Improved "Floating Piston Rod Gland" design

[Roger]

Feb 25, 2020 19:00:04 GMT 92220 said:

Hi Roger.

I have to admit I misread the website. The rods are stainless but not 316. They are F51 stainless which is a Duplex stainless.I still maintain there is no reason for not using 316 stainless, or any stainless rod, for piston rods. In any case there is almost no chance of getting a stainless to stainless sliding contact, as stainless wouldn't normally be used for anything apart from the rods, and a bronze bush in the rear cylinder cover would be quite normal procedure. In fact I have used that combination on my loco......316 stainless rods, mild steel rear cylinder covers, with a bronze bush between the seal and the front face of the cover. So where would the rod be subject to scoring?

Bob.

Hi Bob,
Surely a hydraulic ram needs a guide at the exit point for the ram where the seal is? There shouldn't be a large size load, but it's asking a lot of a hydraulic seal to keep everything in line. I'd be surprised if there wasn't a proper bearing arrangement there to keep the ram central and provide a concentric diameter for the seal to work in.

[don9f]

I know in full size its a completely different scenario but just for interest, the pistons on 92214 had gunmetal "slippers" at the bottom, extending from around "5 o'clock" to "7 o'clock". These were in between the two piston rings and were backed by strong springs, thus helping carry the weight of the pistons and helping keep them central in the cylinder bores. The piston rods pass through a generous clearance hole in the rear cylinder cover....only the cast iron packings make contact with the rods. These packings can "float" like the idea being discussed, to accommodate movement of the rod away from true centre of the cover etc.

I'm aware of other engines that have the same design of pistons, but with the slippers/springs not actually fitted, so maybe the idea didn't work very well and wasn't really needed?

Cheers Don

[mbrown]

And, of course, some designs had piston tail rods for a similar purpose. My prototype, 99 3462 (below), has tail rods with what appears to be a substantial slipper underneath to support the rod. I know that tail rods were common on UK locos in Edwardian times but tended to be omitted or removed in the 20th Century, yet they remained very common in German practice until the end.

P1120878 by malcolm brown, on Flickr


Malcolm

[Gary L]

Feb 25, 2020 20:48:46 GMT don9f said:

I know in full size its a completely different scenario but just for interest, the pistons on 92214 had gunmetal "slippers" at the bottom, extending from around "5 o'clock" to "7 o'clock". These were in between the two piston rings and were backed by strong springs, thus helping carry the weight of the pistons and helping keep them central in the cylinder bores. The piston rods pass through a generous clearance hole in the rear cylinder cover....only the cast iron packings make contact with the rods. These packings can "float" like the idea being discussed, to accommodate movement of the rod away from true centre of the cover etc.

I'm aware of other engines that have the same design of pistons, but with the slippers/springs not actually fitted, so maybe the idea didn't work very well and wasn't really needed?

Cheers Don

I'm guessing that with floating rod seals (which date back a long way in full size) it doesn't matter much if the bottom of the piston/bore wears more than the top. And since piston, rings, and cylinder liner were normally all cast iron (a good wearing combination) then the extra cost and complication of the gunmetal slippers you describe may not have been justified by any small improvement (if any) in the wearing characteristics. -?

Gary

[suctionhose]

I'm aware of 'bearing pads' under loco pistons - white metalled sometimes too. However, I think that may be more often found on modern express types and not used universally.

On mid sized cylinders, certainly traction engine size, a bronze bush or neck ring was used where the rod enters the cylinder with the intention of the Rod supporting the weight of the Piston (some of which were cast hollow to reduce reciprocating weight)

Of those that I have encountered, these neck rings are always worn / scored as are the rods themselves which suggests it wasn't such a good idea other than perhaps that the bushes were replaceable (which probably never happened but was easier in the bush than a rebore). Speculation only...

Point is there are two schools of thought:
Hold the piston clear of the bore by supporting the rod thru the gland OR let the piston rest on the bore and have clearance around the Rod.

I have tried both. By the time the rod passes through a gland and a neck ring there is a substantial length of support. Certainly sufficient considering the mass of a mini piston. If crosshead and slide bar alignment closely agrees then 'all good'.

As mentioned by Julian and others, materials, lubrication and the propensity of an oring to wipe same off are definitely issues to consider. Graphited packing lends itself to the task and really shouldn't give any trouble. Phosphor Bronze is fairly rare around here; Leaded Gunmetal is the common choice.

I have seen stainless piston rods blue from heat. Comments usually allude to superheat but I believe the bluing is more likely a result of heat generated from friction / lack of oil. If that be true then the case for external lubrication of rod / gland is made yet again.

My choice in this case is for piston to bear on the bore + crosshead & slidebars to support outer end and the gland to be clear of the rod. Given this model weighs 1/2 ton in working order and has considerable power, I accepted flexing slidebars, running clearances or even distortion due to expansion would be reasonably likely.

For the gland to be free to self adjust a few thou this way or that was desirable. Devising a satisfactory arrangement, albeit a reinvention of the wheel, was a satisfying process.

[92220]

Feb 25, 2020 20:19:46 GMT Roger said:

Feb 25, 2020 19:00:04 GMT 92220 said:

Hi Roger.

I have to admit I misread the website. The rods are stainless but not 316. They are F51 stainless which is a Duplex stainless.I still maintain there is no reason for not using 316 stainless, or any stainless rod, for piston rods. In any case there is almost no chance of getting a stainless to stainless sliding contact, as stainless wouldn't normally be used for anything apart from the rods, and a bronze bush in the rear cylinder cover would be quite normal procedure. In fact I have used that combination on my loco......316 stainless rods, mild steel rear cylinder covers, with a bronze bush between the seal and the front face of the cover. So where would the rod be subject to scoring?

Bob.

Hi Bob,
Surely a hydraulic ram needs a guide at the exit point for the ram where the seal is? There shouldn't be a large size load, but it's asking a lot of a hydraulic seal to keep everything in line. I'd be surprised if there wasn't a proper bearing arrangement there to keep the ram central and provide a concentric diameter for the seal to work in.

Hi Roger.

Yes they would normally use either a bronze bush or a fluon coated shell bearing. This is a section of a typical hydraulic cylinder (not used to Flickr. Hope it comes out).  As can be seen, the rod is only in contact with seals and the bronze bearing, so there is no chance of scoring, even if the rod is 316 stainless:

 hydraulic cylinder sectional sketch by Robert Shephard, on Flickr

Bob




 

[rrmrd66]

Morning everybody

Just to add my four pennyworth. I may have bored you all before with the fact that I ran a hydraulic sealing company for 30+ years. So here's my bit.

A seal manufacturer from New York told me back in 1973 when I was doing my basic product training in Hamburg Germany that :

"Packings ain't bearings and bearings ain't packings". (Our US friends always referred to hydraulic seals as "packings". Not to be confused btw with braided packing's)

His point being that the seal should do just that and not be subjected to excessive mechanical side loads, conversely adequate bearing surfaces should always be near to the seal to ensure that this happened.

I still say this to myself a lot ( with a New York accent of course!)

Interesting cylinder design Bob. There appears to be an internal seal inboard of the rod bearing as well as the main pressure rod seal plus wiper seal. Some form of primary pressure reducing seal,I wonder?

Cheers

Malcolm

[tony9f]

As an illustration of what happens in full size, during a trip on the PS Waverley I was watching the engines and you could see the piston rods dropping as they stroked out but there was no leakage of steam due to the ability of the packings to float. In this instance they were metallic packings. I can't be certain but I don't think piston slippers were fitted on marine engines as far as I know but bearing in mind how much these things weighed it didn't seem to be an issue. Also bear in mind that marine engines often had steam jacketed cylinders and too high a pressure in them would cause the pistons to rub heavily on the liners known as scrooping. I daresay that adjusting the jacket pressure would also alter the fit of the pistons in the cylinders and help centralize them.

[92220]

Hi Malcolm.

I pulled that picture off the website. I had noticed the internal seal and wondered what it was supposed to do. Pressure reduction would seem to be the most logical reason for it. By enlarging the drawing it can be seen that the inner seal is sealing against the cylinder pressure as is the next one on the outer side of the bearing, with a wiper seal on the outside to stop muck getting inside.

Bob.