Author Topic: Groove for graphited packing  (Read 2398 times)

Offline Cotton

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Groove for graphited packing
« on: February 02, 2021, 01:14:40 PM »
Hello

This is my first post apart from the introduction, so be gentle with me!

The 10v is coming along nicely.  Learning lots as I go, particularly in setting up methods.  I'm about to make the piston.  The book from Stuart suggests that 3mm square graphited yarn is preferable to the oil-retaining grooves of the Stuart plans.  I've decided to go this way and ordered some 3mm square graphited yarn.  Now the questions:

Does the use of the yarn affect the required clearance between the piston and the cylinder?  I have made the cylinder and intended to simply turn the piston to the closest sliding fit I can achieve.

How much compression of the graphited yarn should I aim for?  Or put another way, what width and depth should the groove be for 3mm square yarn?

I've looked through the forum and been unable to find the answers.  That's not to say they aren't there somewhere!

Thank you 

Regards

Ian


Offline Ramon Wilson

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Re: Groove for graphited packing
« Reply #1 on: February 02, 2021, 01:24:58 PM »
Ian - I have posted about using the similar PTFE impregnated material several times now and have used it in all engines built so far with very acceptable results. Graphited yarn would be no different in requirement I feel.

I make my pistons down on the bore size by 2-3 thou - no drag.. The groove for the packing is made a few thou wider than the nominal size of the packing and the depth a similar amount shallower. This gives a very good seal with minimal friction, if hardly any at all - ideal circumstances for a small engine and especially if you desire it to run slowly.

Do scarf the joint as opposed to a butt joint using a very sharp scalpel blade and ensure that any stray strands get nipped off so they don't wedge between piston and cylinder wall

Hope that's of use to you

Ramon (Tug)
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Cotton

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Re: Groove for graphited packing
« Reply #2 on: February 02, 2021, 02:17:17 PM »
It is, thank you.

Regards

Ian

Offline Charles Lamont

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Re: Groove for graphited packing
« Reply #3 on: February 02, 2021, 05:09:50 PM »
Within reason, graphited yarn can be worked to fit. Just make sure you have a fully packed groove and can squeeze the piston into the bore. 

Offline Cotton

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Re: Groove for graphited packing
« Reply #4 on: February 02, 2021, 06:14:25 PM »
Thanks Charles.  So are you suggesting slightly tight in both planes?

Regards

Ian

Offline Charles Lamont

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Re: Groove for graphited packing
« Reply #5 on: February 02, 2021, 09:43:50 PM »
Yes - it will soon bed down. And I think you have the right idea on piston fit.

Offline Ramon Wilson

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Re: Groove for graphited packing
« Reply #6 on: February 02, 2021, 11:26:52 PM »
  I have made the cylinder and intended to simply turn the piston to the closest sliding fit I can achieve.

Yes - it will soon bed down. And I think you have the right idea on piston fit.

If everything is perfectly concentric then I would agree to a degree but there are several tolerances that have to be met to give that situation. By easing the piston fit and allowing the packing to find it's 'level' eliminates one of those tolerances. Turning the piston on the piston rod is another and drilling and reaming the gland set in place in the bottom cover at the same time is another. By concentrating on the latter and easing the bore fit will give a very free running fit but with an exceptional seal.

By easing the width of the groove it allows the packing some expansion as it is being compressed to enter the bore.

This piston was for the Waller Table engine





The engine, made at least ten years ago sits on my sideboard and is turned over every day - just a habit - it is so free it feels as if there is no piston in but close the stop valve and the difference is immediately noticeable and yes, it ticks over very slowly.


Tug

« Last Edit: February 02, 2021, 11:30:00 PM by Ramon »
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline john mills

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Re: Groove for graphited packing
« Reply #7 on: February 03, 2021, 02:08:58 AM »
i have always packed soft packing in quite firmly to run on steam at good pressure  from when i was an apprentice engine reconditioned rear main seals of this type were packed quit tightly it is skill to pack it in tight enough for the shaft still to turn easily .in models is the same my loco 5" gauge gave lots of good service .
 i pack as tight as possible but still get the piston in the bore . 
                  John

Offline Jasonb

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Re: Groove for graphited packing
« Reply #8 on: February 03, 2021, 08:16:47 AM »
drilling and reaming the gland set in place in the bottom cover at the same time is another. By concentrating on the latter and easing the bore fit will give a very free running fit but with an exceptional seal.

For an engine like your 10V that approach may not be ideal as the gland screws in rather than using studs and nuts so if either of your threads are slightly off then you may get a tight spot when the gland is in certain positions. I would make them separate and tighten down the split die a little more than usual for the gland so it has a bit of float and can line itself up with the piston rod.

Offline derekwarner

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Re: Groove for graphited packing
« Reply #9 on: February 03, 2021, 09:46:58 AM »
Could I suggest to wait for your 3mm sq Graphited Yarn packing to arrive as you may well find it to be Imperial of the nominal 1/8" sq

This of course has absolutely no bearing in the functionality of the seal, however if it is the 1/8" section, you will need to allow for the increased width and height of the profile

The same levels of proportionality maintain, however it must be remembered these piston or gland seals are pressure energised by the medium, not by diametrical clearance and attempting to stuff too much cotton or flaxen material into the given cavity

Sufice to say, a graphited packing with the correct nominal cavity height will under pressure of the medium, be energization find a correct set on diameter 

My apprenticeship training 55+ years ago :old:, has led me still be up to date  :happyreader: with the technical aspects of piston or gland packings in these applications........the only additional recommendation is for the angular scarf joint [of any single element] to be in the parallel plane, with the top and  bottom of the piston seal faces

Derek
« Last Edit: February 03, 2021, 10:27:42 AM by derekwarner »
Derek L Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline Jasonb

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Re: Groove for graphited packing
« Reply #10 on: February 03, 2021, 10:16:13 AM »
Derek is a piece of packing in an engine running at 5psi or less likely to be energized much if at all as the compression into the groove when it was fitted may well be More than that?

I can imagine it would be different at 100psi plus on full size but some of my engines will run by just by blowing into them or before the needle even lifts off the stop on my compressor so running on almost no pressure.

Offline derekwarner

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Re: Groove for graphited packing
« Reply #11 on: February 03, 2021, 10:49:05 AM »
If I could explain Jason, if a graphited yarn packing of these styles were installed with a force fit manner into a bore, being of the lightly woven construction, the packing would dislpace itself lengthwise into the cavity until the interference between the woven matt of the seal surface with the bore such that further lengthwise displacement is not a function

If after the initial run up, the packing will attain a certain level of semi permanent set, however the set relaxes when the pressure is exhausted

An example where this does not hold true is with a liquid [fluid], where the gland seal or multiple bands may require a preload to ensure the seal remains fluid drip tight

In a similar manner, a piston packing with a light loading in the bore, will accept low pressures to expand & energise

I am reading this application as the Stuart 10V piston and gland, so suggest a working model would be running a say 20 to 60 PSI.....

Derek
 
« Last Edit: February 03, 2021, 11:27:26 AM by derekwarner »
Derek L Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline Cotton

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Re: Groove for graphited packing
« Reply #12 on: February 03, 2021, 11:35:46 AM »
Thank you all.

I will wait until the packing arrives before I make the piston (may have to spend a little time decorating!  Boo!)

I had intended finish machining the piston diameter on the piston rod but thanks for the reminder.

The gland screw is already made.  I'll see how things are once I've made and assembled the piston/cylinder/rod.  Soon make another gland if needed.  I finished the hole in the gland and cylinder base together so at least there is a chance of no binding there.

The piston itself is an easy enough item to make so I don't mind a little experimentation with diameter, widths and depths of slots etc.  At least now I know where to begin.

Derek you wrote ".... the only additional recommendation is for the angular scarf joint [of any single element] to be in the parallel plane, with the top and  bottom of the piston seal faces...."  To clarify, I understand this to mean that when I look at the packing joint in the groove (looking towards the centreline of the piston) I should see the taper of the joint on both end and not a line at right angles across the groove.  Am I correct?

Thank you again for your support.

Regards

Ian

Offline derekwarner

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Re: Groove for graphited packing
« Reply #13 on: February 03, 2021, 12:52:16 PM »
Yes Ian.......the scarf joint is intended such that pressure is acting on the top [or bottom] full face or annular surface of the seal, so that the increasing pressure forces and squeezes the actual joint surface faces together

In industrial hydraulic fluid systems, the disadvantage with a scarf joint in a rag & rubber multi ring V packing set is that it may weep fluid at zero system or atmospheric pressure, however are proven to seal to 350 Bar [~~5000PSI] dynamic applications

Having said this, split V packings are never 'usually' intentionally designed into such systems........it is only as an emergency where the failed one piece V packing set is removed and the contingency scarf cut V packing set installed

One point to remember with J. Walkers Type Cotton Graphitated Packings, is that you can carefully hammer and change the actual section profile from square to rectulangar .....within limits of course and that is without damaging the actual weave....it is after all only cotton or cotton/flaxen rope weave with high temperature graphite grease impregnated under pressure

Certain Brands now use an MoS2 [Molybdenum di Sulphide] blends.......the originals  :old: [100+ years ago] I believe were cotton rope impregnated with animal fats with Carbon Black + Graphite impregnated   

Derek
« Last Edit: February 03, 2021, 01:09:20 PM by derekwarner »
Derek L Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline Jasonb

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Re: Groove for graphited packing
« Reply #14 on: February 03, 2021, 01:22:46 PM »
I am reading this application as the Stuart 10V piston and gland, so suggest a working model would be running a say 20 to 60 PSI.....
Derek
 

Yes there are certainly difference sin what is needed between a display model and a working one but even so at the lighter pressures I wondered how much full size practice applies. Certainly in the cast of O ring seals the recommended amount of compression ( groove depth) even for working models upto or over 100psi is half or less than the makers recommended amounts where the desire for reduced friction is greater than the need to seal at far higher pressures than the model is ever likely to see.

 

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