Leaky Check Valve

[Charles Lamont]

... but my intuition says your equations are not truly exact.

Your intuition is making you over-think it. Even if you want to consider a ball a few hundred metres in diameter, it is sitting on a horizontal ring seating, and the net force on it is the area enclosed by the ring multipled by the difference in pressure above and below the seating, at the level of said seating. It is not a matter of equations but reasoning. It is the seating that matters. The object sitting on it could be a ball, a banana, or any other shape you care to think of. Any texbook on fluid mechanics will do. The book I used at uni is on the shelf not 4ft from where I am sitting: 'Mechanics of Fluids' B S Massey, but with a publication date of 1970, there must be more modern texts.

 

[Cogsy]

Any texbook on fluid mechanics will do. The book I used at uni is on the shelf not 4ft from where I am sitting: 'Mechanics of Fluids' B S Massey, but with a publication date of 1970, there must be more modern texts.

I'll dig up a textbook from somewhere - I doubt much has changed from the 70's so your book is a good starting point. Thanks.

 

[Steamchick]

Torn it or is correct. The force from the fluid is the pressure in the fluid divide by the cross-sectional area of the circle of diameter equal to the diameter of the sealing surface. A lapped seat is better than a hammered seat. All the internal combustion engines on the planet with poppet valves have lapped seats...
Enjoy!

 

[Steamchick]

Sorry Tornitore45, spell check didn't understand your name!

 

[Charles Lamont]

All the internal combustion engines on the planet with poppet valves have lapped seats...

Really?

 

[Steamchick]

Well, yes! The very precisely ground faces of both the valve seats and valves still have micron sized undulations - when the valves are in use in the engine they actually rotate - that's the beauty of the poppet valve - and the rotation, along with surface abrasion, causes the seat to become "perfect" - effectively by lapping. You can see the seat witness lines on engines after a few dozen hours of running, but not after a strip-down very soon after a new assembly (just minutes of running). This happens during a reasonably short running-in period (few hundred miles?). The witness mark is seen as a greyish line around both seat and valve faces. When you do the lapping of your car's valves with fine carborundum, you are just accelerating the process so you are "good to go" from the first cranking of the engine. But even so, there will still be some incredibly fine imperfections that settle down with running-in... ("Particle sized" views of surfaces are made visible using a Scanning Electron Microscope - not that I have one!). Maybe these links will work? There is a picture of the lines in the surface from grinding. Some are grooves and some are raised lines, and larger-than-molecule lumps. Any high-spots are "worn-off" by the lapping process, reducing the gap between surfaces for gases to pass.. When the gap is smaller than a gas molecule, the perfect seal is achieved. Sorry if I am "teaching grandma."..?

https://www.google.com/url?sa=i&url...hUKEwi3-c323YrqAhUPNRQKHRpRCi4Qr4kDegUIARCkAQ<a href="https://www.researchgate.net/figure...y-scanning-electron-microscopy_fig1_266170047"><img

src="https://www.researchgate.net/profil...-surfaces-by-scanning-electron-microscopy.png" alt="Topographic aspects of the bare stainless steel surfaces by scanning electron microscopy [12]. (R a ) is the arithmetic average roughness determined from a 3D profile."/></a>

Of course, we are talking of a different valve here where I guess it won't lap during service? So to lap the valve (ball?) to the seat is a good idea. Much gentler than a tap with a ball-pein hammer? (Although I use a copper ball for gentle taps. = "a Nudge" as I was taught as a lad.)
Anyway, that is my understanding and an expert can tell me a better story? I learnt this from a re-work shop when I was a teenager, working on motorcycle engines (hobby) and later at a car factory when as an Engine Design Engineer and working with Engine Test cell lads who stripped engines after just a short run as well as after 200 hours of hard Dyno test cycling. I love to learn.. and not all of us are experts. - I'm not. So I consult experts all the time. I am "just practising", as Pablo Cassals said at 70... (but I have been a drip-under-pressure: "Ex" = "has been", "Spurt" = "drip under pressure"!).
Enjoy!
(P.S. - I didn't want to expound at length... but just have! - Any experts able to help further? or correct my errors?)

 

[Charles Lamont]

Ahah, so you are saying poppet valves bed themselves in. Surely that is rather different from lapping, which invoves the deliberate introduction of an abrasive.

 

[Steamchick]

The process of reducing the high spots by rotating valves against the seats - or any similar rubbing action of 2 surfaces is technically lapping. Whether the engine does it or a man is not part of the definition as I understand... but I may be wrong? (I am more often wrong than I prefer to admit!). But I was taught that the final "lapping" is done by the engine when running-in.... - But that was by a 50-years machinist, who was teaching me as a teenager... Sorry if that isn't correct, it is only what I know...
Incidentally, the same applies to crankshafts: Again, I was taught to machine cranks on a dedicated crank grinder - on anything from a 900cc to "large 6-cylinder" diesel cranks. The grinder goes to a certain surface finish (the carborundum stone grade) then the surface is lapped by the finest grade of emery (carborundum on cloth). I was taught the final surface lapping was done "by the detritus in the oil" during running-in as the oil passed through the bearing to crank gap. and the result was the dirt in the oil seen at first oil-change and washout after running-in. Hence, Molybdenum can effectively "damage" an engine as it prevents the "lapping" from happening in the running-in phase.
But Then an Engineer from Glacier-Vandervell Bearings taught me that Aluminium-tin bearings were appropriate on cast-iron cranks, as - from the SEM study - the grinding (Manufacturer's factory finish) left "little hooks" of bent microscopic bits of iron... The Al-Sn alloy contained Silicon - which "lapped" the surface of the cast-iron and removed these "hooks", as well as any other high-spots or hard "lumps" in the surface. If the cranks were ground "the wrong way", then the hooks acted like tools and cut microscopic grooves in the bearing material... But the grinding machines or engines would have to run backwards to do that. Apparently when they first made such Al-Sn bearing materials there were rare cranks made the normal handed way, but which ran backwards ... (e.g. in multi-crank engines?) - which is how they came to understand what was happening (hence the reason to look with the SEM)... Apparently (if my memory is correct?) the crystalline structure of forged steel cranks is different from cast-iron so these "little hooks" do not appear... therefore these journals are more suited to plated Phosphor-bronze bearings. (The running-in of cast iron on the plating of Phosphor-bronze bearings would destroy the plating - so the bearing would be prone to corrosion damage later in life). So crank materials have preferred bearing materials. Anyway, I remember (if imperfectly?) the reference to "lapping" of cranks in the engine...
Sorry if I am wrong, but none of us are perfect learners, nor teachers. What do you know? - I'd like to learn.

 

[Steamchick]

Oh, as you mention "the deliberate introduction of an abrasive", I understood it didn't need to be deliberate - as with the running-in dirty oil scenario. - But "dirty oil", "running-in", "500-mile oil change and flush" were a 1960s thing... Also, for the poppet valves, I understand that it (lapping or other name) happens between the high spots of the 2 surfaces that rub on impact as the valve has a slight rotary motion when it hits the valve seat. The microscopic high spots get rubbed-off without an abrasive as they tend to be harder "lumps" in the crystalline structure. But that was another lesson from a Doctor of Tribology (Friction and surfaces) who was explaining how dry surfaces wear - like valve seats and valves. A similar process to the cast-iron crank journal being "lapped" by the silicon "lumps" in the Al-Sn bearings. (He was the guy discussing various bearing and material matters, as well as Oil, with engine component suppliers , when I spent a few weeks with him in the 1980s). As I said: I am not an expert, but worked with a few and learned some trivia, so please give us your expertise? Knowledge shared is good for us all.

 

[Cogsy]

Those SEM images are almost macroscopic. Molecule size is down to Angstroms and such a surface finish would never be achieved in practice. It's not of metal, but here's an SEM image that I took at a much higher resolution but which is still far above Angstrom sizes. Any guesses about what this image is actually of?

I haven't heard of these ideas about grinding direction or lapping of the surfaces with impurities in the oil. The lubrication barrier is supposed to be an actual barrier. That is, the surfaces do not actually touch when operating but are separated by a thin film (called hydrodynamic bearings). In practice, the small burrs from grinding that penetrate the film thickness experience so much pressure that they wear away during the 'running in' period and this is what we see in the oil. I can't see the geometry of any microscopic 'hooks' having any effect to be honest - they're just too small and for the direction of rotation to have any influence this suggests the cutting action has to be performed by the pointed 'end' of the hook which then machines away a solid surface. At the sub micron scale these marks are just needle points regardless of their orientation.

 

[Steamchick]

Hi again,
I understand the point about the hydrodynamic film... as I remember the discussions about clearances and size of oil particles with the Texaco guy and the Nissan Tribologist. At the distant corner of memory I have a 40micron clearance being the standard gap... but as cooling the bearing (the oil is heated when squeezed though the small clearances) is a major characteristic of the lube system, that may be the reason for the clearance, not the size of the oil particles... 30-odd years and my memory isn't perfect. My boss was ex-GM and he also expressed some surprise at the tribologist's explanations, but as he had extensive papers (in Japanese - which we couldn't read!) with a few pictures, we believed his explanations as the Company Expert. The guy from Glacier (probably retired now?) was their expert, and I do remember that the answers to questions were given a smile and head-nod by the tribologist and he said that was the same explanation as the Japanese bearing maker. Otherwise - apart from memory errors (I am human) - that is the only justification I can use. I feel I am on trial, when I was only trying to offer an interesting "tale" for readers of this thread.

 

[Steamchick]

Oh, it probably isn't relevant, but I do remember (when I was a teenager learning to grind crankshafts) that we had cranks with "a few thou (inch) of wear" - and saw bearing journals with score marks - sometimes down to the base steel - so actually the hydrodynamic film doesn't work to prevent all the time, in the lifetime of the bearing/journal.... Are you a Bearing manufacturer or Tribologist by any chance? - Having access to an SEM I guess you have a big budget! (I was refused 30 years ago)

 

[BaronJ]

Hi Al, Guys,

I'll have a guess at your picture, an oilite bush ?.

As far as engines and poppet valves go, the cam or tappet is usually slightly offset from the center in order to create some rotation of the valve on its seat.

 

[Nikhil Bhale]

These are marine 2 stroke exhaust valves. The valve spindle is fitted with a winged valve rotator. The kinetic energy in the exhaust gas rotates the valve a small amount as it passes. This keeps the valve at an even temperature and helps reduce the build up of deposits on the valve seat .

 

[Steamchick]

Fascinating! I have never heard of that one.
I understood the dynamics of rotation mostly came from the valve springs. A pair of springs is normal, with opposite hand helix. But as one is a smaller radius, there is an imbalance of rotational forces (torque) from the springs, partly compensated by a different pitch and wire size. But there is a net rotation as a result of the torque.
On desmodromic valves, there isn't a spring torque to rotate the valves. I can't explain that one. Similarly with hair spring valve set-up, but maybe that hasn't been used for over 50 years so is irrelevant?
Interesting hearing of your experience.
Thanks.

 

[Cogsy]

I should have given a bit more detail on my image. It is a biological structure taken at around 405 thousand times magnification. The brightest straight lines which run at about a 45 degree angle across the image have a diameter of roughly 50 nanometers (50 billionths of a metre) which is roughly a diameter of only 500 atoms.

The structure itself is made of similar stuff to what your fingernails are made of (and completely clear) but it makes up the wing scales of a species of butterfly. The particular structure here (the spiral holes) are far smaller than the wavelength of visible light yet create wave interference patterns in the visible range which give this butterfly a highly iridescent green colour. Interestingly (to me at least) most of the green, blue and purple colours seen in animals and insects in nature is not created by pigments but is actually structural colour like this butterfly uses. If you look at a bird/fish/snake/beetle/ant/whatever and its colour appears to change shade or intensity depending on what angle you look at it then that animals is using some sort of nano-sized structure to change the white light falling on it to some other colour.

Way off topic to valves though. Sorry for the diversion.

 

[Steamchick]

Really interesting - but I was wrong thinking it was a single layer of human skin.
Must be old age? - I digress often! - But maybe you aren't feeling old yet!

 

[Richard Hed]

I should have given a bit more detail on my image. It is a biological structure taken at around 405 thousand times magnification. The brightest straight lines which run at about a 45 degree angle across the image have a diameter of roughly 50 nanometers (50 billionths of a metre) which is roughly a diameter of only 500 atoms.

The structure itself is made of similar stuff to what your fingernails are made of (and completely clear) but it makes up the wing scales of a species of butterfly. The particular structure here (the spiral holes) are far smaller than the wavelength of visible light yet create wave interference patterns in the visible range which give this butterfly a highly iridescent green colour. Interestingly (to me at least) most of the green, blue and purple colours seen in animals and insects in nature is not created by pigments but is actually structural colour like this butterfly uses. If you look at a bird/fish/snake/beetle/ant/whatever and its colour appears to change shade or intensity depending on what angle you look at it then that animals is using some sort of nano-sized structure to change the white light falling on it to some other colour.

Way off topic to valves though. Sorry for the diversion.

thank yew for that, that is exceedingly interesting.

 

[mcostello]

Diversions are interesting in their own right.

 

[chrsbrbnk]

My understanding is the projected area is what matters the aspect of it being a ball, cone, or a piston with a hex nut all the force vectors in opposition just cancel out and your left with the projected area