Piston rods

[marinesteam]

Just finished the piston rods for the York compound and thought I'd share some pics.

The material called for in the prints is Monel but due to the difficulty machining Monel and the expense I decided to substitute 1045 CPO rod. The CPO designates chrome plated only. This is the rod used in hydraulic cylinder service but supplied unhardened. I was expecting the chrome to make for difficult machining. I could tell that there was some resistance when breaking through the surface but otherwise the machining was no worse than with standard 1045 steel. This material can be somewhat difficult to find but should fit the service conditions very well.

Ken

[Mike Rometer]

Does anyone know a reason why Monel would be spec-ed for this task?

It doesn't have the strength of steel. Its expansion rate is very similar to steel. Cost is considerably higher. Its workability is very difficult (camparatively). It's heavier than steel, though not perhaps the greatest consideration as the size of these parts is not great, so perhaps an ounce or two extra per rod. Its only benefit might seem to be its restistance to corrosion, but as a piston rod would almost always be in an suitably oily environment it shouldn't have too much trouble in that respect.

I'm highly curious!

[marinesteam]

John York is the only person who can answer that question, I didn't ask, as after doing a bit of searching for ground Monel rod stock (couldn't find) had intended to use a substitute for Monel as the piston rods.

I prefer to purchase my tolerance and finish where possible so I had narrowed down my choices to ground & chrome plated hardened 1045/1050 rod or precision ground stainless rod. Both have issues. Stainless, while corrosion resistant tends to gall easily and can be abrasive to the glands so was lower on the list. The hardened chrome rod is of course, hard and not easliy machinable without annealing. A process that while I could do, didn't want to try in fear of damaging the chrome and in due to the lack of control over the process when using a torch. Also, had the option of sending unfinished steel out to be ground and plated but that is what I was trying to avoid with the Monel.

In my searching I ran accross a reference to CPO rod stock and after even more searching found a source on-line. This material can be a bit difficult to source but most hyrdraulic cylinder repair shops should carry it. I used TeamTube and was able to purchase a 4' rod for a pretty decent price.

Ken

[Mike Rometer]

Thanks Ken. It seems a very odd choice to me, though I'm not an expert at these things, I do have some experience, and I wouldn't bother to spec anything above mild steel, or maybe silver steel if the size suited, for this, mainly for strength and weight. 0nce polished, it doesn't seem to bother gland materials of many sorts, and it's cheap(ish). I wondered if I was missing something.

[marinesteam]

I choose the chromed rod over mild steel because the surface finish is much better than I can get even if I use the toolpost grinder, so I'm assured that I'm not going to eat up the glands. Plus, I get extra corrosion resistance that silver steel wouldn't have.

May have been "overkill" but these rods should last a lifetime and then some and the cost was reasonable.

Ken

[Mike Rometer]

As with all these things, each to his own. I've not had a problem with corrosion or the finish with either mild or silver steel, the oil sees to that. I polish both with fine emery cloth and engine oil in the lathe, and the finish can be as good as ground. Nothing wrong with "belt and braces" at any time though.

[fredrosse]

Ground and chrome plated piston rods are about the best for both corrosion resistance and long packing life. Monel has the same advantages, but costs a small fortune. Ground steel rods can work ok, but some of us use no oil in the engine, so rust would be a problem for these engines.

I get ready made hard chromed and ground rods from surplus pneumatic and hydraulic cylinders. A cylinder on e-bay can be found for only a few dollars. On the Margared S. walking beam engine, the 3-1/4 inch diameter piston came from a 3000 psi hydraulic cylinder, with an attached 1-1/4 diameter hard chrome rod. The picture below shows the machining setup which was used to separate the rod from the piston. I did a similar job on a 250 psi pneumatic cylinder piston and rod, only that unit had a 5/8 diameter hard chrome rod, which is what I mated to the 3-1/4 piston. In the second season with the 5/8 diameter chromed piston rod, and packing is holding up excellent, have only tightened the packing slightly one time, and there is virtually no leak in the packing. With good alignment to the crosshead slippers, this packing may last for several years.

The 1-1/4 inch diameter rod makes an ideal plunger for the manual feed pump.

[Mike Rometer]

I can see that running with no steam oil would cause a corrosion (and wear) problem. Have you tried PTFE for gland packing?

[87gn@tahoe]

Monel might have been a bit less expensive in the '70's/ early '80's when John originally designed the engine. He was/is also a professional machinist, so exotic materials may have been more accessible to him at more reasonable prices.

Knowing John, I am sure there was a reason behind him using Monel over other materials. He is a very particular person with very strong beliefs.

[marinesteam]

Monel rods are apparently used in well water pump service so John has a basis for his choice. I'm sure he was going for the corrosion resistance. With the rest of the engine design as any indication, John definitely knows what he is doing.

The guys on Smokstak had some good threads on this subject as well and it seems like the leaning was toward ground & chromed rods as having the best all around balance of corrosion resistance and lack of gland wear if the chrome was applied properly. If the chrome flakes it cuts the glands right up.

Anyway the balance for me was to get as good a surface finish and corrosion resistance as possible without having to send out for grinding or chroming. That's my story and ask me in ten years how it all worked out.