Gallery, Projects and General > Project Logs
Making a Prony Brake to Measure the Power of My Hot Air Engines
vtsteam:
After making the hardware, it was time to split the blocks apart again. My good old Swiss Army knife seemed a better (and longer) wedge shape than an Xacto, so that's what I used.
[ You are not allowed to view attachments ]
The original brake had bolts with heads, unlike my all-thread studs and it was suggested that a tight fit in the lower arm was needed to keep them from turning. Seemed to me that a dab of epoxy on the threads would serve both purposes for my version, so that's what I did.
Then I sorted through my box of springs to find something of the specified size for the adjusment pressure (24 SWG, 3/16" dia) found one and cut off two sections 1/4" long with an abrasive disk in a Dremel tool. These were installed under the adjustment nuts, and the brake arm was reassembled to take a look.
[ You are not allowed to view attachments ]
vtsteam:
Today I started on the stylus that will transmit the force of the brake arm to the digital scale I'll be using. I started with a piece of brass 3/16" and immediately ran into the problem that my 4 jaw chuck won't hold anything that small. That's a feature among others that bothers me about that chuck. Nevertheless I worked around it by taking my square ER collet holder, and clamping that in the 4 jaw. Kind of overkill for a piece this small, but it beats having to change chucks. Funny to see an ER chuck, in a 4 jaw chuck, mounted on a faceplate, but that's what got the job done today. I thinned the brass down to .134" for threading 6-32.
[ You are not allowed to view attachments ]
vtsteam:
And thinking I'd break out my never before used tailstock die holder, ran into problem number two. Despite the fact that it had many different die holder recesses and attachments, it wouldn't accept a plain 1" hex sided die -- which was what I had for cutting a 6-32 thread. Rats! I should have made one myself instead of buying one to... ahem, save time.
Well, the ER collet holder to the rescue again, I started the threads in the lathe by holding a die handle and then transferred the collet holder block to the bench vise and turned the threads the rest of the way.
[ You are not allowed to view attachments ]
vtsteam:
I cut the stylus to length, and rounded the end. Here it is finished:
[ You are not allowed to view attachments ]
vtsteam:
I drilled and tapped the brake arm to accept the stylus at a distance of 10 centimeters from the center of the brake drum, then secured it in place with two nuts. The height of the brake arm is therefore adjustable, and should be set level. A 10 cm moment arm means that every gram registered on the digital scale will be approximately ten times the newton-meters of torque absorbed by the brake. So grams divided by 10 is ~ Newton-meters. From that, and RPM, Watts can be easily calculated.
With this work done, the Prony brake was operable, though I'm not finished. But I couldn't resist setting it up temporarily and trying it out! :zap:
I was disappointyed when at first the engine wouldn't start at all with the brake just sitting, applying no pressure to the drum. And for the first 15 minutes the engine ran I could hardly even touch the drum without nearly stopping the motor. But eventually the engine seemed to loosen up and gradually increase speed to the point where I could attach the brake arm...but without the adjustment springs... just letting the parts run together.
This brake definitely needs to be run in -- it needs more of that. But eventually this evening I was able to adjust the thumbscrews (without springs) for very slight pressure, and get the engine to run at reduced speed, registering a torque reading on the digital scale. I was gratified to see it was a reasonable stable digital reading, probably helped by the inertia of the brass stylus, which, unlike the other parts does not weigh on the drum.
Here's a picture of the first run. I do need to do a little more work to get easier and more consistent readings, to work out a measurement procedure, and to finish it off, but I'm very happy that it works quite well already!
[ You are not allowed to view attachments ]
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version