Two-cylinder, double-acting wobbler

[smfr]

It's been far too long since I've had a good stretch of time in the shop, so I need a satisfying project that I can get done fairly quickly. After making Elmer's Fancy II a while back, I thought that it would be nice to have a double-acting wobbler. Two cylinders is also neat because then it can be self-starting.

So I sat down this morning with a cup of coffee, and whipped up the attached design in Rhino (my favorite Mac 3D software). Nothing too unusual: central column with internal intake and exhaust (I'll see if I can do a ghost view that shows them). Outlier standards will apply pressure on the cylinders via a spring (not shown in the rendering). Cylinder covers will be bolted on with screws or studs. Being double acting, it needs a gland on the top cover. I'll probably shape the crank disks. The cranks will be 90° apart on the actual model so that it self-starts. There are bronze bearings for the crank.

How does that sound? Can anyone see any problems with this design?

Simon

[smfr]

Here's the ghost view showing the air holes. The ports are drilled straight through, then 1/8" holes drilled up from the underside to intersect with both front ports, and both back ports. These will be plugged at the bottom. The inlet and exhaust come in horizontally from the feet of the standard.

This picture shows one immediate problem; if the pin in which the cylinders pivot goes all the way through, it will intersect with the air channels. I'll have to use a shorter pin on each side.

I *think* I've got the "wiring" right  *makes spinny gestures with one hand and pointy gestures with the other while trying to work out the piston motion*

Simon

[zeeprogrammer]

Looks like an interesting and fun project. I'll be watching.
Good to see you in the shop.

Being double acting, it needs a gland on the top cover.

Being a newbie, I don't understand this. Help.

[zeeprogrammer]

Regarding the interference between the port channels and the pivot pin...

This is the same/similar arrangement as the PMR Model 2A/2B. What they did to give more clearance for the pivot pin was have the channels intersect from the outside of the port rather than down the middle of the port.

Let the 0 be the port hole and the | | be the channel...
Here's yours  |0| where the port goes through the channel.
Here's theirs 0| | where the right side of the port and the left side of the channel intersect. That is, the left '|' of the channel is intersecting the port along the port's right side.

Said a different way, move the channel over a bit so it still intersect. Sorry to be so wordy.


Hope that helps.

[tvoght]

Nice proportions on that engine. As another PMR 2 builder, I can agree with what Carl said, although I can't really see the spacing that well in the rendering. The centerline of the vertical passages can be moved almost a full port diameter to the side of the ports and still get good intersection to the ports.

Again, nice looking design.

--Tim

[Don1966]

Hi Simon, good to see you back. Your rendering looks good. I can not see the port opening that good to really make out what you have. I will be watching it t see if you make a move on it.

Don

[Bearcar1]

I was wondering where you disappeared to Simon. I too am glad to see you back in the saddle. That looks to be a very robust design and I really like the addition of the outboard bearing pedestals, as I have never liked the rather unsightly threaded studs and spring approach that is common to most oscillating types. I'll be following along.


BC1
Jim

[b.lindsey]

Simon, any thought to adding a valve which could switch the inlet and exhaust ports to make the engine reversing?

Bill

[smfr]

Looks like an interesting and fun project. I'll be watching.
Good to see you in the shop.

Being double acting, it needs a gland on the top cover.

Being a newbie, I don't understand this. Help.

Hi Zee!

Wobblers are usually single acting, so the end of the cylinder is open; air/steam pressure only pushes the piston out, and it's pushed back in by the momentum of the flywheel. There's just one port at the bottom end of the cylinder.

A double-acting wobbler has a port at each end of the cylinder, so the top needs to be "closed off" with a cylinder cover (with a hole for the piston rod). The gland is a threaded fitting on the cylinder cover through which the piston rod passes, and it tightens down onto some packing (e.g. teflon thread) to seal around the rod. That's going to add some friction, but it does mean that the piston can both "push" and "pull".

Hope that clears it up!
Simon

[zeeprogrammer]

Thanks Simon. That does clear it up for me.
I was looking at my Model 2A/2B and it looked similar to (half of) your drawing.
It has, as you said, a cylinder cover with a hole for the piston.
I couldn't figure out the gland thing as mine doesn't have it.

[smfr]

Nice proportions on that engine. As another PMR 2 builder, I can agree with what Carl said, although I can't really see the spacing that well in the rendering. The centerline of the vertical passages can be moved almost a full port diameter to the side of the ports and still get good intersection to the ports.

Again, nice looking design.

Thanks Tim!

Yes, in hindsight I could have moved the channels further apart (minor confession here: I've already made most of the body . I think it'll be OK though.

Another issue this channel arrangement is that I'll have to avoid the horizontal channels near the bottom when drilling/tapping for the screws to hold the standard down to a base. But there's enough meat to do that; the standard is 3/4" thick, and the channels are only 1/8".

Simon

[smfr]

Simon, any thought to adding a valve which could switch the inlet and exhaust ports to make the engine reversing?

I did think about this, Bill. A reversing lever would simply have to switch inlet and exhaust, but thinking about how that would work made my head hurt! I'd certainly have to reposition things to leave more space for that mechanism.

I saw one similar model that a push lever for reverse, and now I can't find it

Anyway, I'll stick with just going in one direction on this model, I think

Simon

[b.lindsey]

Elmer did one like that...an open column engine but i dont recall the number. It was single acting.

Bill

[smfr]

Time to cut some metal to get this one going!

The first step was to mill a chunk of Al down to size (held in my larger new-to-me screwless vise )



followed by the flycutter:



The fact that the arcs are showing in both directions would suggest that my milling head is pretty well trammed.

Next I laid out the ports, and drilled them. Here I'm drilling the deep hole that will intersect the 4 inlet ports (two in each side); the opening of this hole will eventually get plugged:



I could have drilled the ports through first, but was worried about the drill wandering and losing accuracy on the exit hole, so decided to drill the ports in from each face, and having this connecting hole done made it easier to know when I was at the right depth.

Here I'm reaming what will be the cylinder pivot, after drilling the ports:



This pivot hole doesn't go all the way through because it would intersect with the port connectors, but I have plenty of depth for the pin that the cylinder will pivot on.

Now to profile the sides. I used my rougher to hog away most of the material:



I love this thing! Even my 1HP mill can take off 0.1" in a pass with this in Al.

A 1/4" mill cleans up the finish, and gives me the radii I want:



The outriggers were done similarly, but both as one piece to make clamping easier:



You can see I've scribed the radii which I plan to finish the tops with.

The standard has some overhang bits where the crank is to make the top wider. These I did by gluing on some extra bits with Loctite (yeah, I know 609 is intended for bearings and such, but it's all I have other than superglue).



I also cut to size a bit of sheet for the base, and did something that I've been meaning to try for a while: I used a hardware-store router bit to profile the edge, which worked quite nicely (once I realized that my packing wasn't level and the profile was slanted  ). I ran it at 2000RPM, and did the profile in two passes, with the first pass at less than final depth.



Standard and outriggers were drilled and tapped, and the base drilled and prepped for M4 socket-head screws. That leaves me with this:



Of course the tops of the outriggers need to be finished, and there a bit of fiddly milling on the standard, and a giant slot to cut for the flywheel! Some of that I hope will get done tomorrow.

Simon

[vcutajar]

Good progress Simon.

Good to know that the router bits can be used also.

Vince