|
Post by ettingtonliam on Mar 12, 2022 21:14:40 GMT
I'm putting O rings in Locomotion's pistons. 0.875" dia, 3/32 rings. Tubal Cain, quoting from the late Arnold Throp, 11.3, Table 3, suggests a groove depth between 0.093 and 0.098" for a sliding seal groove. What do you guys use?
|
|
jasonb
Elder Statesman
Posts: 1,209
|
Post by jasonb on Mar 13, 2022 7:24:08 GMT
That is my go to ref when I fit Viton rings and they work for me I always use the sizes under the "ME" column eg the 0.098 deep.
The squash is a lot less than commercial tables give so less friction but good enough to seal at our low pressures as a steam engine is unlikely to see 20bar.
|
|
|
Post by ettingtonliam on Mar 13, 2022 7:27:19 GMT
OK, thats what I'll do. Its only 60psi full boiler pressure, saturated steam, so just over 4 bar.
|
|
|
Post by steamer5 on Mar 13, 2022 8:29:41 GMT
Hi Richard, as usual theres an App for that!
Have a look at O-Ring Master, it has calcs piston, rod( both static & Dynamic) & sealing face. Either metric or imperial
Cheers Kerrin
|
|
|
Post by ettingtonliam on Mar 13, 2022 10:45:48 GMT
Too late Kerrin! I've done them to 0.098" depth in accordance with Tubal Cain. Anyway if it doesn't work, I can always just pack the grooves with graphite string. Its not as if its a loco thats going to get a lot if use. If I can it to work at all that is. There are a number of issues with the design, especially the valve gear and parallel motion, but I think I have them resolved. Ask me again in about 3 years.
|
|
SteveW
Elder Statesman
Posts: 1,396
|
Post by SteveW on Mar 19, 2022 16:02:37 GMT
Remember to polish your bores. Maybe controversial.
Interestingly I recall this one first appearing here in the early days of the original forum back in the 90s. Somewhere out there is a data sheet detailing the necessary grove sizes for a given O ring. What surprised everyone at the time was the need to have O ring roll in its grove. I guess for the same reasons I avoid motorways/dual carriage ways on my motorbike, I want to avoid my rear tyre getting a single flat facet. With single carriage ways the wear is spread around and ultimately removes all the moulding pips into one smooth curve.
|
|
|
Post by chris vine on Mar 19, 2022 16:11:52 GMT
Yes, the room to roll slightly seems to be important.
I guess it gives the ring two places to wear a flat, rather than just one. so an instant doubling of life.
The other part, I think, is that a wide groove allows the steam to get at the side and under the O Ring, so that the steam pressure pushes the O Ring out to seal on the cylinder wall.
A friend had a loco with cast iron cylinders which were not really smooth enough for O Rings. You could hear the steam leaking past to exhaust until you opened the regulator more when they would make a little pop and the leakage would stop.
Now it has iron piston rings which have lasted for years.
Chris.
|
|
|
Post by ettingtonliam on Mar 19, 2022 22:21:03 GMT
Remember to polish your bores. Maybe controversial. Interestingly I recall this one first appearing here in the early days of the original forum back in the 90s. Somewhere out there is a data sheet detailing the necessary grove sizes for a given O ring. What surprised everyone at the time was the need to have O ring roll in its grove. I guess for the same reasons I avoid motorways/dual carriage ways on my motorbike, I want to avoid my rear tyre getting a single flat facet. With single carriage ways the wear is spread around and ultimately removes all the moulding pips into one smooth curve. I took the groove data from Tubal Cain's book. Groove width is 0.125" for a 3/32" o ring. I'm not sure how polished the bores need to be, they are gunmetal, reamed 0.875", and feel pretty smooth. I wish you well with your recovery from the prostate op. When I was diagnosed about 6 years ago, after a discussion with my consultant, I chickened out of the op and went for radio therapy, 5 sessions a week for 7 weeks. Seems to have done the trick so far.
|
|
|
Post by Oily Rag on Mar 25, 2022 23:02:04 GMT
Yes, the room to roll slightly seems to be important. I guess it gives the ring two places to wear a flat, rather than just one. so an instant doubling of life. The other part, I think, is that a wide groove allows the steam to get at the side and under the O Ring, so that the steam pressure pushes the O Ring out to seal on the cylinder wall. A friend had a loco with cast iron cylinders which were not really smooth enough for O Rings. You could hear the steam leaking past to exhaust until you opened the regulator more when they would make a little pop and the leakage would stop. Now it has iron piston rings which have lasted for years. Chris. I wonder why Q rings are not more popular in ME circles with bronze cylinders and valves etc, for all the reasons you mention ? Change conservatism ? or they do not work as well ? I might find out as I have them in m 1/2" cylinders. IMG_7621 by Darrell McCulloch, on Flickr
|
|
jasonb
Elder Statesman
Posts: 1,209
|
Post by jasonb on Mar 26, 2022 7:08:01 GMT
Lack of availability.
I've only built one engine (internal combustion) where a quad ring was specified and gave up in the end trying to find one here in the UK.
7/8" were available from a few US sources but silly money to buy one and ship it back here. Engine runs OK with a Viton O ring and even with no ring fitted so I have not bothered to keep looking.
|
|
|
Post by Oily Rag on Mar 26, 2022 21:01:32 GMT
Lack of availability. I've only built one engine (internal combustion) where a quad ring was specified and gave up in the end trying to find one here in the UK. 7/8" were available from a few US sources but silly money to buy one and ship it back here. Engine runs OK with a Viton O ring and even with no ring fitted so I have not bothered to keep looking. Fair enough. I was fortunate, I went to my local bearing supplier and ordered them.
|
|
jasonb
Elder Statesman
Posts: 1,209
|
Post by jasonb on Mar 27, 2022 6:27:25 GMT
I think it was not helped by needing Viton as it was an IC engine, rubber are easier to come by and the more common 3/4" and 1" easier again than 7/8"
|
|
|
Post by ettingtonliam on Mar 27, 2022 8:08:12 GMT
What advantages do these Q rings have over normal O rings, which are easily available in a range of materials and a huge range of sizes?
|
|
rrmrd66
Part of the e-furniture
Posts: 339
|
Post by rrmrd66 on Mar 27, 2022 8:08:49 GMT
Good morning all Here is the only "O" ring chart you will ever need. It covers ID,OD and face sealing. It did for me for 30+ years working with Germany's two biggest seal manufacturers. www.fst.com/-/media/files/sales%20sheets/merkel%20o-rings.pdfPS Hydraulic seal manufacturers never like using O rings in dynamic applications. An O ring is a static seal. Happy sealing Malcolm
|
|
jasonb
Elder Statesman
Posts: 1,209
|
Post by jasonb on Mar 27, 2022 9:18:28 GMT
Good morning all Here is the only "O" ring chart you will ever need. Malcolm Well as we are talking about dynamic seals on low pressure pistons that chart for static seals may not be the only one we need
|
|
rrmrd66
Part of the e-furniture
Posts: 339
|
Post by rrmrd66 on Mar 27, 2022 18:01:43 GMT
Hello Jason Quite correct to take me to task over this. Have a look at page 559 of the attached. That covers dynamic applications www.powerparts.it/simrit_pdf/o-rings_static_seals.pdfOf course life is never that simple as most oil hydraulic sealing elastomers are not that suitable for steam service. Cheers Malcolm Love the traction engine btw.
|
|
jasonb
Elder Statesman
Posts: 1,209
|
Post by jasonb on Mar 28, 2022 6:19:56 GMT
That's better but as I said in my first reply commercial charts tend to be aimed at much higher pressures which compress the ring more and therefor create a lot of friction which is not wanted in a model, OK if you are working with the 6Mpa (900psi) that they use as the threshold for different hardness of ring. Typical sizes for groove depth from the Tubal Cain book are one third of those in your linked to chart eg 5% compression of ring not 12-15%
This is the same reason that Quad rings were specified on the IC engine I mentioned earlier as they are supposed to offer less friction than a standard O ring
|
|
|
Post by Deleted on Mar 28, 2022 7:00:33 GMT
I used the mobil app 'O-Ring Master' by GMORS...it's an excellent app in which you enter the dimensions and seal type and it will do the rest. It will also list suitable O- rings, their make and code.
Pete
|
|