ~ 1/2) • ~ 2/3 of overexertion claims involved lifting loads, ~ 20% pushing or pulling

  4. Factors affecting manual material handling system • Worker characteristics (Individual) • Physical: age, anthropometrics, posture • Sensory: visual, audit, tactile, proprio etc • Motor: strength, ROM, endurance • Psychomotor: coordination, RT • Personality: job satisfaction, SES • Training/experience: education • Health status: previous, drug use • Leisure time activities: 2nd job, sedentary

  5. Factors affecting manual material handling system • Material/container characteristics (Task & Environment). • Load. • Dimensions. • Distribution of load • 1 vs 2 hand, Moment Arm about back • Couplings (handles). • Stability of load (liquids & bulks).

  6. Factors affecting manual material handling system • Task & workplace characteristics (environment) • Workplace geometry • Frequency/duration/pace. • Complexity • environment: temperature, noise

  7. Factors affecting manual material handling system • Work practice characteristics • individual: speed and accuracy • Organization: teamwork, safety functions, medical staff • Administrative: safety incentives, work shift length, rotation, personal protective devices

  8. 3 strategies to preventoverexertion injury 1) design the task for all workers 2) select workers believed to be at low risk 3) train workers to reduce personal risk levels Often determined by socio-legal-economic considerations

  9. Lifting Limits in Manual Handling • Setting “safe” limits for employees • “gold standard” for workplace • Needs to consider • Epidemiology of MS injury • Biomechanical concepts • Physiological principles • Psychophysical lifting limits

  10. Lifting Limits in Manual Handling Note different limiting factors

  11. 1981: NIOSH equation to evaluate sagittal plane lifting • objective method to determine safe load • Recommendations: • lifting smooth, with no sudden acceleration • objects of moderate width (hand separation of less than 75 cm (29.5 inches) • Good couplings (secure handholds and low foot slippage potential) • Favourable temperatures for lifting

  12. 1981: NIOSH equation to evaluate sagittal plane lifting • objective method to determine safe load • Need to define 4 job attributes • location of CofM (or handgrip center) of the object in horizontal direction (H) • horizontally from midpoint of ankles

  13. 1981: NIOSH equation to evaluate sagittal plane lifting • objective method to determine safe load • Need to define 4 job attributes • location of CofM (or handgrip center) of the object in horizontal direction(H) • location of CofM(or handgrip center) in vertical direction at start of lift (V) • from floor to CofM or handle

  14. 1981: NIOSH equation to evaluate sagittal plane lifting • objective method to determine safe load • Need to define 4 job attributes • location of CofM (or handgrip center) of the object in horizontal direction(H) • location of CofM(or handgrip center) in vertical direction at start of lift (V) • vertical travel distance of the hands (D) • from origin to destination

  15. 1981: NIOSH equation to evaluate sagittal plane lifting • objective method to determine safe load • Need to define 4 job attributes • location of CofM (or handgrip center) of the object in horizontal direction(H) • location of CofM(or handgrip center) in vertical direction at start of lift (V) • vertical travel distance of the hands (D) • Frequency of lifting (lifts / minute) averaged over a period (F)

  16. 1981: NIOSH equation to evaluate sagittal plane lifting • objective method to determine safe load • BUT • limited to sagittal plane • did not consider asymmetry • needs more consideration of width (H) • needed consideration of quality of coupling • needed revision of weight limits based on frequency

  17. 1991 committee to revise:1994 published revision • considered new research findings • biomechanical criteria • physiological criteria • psychophysical criteria • added • angle of asymmetry from sag plane (A) • quality of coupling (C) in 3 classes • still many unknowns and controversies

  18. Biomechanical criteria • Site of greatest stress: L5/S1 • Compressive force: critical determinant • 3.4 kN (3400 Newtons) • safe for most but not all employees • cadaver study & biomechanical models

  19. Spinal Motion Segment Failure Traditional Model Revised Model (McGill, 1997)

  20. Fatigue, by Rodin Physiological criteria • energy expenditure related to repetitive lifting • large energy expenditures required to lift the body and the load • if lifting energy requirements exceed energy producing capacity==>fatigue

  21. Psychophysical criteria • how much an individual will choose to lift if given the choice when lifting for an extended period of time • Guidelines set to meet acceptable lifting capacity of 75% of females (99% males)

  22. Quantifies risk increase when: 1. Heavy objects are lifted. 2. The object is bulky. 3. The object is lifted from the floor. 4. Objects are frequently lifted. 5. Poor grips are provided

  23. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM RWL: Recommended weight limit Identifies the MAXIMAL load for the scenario defined in the equation. Use this value to calculate level of stress. Lift Index (LI): Task load / RWL : percentage of healthy population at risk??? : most healthy population can exceed LI of 1.00?? Compare relative hazard of two tasks/two environments If LI > 3 many workers at elevated risk

  24. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM RWL: Recommended weight limit Identifies the MAXIMAL load for the scenario defined in the equation.Use this value to calculate level of stress. Lift Index (LI): Task load / RWL : percentage of healthy population at risk??? : most healthy population can exceed LI of 1.00?? Compare relative hazard of two tasks/two environments If LI < 1 protective of most workers

  25. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM LC: Load constant Maximum recommended weight for lifting at the standard lifting location sagittal plane, occasional lift, good couplings, <25 cm vertical displacement 23 kg (230N) or 51 lbs acceptable to 75% of female population

  26. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM Multipliers used to adjust (reduce) the recommended load to compensate for less than optimal lifting conditions

  27. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM Horizontal multiplier: increased horizontal distance from spine increases moment arm and leads to increased lumbar stress. HM (metric) = 25 / H HM (english) = 10/ H H: horizontal distance of hands from midpoint between ankles Note that 25 cm (10 in) is about width of body. Measured at origin and destination.

  28. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM Vertical multiplier: reflects increased lumbar stress lifting loads near the floor (What is the cause??) Lifting from near floor requires greater energy expenditure (Why?) Therefore reduce RWL by 22.5% if lift begins at floor More dangerous to lift load to or past shoulder height Therefore reduce RWL by 22.5% for shoulder height VM = (1-0.003 |V-75|) V in cm VM = (1-0.0075|V-30|) V in inches where V is vertical distance of hands from floor Measure at origin & destination, use worst case

  29. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM DM: Distance multiplier reflects increase in physiological demand as vertical distance traveled is increased ( fatigue) DM = (0.82 + (4.5 / D ) in cm DM = (0.82 + (1.8/ D) in inches where D is the total vertical distance moved between origin and destination

  30. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM Asymmetric multiplier: lifting away from sagittal plane Reduce load by 30% for 90 degrees of twist AM = ( 1 - (0.0032 A)) Where A is angle of asymmetry (angular displacement from the sagittal plane) Measure at origin & destination, use worst case

  31. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM FM = Frequency Multiplier Table D-5, p 561 from text Based on work duration (<=1 hr, <= 2hr, <= 8hr) and V (vertical distance of hands from floor, in cm) and Frequency (rate of lifting) lifts/min

  32. Frequency Multiplier

  33. Revised (1994) NIOSH lifting equation RWL = LC x HM x VM x DM x AM x FM x CM CM =Coupling Multiplier Table D-7, p562 from text Based on V (vertical distance of hands from floor, in cm) and quality of coupling Note: penalty is not more than 10% decrease in RWL, so rating not that critical.

  34. Calculate RWL, then what?? • Calculate the Lift Index (LI), as Actual Load Lifted / RWL • Likely that LI > 3 poses a significant risk to many workers (<1 is protective) • a comparison value • multipliers are factors that increase stress • Which multiplier has greatest potential for change? • what changes will reduce the multipliers?

  35. Solve for the overhead • 200 Newton load • 38 cm handles above ground • Ht to press of 160 cm • Assume steps forward the 53 cm to press • Work duration 8 hours • Loads twice during shift • Good grips on stock • Calculate

  36. Solve for the overhead • What if poor handles? • What if unable to step forward, so all is reach? • What if twists 30 degrees to load?

  37. Limitations of equation • Does not recognize individual risk assessment • future include age, sex & Body weight??? • Not for use with one-handed lifting • or seated, or kneeling, or constrained, or hot/cold/contaminated environment, or shovel use, or high-speed lifting • Physiological criteria relate to whole body fatigue, not site specific • relates more to risk of injury?

  38. Summary • Provides a quantitative starting point for comparing tasks. • Links factors associated with risk of LBP in a multiplicative manner • Starting point for ongoing research and validation of assumptions and guidelines

  39. Homework Go to this website by Dr. Peter Keir (York University, Toronto, Canada) and do the assignment (skip the Mital calculations)

  40. NIOSH recommendations to control lifting hazards: • Identify jobs with high musculo-skeletal injury incidence and severity rates by statistical analysis of medical data.

  41. NIOSH recommendations to control lifting hazards: • Observe suspect jobs and for each lift task measure the weight of loads and related H, V, and D data, and note whether lifts are occasional or performed regularly throughout the shift.

  42. NIOSH recommendations to control lifting hazards: • Evaluate the lifting risk levels by computing the LI • Load lifted / RWL

  43. NIOSH recommendations to control lifting hazards: Develop engineering controls such as: • a. Use of manual handling devices. • b. Repackaging load to reduce weights. • c. Rearranging workplace / redesign hardware to reduce H, V, & D factors.

  44. NIOSH recommendations to control lifting hazards: Propose administrative controls: • a.Add personnel to reduce lift frequency • b. Use or modify job rotation to shorten the period of lifting (cross-training) • rotate workers onto other, less physically demanding jobs

  45. NIOSH recommendations to control lifting hazards: • Develop formal training programs emphasizing lift techniques that minimize H, V, D, & F

  46. NIOSH recommendations to control lifting hazards: • Develop worker selection & placement procedures to improve match between worker physical work capacities and specific lifting requirements in problems jobs.

  47. NIOSH recommendations to control lifting hazards: • Implement the most feasible solutions and evaluate effectiveness with follow-up medical and job surveillance.

  48. Load Pushing and Pulling Capabilities: • Approximately 20% of overexertion injuries have been associated with pushing and pulling acts. • One of the leading causes of non-vehicle related deaths in industry is slipping and/or falling.

  49. Load Pushing and Pulling Capabilities: • Vertical height of the handle is critical • About hip height is recommended. • vision • strength in this position • allows development of horizontal force without compromising friction

  50. Material Handling Considerations • Stand/sit erect • Eliminate reaches • Use rollers/conveyors vs. carriers/pivots • Gravity-fed slides/shelves • Keep it close to worker • Tilt bins • Allow access to all sides

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