The Home Machinist!

daveb wrote:I have an A3 I purchased about a year ago. The first time I steamed it things went very well. It was a short run because of the track(under construction) That first run was using Welch Coal.. Since that first run I have tried steaming 4 or 5 times. Each time I had problems. First it likes Welch coal and I could not keep a fire going with other types of coal.. I then had a water feed problem(piping) and then a check valve problem. This engine does not have a high priority with me because I'm in the final stages of building the Kozo new shay but the A3 is ready to go and will probably get it to a track in the next couple of weeks. I now have a good supply of Welch Coal.

Last week I steamed the shay for the first time using a torch. Boiler steamed well but I have several steam leaks one of which was in the steam chest of cyclinder 2. Only one way to get at that, a real pain. The engine itself would barely run(could be because of steam leaks) It ran very well on air.

Where are you on your shay?

A recurring theme is that engines designed to run well on air, will not run well on steam due to heat expansion. Are some of your tolerances too tight?

This isn't a "tolerance" issue, as tolerance is the amount of error that is permissible from nominal. It's an error in engineering, whereby one has not provided the required clearance. Said another way, the same tolerance can be applied to the chosen dimensions--but it's the dimension that must be changed.

That's assuming one actually works to the correct tolerances (which most likely vary from feature to feature). Some folks seem to ignore them, thinking that there is no need to be able to machine to less than a thou.

They're wrong.

Harold

Here is an example what Harold is talking about a .500 Dia Deltronic gage pin will not fit into a .500 Dia ring gage. If it does it will never come out.

You guys are making me nervous. The first time I ran the engine on air it took about 40# to get it moving and it ran in spurts. After many minutes of running it loosened up and would run on 10# or less very smoothly. If I have a problem with tolerances it would be the piston rings,right??

daveb wrote:You guys are making me nervous. The first time I ran the engine on air it took about 40# to get it moving and it ran in spurts. After many minutes of running it loosened up and would run on 10# or less very smoothly. If I have a problem with tolerances it would be the piston rings,right??

Hard to say what the problem is, as it can be any of the things you mentioned. Less than great machining will often assemble tightly, with high spots wearing away, creating a looser fit. That's not the optimum way to make parts fit, but was common practice years ago, when machining was accomplished with spring calipers. Machinists often didn't have the ability to measure closely, nor would machines work to exacting dimension as they do today.

My purpose in posting was not to solve a problem, but to help others understand that tolerance and dimensions are not one and the same. If a dimension is wrong, and tolerance held, the fit will be wrong. Likewise, if the dimension is right, and tolerance is NOT observed, the part may not fit correctly.

Think of it this way. It's common for guys to suggest that machinists don't build good engines, because they work to "too close" tolerances. That's not true. One can work to VERY close tolerances, and get exceptional results, assuming the dimensions are correct for the application. Tolerance allows a given amount of variation from a nominal size. It has NOTHING to do with how a part fits, assuming the part is engineered and dimensioned properly. The tolerance range assures that the part fits correctly so long as it is manufactured within the specified tolerance, working to the DIMENSION.

Harold

Harold right of course, I used the term "tolerance" out of laziness.
Hard to say, and I cannot speak from personal experience, its best one of the experts give advice from this point. My own inclination would be the cylinders, pistons, and packing material

Another reason for an engine to run well on air but not steam is because of the thermal expansion coefficients of different metals. For example, if you have an aluminum piston in a bronze cylinder, the piston will expand more than the cylinder when they are hot. The fit is fine at room temperature.

Fender wrote:Another reason for an engine to run well on air but not steam is because of the thermal expansion coefficients of different metals. For example, if you have an aluminum piston in a bronze cylinder, the piston will expand more than the cylinder when they are hot. The fit is fine at room temperature.

Exactly! An engineering (dimensional) issue, not a tolerance issue, although it could be. If the dimensional spread is correct, but too much tolerance is allowed, there can be an overlap of sizes under extreme conditions, whereby the fits become a snug slip or as extreme as a press fit. Even becoming sloppy. Narrowing of tolerance could then be required, so that isn't possible.

All things must be considered. Temperature involved (coefficient of expansion, which varies according to the alloy involved), required clearances determined, and tolerance of the determined dimension, so that are no overlaps, creating less than acceptable fits.

Harold

very much on the wrong track. please refer to original question.

steamingdon wrote:very much on the wrong track. please refer to original question.

I can't answer the original question, however, I would be inclined to examine LBSC's TICH in 3/4 scale, which we know people can steam well?, and compare the drawings to the A3 and make a pretty good guesstimate if it would steam the same or better? I've been behind 7.5" gauge locomotives that had to stop every 1000ft probably because the engineer didn't know what they were doing.

I've also seen decapods in 7.5" gauge with scaled 50" drivers that would use a ton of steam just to keep up with other traffic. Don't think the engineer was using his Johnson bar effectively.

Just my personal observation, it would seem to me that an Atlantic locomotive with scaled 80" drivers, very large boiler, and spring rigged as an 0-4-0 would be the cats meow...

Daveb ,don't be nervous.you did what many before you have done...run in a new engine.Purinton talks about Harry Sait running in an engine at his shop back in the early days. As far as my original question, I have pretry much given up on getting much of an answer,thanks again to those that actually answered it. My request for the future...READ THE QUESTION...READ IT AGAIN,..then write yor answer on a piece or paper...READ IT OUT LOUD,AND ASK YOURSELF........ Does this actually answer the question?...or is it proof that costipation of thought and diarrhea words exists.....

elm53 wrote:As far as my original question, I have pretry much given up on getting much of an answer,thanks again to those that actually answered it. My request for the future...READ THE QUESTION...READ IT AGAIN,...

You asked two questions in the first posting.

Learn to count.

John.