Little Engines 2.5" Scale Air Pump

[Bill Shields]

Note ..lapped the spool into the bore. Did not use external hone. Used finely ground round stock as the hone to lap the bore, then used a new piece of stock to make final spool.

Oil will help situation. Use spindle oil sight...maybe 10w, no detergents.

[apm]

Note ..lapped the spool into the bore. Did not use external hone. Used finely ground round stock as the hone to lap the bore, then used a new piece of stock to make final spool.

Oil will help situation. Use spindle oil sight...maybe 10w, no detergents.

Bill,

Maybe I am not understanding the lapping process that you are describing that well. The bores were lapped with barrel laps from McMaster. and clover compound. I also have timesaver if needed too.

How would I lap with finely ground rod as the lap and then turn around and use a new piece of the said stock to be the final spool. I would think at the initial starting point either my rod must be slightly larger in dia then my hole (and I would open the hole up) to fit the rod or my rod must be slightly larger in diameter than the hole and I remove material until it fits the hole right? I am not sure how I see that happening in the process you describe above. Am I missing something?

[Bill Shields]

You are correct.

I was unaware that your bores were to size, but should have thought it through a bit better. Sorry

Often when folks are using orings they simply ream a hole and let the ring take up the rest. After, what's a thou it 2 to an oring fit...just clean out the groove a bit

Do you have a set if plug gauge pins so that you can effectively know the actual hole size?

Ever lapped with toothpaste?

If the ground pin is too tight, you can take a piece of scrap rod, put a taper in the end and toothpaste lap the hole open just a bit.

This is how I have fit dozens of oil pumps over the years

[apm]

Hi Bill,

I never heard of toothpaste lapping. I made some more progress tonight on it. My main spool you have to give a gentle push to get it to move thru the bore, I think that spool is pretty good. I had tried to get the piston ring spool back in there but I can't overcome the spool friction of 5 rings working against me on the 5/8" diameter. I think I am going to stick with the solid spool in this bore. As I reinspected the pilot spool that one is where I think most of the leakage was running. Interestingly enough the earlier piston ring spool valve worked pretty well in that spot and the leakage seems to have gone down quite a bit though I am not sure if I have thoroughly stomped it all out yet.

Fortunately the pilot spool is 1/2" so I am thinking of making one more spool and using the following seals from McMaster and maybe these will work even better. https://www.mcmaster.com/catalog/129/3969/1526N11.

One other thing I am thinking of copying off of some of the larger hydraulic spools is that many of the industrial ones have grooves that are about 0.030" deep and wide. I am thinking of putting something similar onto my spools. I think they do 2 things.

First they create a turbulent zone that makes leakage past them harder, and secondly, I am told they tend to center the spool a little as opposed to allowing it to be shoved to one side of the bore for better sealing.

I am not sure how to determine when I have gotten down to an acceptable level of leakage either. Once the thing starts chugging its is hard to tell what's leakage vs what's legit exhaust.

All and all I think I am rounding the corner here. Now the next things to start thinking towards are actual mounting and usage of it.

I am thinking I probably want to lubricate on the steam side (most likely with hydrostatic lubricator, or would mechanical be better?)

Then air side I have an O-ring runnning in a honed cylinder, I am wondering if that needs lubrication? How should I lubricate the incoming air? I am thinking maybe some sort of drip feed on the inlet?

Lastly I am wondering how to govern this whole thing. Ideally I would make a steam valve where air pressure turns it off, however I have been reading lots of people just let the boiler pressure do it's thing. In other words this compressor is a 1:1 ratio. So if my Boiler pressure is 100psig, it would appear that at 100psig air the thing balances out and is a wash. However depending on how bad my spool leakage is, this may be an undesired steam load.

What do others think?

Thanks

Adam

[Bill Shields]

Everything needs lube.

Hydrostatic (displacement) is ok for steam side, but a displacement type will not work on air since they require condensation to lift the oil.

I have seen your groove idea applied, but not in this context.

You are really close. The having to firmly push the piston is problematic and will bite you.

[apm]

Everything needs lube.

Hydrostatic (displacement) is ok for steam side, but a displacement type will not work on air since they require condensation to lift the oil.

I have seen your groove idea applied, but not in this context.

You are really close. The having to firmly push the piston is problematic and will bite you.

Bill what are your thoughts on the firmly push issue? I suppose I could hit it one more time with the lapping tool but I figured we would probably wear it in. Other thought I have is to just quickly look for any more burrs on the spool but I think I have that long since covered. Seem like at the firm push that my clearance is would be pretty good since it's working. I'm thinking the tightness would help me a little especially if the bore expands a little bit with heat. Of course if the spool expands I may have a problem.

[Bill Shields]

Wear in is generally not how it should be approached.

Experience is that these spoiled have to lightly push through the bores. The small one that I have that runs and one of Ben Nixon's that I repaired ..basically you can blow hard with lungs and move the piston side to side.

By the time you add the stiction of the valve under pressure..it all adds up and you get a sticky pump.

Yes the spool expands...is same problem as with main drive pistons that are too tight.

[Bill Shields]

FWIW

The builder's notes on the pump that I have in the basement clearly says that the spool has to just fall through the bore, dry -> under its own weight.

The builder (B&O Bob) confirmed that to me on several occasions as I was updating the drawings.

[Bill Shields]

Stop and think about the workings of the pump.

The spool just pushes the valve back and forth. So what if it leaks a little?

The pressure differential on the ends of the spool is what ...100 psi.

Convert that to thrust on the surface area of the spool...which is what?

at best case, the push required to move the spool cannot exceed the thrust from the pressure differential provided by the steam....which is not much on an area that small.

If the spool is .375 diameter, then the steam thrust available is at max 11# assuming 100# differential -> which assumes full boiler pressure on the steam input, which is not realistic.

Say you put 10 psi on the steam input? You STILL want the spool to trip the valve with what -> a 1# differential on it.

Next, you have to add in (or subtract) the sliding friction of the valve face from the equation.

See why a tight spool is not necessarily a good spool?

[apm]

Well an update here, I wish I could report more progress but I don't know. The entire pump works just fine on air it goes up and down, but the blow by I'm getting is still quite significant of a hiss. Currently I have a solid spool valve is the main driver in the back, and a segmented spool valve using Teflon seals purchased from McMaster that are designed for piston seals.

I test my blow by clamping the cylinder in the upright position and find that a lot of air comes out the exhaust each time. I don't have any way of really quantifying how much air is coming out or measuring it but it seems pretty significant.

To troubleshoot it I squirted some dykem Blue into the airline going into the pump. At first that showed me at least one leak which I was able to fill with our RTV, as it was coming out the bottom of a flange and leaking into the cylinder head. The next leaks were not so simple.

A I squirted more RTV I opened it up to find virtually everything appears blue at which point I got frustrated and decided to call it quits for the night. I suppose with the spool valve I should expect some blow by, and I'm wondering how much better it will be when I switch over to steam.

I know in the past I have gotten myself upset about air leaks in check valves on a boiler only to find that they work fine with steam behind them, so I'm wondering if the next test is just to fill the thing with water and see how much squirts out. Maybe the problem is not as bad as I think it is. Otherwise I just don't know. I suppose if it doesn't consume too much steam in normal operation maybe it's no big deal, some leakage won't hurt too much, but this is definitely getting frustrating. Half of me wants to rebore the heads and put in d-valves which will at least benefit from having steam pressure holding them shut.

Bill or anyone else who's ever played with one of these things how much leakage would be acceptable? I think at this point in time I don't ever want to touch spool valves again in my home shop they appear simple on paper, but seem to be far more frustration than they are worth!

[Bill Shields]

Put steam on it and see what happens...do not be frustrated unnecessarily. If it runs in air you are most of the way there.

One of the tricks of these critters is that should they get a shot of water into the steam section, (which frequently happens when the steam valve is first opened), they have to leak enough to clear it out the exhaust... otherwise they will stall and lock up until the water is cleared manually.

[apm]

Put steam on it and see what happens...do not be frustrated unnecessarily. If it runs in air you are most of the way there.

One of the tricks of these critters is that should they get a shot of water into the steam section, (which frequently happens when the steam valve is first opened), they have to leak enough to clear it out the exhaust... otherwise they will stall and lock up until the water is cleared manually.

I never thought about it that way, come to think about it I don't know if I've ever seen cylinder cocks on any of these compressors out there. I guess the bigger concern I have isn't so much how much blow by happens during the regular operation, so much is how much steam I will lose when the pressure equalizes out between the air reservoir and the steam cylinder. As far as I understand nobody runs these things with a shut off valve that shuts the steam line when the steam rises above a certain pressure and comes back on when the steam falls below a level. Maybe the next test is to just run it on steam at some point in time.