Valve stem clearance

[JPSAXNC]

I had made a knock open valve out of 304 SS. and used a 1/8" alloy steel dowel pin for the valve stem, I thought that would be a good combination of materials. The valve stem hole was reamed to 1/8" and the combination felt very slick. Well it turned out that the SS. was depositing on the stem and the valve stem was seizing intermittently. So I reamed the valve stem hole one thousandth over size. This a valve I was using a heavy valve return spring on trying to control valve bounce. I had the valve set up in a vise and I was pressing the valve stem in a couple of times to see how it felt, and noticed that there was a jolt, and then the stem went forward, there was two motions taking place before the valve opened instead of one. This was do to the one thousandth over stem hole, the stem was moving sideways before moving forward. There was big drop in velocity with the one thousandth over hole, due to the two motions.

[rsterne]

I have never experienced that problem.... and quite often the valve stem holes in my valves are looser than I would like.... Mind you, I never use a really stiff valve spring, as it really has nothing to do with valve bounce.... That is all due to the hammer storing energy from the valve closing....

Bob

[JPSAXNC]

Hi Bob, I just thought it was worth mentioning, because the valve stem will take the path of least resistance, (sideways if there is clearance) and under pressure, I would think the jolt could be great enough to affect accuracy?

[rsterne]

Anything is possible, I guess....

Bob

[PikeP]

Any lateral movement within the gun would likely express itself to your POA, its a complex equation to determine just how much though...

[Motorhead]

304 SS is fairly tough stuff and the poppet pin needs to be HARD ground and polished DRILL ROD as to not gall sliding within the body.
If or when you have similar hardness materials sliding by one another they wont move smoothly requiring a high viscous lubricant to be present.
SADLY in a high pressure system as a PCP valve such lubrication will be BLOWN right out.

A suggestion ... Take the valve body and ream out the poppet pin area to accept a Bronze sleeve to be the bearing surface against the pin material your currently using.

just a suggestion ...

[KnifeMaker]

Yep, exactly, and if no bronze is available for you, fit a brass tube the size needed to be a slip fit for the stem. SS, especially 303-304 316 are very bad about galling as you are seeing here.


Fully polishing the stem will help a LOT!!!



Knife

[HYspd]

Yep, exactly, and if no bronze is available for you, fit a brass tube the size needed to be a slip fit for the stem. SS, especially 303-304 316 are very bad about galling as you are seeing here.


Fully polishing the stem will help a LOT!!!



Knife

what do you about burnishing the stem with Krytox,after polishing?....the pinewood derby guys think that's the bees knees on their alxes

[PakProtector]

Something hard, and something harder, with minimum clearance, and light lube does quite well. It makes things like head squareness important to sealing well enough to pump it up from zero. If I could I wold run for about .0002 consistent, round on round clearance. That is about a mile compared to the head of a diesel injection pump though.
cheers,
Douglas

[JuryRigger]

304 SS is fairly tough stuff and the poppet pin needs to be HARD ground and polished DRILL ROD as to not gall sliding within the body.
If or when you have similar hardness materials sliding by one another they wont move smoothly requiring a high viscous lubricant to be present.
SADLY in a high pressure system as a PCP valve such lubrication will be BLOWN right out.

A suggestion ... Take the valve body and ream out the poppet pin area to accept a Bronze sleeve to be the bearing surface against the pin material your currently using.

just a suggestion ...

Something hard, and something harder, with minimum clearance, and light lube does quite well. It makes things like head squareness important to sealing well enough to pump it up from zero. If I could I wold run for about .0002 consistent, round on round clearance. That is about a mile compared to the head of a diesel injection pump though.
cheers,
Douglas

Yep, exactly, and if no bronze is available for you, fit a brass tube the size needed to be a slip fit for the stem. SS, especially 303-304 316 are very bad about galling as you are seeing here.


Fully polishing the stem will help a LOT!!!



Knife

These guys are all right on the money, in plain bearing design the hardness differential between the bearing and the journal (part of the shaft in contact with the bearing) is a calculated variable differing with the rotational speed of the shaft; the expected loading; clearance; lubrication; and other factors. I am somewhat surprised that a hardened?? dowel pin would gall that badly. They usually have a pretty good finish, but Knife and PakProtector know from experience, which always beats out book work... As a question for Knife, for clarity here; did you harden and temper the drill rod, or use it stock? It is fairly hard as it comes (0-1)... I believe rsterne (Bob) uses it stock...

[JuryRigger]

I had made a knock open valve out of 304 SS. and used a 1/8" alloy steel dowel pin for the valve stem, I thought that would be a good combination of materials. The valve stem hole was reamed to 1/8" and the combination felt very slick. Well it turned out that the SS. was depositing on the stem and the valve stem was seizing intermittently. So I reamed the valve stem hole one thousandth over size. This a valve I was using a heavy valve return spring on trying to control valve bounce. I had the valve set up in a vise and I was pressing the valve stem in a couple of times to see how it felt, and noticed that there was a jolt, and then the stem went forward, there was two motions taking place before the valve opened instead of one. This was do to the one thousandth over stem hole, the stem was moving sideways before moving forward. There was big drop in velocity with the one thousandth over hole, due to the two motions.

Just a thought- .001" is not a terribly sloppy clearance for a pin that size for this type of use, is the valve stem secure in the poppet? Is the size of your reamed hole based of the nominal size of the reamer, or is it measured? If you can feel/sense a large amount of side-to side motion (with poppet seated and held there), you've probably got an oversized hole. (not the end of the world, though. Listen to Bob's post, or sleeve like Knife suggests). Surface finish inside the hole could cause problems even once you update to drill rod... A rough hole in stainless is certain doom to any free motion expected through it... One thought on sleeving, make it a blind pocket, that is leave a "wall" of material at the base of the pocket to prevent (possible) eventual/immediate dislodging from the air pressure when fired. You can always pull the sleeve by tapping to a suitable size and using a bolt/screw to yank it out.

[antithesis]

If i may add my two cents worth, even with the best dry lubes and precise machining, most standard grade stainless doesnt make a great bearing surface if potentially exposed to lateral stresses in linear motion, especially 304 . If it were me i might sleeve with hardened brass or bronze...id avoid stainless and aluminum as bearing surfaces like the plague

[JPSAXNC]

Hi All, Yes, I did put a bronze sleeve in the valve reamed to exactly 1/8" with a 1/8" alloy steel dowel pin valve stem, and got an increase in velocity. When the hammer struck the valve stem with the gun under pressure, the .001 valve stem clearance let the valve stem move sideways, ( robbing some of the hammer energy, before moving forward).

[CableStop]

Hi All, Yes, I did put a bronze sleeve in the valve reamed to exactly 1/8" with a 1/8" alloy steel dowel pin valve stem, and got an increase in velocity. When the hammer struck the valve stem with the gun under pressure, the .001 valve stem clearance let the valve stem move sideways, ( robbing some of the hammer energy, before moving forward).

While this may be true you really need to run some more tests before concluding deflection was the issue.  Try making a sleeve with .001 tolerances and see if velocity decreases.

My guess is the galling action of the two metals is what slowed the poppet down. Maybe deflection caused more aggressive galling, but it was not the root cause. I think if .001 deflection was enough to effect performance o-rings on the valve stem would lead to wildly erratic numbers.

Plus bronze has a low coefficient of friction compared to non galled steel which could contribute to less valve stem loss.

[Motorhead]

issue solved ...