A Simple Uniflow Engine

[gary.a.ayres]

Thanks John.

So do I, even though it will be a while.

My main anxiety at this point is about whether or not these two flywheels are big and heavy enough.

There's only one way to find out though...

[MJM460]

Hi Gary, crankshaft looks like you solved the problem.

With the drain cocks, the issue is that the condensed water is virtually not compressible, so when the piston approaches the top head, very large forces are created in bringing everything to a very sudden halt if there is no where for the condensate to go, and of course, gravity has already made sure it is on the bottom, so that is the best place for the drain valve.

At the Botton centre, I am not sure that the drain is really necessary, as the piston does not trap the condensate at that end.  In part it depends on where the exhaust port is, usually on the bottom with a horizontal cylinder, so any condensate goes out with the exhaust steam.  Otherwise any condensate formed when the engine is running can be a problem.  And you don’t normally leave the condensate drains open once the engine has warmed up and is running.

I can’t really see any reason not to put a drain at the bottom, I am just not sure what it will achieve.  I would save the extra valve for the next project.  If the exhaust is actually at the top, even a small port at the bottom, piped into your separator, will do the job as an auxiliary exhaust, and do no harm as it is closed off by the piston once it is on the way back to the top.  A peculiarity of the uniflow design.

MJM460

[gary.a.ayres]

Thanks MJM - you have given me food for  thought regarding the cylinder drain cocks and the exhaust. I may come back to you on that for further advice when I get back to working on the cylinder.

Meanwhile, a little bit of progress with the crankshaft this evening. The webs were given a further clean up using my cheapo Dremel copy with a small flap wheel. There is still some way to go to make them look really nice but that can be done once the crankshaft is finished.

I decided to use the Loctite and pins approach given that this is my first proper crankshaft. In the photo below, the journal has been fixed into the webs with 638. The actual shaft is still loose and is just in situ to keep everything in line:



Tomorrow night I'll Loctite the shaft in and all being well will do the pins at the weekend.

Maybe my next crankshaft will be silver soldered... 

[gary.a.ayres]

Time for another bit of state-of-the-art CAD:



It's a peculiarity of mine that I prefer to express metric measurements in centimetres rather than millimetres.

This is an actual size drawing of the cylinder based on the original plans by Stan Bray scaled up to x3.5, along with the piston. Note the length of the piston, which removes the need for a piston rod and crosshead guide. This is taken from the plans. Because nature is notoriously reluctant to be scaled, I am a little concerned that the much greater mass of this piston (in the original the diameter is only 10mm)  could make it difficult to drive. I initally considered modifying the design with a shorter piston, piston rod, crosshead and guide, but have decided to stick with the plans as it is one of the characteristics of the design. Having never made a crosshead guide etc. before, I would have liked the challenge, but there will be plenty of opportunities for that in the future.

Should the weight of the piston turn out to be a problem I'll just have to modify it, either as above or perhaps by making it hollow. But to be honest I can't really see it being a problem, as this engine will run on a really good boiler. More of which in due course... 

[gary.a.ayres]

Pinning the crankshaft. Being a novice I needed guidance, and found it in a youtube video by Keith Appleton. The photos below don't need much in the way of comment:





I drilled the holes with some trepidation for fear that the bit would wander when it hit the silver steel bar...



... no problem though. Four good holes:



Matching various diameters of thin silver steel with various small drill bits left me only one option. Fortunately I had just enough of this diameter to make four pins...



... at which point a slight annoyance occurred. I had tested the drill and silver steel rod on a piece of scrap steel and the result was a pin that was satisyingly tight and had to be tapped in with a hammer (as recommended by Keith Appleton). However, in making the actual crankshaft I 'pecked' with the drill to clear swarf. This enlarged the hole a small amount and as a result the pins were much less tight in the holes - not rattling, but a sliding fit. However, I poured plenty of Loctite into the holes before pushing in the pins. I suspect it will be ok. If it isn't, it will just have to be fixed later. The crankshaft in progress will now be left for a couple of days to let the Loctite harden properly.



I then made a start on the piston. All I have to cut cast iron with is a small angle grinder, and this made such a rough cut that when I tried to face the end of the bar the tool kept catching and pulling the bar out of true. Fixed steady to the rescue! First time I have used it in ages:



It did the trick, and I faced both ends of the bar and spotted them with a centre drill, then removed the fixed steady, put the live centre in the tailstock and began turning the bar down towards diameter before calling it a night.

gary

[gary.a.ayres]

The redundant part of shaft between the crank webs was cut away, and the protruding ends of the remaining shaft and the ends of the pins were filed flat. This gives me something which bears some resemblance to a crankshaft:





I hope it runs true! Haven't tested it for that in the lathe yet...

It still needs a bit of shaping, tidying up and polishing, for which this toy - ordered a couple of days ago and still to arrive - should prove invaluable:



30mm belt, variable speed. Looks good to me, but another thing to find space for. Space that I don't have.

I then switched back to the piston. An indexable round-tipped carbide tool seems to create the best finish on the cast iron. I am creeping down to diameter taking very fine cuts with it, alternating with a piece of wet and dry paper wrapped around a square block of wood moving evenly back and forth across the surface, and using the cylinder as a gauge:



Nerve-racking stuff, for fear of overshooting. It's almost there, but the hour was late so I decided to call it a night and leave the final diameter to another day...

[Johnmcc69]

 
 Nice work Gary!
 No rush, take your time, it's all coming together nicely!
 
 John

[gary.a.ayres]

Thanks for your positive words John.

I have often been inclined  to rush things, but this pastime is teaching me to take my time...

[gary.a.ayres]

I have often been inclined  to rush things, but this pastime is teaching me to take my time...

Argh. I should have sufficient self knowledge by now to be aware that when I say something like that I am tempting providence.

The piston makes a nice sliding fit in the bore, and pulls a good vacuum with steam oil. And I'm pleased with that radius at the shoulder, which wasn't in the original plans:



However, as we see more of the piston, machining marks become apparent...



... and the more of the piston we see, the more apparent the marks become (and in the next photo they are actually more apparent than they are when the piston itself is inspected)....



... until we look at the top, say, quarter of the piston and the marks become less apparent again:



I did exactly what John counselled me not to do: I failed to take my time. The piston is now a good fit, but I came too close to overshooting with the tool and it left me with machining marks along the bottom 3/4 of the piston which - if I were to polish out - would probably leave me with the piston too loose in the cylinder. For all I know, the machining marks may not make a difference in functional terms, and the engine may run fine as is. However, at bottom dead centre quite a length of this piston will be visible and it will pain me to see those marks.

I have a few different ideas for possible fixes for this, ranging from leaving it alone through various other options to making a whole new piston. I won't go into these here because I'm going to live with this for now to see what emerges when I eventually run the engine with it in situ before making any modifications. If you have any ideas yourself, please feel free to post them and I'll let you know whether or not they are the same as mine. Remember that as it is this is a long piston which obviates the need for a piston rod, and the connecting rod joins to a hole drilled through the boss at the back end of the piston.

However, there is another isssue: I centre-drilled a small hole in the back end of the piston so that the tailstock centre would hold it true while I turned the piston. This hole may be a bit too deep to the point that it may well encroach on the hole for the wrist pin. I could get round this by - as per plans - milling a slot down the centre of the boss at the back end of the piston to effectively create a fork on the end of the piston which would be drilled through for the wrist pin. The too-deep centre-drilled hole would become history that way. But I really like the idea of having the fork on the connecting rod and a single flatted projection on the back of the piston sitting between the two 'prongs' of the connecting rod fork. In this latter scenario the centre-drilled hole may make mischief with the bearing, unless I shorten the whole thing a bit which I may do but I would prefer not to mess with the original proportions. .

If you get my drift...

Will evaluate both of these issues when deciding whether or not to try to salvage this piston or make a new one.

Suggestions welcome...

[Jasonb]

If you do make a new one don't use that round insert tool as they will tend to get pushed off the work trying to take fine cuts or risk chatter on heavier ones (your marks look like chatter) Either switch to sharp HSS or a **GT insert with 0.2 or 0.4 tip rasius for the last millimeter.

[gary.a.ayres]

I found the round tool was fine on the deeper cuts but it did start to chatter on the final fine ones.

Advice noted, though! Will try what you suggest.

 

[gary.a.ayres]

I almost wish this chatter-marked piston wasn't such a good fit in the cylinder bore so I wouldn't be in a dilemma over whether to keep it, modify it or make a new one!

However, I have decided to press on with it for the time being rather than end up chasing my tail. Decisions on what to do about it will be made later in the build. To be honest, the photos make the marks look worse than they actually are, even though they do remain an issue.

Here, a flat is being milled for the 'crosshead' hole. I used the rotary table with dividing plate and its tailstock as an easy way of flipping the part 180 degrees to ensure parallelism of the two flats:



Both sides done. A light touch with the new belt sander (when it arrives) will tidy things up, but overall I am pleased with this bit:







Then - after marking out the distance from the top end of the piston to the centre of the hole using a height gauge on the surface plate, back into the mill setup to find the centre in the lateral orientation:



I did also get the hole drilled and reamed but my camera ran out of battery so more to follow...

[MJM460]

Hi Gary, more good progress.

Personally, I would not worry about those chatter marks if you are happy with the fit.  There is just a possibility that they will carry a little oil and help with lubrication and sealing of the cylinder.  A bit of a wild guess, but interesting things happen on that scale.   Keep with it until you find a problem.

When it is all complete and working, if you still want to, that is the time to consider making a second one.  My little cylinders seem to work quite well with just a good fit, and a couple of ring grooves which, rightly or wrongly, I consider as labyrinth grooves which help reduce the steam bypassing the piston.  Your marks are a much finer scale, where different hydrodynamic issues rule.  All interesting learning.

MJM460

[gary.a.ayres]

Thanks MJM.

Pretty much how I see it too. I'm sure the piston will work fine as is so it would be disheartening to get bogged down with it at this point.

It had also occurred to me that the marks may help carry some oil.

The thing that annoys me is that with this long piston (which removes the need for a crosshead and guide) some of the marks will be visible at all but top dead centre.

But never mind - as you say I'll come back to it at the end and if all works fine and the marks still bug me I can always make a new one at that point, or possibly modify this one.

[gary.a.ayres]

Have been painting the exterior of the house all week, so progress has been slow for the past few days.

Fortunately it rained today. Returning to the main bearings, which are made of bronze:



A rough trial assembly of the crankshaft with flywheels, pulleys, collars and bearings in situ:



A close-up of one of the bearings in incomplete state:



The part on the left is the bearing itself, drilled and reamed to a nice sliding fit on the shaft. It has two diameters - the flange at left, and the main part of the bearing adjoining it. This narrower part will pass through the frame of the engine and then through the retaining collar as you can see here. A circle of small screws will pass through the flange, the frame and part way through the retaining collar, holding the bearing in position in the frame. The surplus length of bearing (just to the left of the cast iron collar) will be turned flush with the bearing retaining collar once I have gauged the final size using a piece of frame material between the flange and the retaining collar as a guide. The bearing at the other end of the crankshaft will be a mirror image of this one, and the frame will also be symmetrical.

Finally, I decided today that the connecting rod will be a composite structure. The main length of rod will be 12mm EN8 steel, but - like the big end - the crosshead end (including the fork) will be of cast iron. The components will be joined together with M12 threads and Loctite thread seal. The picture below shows that a start has been made on the crosshead end and that there is still quite a way to go with this part. The fork has to be milled and drilled and the whole part will need considerable shaping to make it look leaner and more elegant:



In the lathe chuck the steel rod awaits a corresponding thread to connect it to the crosshead.

Weather due to improve tomorrow, so it's looking like I'll be back on painting duty again...