Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 11)

Load Introduction Principles 7
5.0 Load Introduction Principles
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.
5.1 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 5-1.
Figure 5-1 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 introduced 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
conditions 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
introduction 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.
5.2 Angular Loading
This is a condition where the load F is introduced through the
loading hole, but at an angle to its center line (see Figure 5-2).
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.
Figure 5-2.
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, structure deflection under load,
and the unavoidable deflection of the load cell itself.
5.3 Eccentric Loading
Figure 5-3.
This is a condition where the load F is applied vertically to the
cell, but its line of action is shifted away from the vertical line
through the loading hole (see Figure 5-3). 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/contraction of the scale.
F
F
F