graphite yarn no thanks o-rings yes please

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graphite yarn no thanks o-rings yes please

This topic has 21 replies, 13 voices, and was last updated 19 September 2013 at 13:37 by Ian S C.

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3 February 2012 at 20:14 #83760
michael burton 1
Participant
@michaelburton1
as some off you will know i asked about the pistons being packed with graphite yarn and to be honest im not sure if i have it right as per one off the reply your right it is a black art, so i have orderd some new gunmetal and some o-rings could one of you helpful people explain the method for the fitting o-rings and any tips or guidance

thank you all your all so helpfull many thanks mike
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3 February 2012 at 20:14 #5918
michael burton 1
Participant
@michaelburton1
3 February 2012 at 21:53 #83766
Wolfie
Participant
@wolfie
Watching this with interest, I went for O rings straightaway but haven’t got round to that part yet.
3 February 2012 at 22:01 #83769
Jon
Participant
@jon
Not sure on what your trying to do.

There are guidelines and tolerances for the machining part. I would however ignore they wont last as long if acting as a moving part.

The idea of an o ring is to seal something off, that could be on a cylinder bore and a piston. A certain amount of compression is needed, no compression =no seal.

Books will state clearance in certain areas, meaning a degree of force is needed before the o ring would seal.

Easy fool proof method is to machine the groove, fit the o ring and do a trial and error fit.

If too tight, take a tad more off.

Often better to expand upon the bore of an o ring rather than have clearance.
3 February 2012 at 22:50 #83780
michael burton 1
Participant
@michaelburton1
i was thinking the trial and error method il machine the groves till theres a ittle compresson i guess ike you would have if you coverd one end of a outlet on a air ram?

wiv the graphite yarn pistons that are in the model at the mo when a steam port is coverd you get a restiance and if you slightly leave the port uncovered a smidge u get a nice sucking blowing sound mayby this is right already hmmmm whats your opinions on this?

sorry if this sounds stupid but in terms of loco building im a complete newbie iv built osscilaters an open crank 2 stroke demo model to my own crankcase deign and other parts for other hobbies and a battery loco built out of a 3 1/2 in gauge tich chasiss but this is my first proper venture down the steam avenue so am learning as i go on so all help is greatffuy received
4 February 2012 at 00:17 #83784
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hello again ,

There are published dimensions for O ring grooves but these are intended for mainly hydraulic applications with high pressures . Its best as suggested above to do a trial groove and ‘feel’ the fit . If you haven’t done it before you might like to have a fist try on a trial piston and then copy groove onto working pistons .

Don’t know size of your piston but for say 1 inch diameter ten thou nip would be excessive , three to five thou about right and anything less than three thou too little .

‘Nip’ is the amount that the O ring is squashed between piston and cylinder when assembled .

A small piston correctly fitted with O ring should move much more easily than your graphited yarn packed one .

The actual shape of the groove is basically rectangular with depth slightly less than the O ring section and width slightly greater . Don’t ignore the slightly greater – its very important for the O ring to be able to roll slightly as it moves back and forth .

At the outer corners of the groove put just a bare rounding off , definately not a big chamfer , to avoid cutting the O ring during assembly . Aso ensure that there are no sharp edges in the cylinder for same reason .

Aways avoid moving a dry O ring through the cylinder – apply just a trace of any oil during first assembly .

Ask any questions you like .

MW
4 February 2012 at 07:38 #83789
JasonB
Moderator
@jasonb
Get a copy of “Model Engineers Handbook” the tables in that give sizes specifically for our use on pistons as well as the usual type seals.

I assume your cylinder is lapped otherwise you will soon ware the o ring away.

J
4 February 2012 at 10:04 #83792
Russell Eberhardt
Participant
@russelleberhardt48058
Posted by JasonB on 04/02/2012 07:38:04:

Get a copy of “Model Engineers Handbook” the tables in that give sizes specifically for our use on pistons as well as the usual type seals.

I agree. It’s a very useful book by “Tubal Caine” and full of indispensable info.

Regarding the “O” rings, There shouldn’t be any “nip” between the bottom of the groove in the piston and the bore as suggested by some. This will just give excessive wear.

The bore should be exactly the nominal OD of the ring. This will give slight compression of the ring. There should be less than about 5 thou diametrical clearance between the piston and bore to prevent the ring from extruding between the bore and piston. The depth of the groove should be sufficient to give a slight clearance to the ring, just a few thou deeper than the ring section. The groove width should be about 10 thou greater than the ring section to allow movement.

The seal isn’t produced by pinching between bore and piston. The steam pressure pushes the ring against the side of the groove to obtain a seal, hence the necessity for the groove to have some side clearance.

However, don’t believe me, get Tubal Cain’s book.

Russell.
4 February 2012 at 10:35 #83799
Terry Lane
Participant
@terrylane
… and if you think graphite yarn is a black art, just wait til to try o rings.
4 February 2012 at 19:25 #83863
Another JohnS
Participant
@anotherjohns
When I built my little Tich…

1) The valve and piston spindles had “O” rings on them. I made a “D” bit and cut them out, with a little covers to keep the rings in – flat bits of brass, with screws into the covers. (i.e., not the usual “screw in” graphite yarn packing. It was easy.

2) When I made the pistons, made the rings too tight (so I thought), so I put graphite yarn on. A few years later, my little rich had trouble on some of the steeper grades, so I took her apart; found out that one piston was sans packing, and the other almost clear! I dug out the “O” rings, put them on, put her back together, and the performance was great.

As others have said, you want a bit of “drag”, but not too much; and you want the groove width about 1.5x the ring diameter, so it has a bit of slide and expansion.

“O” rings are great – I’d not bother with the packing, but they DO require a bit of forethought.

Another JohnS.
4 February 2012 at 22:13 #83880
michael burton 1
Participant
@michaelburton1
o rings in slight drag a hiss hiss sound when moving along the floor and rolls nicley so mayby i cracked it, i did test and trim approch as the previous builder of the model machined so many parts at differnt sizes to each other etc. this was the only way to ensure that they both would do the job

many thany thanks for the replys
5 February 2012 at 10:32 #83914
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Michael B ,

Don’t worry about making things to fit other things – its common practice in model engineering and certainly not unknown in industry .

Just for interest :

(1) O rings don’t work at all unless they are touching both bottom of groove and cylinder bore with slight interference as assembled . Purpose of the nip is to ensure that ths condition exists at first assembly and after some wear . The nip is just basically a biased assembly tolerance .

(2) Ordinary steam pressures are too low to distort the O ring enough to make a seal by expanding O ring into the bore .

(3) At modest pressures an O ring is just a sort of compliant piston ring .

(4) Rolling action is desirable (hence wide groove) to allow the O ring to unstick easily when piston changes direction or after a period of non working . Without this design feature there is an instant of high friction at each change which can be a problem in control valves etc and also with larger O rings there is risk of tearing the surface of the ring .

MW

Edited By MICHAEL WILLIAMS on 05/02/2012 10:55:25
5 February 2012 at 12:19 #83924
Mark P.
Participant
@markp
Wolfie,don’t know if this is any good to you but have a look at this http://www.marcorubber.com/sizingchart.htm might be of some use.

Regards Mark P.
17 September 2013 at 17:41 #130000
richard orr
Participant
@richardorr31043
Have read that it is ill advised to use graphite rope packing for piston rings when used in a cast iron cylinder. Am wondering if this is true in all applications across the board or weather it's use is possible in a large , low pressure , low r.p.m. steam cylinder application where the cast iron cylinder could be highly plished?
17 September 2013 at 21:32 #130019
Sub Mandrel
Participant
@submandrel
Hi Richard,

Graphited packing worked well on small engines (bigger ones have almost always used metal rings) for nearly 200 years. then decent temperature-stable o-rings were invented. thou o-rings generally seem to work better, there is no reason why graphited yarn won't work but beware excessive piston clearances that could blow it out in no time.

Neil
18 September 2013 at 03:30 #130030
richard orr
Participant
@richardorr31043
The cylinder I am using to build up a grasshopper steam engine is 12" bore dia. Am wanting to get as good a seal as possible in order to maxamise condensing vacume. O rings seem prefferable but would they hold up in a cast iron application? Will be steaming with graphite introduced into the steam line.Also, I am thinking graphite packing would tend to burnish cylinder where as am thinking o rings might score cylinder if particles in steam are taken up.

Edited By richard orr on 18/09/2013 03:37:05
18 September 2013 at 09:57 #130041
Sub Mandrel
Participant
@submandrel
Hi Richard,

As long as you aren't using superheat (unlikely for a beam engine!) then o-rings ought to last. Yes graphited yarn is said to improve the finish, but I can't speak on o-rings causing scoring.

Neil
18 September 2013 at 10:35 #130045
OuBallie
Participant
@ouballie
On a slightly different tack, but still about sealing cylinders:

Wasn't there an article in ME moons ago about using PTFE rings to virtually eliminate friction, but backed by 'O' rings to provide the needed pressure against the cylinder walls?

Didn't René Etter of RSME, experiment with PTFE and found that it creeped if not suitably contained.

If memory serves, he did overcome the tendency of that material to creep by containing it, only allowing a small amount to 'show', but I cannot remember if he was using it for rings or 'D' valves.

Geoff – Bad day today so no workshop.
18 September 2013 at 10:44 #130046
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Richard ,

If your engine is , as you imply , using vacuum produced by the condensing steam as the main power producer then having minimal cylinder / piston friction will be more important than having a perfect seal .

In any case lots of water inevitably present will produce an excellent seal almost on its own .

Best suggestion therefore is graphited yarn – in square section – very lightly tamped in .

Old engines using vacuum for power stroke usually had rope and tallow seals topped of with loose water .

Some even had leather flap seals – basically big sink plungers .

Is the 12 inch a misprint for 1.2 inch or are you really building a quite big engine . If its big then these detail considerations about sealing matter less .

Tell us more about your engine and we can give better information .

Regards ,

MikeW
18 September 2013 at 12:06 #130056
Ian S C
Participant
@iansc
Piston rings made from sheet PTFE would give good low friction, these type of rings are used in car shock absorbers, and I'v pulled a good few of those to bits, and they never seem to be worn out. When I rebuilt the Stuart S 9 earlier in the year I thought of using teflon, but decided that as I had some cast iron, I used that. Ian S C
18 September 2013 at 14:33 #130060
richard orr
Participant
@richardorr31043
The engine is indeed a 12" bore. double acting steam cylinder. ( Quite a story to my finding it ) It is going into a large sidewheel steamboat that I have been building over the past 14 years. The primary power will come from pressure rather then vacume. However, with a cylinder of that diameter, there is a ot of power to be gained from vacume also

I cut off the steam box and will use the D valve port face to make a type of poppet valve for the exaust that will also serve as a water slug relief port . Intake will be through bored out water drain valves. Intake pressure will be 30 p.s.i.g. expanded down to 10 p.s.i.g. R.p.m. will be from 80 to 120.
19 September 2013 at 13:37 #130138
Ian S C
Participant
@iansc
I think you could make a ring from PTFE(maybe bronze impregnated), a strip of sheetthe thickness of the required ring, and cut to the width, the joint would be best if it were stepped. For even lower friction, carbon impregnated PTFE is available. Ian S C
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