valve for throttling

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Adam Binns
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valve for throttling

Post by Adam Binns » Tue Nov 21, 2017 12:54 am

good morning
I am having trouble tracking down a cheap valve (butterfly or gate) for a diffusion pump and was wondering if it is necessary to have one and if it is possible to machine one.
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ian_krase
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Re: valve for throttling

Post by ian_krase » Tue Nov 21, 2017 2:25 am

For small pumps/fusors including most pipe tee fusors one can potentially get away with a KF40 or KF50 angle valve. These are common and not too expensive on eBay though you need a manual one which are a bit rarer.

Yes, you may be able to machine one. You don't need to seal, just to throttle, so you can get away with a lot - big sheet metal butterflies for example. Magnetic or o-ring seal (maybe an ultratorr fitting) on the valve actuator.


However you still kind of need an isolation valve for your diffpump - this isn't totally needed but is strongly preferred.

Rex Allers
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Re: valve for throttling

Post by Rex Allers » Tue Nov 21, 2017 10:51 am

A gate valve is probably the best for this application but they are hard to find and tend to be expensive, as you have found out. Ian's suggestion of a large bellows valve, NW-40 or NW-50, seems like a reasonable solution to me.

There are some out there with mechanical knob actuators but, as Ian said, the most common and affordable ones are pneumatically actuated. If you tried to use them with the pneumatic actuator, I don't think you'd ever get the finesse desired to modulate the vacuum and internal pressure of D2 in a fusor.

I found myself a gate valve that I intend to use, but before that I was planning to use a bellows valve that I had found fairly cheaply. I had planned to modify the NW-50 pneumatically actuated valve to turn it into a knob actuated valve. I think it is possible without too much work, but I do have a metal lathe which could help. It would certainly be easier than trying to make a high vacuum valve from scratch.

First, to be clear, here's a part of a page from a MKS catalog showing the kind of valves I am talking about.

Bellows valves.jpg

The black cylinder on top has a big piston in it. To open the valve, compressed air pushes the piston up which pulls the bellows up and opens the vacuum valve near the bottom in the picture.

All the valves like this that I have seen have a hole at the top-center of the black piston cylinder and the piston shaft moves out through this hole as the piston opens. I think the main reason is so that sensing switches can be installed on top of the cylinder to sense if the valve is opened or closed.

I thought I could build something to pull on this shaft to open the valve. The actuator piston would still be there but I wouldn't use it. One thing that makes it easier is that all these shafts I have seen either have a central hole that is threaded or they are threaded externally or both.

I have a MKS valve like this with NW-50-sized ports. The actuator shaft is a plain 1/4" rod but it has a central hole that is threaded for an 8-32 screw. This hole is about .6" deep into the shaft. Here's a picture of the top of this piston cylinder with the shaft in the center.

NW50 actuator.jpg

Before taking this picture I put an 8-32 screw loosely into the threaded hole in the shaft. There are also two threaded holes near the opening that are also 8-32 and could be used to mount some part of a mechanism. I put two screws in these holes before the picture. I think these are intended for some kind of bracket to mount position sensing switches.

In the picture you can see that the hole in the top of the piston cylinder, that the shaft projects through, is very much larger than the shaft. The shaft is .25" dia. and I measure the hole as .440".

For this valve, I think the travel between closed and open would be about 1.25". (This is how far the shaft would need to be pulled to fully open the valve.)

The valve has a very heavy internal spring that holds the valve closed. This means a lot of force (pull on the shaft) needs to be applied to open the valve. I tried to measure this with a lever and a digital gauge. The force needed seems to be well over 100 lb., probably around 150 lb. Since the shaft is only 1/4" projecting through a large hole with this large force required, I think it is important to pull very axially. Much lateral force on this shaft could easily bend or distort it.

I haven't done it but my though on how I would first try is this: I would have a 3/8" rod about 2" long that is threaded externally. This would be drilled internally 1/4" diameter except near one end which would only have a smaller bore, just big enough to accept an 8-32 screw. This piece would be slipped over the piston shaft, down through the .440" hole and the smaller-hole end of the piece would be against the end of the piston shaft and screwed on with an 8-32 screw.

Another larger rod, maybe 1/2" dia., would be drilled internally and threaded to match the external thread of the 3/8" piece I just described. This rod would be 1.5 to 2" long. A knob could be attached to one end. This bigger rod is threaded on to the the other piece that was attached over the piston shaft. The end of this bigger piece would press against the top of the piston cylinder. As this is turned it would pull the shaft outward (up through the threaded center of this turning piece).

Or something like that. I'm just trying to suggest that I think it shouldn't be terribly hard to convert a pneumatic bellows valve to some kind of workable mechanical actuation.

----
One other thought if you want to use one of these bellows valves on your chamber. I think the port of the valve to attach to the chamber should be that axial one, not the radial one (the bottom port in the first picture above). This is for two reasons:

1) If you want to use the valve to hold the vacuum in the chamber between runs, this configuration will have the vacuum in the chamber helping to pull the valve closed.

2) To my guesstimation measurements, the volume in the valve around the bellows, open to the radial port, is much bigger than the volume in the short section on the axial port. No need to have extra volume within the valve attached as a virtual addition to the chamber volume.
Rex Allers

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Dennis P Brown
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Re: valve for throttling

Post by Dennis P Brown » Tue Nov 21, 2017 12:00 pm

I doubt one can machine their own butterfly valve - a good vacuum seal is rather important to get in order to achieve the required fine control of gas leak in vs.vacuum pump removal.

I like Rex's approach breaking down and redoing a pneumatic one into a manual (knob/screw) system and I think his approach is doable. I would enjoy seeing his progress on this and posting with photo's if he gets time to do this.

One certainly does not require a gate valve/butterfly system; the 40 or 50 mm bellows valve would work fine - especially with a smaller chamber like a four-way cross.

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Andrew Robinson
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Re: valve for throttling

Post by Andrew Robinson » Tue Nov 21, 2017 1:34 pm

I also have a manual screw type bellows valve in my for sale if you really want to go that route. viewtopic.php?f=20&t=11940
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Andrew Robinson
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Re: valve for throttling

Post by Andrew Robinson » Tue Nov 21, 2017 1:35 pm

Well... I did. Here it is! Older for sale page, but still available: viewtopic.php?f=20&t=11286
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Rex Allers
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Re: valve for throttling

Post by Rex Allers » Tue Nov 21, 2017 6:36 pm

As I mentioned, I found a gate valve that I plan to use, so I don't expect to try to modify this bellows valve anytime soon. It was something I had planned before I found a better option I could buy. I just thought I'd revisit some of the details and share the general idea in case it could be useful to someone else.

I was going to mention that Andrew's post with the valves was back in Feb. but he has already added the proper link. Those valves do seem to be pretty small for this application, though.
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Adam Binns
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Re: valve for throttling

Post by Adam Binns » Wed Nov 22, 2017 2:05 am

thanks for the quick replies
andrew sadly that valve is a bit small as I am using a 2.75-inch chamber but I found one here that is cheap-ish https://www.ebay.com/itm/MDC-Manual-Ang ... SwywRaA76o and if anyone is selling a diffusion or turbo pump to that size than I would be happy to pay for it
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ian_krase
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Re: valve for throttling

Post by ian_krase » Wed Nov 22, 2017 5:08 am

Yeah, that's a good valve to use. Though you will probably only have a Conflat on your chamber side, with the other side going straight to an adaptor. You may want to take it apart and clean all the O-rings, giving them just a thin sheen of vacuum grease.

It's on Make Offer. Most of the time, people selling vacuum stuff on Ebay will accept lowball offers. (Many of them don't bother testing anything and don't really know or care what they are selling.)

Jerry Biehler
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Re: valve for throttling

Post by Jerry Biehler » Mon Nov 27, 2017 8:52 am

Dennis P Brown wrote:
Tue Nov 21, 2017 12:00 pm
I doubt one can machine their own butterfly valve - a good vacuum seal is rather important to get in order to achieve the required fine control of gas leak in vs.vacuum pump removal.
Actually it would be pretty easy to machine one for throttling without isolation. It's usually just one, sometimes two o-rings sealing the shaft into the throttle body. And since it does not need to isolate the butterfly just needs to be a close fit. I have an ISO-100 one I can take a pic of and post. It has a motor and controller built in, just connect to a cap mano for feedback and you can set the pressure and the valve does the rest.

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