chapter 8 manual materials handling limits. introduction robotics has decreased manual labor ...

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


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).


Task & workplace characteristics (environment)Workplace geometry Frequency/duration/pace.Complexityenvironment: temperature, noise


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


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



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





vertical travel distance of the hands (D) from origin to destination





vertical travel distance of the hands (D)Frequency of lifting (lifts / minute) averaged

over a period (F)


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



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



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



Multipliers used to adjust (reduce) the recommended load to compensate for less than optimal lifting conditions


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.



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



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



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



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



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.


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.


Evaluate the lifting risk levels by computing the LILoad lifted / RWL


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.


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


Develop formal training programs emphasizing lift techniques that minimize H, V, D, & F


Develop worker selection & placement procedures to improve match between worker physical work capacities and specific lifting requirements in problems jobs.


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

chapter 8 manual materials handling limits. introduction robotics has decreased manual labor ...

Documents

manual laborrepetitive

manual laborconstruction

niosh equation

safe loadneed

job attributeslocation

niosh study

safe loadrecommendations

job satisfaction