graphite Packing ?

[ozzie46]

How do you install graphite packing on steam engine pistons? My Simplex Loco pistons have 2 grooves, 1/8th wide by 3/16 in deep for packing to get the pistons steam tight but I know nothing about how to go about it.


For instance, how much friction should the piston have on initial insatllation? I assume they would free up some as you work the piston in the cyl.

Can you use a hose clamp (aka jubilee clip?) to squeeze the graphite rope tight enough to get the piston in the bore?

Will the graphite expand some once its in the bore?

I Keep reading about installing packing in steam engine articles but can find nothing on how to do it

Any help will be greatly appreciated.

Ron

 

[arnoldb]

A very good question Ron, and one I'm also curious about as to how it's done.

I know how to go about stuffing cylinder and valve rod pack nuts, but the actual piston packing has me in the dark as well.

Kind regards, Arnold

 

[Ramon]

Hi Ron, Arnold,

Making a sleeve with a tapered portion leading to a parallel section that fits the piston with a slide fit ie just slightly less than the cylinder bore and with a fine surface finish inside will help compress the graphited yarn into the groove. The sleeve should be capable of sitting exactly over the cylinder bore so that once compressed the piston continues into the bore. Judging the exact amount of yarn is difficult but its best to begin with too much - won't compress enough - then reducing the length gradually until it does. For stationary engines I prefer to use PTFE impregnated yarn which is square in section and is cut at an overlapping angle. This is relatively easy to fit and gives a good seal with very low friction. However I have not used this in a 'working' engine such as a loco so have no knowledge as to whether it would stand up to the use a loco would put upon it.

Hope this may help some
Regards - Ramon

 

[SBWHART]

As Ramon said start off more than required then cut back, on my engines I use the graphite yarn, I find that thin stuff is better than the thick, build up two or three turns, I stuff it in the gap using a wooden coffee stirrer and use the same implement to guide it into the bore, aim to fill the gap when it compressed by the cylinder, but be carfull not to make things too tight, if you've only got thick stuff you can unravel it to get thinner strandswide, but I also have never used it on a working engine like a loco.

Stew

 

[ozzie46]

Arnold, Thanks for looking in.

Ramon, Stew;

Thanks for the your comments. I still dont know what would be to stiff of a fit. Will the packing free up some after running it up and down the bore a few times to stabilize it or is it a case of once its in thats as free as it gets?

My piston to bore fit is very close, it will create a slight vacuum with a dry fit and a very good vacuum with some oil added. It will even bounce some.

Ron

 

[lordedmond]

that fit is a wee bit tight for a steam engine , think about when it hot . with a fit that you would not need packing

the packing should just fill the grove, when running with the lubricator on the steam oil will provide the seal , just a nice push should be ok , about like the fit you have now, I do know a lot of loco's with bronze cylinders that use O rings ( silicon ) in place of the packing and they should just rub.

My self I prefer cast iron for the cylinders and proper CI rings , but that me

Stuart

 

[ozzie46]

Stuart, Thanks for the info.

I forgot to mention that my Cyls are cast iron and the pistons are cast iron also. Made from the same chunk so I figured ,(maybe wrongly), that the rate of expansion when at working temp would be the same and the fit would stay the same.
Am I wrong on this?

Ron

 

[lordedmond]

if you have used CI then you must use CI rings , soft packing ( graphite ) will not work vey well. I use a 5 thou diff in sizing on a 1 5/8 cylinder on the loco whose boiler is my avatar


Its a BR Class 4 tank 2 6 4 about 48 inches long and it is a long job as I build true scale ( rivet counter ) all the 12 inches to the foot one has so does my 1 1/16 scale one ( 5 inch G )


FWIW i have a black 5 , Dicot 0 4 2 , BR Class 2 tender and the current build all in 5 inch G


Stuart

 

[Rayanth]

Stuart, Thanks for the info.

I forgot to mention that my Cyls are cast iron and the pistons are cast iron also. Made from the same chunk so I figured ,(maybe wrongly), that the rate of expansion when at working temp would be the same and the fit would stay the same.
Am I wrong on this?

Ron

Ron, you are correct in that the expansion ratio should be pretty much the same. However I think you're picturing it wrong in your head.

The *metal* in both the cylinder and the piston would expand. This means the actual bore of the cylinder *shrinks* slightly, while the piston itself *enlarges* slightly, causing the overall clearance between them to go down. If the clearance was insufficient to begin with, then you'll have rubbing/binding/bad-stuff. Having the piston a bit smaller, with flexible compression rings etc (at least in IC, I'm not ultimately familiar with steam) solves this problem.

- Ryan

 

[mklotz]

This means the actual bore of the cylinder *shrinks* slightly...

Want to try that one again?

Remember, they heat locomotive tires in order to shrink fit them onto the wheels.

 

[Rayanth]

... there's a possibility that i'm just tired and can't think straight myself. but if metal expands when it heats, it should mean that holes shrink. Maybe i'm just being stupid again :'(

- Ryan

 

[mklotz]

From a physics viewpoint, when something is heated the individual atoms move farther away from each other. [OK, it's a bit more complicated than that but I don't think we want to get into any quantum effects.]

Think of all the atoms on the inner edge of the hole. Now imagine them moving away from each other. You should be able to see why the diameter of the hole enlarges.

Incidentally, it's not uncommon to run into this as a test question on introductory physics quizzes.

 

[Lew Hartswick]

Incidentally, it's not uncommon to run into this as a test question on introductory physics quizzes.

Wasn't on any quizzes but it was one of the first demonstrations given in the Physics
235 (I think that was the number, "Mechanics Heat and Sound") back in 1954 or 5
at PSU.
...lew...

 

[Rayanth]

good to know, as i'm starting into a physics major.

At work, we often use bushings as drill guides for pilots (#30 / .1285 dia) -- if we go too long without putting lubricant on the drill bit to cool it down, it will bind in the bushing as it swells. I guess I always assumed this was a combination of both items getting hot and swelling.

In retrospect, the bushings are usually made of steel, while the body is made of alum... this starts to explaining a more complicated situation in which the bushing starts to spin.

The alum's hole is getting larger, loosening the bushing, and the drill bit is 'grabbing' the bushing as the bit expands, and the noise is quite an awful screech...

Thanks for the correction, I am enlightened.

- Ryan

 

[bezalel2000]

Parden my deviation Ozzie

Want to try that one again?

Remember, they heat locomotive tires in order to shrink fit them onto the wheels.

I'm glad you straightened me out on that one Marv, my mind was going nuts trying to figure that out on my own. :big:
Steam is still a black art looking from my place ???

Best I could come up with was
- piston gets hot & expands, with no where to efficiently dissipate the heat it stays expanded.
- cylinder gets hot & expands, but not quite as much because it is surrounded by cool air.
- engine seizes?

but that theory fell apart when I measure the average surface area of the inside of the cylinder exposed to the heat of the steam is greater than the surface area of the head of the piston.

I guess cooling fins on steam engine might be a bad idea? :hDe:



Bez

 

[arnoldb]

Ron / Stew / Stuart, thanks for your original responses

I guess cooling fins on steam engine might be a bad idea?

;D You got it Bez. But not so much because of the cylinder and/or piston expansion, but more because of steam efficiency. A cool engine running on steam is less efficient (or uses more steam) as it cannot use the full expansion effect of the steam as the steam loses a lot of it's stored energy because of cooling down when it enters cold areas such as steam pipes or a cold cylinder. This is why steam supply pipes and most actual steam engines uses lagging.

Kind regards, Arnold

 

[ozzie46]

Marv, Lew Ryan and Bez,
Thanks for your input. Very enlightening.

I see McMacster Carr has 1/8 square packing so I guess I'll try some of that.

I guess there is no hard and fast rule as to the size of the groove as compared to the packing material then. Some things are still just cut and try.

Thanks again.

Ron

 

[mklotz]

If you're still having a problem understanding how the hole in the washer gets bigger with heating, here's a demonstration that may help you to visualize what's going on.

Astronomers have discovered that, whichever direction we look into space, the other stars and galaxies are moving away from us. That's hard to understand for some folks so they use this demonstration.

Take an ordinary balloon and dapple it all over with spots using a Sharpie or crayon. Now begin inflating the balloon. While you do so, pick one of the spots (doesn't matter which) as your reference and mentally estimate the distance from your reference spot to the other spots. Regardless of which other spot you choose, that distance increases as the balloon is inflated. If we regard each of the spots as a galaxy and the reference spot as our own, it's obvious that everything is moving away from us.

[Aside: The clever reader will note the similarity to an expanding "shell" of debris from an explosion. So did the astronomers and thus was born the idea of the Big Bang.]

Using the same spotted balloon, let's now draw two rings around it to form a "belt" around the balloon's equator. We'll imagine that the belt is the locomotive tire and the spots are individual atoms in the tire. Inflating the balloon now corresponds to heating the tire; it forces all the atoms/spots to move away from each other. Of course, when this happens, the inside diameter of the tire increases in size.

 

[doubletop]

if you have used CI then you must use CI rings , soft packing ( graphite ) will not work vey well. I use a 5 thou diff in sizing on a 1 5/8 cylinder on the loco whose boiler is my avatar

Ron

If you want CI rings for your Simplex and aren't into making them Reeves in the UK do them (1.5" x 1/8"). They aren't cheap though.

http://shop.ajreeves.com/cast-iron-piston-ring-1-12--x-18-1-6839-p.asp

or the packing you were looking for

http://shop.ajreeves.com/graphite-yarn-18-18-6760-p.asp

Pete

 

[ozzie46]

Thanks Pete,

I found some packing locally at a hydraulic cyl. repair shop.

The piston was hard to move when I installed it, then I put some oil in the cyl and it moved a lot easier then. I'm hoping it will free up even more when I can run it on air, which I'm hoping to be able to do in the next week or so.

Ron