My first attempt at a test stand was a bit primitive, made from wood and using a fulcrum and springs. While it worked, I really didn't think it would hold up to a J class motor. I looked at many different web sites featuring a variety of load cells, electronic, hydraulic and spring. Taking inspiration again from Richard Naaka's site, I decided on a hydraulic load cell with a simple pressure gauge and a camcorder to record the thrust and time data. It would certainly be the easiest and cheapest to build, and would be strong enough to handle any motor in the foreseeable future. Using Jimmy Yawn's lead on some good software for analyzing the video from the test firing, I downloaded Video Mach, it's freeware for non commercial use, and lets you easily look at the video frame by frame, and capable of quickly capturing individual frames as a JPG image.
The iron for the stand I had laying around. I welded most of it, but I'm sure you could bolt it together if you didn't have a welder. The completed stand weighs in at 31 pounds. At $.50 a pound for iron, that's $15 for iron, the heavy duty Chevy pickup rear wheel brake cylinder was $15, the gauge was $11 and the pipe and fittings were about $8. For under $50 you have a nice heavy duty test stand.
A picture is worth a thousand words, so here are a few thousand words worth.
Above: Completed test stand.
Above: Here is a close shot of the load cell.
Above: Test stand gets its first use.
The base of the stand is just two pieces of 1.5"x3" rectangular tubing welded to a piece of 4" channel iron. The support going up from the channel iron is a 1"x2" piece of rectangular tubing. I welded a small plate to the side of this piece to bolt the wheel cylinder to. This small plate had to be rounded to fit the wheel cylinder, so I just used my bench grinder to round it to the correct profile. Of course, you need to be careful and mount the plate so the wheel cylinder is properly positioned. I made sure the wheel cylinder was expanded to its maximum capacity and the lower boot of the wheel cylinder was just touching the channel iron base. I finished by tack welding a 2" i.d. pipe to the support tube. That of course allows motors up to 2" in diameter to be tested. For smaller motors, I simply cut some PVC pipe to form a collar so they would fit snugly.
To connect the gauge. Buy a short length of brake line when you buy the wheel cylinder. It should be 1/8" pipe. You may need to cut the line in half because most brake lines have flared ends on both ends. You just need the flared end to go into the wheel cylinder, then buy a 1/8" pipe compression fitting that adapts to a 1/4" NPT male end. That end goes into a 1/4" NPT tee fitting, the pressure gauge goes into the top of the tee, and a 1/4" NPT plug goes in the other end. Buy a pressure gauge in the range of the motors you intend to test. For small motors, use a 100 pound gauge, for mid size motors use a 200 to 400 psi gauge, for large motors use a 1,000 psi gauge. Keep in mind, with the wheel cylinder I used, 1 psi on the gauge is about 1.2 pounds of thrust, so you can use that to estimate the gauge size you need. I wouldn't go for the largest gauge, it will lack the detail on smaller motors. You can always buy another larger gauge later on for testing larger motors.
You will need to fill the system with hydraulic fluid. I used transmission fluid, but of course brake fluid is fine, I just didn't have any on hand. Lay the stand on its side with the plugged end of the tee pointing up. Remove the plug. Now open the little bleed screw on the wheel cylinder, it's just next to the line coming out, open it a turn or two. Now start slowly adding your brake fluid to the opening on the tee, when the cylinder is full the brake fluid will start coming out the bleed screw. Close the bleed screw. Tap the cylinder gently to displace any small air bubbles in the cylinder. Be patient here, make sure all the air comes out. I also pushed gently on the cylinder pistons to move fluid and air through the system. After I had all the air out, I made sure the cylinder pistons were fully extended again, then I filled and capped the tee with the end plug.
You will need to run a series of tests to calibrate the test stand. My tests showed some inconsistencies at lower pressure levels. If you want the most accuracy, you will need to find the conversion factor for each range of pressure readings on your gauge. I would simply balance known weights on the load cell and take a reading on the pressure gauge. For example, a 20 psi reading on my gauge was 25 pounds of actual weight. Which is a conversion factor of 1.25 to 1. At 70 psi on my gauge the actual weight was 81 pounds, for a conversion factor of 1.157 to 1. With a cylinder bore size of 1 3/16" the conversion factor should be 1.107 to 1. I assume friction has something to do with the variance, as well, there is a small spring in the cylinder that may increase resistance some. In any event, I intend to calibrate the stand on a regular basis. As the cylinder breaks in, the consistency may improve.
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