Ridder's Stirling Bobber

[bent]

Whittled out my die holders for the lathe tailstock.  Machined a bit of 1117 steel to 1.25" od, then bored and tapped 1/2-20 (to fit the thread on the end of the MT-2 part I got from McMaster), and cut a 13/16" counterbore to fit the dies. (Pics 1 and 2).  Drilled and tapped a set of #6 threaded holes to hold the dies in the pocket, using a clamping v-block from work to index the piece 180 degrees to match the divots on the dies.

Once that was done, made the two pivot screws (shoulder screws) that hold the cylinder in the support fork and allow it to "wobble".  Pic #3 shows the part turned from a piece of 1/4" 12L14 steel hex stock, going dead slow and shallow cuts on the lathe to keep from bending the part and inadvertently making a taper on the skinny end.  Brian used a cylindrical-headed screw with a standard flat screwdriver slot on his drawings, but a hex screw seemed easier to make and looks a bit better in my opinion.  Then used the new die holder to thread the end to #3-48 (Pic #4), by sliding the tailstock on the ways and turning the lathe chuck by hand.  Finally, used the parting tool to create a thread relief where the die taper runs out (Pic 5), before parting the screw off and finishing the head with a flat file and sandpaper.  Very happy with how these turned out, though I need to go back and re-tap the holes in the cylinder to full depth now that I have re-ground the tip of my #3 tap.

Almost done with this one, just need to make a crank and the piston guide bushing, then assemble and test.

[Brian Rupnow]

Carl--when I built the popcorn engine, I redrew everything Stu Hart had done, basing my work in inches instead of millimeters. Although the crosshead guide looks formidable to machine, it is mostly straight lathe work. If you look at this drawing you will see three places that have a 0.313" radius. That is where I used the 5/8" ball nose endmill. also there is one large radius of 0.787".--For that I used  a boring head in my mill. Hope this helps.---Brian

[bent]

That's a fun looking part, Brian.  Dunno why it's posted here, but no skin off my nose.

Finished up most of the bits yesterday, and have started to test assemble to see what needs tweaking/cutting/hammering/filing etc.

Pic #1 - the crank I whittled out from a piece of 1.5" bar (the only stock large enough in my bin) to get a 5/8" diameter, and then parted off the 1/8" thickness needed (had to finish the cut with a hacksaw, my parting tool blade is now dulled beyond use and needs to go to work with me to visit the bench grinder).  Reversed the part and cleaned up the back side, bored and reamed for 4mm shaft (Pic 2).  Tapped #4-40 for the shoulder screw I bought from McMaster (Pic 3), then tig welded to a bit of 4mm shafting I bought (Pic 4 shows the part clamped ready to weld).

The graphite piston guide bushing also had to come from a larger bit of graphite rod.  This time I took Joco's advice and used SWMBO's spare vacuum that'd not been gathering dust (haha) in the back of the closet to suck up all the graphite dust (or at least a pretty fair proportion of it).  You can see the nozzle held off the end of the toolpost in Pic 5. Pic #6 is the part just before parting off - one last check of the fit to the piston, a nice .002" or so running clearance.  Coated the o.d. of the bush with some cyanoacrylate glue, and pressed it home in the rod-end cylinder cap.

Now off to scrounge the garage and find my .050" allen wrench.  I know it's around here somewhere...just not back in the toolbox drawer.  I can't count the number of times I've berated the boys to do that, you'd think I'd heed my own dictums. 

Hope you all have a good holiday, I'm going to be grilling some kebabs later according to SWMBO, along with some corn on the cob.  Oh, and gotta fill the keg with the IPA that's finished fermenting, and make sure there's some bottles of store-bought in the fridge.  Priorities...I think I'll see to the beer first if you don't mind!

[bent]

Well, finished assembly of the engine last night, and went to look for some alcohol to run it, but I'd forgotten I'd used up the last of it (and no, I'm not gonna waste good scotch for a test) 

But, everything fits and spins pretty good, with a bit of tweaking to get everything lined up ok.  I think if I do another build of Jan's engines I will try to make the bearing supports from monolithic blocks machined out, so the bearing bores can be done in one pass and are fixed/aligned at the point of machining.  Trying to manually tweak the bolted on bearing supports to eliminate any binding is a pain.

Also, I need to remember not to buy (never ever again) shafting material from Amazon.  None of the 4mm shafting I got was straight , and the flywheel (so carefully machined, and such a nice snug fit on the shaft, has a noticeable wobble as a result. 

Hopefully I will get it running tonight and post a YouTube video (fingers crossed).

[bent]

Hmm.  No joy.  Suspect I have too much leakage, either past the piston or at the cylinder head.  Will try sealing the latter tomorrow with some silicone, then possibly make a 4th piston with a tighter fit to the cylinder.  Past that, dunno.  Maybe revert to the direct drive that Jan's original engine used, thinking perhaps there is too much friction and windage with the gear reduction and higher flywheel speed?    

[Ian S C]

The piston is the thing. You have to get it to just slip though the cylinder under it's own weight, but virtually stop if you block off the end of the cylinder. If the piston is a wee bit tight, it slides but not quite loose enough, sometimes it's best to run it in by driving the motor from an external power supply, these days I use another Stirling Engine, but a small electric motor is better. If you use a small motor, you'll notice the speed pick up as the piston, and displacer gland frees up.  At the running in stage, and as long as the piston is NOT graphite, a drop or two of oil is OK.  Graphite, polish it down with paper.
Ian S C

[bent]

Thanks for the encouragement, Ian.

[bent]

Well, whittled out a 4th and 5th piston, one with .0005" clearance, the 5th with zero measured clearance and a very tight fit to the cylinder (subsequently run-in by hand).  And it still doesn't run.  Can only conclude there is too much friction in the gear train.  Will rebuild it without the gears, when I'm ready.  For now I am going to set it aside, and start working on the PMR #5 kit I have.
 

[Ian S C]

The gears you have seem to have a fairly high ratio, I wouldn't  go much above 2:1, from the diagram on P1 it looks nearer to 4:1. Your current flywheel would be quite adequate  for direct running. The advantage of gearing up the flywheel is that you can reduce it's size, I'v forgotten the maths that go with this, did well in Mechanics at school in the 1960s, but it seems to have gone now.
Ian S C

[MJM460]

Hi bent,

Following on from Ian's comment, the kinetic energy stored in a flywheel is proportional to the rotational speed squared.  So that four to one gear ratio is like putting a sixteen times larger moment of inertia.  I agree with Ian, the gear ratio is probably too high, as the original design is almost certainly not that far out.

I am not clear why you felt it was needed at all as your early posts imply that it was a decision made before you started building.  Was it in the basis of a U-tube video perhaps?

The simplest thing is to try the flywheel direct driven if the shaft will fit.  Or another flywheel you may be able to borrow.  Or I will look up the topic in my Talking Thermodynamics thread the post number where I discussed using rectangular bars to get an idea of the required moment of inertia.  Gives you a good basis for determining the flywheel moment of inertia required if the air turbulence does not result in too much resistance.

I have to admit to saying the flywheel could not be too big, providing the friction did not exceed the engine capacity, but you seemed to have produced a counter argument.  I assume it would not keep going if you have it a spin to start, so perhaps friction is the issue.

That little engine in common with most other Stirling engines is only single acting, at least when it first starts.  So the power pulse has to give the flywheel enough energy to take the piston and displacer back the other direction.  This is where the flywheel size would be quite critical.  It has to be big enough to run the engine to the start of the next pulse, but not so big that the pulse does not turn it that critical half turn.  I am suspicious that after a few minutes of running, enough air leaks out the displacer rod gland and past the piston, so that the pressure fluctuates above and below atmospheric, and the engine gradually becomes double acting, and so probably a lot smoother running.  But until then, the engine will not overcome much friction.

MJM460

[bent]

Hi MJM.  Yes, it was a decision to try and get a smoother motion than Jan's original machine.  Probably not a good idea, as the apparent "hesitation" in the motion is likely the reason it runs - it takes time for the gas to cool/heat.  In the video of the gear-train version, the motion is still fairly variable in rpm at the flywheel, but he is also apparently using a much larger piston bore and a longer cylinder to boot...have to scratch my head a bit. 

Friction in the gear train, though, is probably the biggest factor in the failure.  Was gone last week on a trip to NYC for #4 son's award ceremony; will probably get back on this one soon.  Patching up the holes in the base plate is one of my concerns, and has me wishing I'd paid more money for an AC tig welder...oh well, nothing that a shot or two of JB weld won't fix. 

[bent]

Ok, rebuilt the engine over the last week, milling down the larger flywheels support that held the gear train, and shifting the mounting holes.  Managed to get everything squeezed up tight enough to re-use the existing crankshaft.  Assembled it all up and spent some time tweaking the alignment to get rid of binding issues.  Also had to replace the marbles - I used the ones that Jan suggested (slingshot ammo) but after the exposure to heat, one cracked and the other two showed signs of soaking up some of the graphite I had used to lubricate the piston during running in.  So, I cleaned the cylinder and glass tube as well as possible with alcohol and some cotton swabs, getting rid of as much of the graphite as I could see.  A couple of assembled engine pics (from two sides) shown below.

And...she runs! 

<a href="https://www.youtube.com/watch?v=8i_aQ742S_g" target="_blank">http://www.youtube.com/watch?v=8i_aQ742S_g</a>

[Admiral_dk]

Great result and you must be very happy that you turned it into a runner 

[bent]

Thanks.  Pretty pleased with the result, after the previous experiment flopped.