Mission Impossible - Designing and Constructing a 3/4" Scale Westinghouse Cross Compound Air Compressor

[Carrdo]

Not only to design and to build it but to make it a working, practical and reliable unit.

Richard is the one who is doing all of the thinking and development work associated with this for the 3/4" scale Big Boy.

But first we have to have a unit that is both working and reliable in any scale. So, we first started with a known 1.5" scale historic design. Even here, to have a practical reliable working unit, there are many problems to overcome some of which have not yet been fully solved. Richard will fill you in on the details and progress made to date in this regard.

[RET]

Hi,

Once in a while Don comes up with an idea and I kinda get roped in. Over the years he and I had been talking about making working cross compound Worthington style air compressors and Big Boy has two of them out on the front of the locomotive. Right now there are two "dummy" compressors in cast aluminum, and I must say they aren't very impressive. Some time ago I made the mistake of saying that I thought it would be possible to make a working compressor in 3 1/2" gauge and Don remembered. About ten years ago I tried to talk Bill Huxhold into making some, but he wisely decided that he didn't want to. About that time he was beginning to work on his turbogenerators and they were very impressive. I have a few of his in each size and the 1/8th scale ones are really something else. I just wanted to have something that he had made.

While the final goal was to do this in 1/16th scale, we needed to have something bigger that worked reliably, so we decided that 1/8th scale was where we should start. From other's experiences, we figured that at least that size was doable.

Don found out that Tom Miller had some left over partly machined castings from the compressors on his Big Boy; not enough to make a complete unit, but everything was there except the main compressor casting. Tom said that the compressors on his Big Boy were quite reliable and ran for many years with no problems. We decided to go ahead and so Tom sent us what he had.

This is what we got from Tom Miller

When you look at the picture, in the top right you can see the sleeve and the shuttle parts partly machined. On the left side you can see the two steam pistons and rods, then at top centre is the top cover casting which includes the shuttle housing. To its right is the bottom cover casting and on the lower left is the intermediate casting which fits between the steam side and the air side. On the lower right, you see the steam cylinder casting. The small parts above and to the right of the pistons and rods are the pilot valve bits.

When we looked everything over, it was pretty obvious that the quality of workmanship on the parts was very high and the casting quality was also very impressive, but some further machining was still required. It also took some time to see how everything fit together and how it all worked. From reading other articles, I knew roughly how things worked, but I still couldn't afford to make mistakes at this stage so I had to be sure. I used very fine wire to see where the passages went and even the binocular microscope helped.

The shuttle valve sleeve and shuttle were partly machined, the internal bore in the top casting was finished as was the bore in the sleeve, but the outside diameters of both parts still had to be turned to size. I used collets and centers to turn the O.D. of the sleeve accurately. I fitted both the sleeve and the shuttle so that they slid easily but with no play ie. about .001" clearance. Both parts had large bosses on one end and these had to be removed and the parts cut to length. There were no drawings so we had to work from the parts themselves.

There is lots more, but this should do for a start.

Richard Trounce.

[Carrdo]

Since I like to delve into the history of everything, I believe that all of the original castings and all of the original machining on these Westinghouse cross compounds were done by Stoney Burke in Australia in the mid 1970's. That is all I know and if anyone knows anything more, please chime in.

[RET]

Hi,

On to part two.

There is a website http://www.danslocoworks.com/Air_Compre ... ration.htm which gives a very detailed explanation of how a cross-compound model would work and also step by step instructions (and drawings) on how to build one in 1/8th scale. This is where I got most of my information before I started this project. If for no other reason than the explanation, I would recommend a visit to the site. Its worth it. If you are interested, another part of the same website http://www.danslocoworks.com/Air_Compre ... ressor.htm gives complete instructions on how to build one.

This next picture shows the underside of the top cover. The picture shows the surface before I used the 400 grit polishing paper/surface plate combination to smooth it out. I have used this process on all the mating surfaces since the intent is to assemble the unit metal to metal without using gaskets. Not to get ahead of myself in the explanation, but I have done this successfully and it works very well.

This view shows the underside of the top cover.

In all these pictures, you can see both the quality of the castings and the high level of accuracy in the machining. I have to emphasize that this is not a project for a neophyte, it needs someone who can work accurately and reliably to .001 inch or less. It is also readily apparent that a lot of thought has gone into the design and building of the unit.

Sorry that this is just a short addition, I'll try to add more later on in the day.

Richard Trounce.

[Dan_M]

Richard - Thank you for the reference to my build of the Westinghouse pump. I do mention on the website that my pump design is NOT a true cross compound pump (at least on the steam side). The air side is compounded. I made this decision after discussions with others who built similar pumps. My pumps have been super reliable and never had a stall problem even after sitting for months over the winter. They run great but the two I have do use up more steam than I thought. I boosted the burners to make up for the difference.

I wish you the best with your build in the smaller scale.

Dan

[Chris Hollands]

Dan did you use a pressure regulator on the start /stop of the pumps or just let them stall , if you used a regulator I would be interested in what you have used .

Chris

[Dan_M]

I just let them stall.

Dan

[Rwilliams]

Full size compressors are prone to condensation issues if not operating most of the time or kept pumping very slowly to keep the condensation to an acceptable level. The pump governor features a small hole in the steam control valve to pass a small amount of steam at all times even when the control valve is closed. The bypass orifice in the steam control valve gives the Westinghouse pumps the signature super slow operation when the need for air is minimal. There is even a small condensate or waste drain in the pump governor to eliminate any moisture that might gather and cause trouble with the operation of the governor. Always a good idea to insulate the steam supply line to a compressor to reduce the condensation levels.

I spoke several times with Barry of Superscale in regards to his plans for an operational pump in 1.6 scale. He had an original set of drawings complete with measurements which he kept hidden away. Would not say where they came from. His biggest problem with the scale pump design was the tiny holes in the full size pump did not scale well for decent long term operation in a scale model pump. He realized that there would have to be some modifications to the internal holes if he did make a pump that was operational.

[Bill Shields]

there is a very good reason where there is a dearth of these units in any small scale...

and why the BvB duplex pumps are so popular

[Asteamhead]

Hello Carrdo,
Maybe this could be of interest to you? Last year I started construction and making of a pair of such CC-air pumps in scale 1 in 10 for my N&W A 1239.
Perhaps we might learn from each other's progress by posting in this forum (Build Log)?

https://www.chaski.org/homemachinist/vi ... &start=180

I'm sorry for I stopped building for some months due to personal reasons, but will continue the project in short. For I built some feedwater- and an air pump squite successful several years ago , I*m looking forward to get them working, too.
But you're right so far that it's not an easy job at all !

Best regards and good luck!
Asteamhead

[Steve Bratina]

Is the big boy finished and running? If so, how is it's steaming ability under load?

[RET]

Hi,

Dan,thanks for the kind wishes. Even for the 1/8th size unit, some luck is needed. Your explanations and fabrication steps on your website for the 1/8th scale size are both instructive and helpful even although our "beast" differs in some aspects.

The more I look at the parts we received from Tom Miller, I think that both Don & I were very fortunate to get parts where all the "hard work" has already been done. My hat's off to whoever did the work, as I said before, both the castings and the machining are very well done. For instance, a lot of fixturing would have been necessary to get all the holes in the right place.

This next picture shows the steam side of the pump from the bottom. As it says in the picture legend, both bores are the same size. For practical purposes, this seems to be the accepted way of making a working model of these pumps. The steam side is made "Simple" but the air side can be "compound."

Steam cylinder assembly. Note that both bores are the same diameter.

If you think about it, there is a very simple reason for this. In full size, the boiler pressure was 200 psi. or more, so a compound air pump was the way to go. You gained in efficiency on both the steam and air sides because of the compounding and you still had at least 100 psi. or more discharge pressure on the air side. In the model world however, because of lower boiler pressure (often 100 psi. or perhaps even less), if both sides are compounded, because of friction losses and air flow restrictions you will wind up with a practical air pressure of between 60 & 70 psi. as an upper limit.

On the other hand, by making the steam side "Simple," some people have achieved practical air discharge pressures of over 100 psi. even with a boiler pressure of 100. psi.

Control Piston with hollow rod and original control rod and shuttle valve end caps.

The above picture shows the control piston and rod assembly together with the original control rod assembly. If you look closely, you can see that there is a cast iron piston ring on the bronze piston. Both pistons had cast iron rings which fit beautifully. The rings have split and overlapping ends the way they should, but the ends of the overlap have hooks to prevent the ring from opening too far. Just a nice little added touch.

The round bronze pieces you see are the end caps for the shuttle. At the lower part of the picture, you can also see the little slide valve that operates the shuttle spool. You think of most slide valves as being flat and running on a flat port face, but in this design, it is easier to make it round since it fits in a 3/16" dia. hole in the top cover. Drilling all those little holes in the top cover casting and having them go where they need to must also be fun. Fortunately, that part was already done for us.

Time to quit for tonight. Lots more to come. I haven't run out of pictures yet.

Richard Trounce.