# Measuring different forces: torque, pressure and bending moments

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An Interview with Kevin Copleston, one of the founders of Sensor Solutions Ltd

Lets just look at modes in a load cell, can use axial, have a straight forward column, put strain gauges on, so measuring axial load, or could look at bending where got a beam, apply a load and put a strain gauge down that beam, putting your strain gauges here and here and here.

Or we can use shear or torque, where you have a shaft where you would put gauges at 45 degrees, and how that is set up in a wheatstone bridge, is looking at either torque on it, or the..

Sensor Solutions: ‘It’s measuring the deformation to the material caused by the torque?’

Yes it’s at 45 degrees so ..

This is a vector diagram, this is a vector diagram of axial load, here’s a vector diagram of bending, this is a vector diagram of ..

This is Poisson’s ratio, this is principle strain here and torque, the principal strain’s at 45 degrees, and they are equal size.

So that’s a vector diagram, that ‘s plotting strain as a vector around this structure.

So those are the 3 modes you use, axial is most simple, simple rules associated with that, usually associated with high loads.

Whereas a moment arm just make the arm longer, so small loads and for large loads, or large values of what is being measured, this has advantages, as in shear, whether the reaction in shear is here down beam, or here down beam, the shear is still the same, independent of where the load is applied. Cantilever beam, if the load is applied there, there is the distribution, the strain level goes down for the same given load. The longer the arm, there is a higher value for strain.

So they can be used in a number of ways, load cells, straight forward, in a silo, or load cell in a testing machine, they just measure compression. If you want to measure tension in a wire, OK, so here’s your wire, to measure tension put a little cantilever beam, put a slight bend in it and as tension increases and decreases, the amount of bending in this beam will go up and down.

And you can have a very long arm and measure very small amounts of tension change, as an example.

But the common bending transducer is the look something like this:

Drawing

Instead of using an axial load, if you have a low load you can use bending, in that element here, as it compresses, if you over load it that little gap will close up.

There and there which means that got overload protection,

It also means this little beam in its own right one will do that, the bottom will do that, and so notice at this point here you have compression, here you’ve got tension, here you’ve got compression, there’s your Wheatstone bridge, up down, up down, you’ve got a very sensitive transducer.

These are what sensor make to go in wind turbines in the blades, are a blade sensor for measuring…., in this case it’s not measuring load, it’s fixed here and the wind turbine blade takes the load and deforms due to that load, this faithfully follows the movement, so it’s a displacement transducer.

So we are now using bending to measure displacement. So there’s a whole load of ways of doing it, thing is coming up with the idea how to do it in the environment you are working in.

So how to measure that, with that, in the environment your working in as efficiently (?) as possible. Ok, so you shouldn’t really have strain in there, strain is stress analysis, strain, you measure strain to calculate stress, to understand what’s happening in your structure. A perfect structure distributes whatever force you put in it with a uniform stress, that’s the perfect structure. So you are looking for the same strain over the whole structure as it is being loaded. If you get high spots, low spots, you need to add material, remove material.

What breaks materials is discontinuity, glass is stronger than steel, what breaks glass is a local discontinuity, flaw, an occlusion, bit of bubble, or scratch in the surface, as it’s brittle it, if you had a pure, if has flaw starts a crack so you have a failure. It’s stress concentrations that kill brittle materials. Thing about steels and things is they are ductile, so stretch a little bit.

And those stress concentrations are taken out when it stresses a little bit.

`Information about: Deflection & Displacement Measurement`