w (recommended weight limit) lifting limits & injury body ... · 2/8/2014 · cm = 0.95 (from...

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):


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 ___ = ___


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


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


Department________________

Job Title __________________


Date______________________


_________________________

_________________________

_________________________


CFAADVHVHMaxAvg

HRSLifts

/min


Object

Coupling

TimeFreqAngleVert.

Dist.

Hand LocationObject

Weight

CFAADVHVHMaxAvg

HRSLifts

/min


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


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


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


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


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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Department________________

Job Title __________________


Date______________________


_________________________

_________________________

_________________________


CFAADVHVHMaxAvg

HRSLifts

/min


Object

Coupling

TimeFreqAngleVert.

Dist.

Hand LocationObject

Weight

CFAADVHVHMaxAvg

HRSLifts

/min


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”


Origin RWL = 23x0.416x0.925x0.884x0.856x0.77x1.0= 5.16 kg

Destination RWL = 23x0.5x0.835x0.884x1.0x0.75x0.95= 6.05 kg


Origin of Lift


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


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)


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.

w (recommended weight limit) lifting limits & injury body ... · 2/8/2014 · cm = 0.95 (from...

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