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Manual handling is a hot topic because?. 27% of all industrial injuries670,000 injuries/yr in the United States60% of all money spent on industrial injuries93,000,000 lost workdays/yr. Approaches to Recommendations. BiomechanicalForces and torquesEsp. L4/L5 and L5/S1Permitted load does not var
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1. Body Strength and Load Handling IENG 321
SDSMT
Spring 2010
2. Manual handling is a hot topic because… 27% of all industrial injuries
670,000 injuries/yr in the United States
60% of all money spent on industrial injuries
93,000,000 lost workdays/yr
3. Approaches to Recommendations Biomechanical
Forces and torques
Esp. L4/L5 and L5/S1
Permitted load does not vary with frequency Physiological
Energy requirements of tasks and effect to cardiovascular system
Greater for occasional or frequent lifting?
4. Strength Factors Individual
Related to gender, age, training, fitness:
Muscle cross-sectional area
Length of muscle
Static or dynamic exertion
Fatigue
Neurophysiological excitation Situational
The amount of strength one will or can apply
Motivation, ego, will to succeed, fear of injury
Skill/experience
Body motion
Ability to brace
Posture
Body segments used
Coupling
5. Strength Exertion Static (Isometric)
Controllable, easy to measure
Rapid onset of fatigue
Reasonably estimate slow motion
Dynamic
More difficult to measure
More common
Isoinertial testing
Maximal
Structural integrity of object (safety factor)
> 95th percentils
Minimal
Ability to perform task
5th percentile
6. Strength Exertion
7. Hand Strength Grasping, holding, pressing, pushing, pulling,…
Extrinsic Muscles
Muscles in forearms, tendons cross carpal ligament
Mostly gross motor control
8. Hand Strength
9. Hand Strength
10. Grips and Grasps
11. Hand Tool Design Provide friction
Gloves may help, but generally increase required grip strength
Mechanical interlocking
Keep wrist straight
Natural grasp centerline 60o-70o
May need to supply left hand specific tooling
~ 10% population left handed
12. Whole Body Strength Pushing
Pulling
Lifting
Lowering
13. Foot Strength Standing
Little force
Infrequent operation
Support body weight on one foot
Alternating feet can help maintain “muscle pump” Sitting
Exert large force with sufficient support
14. Pushing/Pulling Factors Handles
Poor hand/handle surface is the weakest link
One hand vs. two hands
Postural freedom vs. fatigue
Body posture
Standing vs. sitting vs. kneeling
Application height
Where to push or pull from?
Direction
Generally perpendicular to shoulders
15. Push/Pull Summary Two hands are usually better than one.
Force capability goes down as it is exerted more often.
Initial force capability is higher than sustained capability.
Pushing capability is higher than pulling.
Push at waist level; pull at thigh level.
16. Task Modifications Measure the force required to move all wheeled equipment; periodically check the forces.
Force gauges
Install vertical push/pull bars on carts.
Push rather than pull loads.
Spinal compressive loads
Other safety concerns?
Avoid muscle-powered pushing and pulling for ramps, long distances, and frequent moves.
Use mechanical aids and momentum.
Reduce horizontal transfer force by reducing friction (rolling contact).
17. Holding Problems
Holding gives a static load combining body weight and object weight.
Low-back pain arises from spine biomechanics.
Solutions
Reduce the magnitude and duration of the torque.
Use balancers.
Limit high loads to short durations.
18. Carrying Replace carrying with pushing or pulling.
Minimize the moment arm of the load relative to the spine.
Consider carrying large loads occasionally rather than light loads often.
Use teamwork (common in medical facilities).
Consider using balancers, manipulators, conveyors, or robots.
Reduce lifting by raising the initial location.
Avoid carrying objects up and down stairs.
See Liberty Mutual Tables
19. NIOSH Lifting Guidelines 51 lbs is the maximum that can be lifted or lowered (load constant).
Recommended weight limit (RWL) is load constant multiplied by various factors.
Weight 90% of U.S. industrial workers can safely lift
Lifting index = load weight/RWL
Applicable when conditions are met!!
Section 1.2, pg. 9
20. Basic NIOSH Lifting Formula RWL = LC × HM × VM × DM × FM × AM × CM
LC = Load constant (51 lbs)
HM = Horizontal multiplier
VM = Vertical multiplier
DM = Distance multiplier
FM = Frequency multiplier
AM = Asymmetry multiplier
CM = Coupling multiplier
21. Multiplier Formulas Horizontal multiplier
HM = BIL ÷ H
BIL = Body interference limit (10 in.)
H = Horizontal location
Maximum = 25 in. (functional reach)
Measured = midpoint of hand to midpoint of line connecting ankles
Estimated – Consider vertical location from floor and width of container
< 10 in. = 10 + W/2
> 10 in. = 8 + W/2
22. Multiplier Formulas Vertical multiplier
Significant control at origin only?
Determines if destination of hands required.
VM = 1 – VC × | V – KH |
VC = Vertical constant (.0075 in.)
V = Initial vertical location of knuckles
Maximum = 70 in.
KH = Knuckle height of typical lifter (30 in.)
<30 in. = whole body lift
>30 in. = upper body lift
23. Multiplier Formulas Distance multiplier
DM = .82 + DC ÷ D
DC = Distance constant (1.8 in.)
D = Absolute value of vertical travel distance
Asymmetry multiplier
AM = 1 – .0032 × A
A = Angle of symmetry
24. Multiplier Formulas Frequency multiplier
See Table 13.9.
Lifting frequency = mean number of lifts in a 15-minute period
Lifting duration /session in hours may be:
Short = .001 h to = 1 h with recovery time of = 1.2 × duration
Moderate = >1 h = 2 h with recovery time of = .3 × duration
Long = >2 h but = 8 h
26. Multiplier Formulas Coupling multiplier
Depends on:
Height of initial/final hand location and container coupling
Whether coupling is good, fair, or poor (see Table 13.11 or NIOSH guide for classification)
28. Example Load Weight =15 lb
Ho = 12 in
Vo = 33 in
Hf = 12 in
Vf = 22 in
Ao = 5o
Af = 6o
Frequency = .5 lifts/min
Duration = 3 h
Recovery Time = 6 hours
29. Back Problems Wear and tear worse than muscle strain
Low Back Pain (LBP)
Reduces mobility and vitality
Long absences from work
Current main cause of early disability Major “At Risk” Jobs
Nurses
Laborers
Farmers
Baggage handlers
Warehouse workers
30. Intervertebral Disc Strain
31. Basic Biomechanics
32. Intra-abdominal Pressure (IAP) Increase in pressure of abdominal cavity with contraction of abdominal muscles
Helps stabilize trunk
Reduces spinal loading
Back Belts
Thought to increase IAP and minimize risk of injury
33. Safe Manual Handling Material handling aids
Proper techniques
We will visit this again in Workstation Design
34. Guidelines for Manual Handling
Bend the knees.
Don’t slip or jerk.
Don’t twist during the move.
Use machines.
Move small weights often.
Get a good grip.
Put a compact load in a convenient container.
Keep the load close to the body.
Work at knuckle height.