Manual Material Handling Design Criteria

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Manual Material Handling Design Criteria
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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

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• 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)

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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

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Figure 3.1 Schematic Diagram of biomechanics,
modified from Contini and Drillis (1966)
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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

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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)

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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

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Figure 3.2
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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

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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

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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

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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

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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

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Figure 3.3
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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.

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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.

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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

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Figure 3.4
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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)

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Figure 3.5
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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

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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)

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Figure 3.6
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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).

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Figure 3.7
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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

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Figure 3.8
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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

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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

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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.

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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.

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Figure 3.9, 3.10, 3.11
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Figure 3.12
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Table 3.1