# In search of Force Gauge!

1. Nov 4, 2013

### skrat

Hi there,

firstly I have to apologize if this is not the right section of the forum to post my question...

Well, what I need is a more sophisticated newton meter, to measure the force on the line. Let me describe the whole thing: I am a paragliding pilot and we use gliders that have lots of lines which connect me and the canopy above my head. Well, every now and than tho whole glider has to be checked if it is still safe to fly with and part of that test is also to measure the force at which one of randomly selected line breaks. (that line is later replaced with a new one).
So I build a system where one end of the line is fixed to the wall and the other is fixed to the motor which moves at a constant speed away from the wall. Now to measure the maximum load before the line breaks I need a force gauge that will after the experiment is finished give me the information I need.
There is only one problem. Force meters are really cheap if you are looking at 5N of force, but as soon as the force gets larger, so does the price. For a brand new force gauge with maximum load of 1000 kg I would have to say goodbye to about 1000 eur.

So my question to all of you here is: Where could I possibly get a used (second hand) force gauge with maximum load of at least 360 kg that would tell me the maximum measured force during the experiment or show the graph on the computer?

I also thought about making a force meter like Newton did: using one strong spring, but this would look really unprofessional to the people that don't know physics - and I don't want to explain to each and every one the whole idea behind the spring.

Thanks for all the help!

2. Nov 4, 2013

### UltrafastPED

So design something specific to your application!

For example, weld a lever to an axle; the short end of the lever connects to your line, while the long end is connected to a cheap push-pull force meter.

The ratio of lengths of the lever arms gives your force-reduction ratio.

Because the force meter will have a limited length you will have to rig an adjustable - but high tensile strength - "stretcher" so that it will reach; perhaps a steel cable with a clamp.

Or you can occasionally calibrate the lever and just use the maximum angle - record the session with a digital camera movie to find the maximum angle.

3. Nov 4, 2013

### UltrafastPED

Reviewing the OP, it should be possible to take measurements based on the current draw of the constant-speed motor: review the spec sheets and see if they provide load-current tables for your motor. If not, you could calibrate the load-current for yourself.

Then all you would need is a amp meter and the conversion table!

4. Nov 4, 2013

### AlephZero

Really? Here's a 1000 kg load load cell that costs $50. http://www.phidgets.com/products.php?product_id=3141 Plus another$50 for a computer interface, if you don't want to make something cheaper yourself: http://www.phidgets.com/products.php?product_id=1046

But either way, IMO you ought to ask yourself if your life is worth less than 1000 eur (or even \$50) before you start on this sort of DIY measurement project.

5. Nov 4, 2013

### skrat

UltrafastPED:

Well the last idea is simply amazing one! :O

Let's say that the load-current tables are not provided for my motor (in reality I don't have it yet)... It would probably too naive to think that the relation between the load current and force is linear, right?

AlephZero:

THANK YOU VERY MUCH for this link! I've got absolutely NO idea why these are so "cheap".

To calm you down: Most of the lines have a breaking point of 250+ kg, keep in mind that on a simple glider there are 18 lines, which are all tested to hold at least 8g before the serial production starts. So, no worries!

6. Nov 4, 2013

### UltrafastPED

The work performed is proportional to the current for a fixed voltage ... but for an induction motor you have reactive current that flows in and out to provide the magnetic field.

So it can be sort-of proportional, but would require proper averaging. Sounds like an experiment to me!

Or better is to carefully select the motor with your requirements in mind. If the motor has permanent magnets then there should be minimal or no reactive current.

7. Nov 5, 2013

### AlephZero

It's your risk assessment not mine. But FWIW we had a guy at work who learned hang-gliding (with professional instruction) and then built his own glider.

He doesn't work with us any more, after a "head down" landing when something broke. He didn't break any bones, but stringing two or three words together coherently is about the limit of his intellectual ability now.