# Load cell behaving weirdly. Output magnitude of force depends on which side is held and which side force is applied

I have a generic load cell (image) and am using Hx711 which is connected to arduino. The code is from Instructables.

When I fix and apply the load as in case 1, it shows positive force. But when I do as in case 2, it shows negative force. I haven't flipped the part just rotated about Z axis. Here is my question: Shouldn't both cases show same force considering region of 'strain gauge 1' is at tension and region of 'strain gauge 2' is at compression? Am I missing something?

• case 2 has the same effect as lifting the left end in case 1 Commented Mar 12, 2020 at 16:19
• @jsotola can you explain a bit more why pulling left end of case 1 rather than pushing gives same output as case 2? Because wouldn't pushing down in both cases would result in tension of upper region? Commented Mar 13, 2020 at 4:06
• Commented Mar 13, 2020 at 8:03

Shear vs bending

The main reason for the change in signal magnitude when held at left and force applied at right vs when held at right and force applied at left is because it is a shear beam rather than a bending beam.

This is a cad model of shear beam.

held at right and force applied on left

held at left and force applied on right

In the first simulation figure, left is held at stationary and force is applied at right, if we look at the upper region, the region above right hole has convex shape i.e. lengthening of the region and the region above left hole has concave shape i.e. compression of the region.

In the first simulation figure, right is held at stationary and force is applied at left, if we look at the upper region, the region above left hole has convex shape i.e. lengthening of the region and the region above right hole has convex shape i.e. compression of the region.

This is a cad model of bending beam

held at right and force applied on left held at left and force applied on right

Whereas, in case of beam with no holes, we can see that in both cases, the upper region is in tension i.e. elongation and lower region is in compression.

In case of shear beam, the strain gauge is above the hole region. The elongation and compression of region is dependent on which side it is held and force is applied. But for bending load cell, it is not the case.