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Experiment: Force transducers

Forces generated in an earthquake can destroy entire cities, so it might seem odd to say that they are just an abstract construct.
We’re in the Mechanics of Solids Laboratory, with its machines for testing materials for strength and stiffness. They can test components, too, like a rubber bearing for a bridge support. A bridge bearing would need a large machine - we can use this smaller one. We’re going to test this wire strap. The machine will give us a plot of load versus deflection, right up until it breaks. Each cable you’ll specify for suspending the loudspeaker will be like this. We’ll start the test.
The strap failed at the clamp. We expected that. The cable for suspending the loudspeaker would probably fail at the clamp or swage, too. From the chart, the maximum load before it broke - it’s ultimate strength was 3.2 kilonewtons. The ultimate load specified for the wire was 3.7 kilonewtons. It probably failed before it reached this value in the test because of the clamp. See how the clearance in the system is gradually taken up. Then the curve becomes approximately linear before decreasing in the slope, until finally the wire fails suddenly. There are seven strands in the wire, but only four strands broke. After a while the others took up the load again, but it had definitely failed.
Different materials produce different shapes of curves. For example, here’s our chart from week one of load versus deflection for a rubber band. Now to this week’s experiments, force transducers. In this experiment, you will learn how an elastic component can be used as a force transducer. A transducer is a device that converts something we can’t measure into something else that we can. This spring balance is a transducer. When you load it up the spring stretches, and we measure the stretch using a calibrated scale that reads directly in force units. The load applied by the test machine is measured by a transducer.
It uses a spring element, too, but in this case the deflection is tiny and is measured electronically by an electrical strain gauge. But the principle is the same. It has to be calibrated, too. This luggage scale also uses a strain gauge. Some bathroom and kitchen scales do, too. They all have to be calibrated in some way. In this experiment, we’ll make a matched pair of transducers out of our chains of rubber bands. We’ll use our two-bands chain, so we can measure moderate loads more accurately. Then, in the next experiment, we’ll use them to investigate how forces add.
Of course, we’ll have to calibrate our transducers, and for this we’ll need to find the weight of the pans in terms of washers - because one washer is our unit of weight. Let’s get started and weigh the pans. We’ll load the chain of bands until the initial stretch is taken up, then hang the second pan from the chain and note the extension. Then we’ll take the pan off and add washes until the extension is the same as with the pan. We’ll repeat this for the other pan. To get a calibration curve we’ll load up a transducer in stages, recording weight and extension. We’ve done this already for one chain of bands.
All we need to do for that one is add the weight pan into the total load, and plot total weight against length. Then, we will repeat this for a second chain of rubber bands. Here are the calibration curves. They’re not straight lines, so we’ll have to read off the force for each extension measurement. Commercial force transducers are more linear, but even they might use a calibration curve for precise work. We’ve got our transducers. In the next video, we’ll use them to see how forces add.

Forces generated in an earthquake can destroy entire cities, so it might seem odd to say that they are just an abstract construct. More on that in the analysis activities.

For now, we need a way of detecting a force, measuring how big it is and specifying the direction it acts in. For this we need a transducer – a device that converts something we can’t measure into something else that we can.

Our force transducers will be based on the chain of rubber bands that we made during the experiment in Week 1. To help keep creep under control we will minimise the time between loading up and measuring.

If you didn’t do the Week 1 experiments it’s not too late to do them now. It would be a good investment because you can use the transducers this week and next.

Before SI units were common the unit of force was often the weight of a standard mass. In some places it still is, for example the unit of mass in US customary units is the pound (the pound mass), and the unit of force is the weight of a pound mass (the pound weight). We’ll have more on weight later. Mostly with US customary units the term ‘pound’ is used for both mass and weight, leaving you to work out what is meant from the context. This can be confusing but it won’t affect us much because we’ll generally use SI units, where the unit of mass is the kilogram and the unit of force is the Newton.

Once we have our transducers we’ll use them to find out how forces add.

You can download instructions to the experiment in the Downloads section below.

Talking points

  • Did you think of the effect of creep when the transducer was calibrated and used?
  • Was I being hopeful when I suggested that a precision of +/- 1 mm was possible using the rule?
  • Did it seem odd to have the weight of a “washer” as the unit of force?

Share your experiment

If you attempt the experiment, take a photo and upload it to our Through Engineers’ Eyes Padlet wall. You can include a link to your photo in the comments for this step (click on your post on the Padlet wall and then copy the web address).

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Through Engineers' Eyes: Engineering Mechanics by Experiment, Analysis and Design

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