Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 8)

SYSTEM DESIGN
1-6
Load Introduction Principles
L
C
L
C
Figure 1-14
If the direction of the force is constant, calibration will compensate
for this and the scale will weigh accurately. However, if the angle
changes as the force is applied, it will cause nonlinearity and if
there is friction in the mechanical system, hysteresis will also be
present. Angular loads can be caused by mounts that are out of
level, a nonrigid foundation, thermal expansion/contraction, struc-
ture deflection under load, and the unavoidable deflection of the
load cell itself.
Eccentric Loading
Figure 1-15
This is a condition where the force F is applied vertically to the cell,
but its line of action is shifted away from the vertical line through
the loading hole. This is not a detrimental condition if the force is
applied consistently at the same point, since calibration will
compensate for this effect. However, if the point of application
moves horizontally as the scale is loaded, it will cause nonlinearity
and possibly hysteresis. Eccentric loads may be caused by poorly-
designed mounting arrangements and thermal expansion/con-
traction of the scale.
A clear understanding of the exact manner in which a load must be placed on a load cell will assist you in both designing a vessel that is
to be equipped with load cells, and in choosing the correct type of load cells and mounts for your application.
The Ideal...
Load cell specifications are derived under laboratory conditions,
where load is applied to the cell under near-perfect conditions. The
performance of load cells in an actual process weighing application
can be greatly degraded if care is not taken in the means by which
the load is applied to the cell.
Figure 1-13
Figure 1-13 shows a typical mounting arrangement for a single-
ended beam. The fixed end is fastened to a “rigid” foundation, while
the free end is cantilevered to allow downward deflection as load
(F) is applied. Under ideal conditions, the mounting surface would
be flat, horizontal and perfectly rigid. The load F would be intro-
duced vertically with minimal extraneous forces applied, and the
load cell would be totally insensitive to all forces other than
precisely vertical ones.
However, in the real world, load cell mounting and loading condi-
tions are far from ideal. Incorrect loading is by far the most
common cause of accuracy problems encountered by service
technicians. Understanding the following common load introduc-
tion problems will prevent loading errors in your vessel weighing
application.
Though the discussion is confined to single-ended beams, many of
the principles apply equally to other load cell types.
Load Introduction Principles
Angular Loading
This is a condition where the load F is introduced through the
loading hole, but at an angle to its center line. This angular force
can be broken up into its vertical component along the loading
hole center line which the cell will register and its horizontal
component at 90
°
from the center line. This horizontal component
is a side force to which, ideally, the load cell would be totally
insensitive. For example, if force F is inclined to the load hole
center line at an angle of 5
°
, then the force registered by the cell is
reduced by .4% while a side force of .01F is also applied.