What material are oilless seals made of?

[sid_galt]

What material are oilless seals made of?
Do they generate more friction than oiled seals?

 

[FredGarvin]

What kind of seal are you referring to? Lip seals, o-rings, gaskets...

 

[Q_Goest]

I assume you're referring to reciprocating or rotating (ie: dynamic) seals typically found in oil-less pumps and compressors. Is that right?

 

[sid_galt]

Yes, I'm talking about rotating seals, e.g. a vane compressor.

 

[Q_Goest]

Hey Sid, you ask a lot of questions about vane compressors. What's up with that? Are you working on a design or something?

I'm not sure what the blades of a vane compressor are made from, but they're probably NOT the same material as piston rings or rotary seals because they're under relatively high stress (bending loads). Piston rings and rotary seals are made from reinforced plastics such as carbon or glass filled Teflon and other plastics.

I'd suspect vanes in a vane compressor are a coated metal but I'm not sure. The problem with metals are they don't wear well when rubbing against each other. Lots of galling, nasty stuff, especially when un-lubricated. Lubed might be ok, and as I understand it, most vane compressors are oil lubricated.

 

[FredGarvin]

Yes, I'm talking about rotating seals, e.g. a vane compressor.

To get a bit more specific, what area of the compressor? I think the type used is going to depend if you're talking about sealing around a shaft or a vane tip, etc...

A lot of times, you will see labrynth seals used. They are eventually non contact. I say eventually because they start out life with a bit of an interference until they wear in. Then they are non contact. Theoretically, they last forever.

You can also have carbon seals which use a carbon element with a very hard runner. The seal is the mating face between the two. The runner is attached to the shaft and the carbon is static.

 

[sid_galt]

Hey Sid, you ask a lot of questions about vane compressors. What's up with that? Are you working on a design or something?

An engine which works on the vane compressor principle to be exact for my Intel competition. Sealing is of course again a problem as well as the excess oil consumed.

[quote
most vane compressors are oil lubricated.[/QUOTE]
Is the oil reextracted from the air after its compressed?

 

[sid_galt]

To get a bit more specific, what area of the compressor? I think the type used is going to depend if you're talking about sealing around a shaft or a vane tip, etc...

Vane tip.

You can also have carbon seals which use a carbon element with a very hard runner. The seal is the mating face between the two. The runner is attached to the shaft and the carbon is static.

What is a runner?

 

[Q_Goest]

Very cool, Sid! That's quite an adventurous project, not easy at all. Post some pics when you get it done!

I'd suggest you forget about comparing this to a compressor, compare it to a 2 stroke IC engine instead. Run it with oil in the gas and make the vanes the same material as piston rings. They can be simple iron or steel on cast iron cylinder. I've seen high performance rings that are hard chrome plated, but you probably don't want to go to that expense. I'd suggest looking to see what materials piston rings are made from and use that. Also what material the cylinder liner is made from and use that.

How are you going to seal the edges of the vane? Just a close fit might work, but that might be improved upon by making the vane segmented somehow. Also, the end of the rotating cylinder needs to be a very close fit. How could that be sealed better? Don't know - thoughts?

 

[FredGarvin]

I don't know if you have seen this, but take a look here:
http://www.cashflo.co.uk/Vane.html

Rotary vane compressors use either carbon vanes for low pressure oil-free duties, or asbestos/steel vanes for lubricated duties. Various asbestos (and similar performance asbestos free) compounds are widely used for low pressure applications, up to about 3 barg. Over and above this pressure, the vanes are steel or cast iron because of the substantial bending forces present when the machine is running.

In compressor service, a lubed machine will produce service air that has a certain amount of oil in it. Some applications can allow this oil content. Others that can not, must filter that oil. Usually a coelescing filter in line with a praticulate filter can get you above 99% oil free.

 

[FredGarvin]

What is a runner?

In a carbon seal arrangment, there are two components; the carbon seal itself and the runner. The carbon seal is usually the stationary object. The runner is attached to something that is moving, like a rotating shaft. The runner (moving) presses against the carbon element and the mating faces is where the seal takes place. Just like anything, they have their advantages and disadvantages.

 

[sid_galt]

I'd suggest you forget about comparing this to a compressor, compare it to a 2 stroke IC engine instead. Run it with oil in the gas and make the vanes the same material as piston rings. They can be simple iron or steel on cast iron cylinder. I've seen high performance rings that are hard chrome plated, but you probably don't want to go to that expense. I'd suggest looking to see what materials piston rings are made from and use that. Also what material the cylinder liner is made from and use that.

Unfortunately, only one month is left for my presentation in the national fair so I can't get the engine made on such short notice. But if selected, I'll probably make one for the finals.

Any suggestions on the presentation?

How are you going to seal the edges of the vane? Just a close fit might work, but that might be improved upon by making the vane segmented somehow.

Maybe making three segments with two segments on the sides with one end on the top shaped like an elbow?

Also, the end of the rotating cylinder needs to be a very close fit. How could that be sealed better? Don't know - thoughts?

I haven't given much thought to that. I am concentrating on the vane tip sealing.

 

[sid_galt]

In a carbon seal arrangment, there are two components; the carbon seal itself and the runner. The carbon seal is usually the stationary object. The runner is attached to something that is moving, like a rotating shaft. The runner (moving) presses against the carbon element and the mating faces is where the seal takes place. Just like anything, they have their advantages and disadvantages.

Does that mean that the seal has essentially a line contact with the housing?

 

[FredGarvin]

Does that mean that the seal has essentially a line contact with the housing?

I'm not quite following you on that one. The seal ends up being a surface. Take two cylinder ends of the same size and butt them up together. The seal would be the interface between the two ends with one cylinder rotating and the other stationary. I'll see if I can find a picture.

 

[FredGarvin]

Take a look here:

http://www.powermizer.com/gas_booster/shaft_seal_design.htm

http://www.azom.com/details.asp?ArticleID=113

 

[Q_Goest]

Any suggestions on the presentation?

What is the intent of the presentation? Is this website the "Intel Science Talent Search" you're entering?
http://www.sciserv.org/sts/
Whatever presentation you have to come up with there must be some guidelines. If they're on the web somewhere I'd be glad to take a look and offer suggestions. It would make it easier for others to make suggestions too.

The seals Fred is showing you are shaft seals. They'll be needed to seal a shaft from the environment. As they wear, one of the two parts moves to maintain contact between the two faces.

In comparison, the ends of the vanes that contact the ID of the circular housing are intended to seal a higher pressure cavity within the housing from a lower pressure cavity. Again, as they wear, the vane moves outward to maintain contact with the ID of the circular housing to close up any gap and prevent leakage.

Both of these seals are fairly straight forward in design and relativly simple. Sealing the ends of the vanes where they meet the ends of the cylinder housing is a bit more tricky. Wear of either contacting surface will open up a gap that allows leakage. A single piece vane can't make up for wear, so a segmented vane would have to be used, though it may not be needed if leakage is minimal. I don't think vane compressors worry about this leakage much, so they make them with a single vane. But the temperature of the gas inside an engine is much higher, as is the pressure, which means much more leakage per unit mass. I think the increased leakage would be the downfall of any practical vane type motor.

Even if you had segmented vanes to make up for wear around the edge of the vanes, I think the most difficult part is to seal between the rotating cylinder and the wall. If you imagine there's a shaft passing through the wall and is then attached to a larger diameter cylinder into which the vanes are mounted, that larger cylinder is sealed to the ID of the housing by the vanes. But the ends of that larger cylinder are not sealed anywhere. Gas on one side of the compressor can move over to the other side by skirting around the end of the cylinder. If the gas can't get past the ends of the vane, it can get around or under the vane where the cylinder isn't touching the wall. You might use a face/shaft seal type as Fred is showing at that area, but realize it won't be a perfect seal.

Ok, forget seals for a minute, how are you proposing to inject fuel/air and exhaust it? Is this all going to be done with a single vane compressor/motor or two separate ones? I suppose you could use two separate ones, one that acts as a compressor and one that acts as a turbine to drive the compressor and take power off just like a conventional turbine engine. Doing the compression, expansion, evacuating spent gasses, and putting fresh fuel/air back into the chamber might be difficult with a single vane machine.

Have you considered a rotary machine like this one?
http://www.affordair.com.au/images/whatisaircon/rotary.gif

I think the advantage of this one would be the highly reduced wear of the vane and easier to seal at the ends of the rotating cylinder. Just a thought.

 

[FredGarvin]

Carbon seals are indeed shaft seals. I wouldn't hink they could be used for the sealing of the vanes. I was just completing the explanation of them since Sid asked what a runner was.

Not being terribly familiar with rotary vane compressors, I should ask this: Is there any reason why the vanes can't stay in contact with the wall via. Q's suggestion of a segmented vane? Also, what kind of pressure ratio are we talking here?