Sticky Piston Valves

[Rick Denney]

Because of the larger surface area to wear, quite a bit more IS worn, along with guides, casings, and ports.

No, that isn't really how wear works.

Wear is a function of friction, which is a function of the normal force divided by the surface area. Thus, more surface area means less wear for a given force, because the force is spread over more wear surface. That's why big brake pads last longer than little ones.

The other question is the force. A rotary valve sits with its axle spindles resting horizontally between two journal bearings. The spindle bearing surfaces are quite small--probably less than a 20th of a square inch. And rotors are relatively heavy. So they actually put more wear stress on the spindles and casings, and that's why we use a thicker oil to lubricate those points.

A front-action tuba's piston also rests on the casing, but in this case the surface area is more like a couple of square inches. Thus, the oil can be much lighter and still allow the piston to rest on a film of oil.

A top-action tuba's pistons don't have a wear force between the piston and the casing. The piston is perched on the spring on those instruments.

Damaging wear force on a piston is caused by applying a lateral force when moving the valve. That's why we learn to put our fingers squarely on the button and push straight in, within reason.

But even that wear force would likely have a negligible effect if we kept the valves properly cleaned and lubricated.

I suspect that rotary valve wear out faster than piston valves, and the clanking of older rotary tubas, even those with updated linkages, bears witness to that. They are more like to work when worn, I suppose. I also suspect that both piston and rotary valves are unlikely to wear at all if they are kept properly lubed.

Rick "whose piston tubas never seem to show more valve wear than they did on the day they were bought" Denney

[Dan Schultz]

.... I suspect that rotary valve wear out faster than piston valves, and the clanking of older rotary tubas, even those with updated linkages, bears witness to that. They are more like to work when worn, I suppose.

Rick "whose piston tubas never seem to show more valve wear than they did on the day they were bought" Denney

I normally aggree with you on most points. But... this is statement is a bit confusing. A properly operating rotary valve never makes contact between the outer diameter of the rotor and the inside of the house. Only the very tiny bearing surfaces (rotary and axial) are supported on a film of lubricant. The ENTIRE outside diameter of a piston is supported on a film of lubricant.

1) Worn piston valves have INCREASED clearance.
2) Worn rotor bearings have DECREASED clearance.

Pistons will continue to operate but will be sloppy. Rotors will bind.

[windshieldbug]

No, that isn't really how wear works.

I am no engineer, nor do I even play one on the web.

But it seems to this tuba jockey that the lateral force necessarily applied to 3+ pistons that are NOT placed in linear concert with the applying digits will by definition create more wear points than remote rotors, which have the force translated into a consistent actuating device.

In every case that I've detected noisy rotors on my horns, it has been as a result of worn linkages, NOT worn valves. As linkages improve, so do the reliability and usable lifespan on rotors.

Unfortunately, despite advances in lubrication I cannot say the same for my anecdotal observations of my piston valve horns. Regardless of viscosity, one would assume that one molecule should remain sufficient.

And no, my bigger brake pads didn't last longer. In fact, they lasted less long. They just transferred more force over a wider area to stop faster. If you REALLY want to stop fast without using pad area, drive into a mountain!

Bottom line for me is that with proper maintenance, valves is valves, and all CAN be reliable. The trade-off for me is the bore-size and placement in the bore profile, and all things being equal, I prefer bigger, later!

[jonesbrass]

But it seems to this tuba jockey that the lateral force necessarily applied to 3+ pistons that are NOT placed in linear concert with the applying digits will by definition create more wear points than remote rotors, which have the force translated into a consistent actuating device.

In every case that I've detected noisy rotors on my horns, it has been as a result of worn linkages, NOT worn valves. As linkages improve, so do the reliability and usable lifespan on rotors.

Your first point has been the source of my dislike for pistons- I was always the umpteenth guy to play the horn, and I think the minor differences in the angle and pressure used to push the valves down caused the pistons to stick on me. OTOH, I have never played a rotary tuba with "bound" valves, even when I was the umpteenth guy to play it. Of course, YMMV.

Still hopeful to find a good (for me) piston horn someday.

[Wyvern]

I think the minor differences in the angle and pressure used to push the valves down caused the pistons to stick on me.

I wonder if that is the "proximity effect" (quoting Rick Denney) as to why pistons seem to stick on me too?

[windshieldbug]

I think the minor differences in the angle and pressure used to push the valves down caused the pistons to stick on me.

I wonder if that is the "proximity effect" (quoting Rick Denney) as to why pistons seem to stick on me too?

If pistons are sticking on you, I would perhaps suspect ferrous metals and magnets...

[Wyvern]

If pistons are sticking on you, I would perhaps suspect ferrous metals and magnets...

[Rick Denney]

1) Worn piston valves have INCREASED clearance.
2) Worn rotor bearings have DECREASED clearance.

Pistons will continue to operate but will be sloppy. Rotors will bind.

It seems to me that they'll rattle around in the bearings first, before they wear to the point where the rotor contacts the casing enough to cause damaging friction.

Rick "who has rattled lots of loose rotors that still worked, but noisily" Denney

[Rick Denney]

No, that isn't really how wear works.

I am no engineer, nor do I even play one on the web.

But it seems to this tuba jockey that the lateral force necessarily applied to 3+ pistons that are NOT placed in linear concert with the applying digits will by definition create more wear points than remote rotors, which have the force translated into a consistent actuating device.

In every case that I've detected noisy rotors on my horns, it has been as a result of worn linkages, NOT worn valves. As linkages improve, so do the reliability and usable lifespan on rotors.

On the brake-pad subject, I'm not guessing, and yes I know you once raced cars, as did I. Frictional force is the normal force times the coefficient of friction, and a given normal force reduces the frictional force when applied over a larger area.

But back to piston valves. If you put your finger on the side of the button, you'll impart a significant lateral force that will cause additional wear. But if your finger is on the top of the button, even if off-centered, the lateral force will be pretty small. Consider that your finger might be a quarter inch off-center, but the piston is 5 inches long, which means that the lateral force is one twentieth of the moment caused by the off-center finger. If this was sufficient to break through the film of a lubricant, the valve would bind or feel rough. In practice, valves that stick from dirt do so on the way back up, not on the way down, and on the way up they are pushed by that nicely axial spring.

I'd much rather have off-center buttons so that they are under the fingers than having short fingers pushing on the edge of the button.

Rick "noting 20K valves that survive years of abuse with off-center buttons" Denney

[Donn]

But back to piston valves. If you put your finger on the side of the button, you'll impart a significant lateral force that will cause additional wear. But if your finger is on the top of the button, even if off-centered, the lateral force will be pretty small. Consider that your finger might be a quarter inch off-center, but the piston is 5 inches long, which means that the lateral force is one twentieth of the moment caused by the off-center finger. If this was sufficient to break through the film of a lubricant, the valve would bind or feel rough. In practice, valves that stick from dirt do so on the way back up, not on the way down, and on the way up they are pushed by that nicely axial spring.

I'd much rather have off-center buttons so that they are under the fingers than having short fingers pushing on the edge of the button.

Rick "noting 20K valves that survive years of abuse with off-center buttons" Denney

I recently have been been spending a lot of time with a valve trombone that's a few decades old. The first valve has an annoying tendency to come up slow - if I play it the way I naturally wanted to. I've learned to move my hand out farther, changing the angle of attack so to speak, and this seems to have solved the problem.

I have no idea how the piston binds, or what causes this slow return (not just on this instrument, but other similar cases with oldish valves.) It's actually kind of hard to make it happen (for example, if you're in the instrument repair shop "look, I know it looks fine right now, but it sticks all the time and it drives me nuts, so please fix it!") It's definitely more likely to happen while playing than just fingering, and maybe more likely to happen when the sticky valve has been down for a note or two. Moisture or other effects from breath might play some role. Oil helps, but not much.

[windshieldbug]

Frictional force is the normal force times the coefficient of friction, and a given normal force reduces the frictional force when applied over a larger area.

Yes, I was implying that one could transfer MORE force, more effectively. But that's why I wasn't a mechanical engineer!

I'd much rather have off-center buttons so that they are under the fingers than having short fingers pushing on the edge of the button... noting 20K valves that survive years of abuse with off-center buttons

A much better way of saying what I was trying to get at, thanks!

[tuba_hacker]

And remember that new piston casings must have time to build up a uniform layer of oxide. Frequent use is the best procedure, plus keeping it clean and oiled. DON'T, however, polish the insides of the casings to bright brass, or you'll have to start the process over again.

Rick "13" Denney

Perhaps this is why the first valve on my Weril became worse after a chem clean?

Personally, I'm with Neptune. In all my years of playing trombones with an F-attachments, I've never had a problem with a rotary valve sticking or working slowly.

[Donn]

In all my years of playing trombones with an F-attachments, I've never had a problem with a rotary valve sticking or working slowly.

I have played a tuba with clock spring rotors that had a sticky valve. It would lock down, in a mechanical sort of way. I felt that it had something to do with the rotor linkage pivot, but couldn't prove anything.

[Dan Schultz]

1) Worn piston valves have INCREASED clearance.
2) Worn rotor bearings have DECREASED clearance.

Pistons will continue to operate but will be sloppy. Rotors will bind.

It seems to me that they'll rattle around in the bearings first, before they wear to the point where the rotor contacts the casing enough to cause damaging friction.

Rick "who has rattled lots of loose rotors that still worked, but noisily" Denney

You're missing my point, Rick. It takes very little wear on the bearings before the rotors will drag inside the casing.

[Dan Schultz]

I suspect Rick "gets it".

The topic, though, is "sticking".

I, like Rick, haven't encountered very many worn/noisy rotary valves that actually "stick".

You mean that a rotor binding because it's rubbing the inside of the casing isn't called 'sticking'?

Even though the original post aluded to sticking PISTONS, the 5th post spoke of a Mirafone 186... which of course, has rotors.

The point I'm trying to make here is that the elements involved in pistons and rotors are two TOTALLY different sets of dynamics.

[Rick Denney]

The point I'm trying to make here is that the elements involved in pistons and rotors are two TOTALLY different sets of dynamics.

I'm gonna get picky with you, in full knowledge that you know what you are talking about.

The dynamics of lubrication are the same. The bearing surfaces float on a film of oil. If the oil is wiped or washed off, the valve will stick because of metal-to-metal friction.

It's easier for the small surfaces of a rotor bearing to lose the film, because the force is higher. We counteract that by using heavier oil that builds a stronger film. But they'll wear faster if you run them dry. The heavier oil, even though it requires more force to overcome viscosity, still works because the bearing diameter is small and the linkage has considerable mechanical advantage on it. Pistons, on the other hand, have no mechanical advantage and large bearing surfaces and the oil must therefore be very light. And that means it has to be replaced frequently.

Your original statement (was it yours? I've lost track) suggested to me that you (or whoever) were arguing that rotary valves had less wear potential because the bearing surface was smaller. It's just the opposite.

Rick "whose Miraphone valves and worn and need heavyish oil to run quietly (no, it's not the linkages), but they do not stick" Denney

[Wyvern]

If well maintain, rotors, do not have to wear. My old Haag (Swiss made) Eb dates from the first half of 20th century and there is no discernable slackness in its rotors which still run smoothly and quietly (never sticking!) using old clockwork springs.

Having only three valves, its use is limited, but it comes out each Christmas carol playing.

[Dan Schultz]

The point I'm trying to make here is that the elements involved in pistons and rotors are two TOTALLY different sets of dynamics.

.... The dynamics of lubrication are the same. The bearing surfaces float on a film of oil. If the oil is wiped or washed off, the valve will stick because of metal-to-metal friction..... It's easier for the small surfaces of a rotor bearing to lose the film, because the force is higher. We counteract that by using heavier oil that builds a stronger film. But they'll wear faster if you run them dry.

That's very true, Rick.... and I'm not looking for an argument here. However... the can hear the wheels of some of the poster/readers turning. I suspect that folks are trying to apply the same theory to the outside of a rotor and to the outside of a piston. The outside of the piston needs a film of oil to isolate it from the casing and allow it to move freely. The rotor only requires a film of oil on the bearing surfaces. Even if the rotor bearings run dry, the rotor will never actually contact the inside of the casing..... for a while, anyway.

Now... there IS a matter that applies to BOTH pistons and rotors... a film of something (oil, moisture, etc) IS necessary to maintain a air seal between the rotors/pistons and the casings.