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Load Cell/Test Stand

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edwardw

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I thought this might be a good place to post this - but I don't know. Does anyone out there have a load cell /test stand that they made? If so, what are the specs and how much $ did it cost. I am looking to build one to test some RMS motors to see how the curves stack up to what they tell me.

Edward
 

gerbs4me

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I was thinking of getting one, the $25 one seems good enough for me for now.
 

Stymye

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keep in mind that the $25 one is electronic readout
it will require more components
including a data acquasition unit that is $120....
 

gerbs4me

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you do have a point styme
couldn't you just use a bathroom scale and a stop watch? but you wouldn't get as much data, just thrust and burn time.
 

edwardw

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I was looking at the aerocon system - $160 for a 44lb load cell. I think that I could make it work though. If instead of having the engine thrust against the cell I used a lever to reduce the force by 1/5 then I could test engines up to 220 lbs. Any ideas on doing that? I also could have interchangable arms to get more capacity out of the load cell.
 

Bullpup

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I picked up a 500lbs load cell for $40
Dataq 10bit A/D was $25
INA125 was a few bucks plus some parts from Radio Shack.

The rest was scrap metal and a friend did the welding.

Total of about $75-$80

There is a photo of it on the RCS web page:

http://www.rocketmotorparts.com/

Under Users,

G
 

edwardw

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Thank you all for your advice and this is our current setup.

We have two 2500 psi pressure transducers - one for chamber pressure and one for thrust.

We have a 4 ton hydraulic jack from Harbor freight that is connected to one of the transducers.

We got 6 INA125 chips as samples from TI - Free!!!!

Our transducers output 0 to +5 volts so we do not need an amp after all.

The DataQ RS-194 is our A/D converter and software.

We will be able to measure chamber pressure up to 2500 psi and thrust to 2375 lbs.

The total cost was $150

Edward
 

Stymye

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Edward how are you integrating the pressure transducers?
do you have any drawings to share?
I have access to the type that tap into a hose or tube
( measures air or fluid pressure ) with a small digital readout
or you can send the info to a card
 

edwardw

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Here is how I did it.

The hardest part is getting the jack to work the way you need it, as a force transducer. As set up you pump on the small cylinder and then the big jack part moves up. You need both of these to be connected. That means when you press down on the little one the big one goes up, and when you press down on the big one the little one goes up.

First you have to unscrew the nut holding the little pump down. Take the one way valve assembly out. You will notice there are two holes on the top, one on the bottom, and one in a groove on the side. This one on the side goes to the fluid reservoir. We need to plug that one so that we are just connected to the main jack cylinder.

Take a punch and knock out the ball bearings to both that act as one way valves. Make sure you don't scar up the surfaces as they also serve to seal the chambers.

For the side hole you must plug it. The easiest way I found was to buy a small metal taper dowel ( I think number 0 or 1) and cut it off about 1/3 of the way down. Once you have the ball bearing out insert the 2/3 piece in the hole. It should fit snug. Hammer it in until it will not go in any more and makes a nice tight seal.

Now we just run some teflon take around the sides and top to help it seal. Drop it back down, two holes up.

Now we need to get a way to hook the pressure transducer up to this cylinder. It just so happens on the Harbor Freight 4 ton jack the cylinder diameter is 7/16 inch. That is the same size required by a 1/4 NPT tap. That means we can use 1/4 NPT brass to connect to the pressure transducer that is 1/4 NPT.

It takes a while to tap the cylinder and it is best held down in a vise and tapped. A local plumbing supply store had the tap for $7.29.

Once we had it tapped we arranged some fittings to get it to the pressure transducer. We had already filled and bled the air out of the system. Remember, you have to have the cylinder about 1" up off of the bottom or it will hit the bottom of the cylinder and you won't get any force readings.

The area of the cylinder is .9512 inches squared. This means that for every pound you push down on the cylinder with you get this formula..


F = (.9512 in^2 * PSI)

The force your motor is thrusting with is F, .9512 is the area of the cylinder, and PSI is what your gauge or Pressure transducer is reading.

So if you have a 100 psi guage you can read a maximum thrust of 95.12 LBS. We have pressure transducers that go up to 2500 psi, and the data is recorded by DataQ on a laptop.

I have attached a sketch of what it looks like - just a schematic, not the finished product.

Another benefit of using a jack stand is that your stand doesn't have to take the load of your motor firing, the jack stand transfers it right to the ground. All your stand has to do is steady the motor and keep it upright.


Any questions I will be more than happy to answer.


Edward

www.stlrocketry.com
 
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