w (recommended weight limit) lifting limits & injury body ... · 2/8/2014 · cm = 0.95 (from...
TRANSCRIPT
15/04/2013
1
RWL (RECOMMENDED WEIGHT LIMIT)
Winda Halim, ST., MT
IE-402 Analisis Perancangan Kerja dan Ergonomi 1
Jurusan Teknik Industri
Fakutas Teknik
Universitas Kristen Maranatha
Outline• Lifting Limits & Injury
• NIOSH Principles
• NIOSH Analysis
• NIOSH Graphs
• NIOSH Examples
Lifting Limits & Injury
Lifting
H
H
D
V
V
W
Ankles
Body Midline
H
H
D
V
V
W
Ankles
Body Midline
START
FINISH
Some Low Back Injury Stats
• Overexertion claimed to be cause of low back pain
(LBP) by over 60% of sufferers; less than 1/3 of these
return to work
• 2/3 overexertion injuries involve lifting
• 1/5 overexertion injuries involve pushing or pulling
• Lost wages, medical treatment, & lost productivity
• $72 billion/yr (U.S.) and $5.7 billion/yr (Canada)
• need for scientific research to understand the mechanics
of LBP & to highlight its predisposing factors
Types of
Failure
Traumatic
Cumulative
[McGill, 1997]
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Lifting Limits
Frequency (Lifts per minute), or
Time Duration of Lift & Carry, or
Distance of Lift, or
Style of Lift
Recommended
Weight Lifting
Limit
Biomechanical (weight & force lift limits), or
Physiological (“calorie” energy expenditure), or
Psychophysical (subjective “I can or cannot lift it”)
NIOSH Principles
What is NIOSH?
National Institute for Occupational Safety & Health (U.S. government agency)
Standard guidebooks◦ Work Practices Guide to Manual Lifting (NIOSH, 1981)
◦ Applications Manual for the Revised NIOSH Lifting Equation (Waters et al., 1993 and 1994)
Developed an “occupational lifting” formula to compute Recommended Weight Limit
Big influence on worker safety and health issues
NIOSH Recommendations
1. Smooth Lifting: no sudden jerky motions
2. Objects Size: moderate width with hand separation < 75 cm
3. Posture: unrestricted with no torso bracing
4. Coupling: secure handles & low shoe-floor slippage
5. Temperature: favourable for lifting
6. Horizontal Location: Center-of-Mass of object/handles to Ankles
7. Vertical Location: Center-of-Mass of object/handles to Floor
8. Vertical Travel: distance traveled by hands from start to finish of lift
9. Frequency of lifting: average lifts / minute during the time period
10. Asymmetry: angle from center-of-mass to body’s midline from start to finish of lift
11. Coupling of load: three types of object “grasping” (good, fair, poor)
Basis for NIOSH
NIOSH analysis was developed for 3 different population norms that would protect 90% of
workers:
Biomechanically: L5/S1 forces should be < 3400 N. Physiologically: metabolic energy expenditures
should be below predicted levels (e.g. < 4.7 Kcal/min for lift that has duration < 1 hour and object height < 75 cm from ground).
Psychophysically: subjective worker estimates would accommodate 75% of women and 99% of men (or 90% of a mix of men and women performing a lifting task).
NIOSH Analysis
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NIOSH: The Lift
H
H
D
V
V
W
Ankles
Body Midline
H
H
D
V
V
W
Ankles
Body Midline
START
FINISH
NIOSH: The Equation
RWL = LC x HM x VM x DM x AM x FM x CM
Multipliers
Load Constant
Recommended Weight Limit
NIOSH: The Factors
Symbol Name Source
RWL Recommended Weight Limit [kg or N]
LC Load Constant 23 kg or 226 N
HM Horizontal Multiplier 25/H {H [cm] at start & end of lift}
VM Vertical Multiplier 1 – (0.003|V-75|)
{V [cm] at start & end of lift}
DM Distance Multiplier 0.82 + (4.5/D) {D [cm] is total
vertical distance lifted}
FM Frequency Multiplier Graphs: lifts/min & work duration
AM Asymmetric Multiplier 1 – 0.0032A {angle A [deg] at start &
end of lift with respect to sagittal body
midline}
CM Coupling Multiplier Graphs: quality of “grasping” or
“gripping” the object to be moved
Note: 0 < all multiplier values < 1. If calculated value >1.0, then use 1.0
Multiplier Abbreviation Metric U.S.
Load Constant LC 23 kg 51 lbs.
Horizontal HM 25/H 10/H
Vertical VM 1 – (.003|V-75|) 1 – (.0075|V-30|)
Distance DM 0.82 – (4.5/D) 0.82 – (1.8/D)
Asymmetric AM 1 – (.0032 * A) 1 – (.0032 * A)
Frequency FM Table 5 Table 5
Coupling CM Table 7 Table 7
Lifting equation for calculating the Recommended Weight Limit (RWL):
RWL = LC x HM x VM x DM x AM x FM x CM
Lifting Index
Actual Weight of Object W
Recommended Weight Limit RWL=LI =
Actual Weight of Object W
Recommended Weight Limit RWL=LI =
• After RWL is computed, it is compared with the actual weight W
of the object being lifted.
• Comparison is made at the origin and destination of a lift
• Larger of the two values is considered the “stress level” of a lift
• LI < 1.0 …. protective of most workers
• LI > 3.0 …. poses significant risk to most workers
• 1.0 < LI < 3.0 …. many jobs fall in this region
• Jobs need to be either redesigned to minimize LI, otherwise need
increased job screening, more careful training, and medical
monitoring
Job Analysis Worksheet
Department________________
Job Title __________________
Analyst’s Name ____________
Date______________________
Job Description____________
_________________________
_________________________
_________________________
Step 2. Determine Multipliers and Compute RWL
Origin RWL = ___ x ___ x ___ x ___ x ___ x ___ x ___ = ___
Destination RWL = ___ x ___ x ___ x ___ x ___ x ___ x ___ = ___
RWL = LC x HM x VM x DM x AM x FM x CM
Step 3. Compute the Lifting Index
Origin Lifting Index = Weight / RWL = ___ / ___ = ___
Destination Lifting Index = Weight / RWL = ___ / ___ = ___
Step 1. Measure and Record Task Variables
CFAADVHVHMaxAvg
HRSLifts
/min
Dest.OriginDest.Origin
Object
Coupling
TimeFreqAngleVert.
Dist.
Hand LocationObject
Weight
CFAADVHVHMaxAvg
HRSLifts
/min
Dest.OriginDest.Origin
Object
Coupling
TimeFreqAngleVert.
Dist.
Hand LocationObject
Weight
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NIOSH: Pros
• combination of data from 4 disciplines:
epidemiology, biomechanics, physiology, and ergonomics
• comprehensive review of the literature
• multiplicative nature of equation makes estimates
conservative
• easily measured parameters
• each factor can be measured independently
• useful tool in industry
• good starting point for more complex cases
NIOSH: Cons
• single equation for all populations
• suitable for most, but too high for some
subjects (advanced age, weaker spines)
• does not consider gender differences
• focus only on lumbar spine
• speed of lifting & duration neglected
• frequency multiplier is a physiological limit
• only for “two handed” lifting technique
NIOSH Graphs
Horizontal Location
(cm)
(in)
Ho
rizo
nta
l M
ult
iplie
r (H
M)
Vertical Location
(cm)
(in)
Ve
rtic
al M
ult
iplie
r (V
M)
Lift Distance
(cm)
(in)
Dis
tan
ce
Mu
ltip
lie
r (D
M)
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Frequency Multiplier (FM)(Below Waist Height, V < 75 cm (30 in)
Lifting Frequency (Lifts/min)
1 hr1-2 hrs2-8 hrs 1 hr1-2 hrs2-8 hrs
Frequency Multiplier (FM)
(Above Waist Height, V > 75 cm (30 in)
Lifting Frequency (Lifts/min)
1 hr2-8 hrs 1-2 hr 1 hr2-8 hrs 1-2 hr
Asymmetric Multiplier (AM)
Asymmetric Angle (degrees)
Coupling Multiplier (CM)
Couplings V < 75 cm (30 in) V >= 75 cm (30 in)
Good 1.00 1.00
Fair 0.95 1.00
Poor 0.90 0.90
Good = fingers wrap completely around object or handles
Fair = only a few fingers grasp around the object firmly
Poor = only a few fingers or fingertips are partially under or around object
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NIOSH Examples
Example 1
Other Items
• feet remain fixed
• 1 lift / 4 hrs
• 8 hour shift
• very firm grip
• no twist motion
53 cm
160 cm
20 kg
70 cm
38 c
m
END
START
Job Analysis Worksheet
Department________________
Job Title __________________
Analyst’s Name ____________
Date______________________
Job Description____________
_________________________
_________________________
_________________________
Step 1. Measure and Record Task Variables
CFAADVHVHMaxAvg
HRSLifts
/min
Dest.OriginDest.Origin
Object
Coupling
TimeFreqAngleVert.
Dist.
Hand LocationObject
Weight
CFAADVHVHMaxAvg
HRSLifts
/min
Dest.OriginDest.Origin
Object
Coupling
TimeFreqAngleVert.
Dist.
Hand LocationObject
Weight
20 20 53 38 123 160 122 0 0 0.2 8 Good
Horizontal
Body-to-Hand
Distance (feet
are locked in
place)
= 53 cm + 70 cm
= 123 cm
Total
Vertical
Lift
= Dest. – Origin
= 160 cm – 38 cm
= 122 cm
Minimum
NIOSH
Value
Reportable
Step 2. Determine Multipliers and Compute RWL
Origin RWL = 23 x 0.47 x 0.889 x 0.856 x 1 x 0.85 x 1= 7.02 kg
Destination RWL = 23 x 0.203 x 0.745 x 0.856 x 1 x 0.85 x 1= 2.53 kg
RWL = LC x HM x VM x DM x AM x FM x CM
Origin of Lift
LC = 23 kg = fixed factor
HM = 25/H = 25/53 = 0.47
VM = 1 – (0.003|V-75|) = 1 – (0.003|38-75|) = 0.889
DM = 0.82 + (4.5/D) = 0.82 + (4.5/122) = 0.856
AM = 1 – 0.0032A = 1 – 0.0032(0) = 1
FM = 0.85 (since 1 lift/4 hrs = 0.004 lifts/min = approx. 0 on graph)
CM = 1.0, since V < 75 cm and “good” grip
Destination of Lift
LC = 23 kg = fixed factor
HM = 25/H = 25/123 = 0.203
VM = 1 – (0.003|V-75|) = 1 – (0.003|160-75|) = 0.745
DM = 0.82 + (4.5/D) = 0.82 + (4.5/122) = 0.856
AM = 1 – 0.0032A = 1 – 0.0032(0) = 1
FM = 0.85 (since 1 lift/4 hrs = 0.004 lifts/min = approx. 0 on graph)
CM = 1.0, since V >= 75 cm and “good” grip
Step 3. Compute the Lifting Index
Origin Lifting Index = Weight / RWL = 20 / 7.02 = 2.85
Destination Lifting Index = Weight / RWL = 20 / 2.53 = 7.91
Conclusion
• Origin: the start of the lift is acceptable and safe since LI < 3
• Destination: the end of the lift is dangerous since LI > 3. The
“stress level” is LI = 7.91, the larger of the values. This could be
the point where serious low back injury will occur. The task setup
must be changed at the destination, or increased job screening,
medical monitoring, and training must be introduced.
Example 2
Task
Moving trays from
conveyor belt and
putting them on
the cart
Other Items
• 10 kg trays
• 1 lift/min
• 4 hour shift
• feet are fixed
• “fair” grip
• upper body twist
motion at START
• tray placed
straight down
onto cart at END
50cm
60cm
45deg
20cm
90cm
(End)
(Start)
Sagittal
Body
Midline
90cm
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Job Analysis Worksheet
Department________________
Job Title __________________
Analyst’s Name ____________
Date______________________
Job Description____________
_________________________
_________________________
_________________________
Step 1. Measure and Record Task Variables
CFAADVHVHMaxAvg
HRSLifts
/min
Dest.OriginDest.Origin
Object
Coupling
TimeFreqAngleVert.
Dist.
Hand LocationObject
Weight
CFAADVHVHMaxAvg
HRSLifts
/min
Dest.OriginDest.Origin
Object
Coupling
TimeFreqAngleVert.
Dist.
Hand LocationObject
Weight
Total Vertical
Object Movement
= Start – End
= 90 – 20
= 70 cm
10 10 60 90 50 20 70 45 0 1 4 “fair”
Step 2. Determine Multipliers and Compute RWL
Origin RWL = 23x0.416x0.925x0.884x0.856x0.77x1.0= 5.16 kg
Destination RWL = 23x0.5x0.835x0.884x1.0x0.75x0.95= 6.05 kg
RWL = LC x HM x VM x DM x AM x FM x CM
Origin of Lift
LC = 23 kg = fixed factor
HM = 25/H = 25/60 = 0.416
VM = 1 – (0.003|V-75|) = 1 – (0.003|90-75|) = 0.925
DM = 0.82 + (4.5/D) = 0.82 + (4.5/70) = 0.884
AM = 1 – 0.0032A = 1 – 0.0032(45) = 0.856
FM = 0.77 (from graph, since 4 hr shift and V >= 75 cm)
CM = 1.0 (from table, since V >= 75 cm and “fair” grip)
Destination of Lift
LC = 23 kg = fixed factor
HM = 25/H = 25/50 = 0.5
VM = 1 – (0.003|V-75|) = 1 – (0.003|20-75|) = 0.835
DM = 0.82 + (4.5/D) = 0.82 + (4.5/70) = 0.884
AM = 1 – 0.0032A = 1 – 0.0032(0) = 1
FM = 0.75 (from graph, since 4 hr shift and V < 75 cm)
CM = 0.95 (from table, since V < 75 cm and “fair” grip)
Step 3. Compute the Lifting Index
Origin Lifting Index = Weight / RWL = 10/5.16 = 1.94
Destination Lifting Index = Weight / RWL = 10/6.05 = 1.65
Conclusion
The stress level is the larger value, LI = 1.94. But, at both origin
and destination the lifting index, LI < 3. Thus, most workers will be
safe from any potential back injury. The task can remain as is.
Example 3Task
Moving boxes from
conveyor belt &
placing them onto
a cart
Other Items
• 15 kg boxes
• 3 lifts/min
• 3 hour shift
• feet are fixed
• “poor” grip
• upper body twist
motion at START
• boxes placed
straight down onto
cart at END
35cm
50cm
30 deg
30cm
100cm
(End)
(Start)
Sagittal
Body
Midline
100cm
Answers: LI (start) = 3.57, LI (end) = 2.41
Sources
Chaffin et al., Occupational Biomechanics, 1999.
NIOSH, Work Practices Guide to Manual Lifting, 1981.
McGill, “The biomechanics of low back injury: implications on current practice in industry and the clinic”, J.Biomechanics, 39(5):465-475, 1997.
Waters et al., “Revised NIOSH equation for the design and evaluation of manual lifting tasks”, Ergonomics, 36(7):749-776, 1993.
Waters et al., Applications Manual for the Revised NIOSH Lifting Equation, 1994.