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Making a Prony Brake to Measure the Power of My Hot Air Engines

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vtsteam:
It was a lot of fun when rehabilitating No. 83, my hot air engine to see how fast it would go after each change. But top RPM is not a very good measure of a hot air engine. Since I want to go further with modifications, I really need a way to look at power output. And I'd like to build other hot air engines as well (in fact I've already started on No. 84, a Rider type engine) so a tool for the job is needed.

Enter Professor Dennis Chaddock and his Prony brake dynamometer for hot air engines, featured in September 1976 Model Engineer. This simple device was used to compare model hot air engines submitted for a new competition slated for the upcoming 1977 Model Engineer Exhibition.

This device used a steel brake drum and brake shoes of hardwood, with a notched and graduated aluminum balance arm with moveable weights. The moment was used to figure momentary torque, and in conjunction with a clockworks tachometer the power output could be arrived at arithmetically.

I'm going to use some of Professor Chaddock's design, but use a digital tach, and scale, since I already have both. Somewhat less visually elegant, but a lot easier to work out the power curve -- particularly since I can do it in Watts with minimal units conversion, by comparison with his original inch and ounce torsionometer.

Another slight variation, his was designed for a standardized motor shaft of 5/32" diameter and mine will be for 8mm, since that's what No. 83 has, and is a standard I intend to continue. Also he modestly referred to his cool little dynamometer as "a bit of oak" because of the brake shoes. Mine will be "a bit of cherry," in that case since I have a lot of that hardwood, which I cut and milled on my own property.

Here's a start, parting off the brake drum on my homemade lathe:

 

And here's the cherry wood I'll be using, milled out. It's kind of surprising how small this all is, after having looked at the plans many times over the years in the magazine.  :coffee:  It really is just a little bit of... whatever!


 

vtsteam:
First step in making the torsion arm is gluing the two blocks together temporarily with tacks of super glue. I put two dots at either end on both sides and just let them wick a small way into the joint from the side. I used a toothpick to pick up a small amount of glue from a drop that was placed on the waxed paper. I really didn't want the blocks to be glued together permanently, and they would have been impossible to separate if I'd applied it directly from the tube.

 

I don't have accelerator, and wood sometimes slows down superglue's cure. So I used a couple small pinches of bicarbonate of soda, which effects an almost instant cure.

 

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vtsteam:
I marked out and drilled all of the hole locations in the torque arm, and drilled the smaller ones full size.

The large hole for the drum was drilled undersize with a Forstner bit.

 

Professor Chaddock recommends boring on the lathe, but I find centering an odd shape like this one difficult without some kind of initial pilot hole, particularly on wood. Much easier to pre-drill on the drill press. A large hole in the piece was actually easier to center on the lathe than a small pilot would have been. I just clocked in the ID with a gauge. Then I did the final boring to size.

 

vtsteam:
The Chaddock design calls for two 1-1/2" long 8 BA adjustment screws nuts and washers. These are approximately equivalent to US 2-56. To me that's pretty thin -- I have stock small screws of all common US sizes down to 4-40, but nothing in 2-56, and certainly not anything near that size 1-1/2" long. So time to make some. Luckily I do have 2-56 taps and a die.

The other problem however was what to use for wire stock? I eventually found some odd size brazing rod, slightly oversize for the job, but it did accept the die without complaining So I started the slightly laborious job of hand turning 3" of 2-56 screw thread. That's 336 half turns of the die handle, not including backing off twists! Actually, of course, all part of the fun: making something from nothing.

 

vtsteam:
Then it was on to the adjustment nuts. Finally something straightforward in the lathe, out of metal! These were drilled tapped, knurled, and finally parted off in my homemade lathe, the most enjoyable part of today's work:

 

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