LINK Systems System 1100 Tonnage Monitor - Manual (Page 38)

3-1
3.1 General
Load bearing structural members of machines are elastic bodies --
stretching, compressing, bending, and/or twisting depending on
applied forces. Externally applied forces or moments (torsion)
applied to a solid body cause internal stresses (forces per unit
area) in that body, resulting in dimensional changes of the body.
Such force induced dimensional changes are referred to as strain,
and are expressed as changes in length per unit length.
The strain induced in a metal structural member depends on the
externally applied force, the physical properties of the particular
metal used, and the geometry of the structural member. As long as
stresses within a structural member are less than a certain value,
(dependent on type of material) called the limit of
proportionality, strain is proportional to stress, and for a given
geometric shape and material, to the size, direction, and point of
application of externally applied force. Within this "elastic
region" of the material, the structural member will return to its
original dimensions when an externally applied force is removed.
External forces of such magnitude as to induce stresses in the
material somewhat above the limit of proportionality will cause the
material of the structural member to reach the yield point, causing
permanent distortion of the structural body when the force is
removed. Still greater applied forces will cause the material to
reach the point of fracture, breaking the structural member.
Long term fatigue of structural members is also related to force
induced stress levels and the number of stress cycles (applications
and removals of force on the structural member). Lifetime of
structural members operated well within the elastic region of the
structural material (at low stress levels relative to the limit of
proportionality) is extremely long in terms of stress cycles (and
hence machine cycles). Lifetime is significantly reduced as
structural member stresses near the limit of proportionality.
Structural members of machines that use force in production
processes normally are designed so that induced stresses at rated
load are well below the limit of proportionality, avoiding
stretched, bent or "sprung" machine components and short machine
cycle lifetimes. Thus, the machine operates in the elastic region
of its structural members, with stresses and strains that are
related to applied loads. The stretch or compression of certain
Section 3. Theory of Operation