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
