Does anyone have any thoughts on using a spring washer in place of the threaded bar/spring usually employed to tension the cylinder on a simple 'wobbler'? Might it be a little too limited in adjustment? I have an idea for an engine but space would be tight
I have seen a rectangular 'U' shaped piano wire spring used to seat the cylinders of a twin oscillator. I guess it would work for a single cylinder arrangement as well and save some space by not having the coil spring sticking out behind the cylinder mount.
Do you mean a twin with cylinders on opposite sides of a column/combined port block with the spring around the outside? I think the second engine on this page is a bit like what you describe... www.acs-engineering.co.uk/id19.htm
Paul, That is quite a common way of seating the cylinders but it is not what I was trying to describe.
Take a length of piano wire and shape it into a U so that the ends of the wire press against the outside of the cylinder and the port block touching them on the pivoting axis.
______ l l l l______
I hope this crude drawing helps to make this a bit clearer. The thing about this way of springing is that it takes up very little space, unlike the coil spring behind the port block. Finding the right tension is probably more difficult with the U spring.
You may find a dished spring like a Belleville washer or make a flat spiral spring like a volute spring.
The alternate suggested by others is a yoke over the cylinder to a fulcrum on the opposite side.
A bit left field but I have considered using a counter balance piston on the back of the pivot. This piston would be supplied from the steam supply line to force the cylinder against the port face. Sounds so simple I might yet try it on a G1 loco I am building. One of Geo Stephenson's early locos, "Blucher", used steam piston springing for the wheels.
Ian: Co-incidentally I was looking at some of those Belleville washers earlier today but wasn't sure how much adjustment would be available.
Ideally I'd like to put the cylinders either side of a solid vertical column (as in that link I posted further up the thread) but I can't think how they could be tensioned other than with external springs/wire so I'll probably put each on it's own column.
that way the pivot could go straight through the column and both cylinders and be tensioned with spring washers. I need to think about port positions for that but my gut feeling is that the calculation will be the same as when the pivot is 'in the middle'.
Moving the pivot to the end of the cylinder would allow one pivot spindle for both cylinders and just one spring would be needed. It would make a double acting arrangement more difficult but not impossible.
I'm not sure how much effect it would have, but with the pivot at the top you might need a greater crank pin throw. If you don't the ports will certainly need to be closer to each other. Give it a go and let us know! John
A quick bit of calc-u-sketch shows that the ports would indeed be close together for the sort of size engine I'm thinking of - it'd also mean the cylinder being longer.
I'm going knock up a 'dummy' in steel and see how it looks... brass is just too expensive to waste these days!!
EDIT: I think the ports would have to be as far from the pivot as possible to get some separation between them and the throw would need to be greater. I just thought, there's no reason that the pivot has to be 'in' the cylinder, it could just as easily be on a (flat?) extension behind it.
G'day Paul I take it you are intending a single acting cylinder. Some options are put the pivot at the bottom of the cylinder to increase the sweep over the ports or put the pivot to one side. The later option could have two cylinders with pistons joined by a yoke.
I reckon there's enough options here; my mind is already dreaming up strange linkages to get a non dead centre arrangement
Hi Paul, Here is the link to a picture of my double acting oscillator. Sorry it is not bulled up, I am still working on it. You can see the cylinder seating spring I tried to describe the other day. The crank shaft is the one I wrote about in the general section.
John, upload to photobucket or wherever then just put the url (address of image) in "image tags" (img) and (/img) but in square brackets like this...
That's a great little engine! What bore/stroke is it? I need to read up your other thread later. I wasted about a foot of brass yesterday trying to make two identical cylinders - each time the port/pivot holes were in marginally different places - so I'm going to make a jig!!
Paul, It is 12X16 bore/stroke. Not all my design and very much a mongrel.
I made a jig to locate the holes in the port block. It was a strip of metal with the pivot hole and steam ports at one end and a slot at the other end whose width was the same as the con rod bearing diameter. Then as the crank is turned to the about 90 degree positions the port locations can drilled using the jig as the guide.
I thought I had tried to post the picture as you said but must have got something wrong. I'll try again sometime.
Well I think it's excellent John, mongrel or not! I see now how the U-spring works and understand how it must be tricky to get the tension just so. I read your other thread about loctite-ing the crankshaft - looks good too.
What's going on at the back there where the cheese-head screw is, is it an on-board regulator? Are you planning to run it on steam or air (or is that a silly question)?
I'd like to see more pics either as you progress or just some of the components if you disassemble it anytime.