Preservation Loco 'Exchange' Comparisons

Sounds as if the ''Friends of Dugald Drummond and William Pickersgill'' have had a Chapel meeting

 

But the force on the track is from the total weight of the engine, not just the un-sprung weight of the axles; the total weight will not vary that much between the two bearing types (certainly less than between different engine types)?

Maybe there's some sort of resonance thing going on with the springs (and maybe the track deflection too), in which case the un-sprung weight of the axles could well be a factor in that resonance?

It would probably take detailed dynamic modelling to say for sure - something not really possible even in the more technically advanced latter operational steam days, that probably takes a computer (much as calculations concerning aerodynamic flutter were one of the first applications for Konrad Zuse's early computers - although his calculations were not detailed finite analysis, which I won't get into here). The re-work for the new P2 involved that sort of work, but I don't know if they looked at the bearing type issue.

Noel

 

Or possibly that the demands of working an intermittent service on a short heritage line lead to a different idea of optimal than working on a mainline with much longer duty cycles, less stop and start operation and massive workshop capacity in the background?

Tom

 

Or the ''don't 'old with them improvements tendency'' just as probably.

 

I would agree with you if we were talking about perfectly smooth track. If there is a bump in the track, or other unevenness, the unsprung weight would have to ride over the bump, with the sprung weight being isolated by the springs. Once it has gone over the bump the resonance between the unsprung weight and the flexibility of the track could continue for a little while.

If there is any data on the stiffness of the track, as well as typical springs, and the relevant sprung and unsprung weights, it should be possible to do some calculations.

 

The imortal Chapelon is cited somewhere for stating that most of claimed superheat improving cylinder efficiency could be won back by rearrangement of heating surfaces in a saturated boiler and then having longer liner and ring life.

 

Was that an idea ever tested in hot metal?

 

I had been mulling over Chapelon's logic for reheating, but not superheating some of his compounds. For a similar superheat temperature, you would expect the HP cylinder in a compound to be considerably hotter than the cylinder in a simple engine, and this might have lead to difficulties. If you could then also extract some intermediate pressure steam, before it passed through the reheater, and do something useful with it such as using it in an injector, there might be some thermodynamic advantage in the arrangement.

 

Steve, yes I think E4s were very good, or more to the point well matched to the work required of them. I spent a lot of time on 473 when we were running it after its last overhaul and it felt quite punchy and smooth running. There may well be an element of NIH which affects all engines which were novel or ‘foreign’ to the crews of the day. Brighton engines with an air brake were liked by crews familiar comfortable with it. A lot of the radials lasted until quite late especially after the cull in the 50’s which got rid of a lot of frankly poor or mediocre Brighton stuff which implies they were useful things. I knew a fitter who rated the E6s highly. The slide valves were below the cylinders and they would run very freely when coasting, but were a pain to work on. I think Tom is right, horses for courses and heritage railway operation has its own set of conditions not necessarily met by ‘yer flashy modern gadgets with all the gizmos on ‘em’. Back in a bit, I’m off to set fire to a wicker man…

Right that's the incomers, with their fancy London ways dealt with.
As for ride quality, I’d overlooked the components below the tyres. It is noticeable when you go over some bridges that you feel the ride firm up. I think the answer is a complicated one with many variables. If there are any clever engineers who can do the sums it would be very interesting.

 

The last CEGB Steam Colliers had triple expansion engines with superheat, the superheated steam then heated the steam going into the LP cylinder cooling it before it went into the engine so it doesn't carbonise the lubricating oil.

 

That makes my head hurt!

 

The 6 cylinder 2-12-0 was tested with interstage superheat. The h.p. cylinders worked at a long cut off and so were not particularly prone to losses through condensational artefacts. It was found that the h.p. superheat could be removed with negligible impact on efficiency. So far as Chapelon's engines go the only time they gave trouble due to lubrication issues was on the 4-8-0s when the wrong grade of oil was supplied for the bogie axleboxes. While the use of atomisers continues the quality of lubricants will always be degraded before it reaches its area of application.

 

OK, my explanation leaves something to be desired, but look at

https://www.shipsnostalgia.com/threads/triple-expansion-steam-engine.26717/

Penultimate post

These reheat engines interest me because the principle is clever and simple. On NEM engine with the full reheat system, the boiler steam going to the HP is used to re-heat steam leaving the HP (via a tubular heater). This means the steam temperature can be higher than would otherwise be possible on a reciprocating engine because the boiler steam is de-superheated by the reheat system, yet at no loss because the heat 'lost' is added to the IP steam. I believe the same principle is applied to the last of the Skinner marine engines, the compound uniflows (one still running commerically I believe).

 

A few thoughts on some of the topics of the day...

Resuperheat without HP superheat in compounds was tried as a variation on 160A1, with I believe acceptable results, and the obvious advantage that where you would otherwise need very high steam temperature for acceptable expansion performance, the peak steam temperature was the highest of either HP saturation or LP resuperheat. 160A1 also had steam jackets on the cylinders, so if you put HP steam through the steam jackets, with a restriction on the exhaust passing to the intermediate receiver, the steam pressure in the jacket can be above the average of the steam in the cylinder, which keeps you out of condensation losses.

The harsh ride with roller bearing axleboxes is interesting. I believe the extra unsprung weight, particularly of cannon boxes, is probably a strong contributing factor - unsprung weight is bad for ride on any wheeled vehicle. However, one other consideration is that plain bearings, once on the move, ideally support the load on a film of oil with no metal to metal contact at all, whereas rollers are lubricated but not intended to establish a hydrodynamic film. A thin oil film is suprisingly stiff, but also will tend to dampen vibration. Same goes for the white metal layer to an extent; no it shouldn't be deforming, but it won't ring like a piece of hard steel. In practice the operation is more likely to be a mixed regime, some metal-to-metal contact with a partially established oil film, but it's still fundamentally more likely to dampen vibration than rollers, so will feel less harsh.

 

34105 had a reputation as being a bit of a poor steamer, according to Peter Smith.