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Title: Manual Material Handling Design Criteria


1
Manual Material Handling Design Criteria
2
Introduction
  • Stress/Strain Concept
  • Workers affected by two types of forces
  • Immediate physical environment
  • Biomechanical forces of workers body
  • Both forces fundamental to basic ergonomics
    principles and described by the laws of Newtonian
    mechanics and biological laws of life
    (musculoskeletal and physiological systems
    response to task demands
  • Stress/Strain involves external forces and effort
    upon the worker which potential produces strain
    on workers musculoskeletal and physiological
    systems
  • Goal of ergonomicist is to reduce stress
    sufficiently to minimize musculoskeletal and/or
    physiological strain

3
  • MMH Stresses
  • Three approaches used
  • Biomechanical
  • Studies the musculoskeletal structure such that
    the physical, or mechanical, limits of the
    individual are determined.
  • Physiological
  • Studies the circulatory responses and the human
    bodys metabolic response to various loads.
  • Psychophysical
  • Establishes acceptable lifting weights to the
    individual. The individual subjectively
    quantifies his tolerance of stress (NIOSH, 1981)

4
Biomechanical Approach
  • Biomechanical Analysis for MMH
  • Definition and Applications
  • Biomechanics defined by Contini and Drillis
    (1966) as the science which investigates the
    effect of internal and external forces on human
    and animal bodies in movement and at rest.
  • Winter (1979) defined biomechanics of human
    movement as the interdiscipline which describes,
    analyzes, and assesses human movements.
  • Frankel and Nordin (1980) defined biomechanics as
    the discipline which uses laws of physics and
    engineering concepts to describe motion undergone
    by the various body segments and the forces
    acting on these body parts during normal daily
    activities

5
Figure 3.1 Schematic Diagram of biomechanics,
modified from Contini and Drillis (1966)
6
Biomechanics Definitions(Continued)
  • General Biomechanics
  • Concerned with the fundamental laws and rules
    governing organic bodies at rest or in motion
  • Biostatics
  • Considers those situations in which only analysis
    of dobdies at rest or bodies moving in a straight
    line at uniform velocity (i.e., no acceleration
    generated, thus no force yield) is involved
  • Biodynamics
  • Concerned with the description of the movement of
    the body in time without consideration of the
    forces involved (kinematics) and motion caused by
    forces acting on the body (kinetics).
  • Both internal and external forces are included in
    kinetic analysis of motion

7
Occupational Biomechanics Definitions(Continued)
  • Division of applied biomechanics that involves
    applying the principles of biomechanics towards
    work in improving everyday activities, especially
    dealing with human disorders and performance
    limitations which exist at present in a variety
    of manual tasks in industry
  • Can be defined as the study of the physical
    interaction of workers with their tools,
    machines, and materials so as to enhance the
    workers performance while minimizing the risk of
    musculoskeletal disorders (Chaffin and
    Andersson, 1984)

8
The Body as a System of Levers
  • Biomechanics based on the disciplines of
    anthropometry, engineering science,
    bioinstrumentation, and kinesiology.
  • Requires criteria for application of measurements
  • Complexity of measurements and need for safety
    have resulted in extensive use of modeling
  • Modeling allows simplification, eliminates much
    of the experimentation and elaborate data
    collection/analysis

9
Figure 3.2
10
The Body as a System of Levers
  • Biomechanical approach requires evaluating the
    body as a system of links and connecting joints
  • Each link has the same length
  • Each link has the same mass and moment of inertia
    (Center of Mass)
  • Various researchers use various links increases
    in computing power has increased complexity of
    modeling
  • Torso is often considered as a single or two-link
    system for simplicity although more complex
    modeling of the spine has been developed
  • Body segments rotated around joints by skeletal
    muscles attachments of same are close to the
    joint
  • Small contractile distance transformed into large
    resistance and mechanical advantage large muscle
    forces for small loads

11
Stress on the Musculoskeletal System
  • To estimate mechanical stress imposed on body
    while at rest or in motion, must use various
    mechanical properties of body segments to perform
    mechanical analysis
  • Simplification and assumptions necessary use
    biomechanical models of various degrees of
    sophistication
  • Remember that forces are vector quantities with
    four characteristics
  • Magnitude
  • Direction
  • Line of Action
  • Point of Application

12
Stress on the Musculoskeletal System
  • Three types of Forces on Total Body System
  • Gravitational Forces
  • Those forces acting through the center of mass of
    each segment with magnitude equal to the mass
    times gravitational acceleration
  • Ground Reaction or External Forces
  • Due to applied workload and body segment weights
  • Muscle Forces
  • Expressed in terms of net muscle moment acting at
    a joint. Some other forces such as joint
    friction and forces within the muscle also
    contribute to net moement

13
Static Analysis
  • Used to study the rotational moments and forces
    acting on the human body when no movement is
    involved
  • Physical forces can be analyzed as if executed
    statically (even when involve movement)
  • Dynamic considerations important (mechanically)
    only when motion involves significant linear or
    angular accelerations
  • If this is not the case, static analysis
    techniques are useful for studying static and
    quasi-static (quasi-isometric) physical activities

14
Analysis of One Segment Link
  • Example is forearm free body (see figure 3.3).
  • Assumes no significant joint exists
  • Assumes 8kg load held in hands
  • Load acting at hand produces a torque at elbow as
    does the weight of the forearm and hand
  • Involved muscles contractile activities produce
    necessary torque to counterbalance aforementioned
    torques
  • Since no body movement, static analysis is assumed

15
Figure 3.3
16
Analysis of One Segment Link(Continued)
  • Static Equilibrium
  • ? Moments (M) 0 ? Forces (Fx or Fy) 0
  • Relative force and torque at elbow joint
  • Yt 0 W1 Wa Fy (Force equilibrium)
  • Where
  • W1 the weight of the load
  • Wa the weight of the forearm and hand
  • Fx the reactive force at elbow joint in the
    x-direction
  • Fy the reactive force at the elbow joint in the
    y-direction
  • Yt the force in the Y direction
  • Xt the force in the X direction.

17
Analysis of One Segment Link(Continued)
  • Reactive force at elbow joint in X direction
    with no force developed in horizontal direction
    is 0
  • Reactive torque at elbow joint necessary to
    counterbalance forces produced by the load
    (weight body segment x distances from Center of
    Rotation)
  • ?M 0 -W1(D1) Wa(Da) ME
  • Where
  • D1 the length of the link
  • Da the distance from the elbow to the link
    Center of Mass (CM)
  • ME the reactive moment at the elbow joint.

18
Analysis of One Segment Link(Continued)
  • Assume forearm no longer held horizontal but at
    angle as in figure 3.4
  • Reduction in the moment arm
  • M 0 -W1(D1)(cos a) Wa(Da) (cos a) M1E
  • Or
  • M1E (cos a) x ME
  • So, the load and the weight of the arm has an
    additive effect on the elbow moment, with its
    maximum moment value when the arm is horizontal
    and a minimum effect when the arm is vertical

19
Figure 3.4
20
Analysis of Two Links
  • Effects of loads on hand and accumulated body
    segment weights transmitted to feet where
    reaction force takes place
  • Can treat two-link model as two separate one-link
    systems with same torque and force analyses
  • Static equilibrium condition, reactive forces and
    torque should be equal but opposite (distal end
    of the upper arm link)

21
Figure 3.5
22
Analysis of Two Links(continued)
  • Muscle group should produce force/torque at the
    shoulder joint to counteract force/torque of body
    segment and reactive force from previous link
    (calculated as 5.65kg)
  • Yt
  • -2.1kg 5.65kg Rs
  • Rs 7.75kg
  • And
  • ?M 0 -Wu(D2) Re(Du) - ME Ms
  • -2.1kg x (0.13m) 5.65kg (0.33m) 3.92kg
    m Ms
  • Ms 5.86kg m

23
Analysis of Two Links(continued)
  • Arm posture changes have a great effect on the
    moments at the elbow and shoulder but no effect
    on external reactive force
  • Previous two-link model is effective, except
    vertical distance from point of rotation to
    action line of force is used to calculate moments
    (see figure 3.6)

24
Figure 3.6
25
Analysis of Multiple Links
  • Use same method to calculate reactive force and
    torque at each joint
  • Changes from the horizontal will change (increase
    or decrease) the moment but not the force.
  • Different posture can have different reactive
    moments and forces
  • Keep in mind Cartesian coordinate mapping systems
    and force terms (positive or negative).

26
Figure 3.7
27
Analysis of Internal Forces
  • Possible to look at models of internal muscles
    (see figure 3.8)
  • Resulting moment equation can be expressed as
  • ?M 0 - W1(D1) Wa(Da) Fm(Dm)
  • Where
  • ?M the sum of the moments about the elbow
  • Fm the force due to muscular contraction
  • Dm the distance from the elbow to the point of
    muscle action on the link

28
Figure 3.8
29
Analysis of Internal Forces(continued)
  • Substituting those previous values into the above
    equation and isolating the unknowns produces
  • ?M 0 - 4kg(0.36m) 1.65kg(0.15m) Fm(Dm)
  • Fm(Dm) 1.69kg-m
  • The torque of the muscle contraction must equal
    1.69kg-m for the link to maintain static balance.
    If the value for Dm is assumed to be 0.05m, the
    magnitude of the muscle force can then be
    determined.
  • Fm(0.5m) 1.69kg m
  • Fm 33.75 kg

30
Analysis of Internal Forces(continued)
  • Next, the horizontal and vertical force can be
    determined. Since there are no horizontal forces
    acting in this example, the horizontal force, Fx,
    drops out of the analysis. Substituting the
    known values into the following equation and
    isolating the unknown, Fy, produces the vertical
    force on the elbow.
  • Yt 0 -W1 Wa Fy Fm
  • - 4kg 1.65kg Fy 33.75kg
  • Fy -28.1kg

31
Example 20kg Industrial Task
  • Assume frequently handled 20kg load
  • Creates a muscle contractile and vertical force
    at the elbow as follows
  • M 0 -10kg(0.36m) 1.65kg(0.15m) Fm(0.05m)
  • Fm 76.95kg
  • Yt 0 -10kg 1.65kg Fy 79.95kg
  • Fy -88.6kg
  • Demonstrates the forces sustained in holding
    common industrial load can be quite large.

32
Dynamic Analysis
  • See extensive notes on calculations of static and
    dynamic forces, moments as a function of various
    variables in notes
  • Note the effect of static versus dynamic loading
  • Note the effect of various angular (e.g.,
    postural) changes on the overall loads (forces,
    moments, inertia) created when handling a
    relatively small weight
  • Notice how much more complicated the modeling
    becomes moving into a multivariable domain.

33
Figure 3.9, 3.10, 3.11
34
Figure 3.12
35
Table 3.1
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