Measuring Solenoid Force

When designing the V-Twin solenoid I thought I should try and improve the solenoid. One area of particular concern was that for this one I used small permanent magnets in the armature, useful as it means I can make the engine double acting, but the downside is that it only acts over half the stroke and the force drops to zero when the armature is centred in the solenoid.

So first thing is to measure the force. So I disconnected the conrod on my solenoid engine and fixed the other end of the wire to it that was attached to the weight.

As you can see, the contraption I came up with: 2x pulleys, some digital scales and a weight.

The idea is you place the weight on the scales and zero them, this means that when you turn the solenoid on you get a negative weight on the scales.

I made some small pulleys with ball races so that they are very free running. Quite frankly these could be just plain bearings.

These pulleys are fitted to the fixture and fittings in my workshop – you never know when they will be useful for something.

As the pulleys are fixed I had to adjust the position of the piston in the solenoid by jacking the motor up. You could just as easily adjust the height of the scales and achieve the same effect.

The measurement. With a fresh battery the peak was 33g, this dropped to 22g max with use and as the armature moved to the centre of the solenoid this rapidly dropped to 0g.

33g is 0.033kg, multiply this by 9.8ms-2 to calculate the force. This gives 0.32N

22g = 0.022kg -> 0.22N

So there is my new target I have to beat. To make this simple you can just divide the mass in grams by 100 to get the force in Newtons – rough, but pretty good.

All of the parts for this can be made with hand wood working tools and a small lathe such as a Unimat 3.

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