Tension load cell options

Question

I'm designing something similar to an electronic hanging scale (think a heavy object on the bottom, with a scale attached immediately above it). I am not that familiar with what the most typical way to achieve this would be. From my initial research it seems like a load cell with an HX711 load cell amplifier plus a microcontroller could be a good choice. I don't know that much about load cells, but there seem to be more options for compression than tension load cells. S-type load cells might be a good option (but might be pricey). It has also occurred to me that even though I would be measuring tension across the entire scale, I could design the scale such that a compression only load is used. Anyone have any input about integration of a load cell into such a device would typically be achieved? Thank you.

Answer 1

Here is what the inside of a luggage scale looks like:

The load cell looks like a cantilever beam load cell. The shear element is an aluminum shape with strain gauges probably bonded to the aluminum top and bottom. Image from here

The strain gauges are alloy serpentine foil shapes, generally supplied on a polyimide backing. They look like this (image from Wikipedia):

By connecting two similar strain gauges in a half-bridge (one in tension and one in compression) the temperature coefficient of output voltage is minimized and the output is doubled.

Answer 2

From a mechanical perspective, as you say, the active element of these devices is almost always a ceramic element that is much stronger in compression than tension, and is therefore designed to operate in that mode. More importantly, in compression you don't need attachment points on the base measurement device, you just need to sandwich it into a load cell. So the bare devices are almost all compression only (or at least have much lower tension ratings).

You therefore either need to design a pair of tension elements which are interlinked via a compression cell, or buy a complete "tension" module that does this for you.

I've used self-design tension cells in the past, but I don't have the design files or photos to hand. They looked like two interlinked rectangular eye bolts, with the load cell sitting at the interface point.