Sticky Throttle

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daves1459
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Sticky Throttle

Postby daves1459 » Thu Aug 03, 2017 8:09 pm

I have a flat slide valve throttle in the steam dome of my boiler. When the boiler is cold and so no steam pressure on the valve the throttle lever action is smooth with about the right amount resistance. When the boiler is up to pressure, 120 psi, the throttle lever gets much stiffer and exhibits a sort of stick/slip behavior when trying to move it is small increments. I was wondering if any of you readers have had a similar problem with a flat slide type throttle and how you improved the throttle lever action, if anything?

Some sort of pressure balancing would help. But, given the space in my steam dome I don see how.

I had thought some sort of lubrication would help. But, oil and boiling water don't mix. I have a product called "DriSlide" that contains naphthenic oil, molybdenum disulfide, and graphite that the manufacturer claims is good to 750 degrees F and is unaffected by water.

I have also thought about coatings. For example like the hard Teflon powder coatings applied to piston skirts that have molybdenum disulfide in them.

Any suggestions are welcome. I can deal with the stiff throttle lever, but, the stick/slip is a bit unnerving and makes slow speed control difficult.

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Marty_Knox
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Re: Sticky T, hrottle

Postby Marty_Knox » Thu Aug 03, 2017 9:09 pm

You didn't say what materials your throttle is made from. In a situation like this I would go with a stainless steel plug riding on bronze. The plug needs a little bit of 'float' so it can conform to the seat, then it needs to be lapped in. You need to use a non-embedding lapping compound like TimeSaver. You might be surprised how smooth a properly lapped slide valve throttle can be.

daves1459
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Re: Sticky Throttle

Postby daves1459 » Fri Aug 04, 2017 10:09 pm

I did forget to give the materials didn't I. Silly mistake. Both the valve plate and valve are bronze. I'm not sure of the alloy. They are a darkish yellow bronze so probably phosphor or tobin bronze. I lapped both pieces on a lapping plate. But, didn't lap them to each other. I have since surface ground both parts. Would that be adequate?

I was thinking of reducing the valve seal area. The current valve contact area is 1.1 sq. in.. I thought it could safely be reduced to around .65 sq. in..
The force to move the valve should go down directly proportional to the reduction in seal area. Do you have any idea what optimum seal area ratio to port size and minimum seal width around the port should be?

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Bill Shields
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Re: Sticky Throttle

Postby Bill Shields » Sat Aug 05, 2017 11:40 am

lapped bronze on bronze should be fine.

reduction in area is always a good idea as long as you get a seal (valve does not leak).

1/8" on the edges is generally enough, and that much on each end at full travel should be good.

some (much) of the stick / slip could also be from flex in the rods / mechanism controlling the throttle.

Did you think of going to larger diameter rods and / or stiffening up the joints?

Since we don't have a picture of what you are doing, it's a bit of pie in the sky...

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Fender
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Re: Sticky Throttle

Postby Fender » Sat Aug 05, 2017 2:01 pm

Tom Artzberger uses a similar valve on the RGS 20 4-6-0. I believe the valve is made from ptfe (teflon) sliding on a metal seat.
Dan Watson

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Re: Sticky Throttle

Postby BillF » Sat Aug 05, 2017 3:55 pm

Tom Artzberger uses a similar valve on the RGS 20 4-6-0. I believe the valve is made from ptfe (teflon) sliding on a metal seat.

I was going to mention this, and I can add a bit of evidence about how smooth it is: The first couple of times I test-fired my locomotive I found that the throttle would slowly creep open. The boiler pressure was working to eject the throttle rod. I had to tighten the rod packing gland to put enough drag on the rod to keep it in one position. Exactly the opposite problem to a sticky valve.
- Bill Frensley

daves1459
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Re: Sticky Throttle

Postby daves1459 » Sat Aug 05, 2017 11:10 pm

Spring in the actuation linkage is an excellent thought. I'll bench assemble the throttle and post a picture.

Hmmm, Teflon aye! Teflon is a plastic so tends to flow under load and it has a poor memory so once flowed stays distorted. At 100 psi and 350 degrees F wouldn't the Teflon be softer than at room temperature. Are Tom's and Bill's valve made of virgin Teflon or a Teflon with glass or other filler? Are the Teflon valves inserted into some sort of retainer that encapsulates the valve except for the sealing surface? A picture or design please?

I was thinking of making the valve out of Vespel SP21 which is also a Dupont product. It is a sintered polymer made much like powdered metal. It is very stiff, good for elevated temperatures, machines well, is used often for bearings as it has a lot of graphite in it, and I've got some. Anyone have any experience with any of the Vespel concoctions?

-Dave Smith

daves1459
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Re: Sticky Throttle

Postby daves1459 » Sun Aug 06, 2017 11:03 pm

Here are a few pictures of the throttle in question. As you can see there is a lot of surface area at the valve to seat interface. I can reduce that surface area by 40% and reduce the throttle lever effort from about 4.5 pounds to 2.3 pounds. But, that may not do much for the stick/slip problem. That makes me very curious about the design to use Teflon.
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Fender
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Re: Sticky Throttle

Postby Fender » Mon Aug 07, 2017 8:36 am

No experience with vespel, but it sounds like it would work.
Not sure from your photos how thick the existing metal valve is, but if you could machine a cavity into its face that would be deep enough to tightly contain a plastic valve insert, this would serve two purposes: to capture the valve, and keep it from deforming under pressure, and to reduce the area of the valve seat. The plastic would need to stand a little above the metal, so that only it would be in contact with the valve seat.
Appears that the valve slides vertically to open and close. What holds the valve against its seat, besides boiler pressure?
Dan Watson

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Fender
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Re: Sticky Throttle

Postby Fender » Mon Aug 07, 2017 9:58 am

According to some engineering tables I have, the static coefficient of friction between bronze and steel or cast iron is between 0.16 and 0.22; however, by my reading, this is for lubricated surfaces, so expect it would be higher in the (unlubricated) boiler This means that the force required to move the bronze would be between 16 to 22% of the force pressing it against the other surface. The table doesn't list bronze to bronze coefficient, but I assume it would be greater.
By contast, for PTFE it ranges from 0.05 to 0.20, depending mostly (I believe) on the smoothness of the metal surface. Lubrication is not an issue. So it would be important that the seat have a smooth finish for a PTFE valve.
Dan Watson

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Fender
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Re: Sticky Throttle

Postby Fender » Mon Aug 07, 2017 10:23 am

Another consideration: for most materials, the coefficient of static friction is considerably higher than that for dynamic friction. This is why you see the "stick-slip" phenomenon, because after the valve breaks loose, it slides more freely. With PTFE the static and dynamic coefficients are almost the same.
P.S. I have no connection to a company that sells PTFE! :wink:
Last edited by Fender on Mon Aug 07, 2017 10:26 am, edited 3 times in total.
Dan Watson

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Bill Shields
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Re: Sticky Throttle

Postby Bill Shields » Mon Aug 07, 2017 10:24 am

Looking at the linkage, I see some areas for possible 'stiffness' improvement

When the first rod PULLS through the angle of its connecting link on the right, it is going to tend to want to 'straighten' because the rod is not infinitely stiff. This will create some of the 'slip / stick' you are describing -> pull and straighten the linkage instead of pull and move the valve.

Also, there are a many pivots there, and a few .001" on each will add up to lost motion at the throttle handle.

It all adds up.

Reducing contact surface area will certainly help.

Reducing friction will help but getting the PTFE into place and getting it to stay there is annoying.

Imagine if this were a SLIDE VALVE at the cylinders....


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