Swede Posted March 4, 2009 Posted March 4, 2009 (edited) I know it's been beaten to death, but I'd like to discuss ways of measuring the pressure of a ram when using hydraulics to ram your rocket motors. Being able to accurately repeat a process is THE way to make reliable motors, and simply stroking a handle until it "feels right" isn't good enough. Passfire has an article about tapping a bottle jack for a gauge, but it is complicated and not a trivial thing. There must be an easier way. Wolter sells their "Pressure Conversion Gauge" for about $107 that apparently works well. http://www.wolterpyrotools.com/image/pyrotool_750t.jpg Going cheaper, would it be possible to use a short ram in reverse? Something like this, from Harbor Freight, $30:http://www.harborfreight.com/cpi/photos/42800-42899/42830.gif You'd have to add a short section of coupled hose, and a gauge. But honestly, I'm not sure it would work in reverse... my knowledge of hydraulics is pretty limited. Then I thought of electronics, since I'm deep into my data acquisition project right now. A load cell (compression) transducer might be ideal, if one can be found cheaply enough. Here's a picture of one from Omega Engineering, an excellent but not especially cheap company: http://www.omega.com/Pressure/images/LCGC_s.jpg There's a whole slew of these things on this page among others. The problem is the cost when new. The idea with a transducer is simple... you apply a voltage to the device, typically 5V or 12V DC, and the transducer outputs a voltage proportional to the pressure exerted on the unit. Measurement can be as simple as hooking up a voltmeter to the output of the load cell. These things usually come with documentation that will tell you that "5.3V = 2,350 pounds" (for example) so you can know exactly what amount of force is being applied. I'd mount a transducer like this on some sort of base fitted to a hydraulic press, homemade or commercial. Any thoughts? Is there a better/cheaper/easier way to do this? Edit - after I posted the above, I found an inexpensive load cell on eBay:eBay Load Cell It should be worth a bit of experimentation. Edited March 4, 2009 by Swede
jacob Posted March 4, 2009 Posted March 4, 2009 Swede, those short rams work just fine in reverse, take a look at this.
Aquarius Posted March 4, 2009 Posted March 4, 2009 Swede, is your press a motor driven hydraulid press with a piston and all the whistles and bells on, or is it a "jack-in-a-frame"?
Swede Posted March 4, 2009 Author Posted March 4, 2009 (edited) Swede, is your press a motor driven hydraulid press with a piston and all the whistles and bells on, or is it a "jack-in-a-frame"? Jack-in-a-frame. Air-over, which I thought would solve my problems but which doesn't help. Otherwise, I'd slap a gauge on there quickly. Jacob, thanks for the link! That is EXACTLY the mod I was looking for on the short ram. For $30 for the ram, plus $10 for the gauge, and a bit of effort, you have a nice tool. Edited March 4, 2009 by Swede
FrankRizzo Posted March 5, 2009 Posted March 5, 2009 Jack-in-a-frame. Air-over, which I thought would solve my problems but which doesn't help. Otherwise, I'd slap a gauge on there quickly. Jacob, thanks for the link! That is EXACTLY the mod I was looking for on the short ram. For $30 for the ram, plus $10 for the gauge, and a bit of effort, you have a nice tool. Here's my reply to a thread over on UKR: http://www.pyrosociety.org.uk/forum/index....ost&p=45431
Swede Posted March 5, 2009 Author Posted March 5, 2009 Thanks again, Frank. I've got that electronic transducer coming and the price was right... we'll see if it can be used. If not, one of those modified rams seems to be a good option. In either case, it shouldn't be too tough to come up with tables for loading pressures, which will vary by ram diameter. This is one of those things that I've needed to do from day one, but am only now getting around to it. No more guessing at pressure... nice!
Swede Posted March 10, 2009 Author Posted March 10, 2009 (edited) The fleabay transducer arrived, and looks nice. It is an unamplified strain gauge mounted inside this stainless biscuit. I have the components to create a nice instrument amplifier for it. My question is more mechanical. I am not all that familiar with these devices. What's with the nipple? Should I allow pressure on the nipple, or should I machine a shielding plate with a cutout for the nipple, so the weight bears on the rest of the surface? http://www.5bears.com/chem1/5bt03.jpg On the bottom side is a ring. (locating ring?) With this one, I'm wondering if I should machine a nest for the ring, or just set the unit on a block of steel. Anyone with any experience with this sort of thing, let me know! I'd appreciate it. http://www.5bears.com/chem1/5bt04.jpg Edit: I emailed the company, and despite the age and obsolescence of the unit, they kindly sent me the data sheet. The pressure goes directly on the nipple. I'll have to machine some sort of plate or other interface that presses the nipple, but still provides a stable platform for the rocket tooling. Edited March 11, 2009 by Swede
Swede Posted April 9, 2009 Author Posted April 9, 2009 I finally got around to creating a circuit and setting up the strain transducer for use in a hydraulic press. It works really slick. I created a blog entry with gobs of detail: Rocket press Transducer blog For those who don't care about the nuts and volts, in a nutshell, I made an amplifier for the strain gauge signal, and displayed it on an analog voltmeter. http://www.5bears.com/chem1/hpt003a.jpg There's only one thing I am not sure of, and maybe someone can clear it up. Here's the theory from the blog entry: In actual use, there are two basic ways I can use it. The first and most likely way is to keep good notes so that I can replicate a process at a later date. For example, if I pressed a 4 ounce rocket to 3V and it CATO's consistently, but one pressed at 5V works perfectly, then future rockets will all be pressed to 5V. The other way would be to calculate the actual pressure in PSI, and create a graph or two, plotting voltage vs. PSI for a given diameter ram. Example: The cell can handle 7,500 lb, and the ram is 1 inch in diameter. I have a surface area of the ram of Pi R-squared, or 0.785 square inches. If 10V on the meter is 7,500 pounds, then the pressure under the ram is 9554 PSI. A smaller diameter ram will have a correspondingly larger PSI. Is the second thought accurate? The entire transducer can handle 7500 lb, but if a 1 inch ram is exerting 7500 lb, the pressure under the ram is, as noted, 9554 PSI. A 1/2 inch ram would produce 38,000 PSI at 7500 lb indicated due to the surface area of 0.196. True?
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