Chapter 8 Manual Materials Handling Limits
Dec 29, 2015
Chapter 8
Manual Materials
Handling Limits
Introduction
Robotics has decreased manual laborrepetitive and structured jobsmostly successful industries
CATCH 22: capital investment for robots, need to be successful to get investment
Unstructured jobs still manual laborconstruction, assembly, equipment repair,
fire fighting, police, nursing
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
Factors affecting manual material handling system
Worker characteristics (Individual)Physical: age, anthropometrics, posture Sensory: visual, audit, tactile, proprio etcMotor: strength, ROM, endurancePsychomotor: coordination, RTPersonality: job satisfaction, SES Training/experience: education
Health status: previous, drug useLeisure time activities: 2nd job, sedentary
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).
Factors affecting manual material handling system
Task & workplace characteristics (environment)Workplace geometry Frequency/duration/pace.Complexityenvironment: temperature, noise
Factors affecting manual material handling system
Work practice characteristicsindividual: speed and accuracy Organization: teamwork, safety functions,
medical staffAdministrative: safety incentives, work shift
length, rotation, personal protective devices
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
Lifting Limits in Manual Handling
Setting “safe” limits for employees“gold standard” for workplace
Needs to considerEpidemiology of MS injuryBiomechanical conceptsPhysiological principlesPsychophysical lifting limits
Lifting Limits in Manual Handling
Note different limiting factors
1981: NIOSH equation to evaluate sagittal plane lifting
objective method to determine safe load Recommendations:
lifting smooth, with no sudden accelerationobjects 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
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
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
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
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)
1981: NIOSH equation to evaluate sagittal plane lifting
objective method to determine safe load BUT
limited to sagittal plane did not consider asymmetryneeds more consideration of width (H)needed consideration of quality of couplingneeded revision of weight limits based on
frequency
1991 committee to revise:1994 published revision
considered new research findingsbiomechanical criteriaphysiological criteriapsychophysical criteria
addedangle of asymmetry from sag plane (A)quality of coupling (C) in 3 classes
still many unknowns and controversies
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
Spinal Motion Segment Failure
TraditionalModel
RevisedModel(McGill, 1997)
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
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)
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
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 theequation. 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 environmentsIf LI > 3 many workers at elevated risk
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 theequation.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 environmentsIf LI < 1 protective of most workers
Revised (1994) NIOSH lifting equation
RWL = LC x HM x VM x DM x AM x FM x CM
LC: Load constantMaximum 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
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
Revised (1994) NIOSH lifting equation
RWL = LC x HM x VM x DM x AM x FM x CMHorizontal multiplier: increased horizontal distance from spineincreases moment arm and leads to increased lumbar stress.
HM (metric) = 25 / HHM (english) = 10/ H
H: horizontal distance of hands from midpoint between anklesNote that 25 cm (10 in) is about width of body.
Measured at origin and destination.
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 floorMore 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 cmVM = (1-0.0075|V-30|) V in inches
where V is vertical distance of hands from floor Measure at origin & destination, use worst case
Revised (1994) NIOSH lifting equation
RWL = LC x HM x VM x DM x AM x FM x CM
DM: Distance multiplierreflects increase in physiological demand as
vertical distance traveled is increased ( fatigue)DM = (0.82 + (4.5 / D ) in cmDM = (0.82 + (1.8/ D) in inches
where D is the total vertical distance moved between origin and destination
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 planeReduce load by 30% for 90 degrees of twist
AM = ( 1 - (0.0032 A))
Where A is angle of asymmetry (angular displacement from thesagittal plane)
Measure at origin & destination, use worst case
Revised (1994) NIOSH lifting equation
RWL = LC x HM x VM x DM x AM x FM x CM
FM = Frequency MultiplierTable 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
Frequency Multiplier
Revised (1994) NIOSH lifting equation
RWL = LC x HM x VM x DM x AM x FM x CM
CM =Coupling MultiplierTable D-7, p562 from text
Based on V (vertical distance of hands from floor, in cm) and quality of couplingNote: penalty is not more than 10% decrease in RWL,
so rating not that critical.
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 valuemultipliers are factors that increase stressWhich multiplier has greatest potential for change?what changes will reduce the multipliers?
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
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?
Limitations of equation Does not recognize individual risk
assessment future include age, sex & Body weight???
Not for use with one-handed liftingor 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?
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
Homework
Go to this website by Dr. Peter Keir (York University, Toronto, Canada) and do the assignment (skip the Mital calculations)
NIOSH recommendations to control lifting hazards:
Identify jobs with high musculo-skeletal injury incidence and severity rates by statistical analysis of medical data.
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.
NIOSH recommendations to control lifting hazards:
Evaluate the lifting risk levels by computing the LILoad lifted / RWL
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.
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
NIOSH recommendations to control lifting hazards:
Develop formal training programs emphasizing lift techniques that minimize H, V, D, & F
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.
NIOSH recommendations to control lifting hazards:
Implement the most feasible solutions and evaluate effectiveness with follow-up medical and job surveillance.
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.
Load Pushing and Pulling Capabilities:
Vertical height of the handle is criticalAbout hip height is recommended.
vision strength in this position allows development of horizontal force without
compromising friction
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