Psyc 552 Ergonomics & BiomechanicsLecture 14

Evaluating Lifting with NIOSH National Institute of Occupational Health &

Safety. Created Lifting Equation in 1994. The multiplicative model that computes a

Lifting Index (LI). LIs > 1.0 pose greater risk of low back pain.

NIOSH Equation Components

Vertical locationHorizontal location

Load weightContainer characteristics

NIOSH Equation ComponentsAsymmetry angle

Other Task Measures Frequency of lifts Lifting duration

Components to Multipliers

RWL = Recommended Weight Limit LC = Load Constant (51 lbs) HM = Horizontal Multiplier VM = Vertical Multiplier DM = Distance Multiplier AM = Asymmetry Multiplier FM = Frequency Multiplier CM = Coupling Multiplier

RWL = LC x HM x VM x DM x AM x FM x CM

Variables – Horizontal Component Horizontal Location (H) is measured from the mid-

point of the line joining the inner ankle bones to a point projected on the floor directly below the mid-point of the hand grasps (i.e., load center), as defined by the large middle knuckle of the hand.

H = 8 + W/2 for V => 10 inches H = 10 + W/2 for V < 10 inches W = width of the container in the sagittal plane and

V is the vertical location of the hands from the floor.

Variables – Horizontal Multiplier HM = 10/H When H < 10, HM = 1 When H >25”, HM = 0

Variables – Vertical Component The vertical location should be measured at

the origin and the destination of the lift to determine the travel distance (D)

Variables – Vertical Multiplier VM is based on the absolute deviation of V

from the optimal or knuckle height of an average worker.

VM = 1(.0075|V-30|) – for inches When V is at 30 inches (75 cm), the vertical

multiplier (VM) is 1.0. If V is greater than 70 inches, then VM = 0

Variables – Distance Component Vertical Travel Distance variable (D) is

defined as the vertical travel distance of the hands between the origin and destination of the lift.

DM = (.82 + (1.8/D)) – for inches The DM is 1.0 when D is set at 10 inches;

DM is 0.85 when D = 70 inches

Variables – Asymmetry Component Asymmetric angle (A) defined as the angle

between the asymmetry line and the mid-sagittal line. The asymmetry line is the horizontal line that joins the mid-point between the inner ankle bones and the point projected on the floor directly below the mid-point of the hand grasps, as defined by the large middle knuckle.

Variables – Asymmetry Component The asymmetry angle (A) must always be

measured at the origin of the lift. If significant control is required at the destination, however, then angle A should be measured at both the origin and the destination of the lift.

Variables – Asymmetry Multiplier AM = 1-(.0032A) The range is from a value of 0.57 at 135

degrees of asymmetry to a value of 1.0 at 0 degrees of asymmetry.

If A is greater than 135 degrees, then AM = 0

Variables – Frequency Component Frequency is:

The number of lifts per minute (F) The amount of time engaged in lifting (duration) The vertical height of the lift from the floor.

Lifting frequency is the average number of lifts per minute over a 15 minute period.

When work does not require lifting for 15 minutes and the lifting frequency does not exceed 15 lifts per minute then: Compute the total number of lifts for a 15 minute

period – (lift rate X work time) Divide the total number of lifts by 15 Use the quotient as the frequency F for the table.

Frequency Special Considerations

Lifting Example A job requires:

Lifting for 8 minutes Light work for 7 minutes Lift rate for the 8 minutes is 10 lifts/min

The lift frequency F would be: (10 x 8)/15 = 5.33 lifts/minute

Lifting Duration – Short Short: <1hour, followed by a recovery time

equal to 1.2 times the work time (Rest time / Work time = 1.2).

To be classified as short-duration, a 45-minute lifting job must be followed by at least a 54-minute recovery period prior to initiating a subsequent lifting session.

Lifting Duration – Moderate Moderate: > 1 hour < 2 hours, followed by a recovery period

of at least .3 times the work (Rest time / Work time = .3).

If a worker continuously lifts for 2 hours, then a recovery period of at least 36 minutes would be required before initiating a subsequent lifting session. If the recovery time requirement is not met, and a subsequent lifting session is required, then the total work time must be added together. If the total work time exceeds 2 hours, then the job must be classified as a long-duration lifting task.

Lifting Duration – Long Long: 2 to 8 hours, with standard breaks

(morning, lunch, and afternoon).

Variables – Frequency Multiplier The FM value depends upon the average

number of lifts/min (F), the vertical location (V) of the hands at the origin, and the duration of continuous lifting. For lifting tasks with a frequency less than .2 lifts per minute, set the frequency equal to .2 lifts/minute. For infrequent lifting (i.e., F < .1 lift/minute), however, the recovery period will usually be sufficient to use the 1-hour duration category.

Variable – Coupling Component An optimal handle design

has .75 - 1.5 inches diameter, > 4.5 inches in length, 2 inches clearance, cylindrical shape, smooth, non-slip surface.

An optimal hand-hold cut-out has the following approximate characteristics: > 1.5 inch height, 4.5 inch length, semi-oval shape, > 2 inch (5 cm) clearance, smooth non-slip surface, > 0.25 inches container

thickness (e.g., double thickness cardboard).

Variable – Coupling Component An optimal container

design has: < 16 inches frontal

length, < 12 inches height, a smooth non-slip

surface.

A worker should be capable of clamping the fingers at nearly 90 degrees under the container, such as required when lifting a cardboard box from the floor.

Variable – Coupling Component A container is considered

less than optimal if it has: A frontal length > 16” height > 12” rough or slippery surfaces, Sharp edges, asymmetric center of mass, unstable contents, requires the use of gloves. A loose object is considered

bulky if the load cannot easily be balanced between the hand-grasps.

A worker should be able to comfortably wrap the hand around the object without causing excessive wrist deviations or awkward postures, and the grip should not require excessive force.

Variable – Coupling Multiplier The coupling multiplier (CM) is determined

from decision tree and a tabled value.

Tabled Values

Tabled Values (cont.)

Tabled Values (cont.)

Coupling Quality

23”

15”

44#

23”

63”

30”

Manufacturing

Punch Press

44 44 23 2315 63 48 0 0 <.2 <1 Fair

Origin Destination

.44 .89 1.0 1.0 .95.86

.44 .75 1.0 1.0 1.0.86

16.3

14.5

Computing RWLs

•The RWL for the origin and destination is computed because significant control is required.

•Significant control: Precision placement where the worker 1) re-grasps the near the destination, 2) momentarily holds object at destination, or 3) carefully positions load at destination.

Computing Lifting Index

16.344

4414.5

2.7

3.0

•LIs > 1 indicate increased risk of low back pain.

Redesign the Job

Bring load closer to the body – rotate it 90 degrees.

Lower destination height. Reduce travel distance. Eliminate significant control.

.44 .89 1.0 1.0 .95.86

.44 .75 1.0 1.0 1.0.86

16.3

14.5

23”

15”

44#

23”

63”

30”

The reel is 12” wide.

Modifications

1.0 .89 1.0 1.0 .90.86

.83 .75 1.0 1.0 .90.86

35.1

24.6

35.144

4424.6

1.3

1.8

NIOSH Equation Limits The model DOES NOT apply when lifting or

lowering: With one hand Over 8 hours While seated or kneeling In restricted work space Unstable objects Wheelbarrows or shovels With high speed motion With unreasonable foot to floor friction In unfavorable environments (66-79 degrees and 33 to

50% humidity)