Help sourcing a brass replacement material urgently required...

[JonGD]

Hi guys,


I am are looking for a material for use in the semi-conductor industry i.e. Cu free.

The material is to be used to replace the brass valve-seat used in one of our conventional pneumatic valves. At the moment we use a brass half ball-bearing and a brass seat, coining (plastically deforming) the valve-seat to provide a tight seal against the half-ball.

We have tried using stainless steel for both components, but the valve-seat proved too hard to give a reliable seal and we have tried stainless steel half-ball against a delrin valve-seat, but the delrin will not plastically deform and give a reliable seal.

We believe that Aluminium will probably be too soft, as the units will be expected to have a reliability level of 100 Million MCBF although we may be wrong and there may be a suitable Cu free alloy out there.

Also would you reccomend making both parts from the same material or sticking with stainless steel for the half-ball?

In summary i am looking for any suggestions for a material that can be used in the semi-conductor industry that will be soft enough to easily plastically deform and provide a good seal but hard enough to have a low-wear rate and last for 100M cycles of impact? Answers on a postcard please...


Cheers guys,


Jon

 

[israelkk]

As I understand you are looking to modify a "conventional pneumatic valve" into a clean valve that will suit for semi-conductor industry. Did you consider using off the shelf valve designed for the semi-conductor industry such as a valve from Swagelok

http://www.swagelok.com/

or Ham-let

http://www.ham-let.com/

?

 

[JonGD]

Hi Israelkk,


I work for a pneumatics instrumentation company, we design and manufacture precision proportional pneumatic control valves. To be honest it probably wouldn't go down too well to use a competitor's valve!! The valves required are precision valves (down to mbar control) and the company we are supplying to are happy with the performance of our valves, but they need to be adapted to remove any traces of brass.

As we have limited experience within the semi-conductor industry i am looking for some hints or reccomendations with respect to material selection, to avoid the trawling through thousands of material spec sheets to find a suitable substitute.


Kind regards


Jon

 

[diamondjim]

Just curious if Viton instead of
Delrin will work in this application.

 

[israelkk]

I would look into Swagelok

http://www.swagelok.com/

and/or Ham-let

http://www.ham-let.com/

catalogs to see the materials they use.

Are you trying to use a metal to metal seal?
Is this a manual valve?
You say "coining (plastically deforming) the valve-seat to provide a tight seal against the half-ball" do you mean that the half ball is pressed against the seat by force to create the sealing?
Is the ball too experience plastic deformation?
Are you sealing against sharp edge or against a cone surface?
Does it seal gas or liquid?

 

[JonGD]

Hi Israelkk,


We normally use a metal to metal seal using a brass half-ball against a brass valve-seat, and the "coining" is performed by pressing a hardened ball bearing (of the same diameter as the half-ball) into the valve-seat, deforming the sharp edge into a radius that should then seal against the half-ball. We were hoping that the softer plastic material used in this application would negate the need for this step but it has not provided a sufficient seal long-term. The medium we are working with is clean (5um filtered), dry, compressed air. The valve is an electronically controlled valve, the sealing force being supplied by air-pressure on a diaphragm or by a spring.


Regards


Jon

 

[israelkk]

Is this a leak tight (zero leak) seal?
How much leakage if any you allow?
From my experience, I find it very difficult (or even impossible) to seal for zero leak using metal to metal seal unless the sealing force is quite high resulting in a large solenoid.

Secondly, after "coining" how do you assure that the actual ball in the valve will always seat exactly at the "pre-coining ring" with even a slight offset?

Can you provide what is the seat diameter and how much the ball move to fully open the valve?

What is the working pressure?

 

[BillPSU]

You may have to compromise but I would think either teflon or some urethane would be candidates. Getting the the zero leak will have be to gotten by reducing the durometer unitl you get what you want. The wear resistance would have to determined and maybe a glass filler will meet your needs.

 

[JonGD]

Comparing numerous mechanical properties (especially hardness!) for metals, Titanium Grade 2 seems a reasonable match to Brass, so samples are on the way. Any reasons why any of you would feel Ti is not a suitable replacement?

 

[GregLocock]

Galling

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JonGD]

Hi Greg,

I'm aware of machining problems associated with Ti, but am going to check and see that our machining sub-contractor thinks he can handle it.

One of the suppliers that I have contacted (Nigel @ Titanium International, Birmingham, England) has been brilliant and is sending us a sample quantity for turning. He has also offered to give advice to our sub-contractor if necessary, to stop them jacking the price up too high or cocking the machining of the sample components up!

From what i have read that advice will mainly be: ensure the cutting tools are sharp, the speeds are low and the coolant plentiful!

Our machining sub-contractor (Geoff @ Cliffe Hill, Wakefield, England) is a good guy and i'm sure he'll be able to handle it - adds another string to his bow as well if he can state that he has Ti experience!

 

[unclesyd]

I think GregLocock's caution was concern about the potential, very high, of galling while operating. Titanium has one of the highest potentials to gall, especially in a self-mated couple.

 

[JonGD]

Right, sorry - i thought it meant galling with the tooling during manufacturing!

I think that we are going to go continue using the stainless steel half-ball to reduce the chance of occurence. Does this sound like a feasible plan, or is there any other info I should be aware of? The prospect of post machining coating has alo been mooted, but i'm concerned that the coating will be damaged during coining.

 

[unclesyd]

Titanium likes to gall with anything that touches it.
Here are several possibilities that you could give a look at. They could be coupled or self-mated.
Like Nitronic 60 self-mated is very good. Waukesha 88 is good with most anything. The Stellites are very common. It will take a little comparing to get the cost vs benefits down pat.

Talonite
Waukesha 88
Stellite 6B or 6K
Nitronic 60
Gall Tough

 

[btrueblood]

Agree with diamondjim - try a teflon (PTFE) seat, it will plastically deform quite nicely.

 

[JimCasey]

If you can't use red metal, You have not said why TFE is nit acceptable. It's good to around 500F, and if you are clever with the seating design it can handle quite high pressures. If you just make a TFE orifice I wouldn't trust it much past 100 psi. Different aluminum alloys might be worthy of consideration, selected for hardness similar to brass.

You might think about a redesign, too. Instead of a half-ball, think of using a 30-degree cone. Face the seat with a very narrow 29-degree cone, so there is essentially line contact and the specific load will be very high, ensuring deformation of the eating surface.

 

[nigelt]

I have only one comment about TFE, and that's it's high creep rate (when loaded), so it will quickly deform away as the valve repeatedly cycles - it is particularly poor when hammered!
At this stage, as the focus has been on finding a material to suit the configuration of the valve design, do you need to step back and ask if there's a more suitable valve layout for the task?

"The ideal client is one possessed of great good sense and perfect judgement;
that is to say, one who agrees entirely with the designer at all times and in every respect."
Fenwick Williams, Naval Architect.