Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 9)

1-7
SYSTEM DESIGN Load Introduction Principles
Side Loading
Figure 1-16
This is a condition where the vertical force F (which you are trying
to measure) is accompanied by a side force R applied at 90
°
to F.
This force can be constant, but more typically is a force that varies
over time and hence affects the linearity and possibly the hysteresis
of the scale. The ideal load cell would be totally insensitive to side
loads. However, in practice these extraneous forces do affect the
output of the cell and two seemingly identical cells can react
differently to the same side load. A related condition is the END
FORCE, P, which is similar to a side force, except that it acts on the
end face of the cell. Side forces are the result, typically, of thermal
expansion/contraction, mounts which are not level, and vessel
dynamics (caused by mixers, etc.).
Twisting Loads
(a) (b)
h
T h 1
L
C
T
Figure 1-17
Typically, a side force does not act exactly at the neutral axis and
hence produces a torque or twisting effect in addition to the side
force. A load cell can be subjected to a torque (T) in a number of
ways. Figure 1-17 (a) illustrates a condition where the line of action
of a side force is moved away from the neutral axis by a distance h
resulting in a torque of Rh. Figure 1-17 (b) illustrates a situation
where the load is hung from the cell using a bolt. Any side force
applied by this arrangement has a much larger twisting effect on
the cell because of the increased distance h
1
to the neutral axis.
Figure 1-18 illustrates a torque of magnitude F
y
exerted as the
result of the load F being applied at a distance y from the loading
hole center line.
y
T
Figure 1-18
Mounts which are out of level, thermal expansion/contraction,
structure deflection under load and dynamic side forces (caused by
rotating mixers, etc.) all cause twisting of the load cell. Since these
forces tend to vary in magnitude as a function of time, temperature
and/or load, the effects are not predictable, and will degrade system
accuracy.