8
4) WORKING PRINCIPLE OF LOAD CELLS
4.1
MEASURED FORCE:
All the load cells (single - axis) measure the force only in the direction of their weighing
axis (= measuring axis).
The component vector-forces on other directions not only are
not measured but they can deform the cell, they introduce supplementary errors and they can
destroy it. When a cell is used in a scale or in a weighing system, it is submitted to the
vertical gravity-force due to the weight (= mass) to be measured, therefore the weighing axis
of the cell has to remain perfectly vertical in all the loading and measuring conditions.
4.2.1
In a load cell are acting 2 vector-forces: the force to be measured applied to the cell
and the reaction force of the fixing plane. Both these forces are the resultant (as
value and as direction) of many distributed forces.
Important: the positions of the barycentre (= point of origin) and of the directions of
these two forces must not change during the application of the load; moreover both
these two forces and the axis of measure of the cell must coincide or must be parallel
among them depending on the principle of working of the cell.
Conditions different from the over listed decrease of many times the measure quality.
4.2
DISTANCE BETWEEN THE FORCE AND THE WEIGHING AXES:
The force to be measured is to be aligned along the load cell measuring axis but for some
load cell series it is enough to be parallel, therefore there are four types of load cells:
4.2.2
4.2.1 Series QB 700 - Series 500 QD – Series EG: (for scales)
with
a
flexing parallelogram.
The point of application of the force can be far, from the weighing axis of the
transducer, up to several centimeters in the QB 700 and EG (see bulletins) and few
centimeters in 500 QD.
This distance is increasing with the area of the flexing parallelogram.
4.2.2. Series MB 400 - MR 400 - MD 5000 - PT500 – 560QDT - KCI: with working
principle = shear.
The point of application of the force can be far 2÷3 mm from the weighing axis of
the transducer, with an error still acceptable for most of the industrial applications.
4.2.3 Series LD - LT – HC7000 - HC 2000 - BC 300 – ME – MF – LCR 200 – MB490 –
MP- AP 7000: with working principle = flexion.
The point of application of the force can be far 1÷2 mm from the weighing axis of
the transducer, with an error still acceptable for most of the industrial applications
Remarks: from the over listed: category 4.2.1. can accept the weighing pan directly
connected to the cell. For the other categories (from 4.2.2 to 4.2.5) it is
always important the use of accessories (spherical joints, chains, etc.) in order
to obtain that both axes always coincide.
Summary of Contents for 500QD Series
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