Little Engines 2.5" Scale Air Pump

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tomc
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Re: Little Engines 2.5" Scale Air Pump

Post by tomc »

We bought those castings and had them machined for us. i found the umbrella valves are very finicky. Make sure you have no burrs or sharp edges on the plate or they will ruin the valve.

Tom C.
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apm
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Re: Little Engines 2.5" Scale Air Pump

Post by apm »

djjh87 wrote: Sat Mar 11, 2023 7:01 am Mike and apm,
Any updates on the air compressor I would like to tackle one in the future. Thanks Jim
Well my oldest daughter was born not long after I posted here and then my next came in 2019, that set me back sometime in the shop for sure. Since then I am finally one last step away from trying it all out. All that's needed is the central puller rod. I need to figure out length of the rod and the bobbin at the bottom, that I'm ready to put it all together.

Tell you the truth I've been a little nervous and putting this off for some time though I think it'll finally happen in another week or two as I have one little last project in front of it.

The scary thing for me is that it's kind of going to be a groundhog Day moment for me. The design works flawlessly on the computer but as one who earns his living in engineering it won't be the first time I've seen a perfect design on the computer fail in the real world. So much like the groundhog I will finally see if I have a bunch more machining to do or if I'm finally done and it's kind of scary.

I wish I was working with a proven design at this point. The spool valves I went with proved to be a lot more challenging to get to work. The o-rings I planned to use never really worked out and slipped smoothly so by the time I was done I wound up going with segmented spool valves and custom made piston rings. They became very complicated but I think they're going to work.

As noted earlier in the thread when I finally mocked everything up in 3D it was clear that the original prints had some serious issues. At this point if the head fails to run I'm not quite sure what I plan to do going forward. I guess you'll have to check in in another week or two.
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Bill Shields
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Re: Little Engines 2.5" Scale Air Pump

Post by Bill Shields »

The trick to making the head work, at least on a smaller 1" BvB pump, is a spool valve that just drops through the lapped bore under its own weight when totally dry.

This from the fellow who built the one that I drool over every time I see it run.

It's been running for best part of 45 years.

Ben Nixon's assembly notes for his 1.5" pumps (think he did 50 +/-) are similar...and they run like watches.

It's doable-> you are headed on right track, but do not be discouraged if at first it does not run as hoped...and running on air with these things is not the same as steam.
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apm
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Re: Little Engines 2.5" Scale Air Pump

Post by apm »

Well an update here, I got the whole thing together tonight and it doesn't work. I have a few troubleshooting ideas for either tomorrow night or Thursday night and if I can't get those ideas working I may go back to the original design.

Does anyone have a good drawing of a working head per the plans that I could go over? I think I now have it all modeled in Inventor but it wouldn't hurt to compare with someone before I start remaking the head.

At this point I think my history with it is I tried to get clever with spool valves but every step along the way it seems like I may have overcomplicated it and it is probably time to just start over on the head.

Thanks,

Adam
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Bill Shields
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Re: Little Engines 2.5" Scale Air Pump

Post by Bill Shields »

Read my comment about the spool.

The other critical dimension is the free travel of the vertical trip rod and where it places the valve it drives

Assume you are squirting in oil with the air on both ends...
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apm
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Re: Little Engines 2.5" Scale Air Pump

Post by apm »

Bill Shields wrote: Wed Mar 29, 2023 4:43 am Read my comment about the spool.

The other critical dimension is the free travel of the vertical trip rod and where it places the valve it drives

Assume you are squirting in oil with the air on both ends...
In my latest version, I have piston rings on the spool valve which I figure should do a decent job of sealing. Yet still when I tried it out I have a major short circuit from inlet to exhaust. I plan to take another look at it tonight, as I'm wondering if the pilot spool is sitting a little too low, but outside of that I'm wondering if I made some sort of mistake in my porting.
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Bill Shields
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Re: Little Engines 2.5" Scale Air Pump

Post by Bill Shields »

The original iirc spool is one piece, no rings.

This puts the portvspacine edge to edge of the spool.

When you put orings in, did you account for the fact that the sealing edge of the oring is not the same location as it would be for a solid piston?

This affects porting.
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apm
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Re: Little Engines 2.5" Scale Air Pump

Post by apm »

Well an update here, what a difference one night can make. Sometimes when you get frustrated you just need to walk away instead of completely surrender as I felt like doing last night.

After preemptively admitting defeat little too early I took Bill's advice and redid my spool valves, with running fits that just barely slide freely inside of the bores. Still to be determined as if it's good enough of a fit in the long run. You can feel a little bit of air coming out of the exhaust when it really should not be doing so. I'm wondering how it will operate under live steam.

One of the nice things about using spool valves is I was able to isolate each side one at a time first the pilot stage then the shuttle valve stage and verify each was working correctly before moving on. As of tonight my pilot valve moves the shuttle valve back and forth, and the shuttle valve appears to correctly connect the pressure supply back and forth between between the two sides of the cylinders. I believe the exhaust is also working correctly but I have no real way of testing that other than running it.

I got a little ambitious and tried putting the whole thing together as seen in the photo below to see if I could get it to go up and down and I came close. Down stroke is working quite well but sending it back up is a different problem.It looks like right now my problem is that on the top end of the stroke it pushes the valve completely to the top to reverse directions however I think on the bottom side I'm not quite pulling down the valve as much as we need to for it to all work. Hopefully this is just a simple adjustment of some screw threads but we'll see.

It's kind of a pain in the butt how the whole thing goes together with the reach rod and all so probably try it again tomorrow night if I get time or another night in the garage to work on this but I think I'm finally turning the corner after about 5 years of on and off working on this project.

I wish in hindsight with the spools I just started with what I have now as they're a hell of a lot less complicated than what I was trying to do. I had gone to segmented spools with piston rings in the whole thing took quite a bit of time and got very complex.

So I guess I'll end with a photo of where I'm at to date. I am hoping I'm only about one night away from having this thing all working.
PXL_20230330_025712642_copy_756x1008.jpg
PXL_20230330_025712642_copy_756x1008.jpg (134.34 KiB) Viewed 3014 times
apm
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Re: Little Engines 2.5" Scale Air Pump

Post by apm »

Well tonight was the first night that I saw it stroking back and forth back and forth. That was the good news now for the bad news, I have a continual leak from Air supply to the exhaust port. I'm pretty sure the problem is leaking in the spool valves but I could be wrong. I know that spools are supposed to leak somewhat but I'm afraid it may be leaking a little too much.

Bill when it comes to spool valves how much air would you expect to see leaking out while testing on air? The only other issue I can't really wrap my head around, is how much air should actually be coming out of this thing during normal operation? It's quite hard to actually rationalize it because one side's always going to be exhausting while the other side is filling, yet at the same time when I run the pressure down somewhat low, and fight to hold the piston in the top most position I still hear a lot of leakage. I am wondering what the next step in the troubleshooting will be.

I'm running out of ways to isolate things down to try to figure out where the leaks are coming from. But I guess it'll have to wait for another night. Hopefully the problem is somewhere that is actually solvable and not an inherent design flaw. Time will tell.
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Harold_V
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Re: Little Engines 2.5" Scale Air Pump

Post by Harold_V »

The harsh reality is unless you have a rigid hone and a cylindrical grinder, the likelihood that you'll be able to create a leak free assembly is not good. The average lathe simply can't produce round and straight enough surfaces to create the desired fit. That's why rings are employed.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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Bill Shields
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Re: Little Engines 2.5" Scale Air Pump

Post by Bill Shields »

Bob T lapped the spool into the head to get it correct.

Says the dry, clean spool needs to be such that it will not fall through under own weight, but can be blown through with light "lung power"

Are you lubricating the incoming air?
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apm
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Re: Little Engines 2.5" Scale Air Pump

Post by apm »

Bill Shields wrote: Fri Mar 31, 2023 5:18 am Bob T lapped the spool into the head to get it correct.

Says the dry, clean spool needs to be such that it will not fall through under own weight, but can be blown through with light "lung power"

Are you lubricating the incoming air?
No lube on incoming air right now, I lapped the bores on both holes but maybe I have more work to do. Anyone here have a sunnen hone I could send this to who could maybe clean the bores out further? Or know of a shop that does low volume honing?

My guy who used to do my honing passed away. The bores are 1/2 and 5/8".

I am probably a little looser than won't fall thru on it's own but still rather tight. It's really hard to test the 1/2" hole as it's blind.

Any suggestions for accurately measuring and sneaking up on the od? I have just tried 0.001" at a time on the lathe from where it won't fit till it will. Maybe I need a better strategy or need to turn to lapping?
Last edited by apm on Fri Mar 31, 2023 5:53 am, edited 1 time in total.
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