I always read that the cylinders needed to be in front of a steam locomotive because otherwise the more exposed push rods would suffer from dirt, grit, and grime. Sounds reasonable. But of course the famous Cab Forward articulated engines did exactly that.
So my question is this: did the cab forwards indeed suffer increased wear and tear on the drive gear, or was the warning more of a "potential for harm" kind of thing. Did the running gear of the cab foprwards require more repair and maintenance than the same basic loco running the other direction?
The SP cab-forwards were hardly the only steam locomotives with the cylinders behind the driving wheels. Also consider B&O's N1 duplex drive and the PRR Q1, the rear cylinders were behind the drivers. For further consideration, look beyond our borders at Garratt locomotives.
Call them 'piston rods'; pushrods are found in IC engines working the valves.
There are two specific problems with 'forward-facing' cylinders: the polished surfaces of the piston rods are more prone to pitting and compromise of their lube film, and the glands in the cylinder head are more prone to external damage as well as accelerated wear. The result, of course, is increased steam leakage and, to an extent, required oil consumption for a given level of maintenance. (Remember that more oil on the piston rod means more oil in the exhaust steam, and by extension that may mean problems with feedwater heating and boiler priming...)
The effect of piston thrust is different, as the rod angularity now acts to move the main up rather than push it down -- this has some implications for placement of balance weights. For technical reasons it is now the 'opposite' side of the main crankpin journal that takes the 'push' and 'pull' of the mainrod (compared to the situation with cylinders ahead), but I doubt there is any meaningful compromise in performance for that reason alone -- of course, there is no effective change in the rod big end stresses, which remain reversing once per revolution.
There is a recognized situation with Garratts, but it involves the valve gear and not the pistons. There are different stresses associated with putting Walschaerts in 'reverse' (with the reach rod above center in the link, rather than below) and I believe most Garratts, to achieve maximal parts compatibility, had one engine forward and one reverse in either direction. On a road which habitually turned its Garratts "properly" (e.g. put the exhaust behind the cab, if the fuel bunker is designed for visibility) the result will be some accelerated wear and tear in the 'rear' engine's valve gear if the cylinders are, as most often found on Garratts, toward the outer ends of the locomotive. A locomotive that operates continuously with an engine 'backpedaling' will be (properly) designed to have its gear events reversed so the reach rod for that engine can be 'down' when the locomotive is moving forward.
You did not mention, by the way, any increased problem with crosshead surfaces. This is a reported difficulty with some designs of crosshead, as in 'conventional' drive the crosshead is pulled up against the upper guide surfaces, but in reverse pulls it down and away -- this augments problems with wear and contaminants particularly in multiple-bearing crossheads. There is also of course somewhat increased dust and dirt thrown up against the crosshead guides. The same considerations as for piston rods apply to the valve rods in some degree.
Any exposed drivetrain will suffer more than an enclosed one...right? I know of no non-exposed drivetrains on steamers. From the stuffing boxes in various places all the way to the bearing covers below and further away from them, piston rods, crossheads, links' pivots, lubricators, pumps, main rods, side rods, crank bearings/bushings...they all suffer from exposure unless they can be kept liberally lubed or spray-coated by the conscientious crew...or faired. But I think I'd have to agree that it gets somewhat worse the farther rearward on an essentially exposed reciprocating drive one places oneself.
There have been locomotives with enclosed running gear and valve gear drive. The principal issue is that as soon as there's any steam leakage -- and that can happen pretty quickly without advance warning -- the condensation and oil in the confined space make a mess.
Personally, I like the idea of using an elastomer 'bellows', like the boot on an offroad shock, to go over the exposed portion of the piston rod from gland to crosshead. This can be vented as needed or even given treatments against certain contaminants.
The rest of the running gear is cleaned and then sprayed with one of the modern 'hygroscopic' clear coatings. This repels water and most of the waterborne contaminants such as road-dust emulsion. It is not difficult to make up and use the equivalent of hub-liner seals to go between rod eye sides, crank cheeks, eccentrics, etc. to keep crap out of bearing areas while allowing the kind of 'total'loss' lubrication so often used -- hey, Mr. Alemite Man! -- and as noted there have been periodic experiments with pressure-circulated oil in rods and pins (not successful up to now, but if EPA applies pollution controls to steam locomotive lubrication it might 'have to be.')
I am in favor of fairings that exclude dust and grit from contact with critical running-gear components. That isn't nearly as simple an actual design exercise as it might appear. Most 'streamlined' locomotives had considerable vacuum effect underneath that actually sucked dust into areas from which it ought to have been excluded...
There are also these wonderful little 0-4-4-0T Meyenberg Mallets that run on the narrow gauge lines in Eastern Germany, they have the cylinders in the middle on both sets of drivers. Sometimes they are both in front, though.
RMEPersonally, I like the idea of using an elastomer 'bellows', like the boot on an offroad shock, to go over the exposed portion of the piston rod from gland to crosshead. This can be vented as needed or even given treatments against certain contaminants.
The problem I see there is how to install it. Either something has to come apart, or the bellows has to have a seam in it which can then be opened to place it over the rod.
If the service life is such that the bellows will last as long as the rod will remain in place (and I have no idea how often that gets disassembled), then it becomes a non-issue.
If this were to become a mandatory item, then ease of replacement would be important. Some steam locomotives never get far from home. Others range pretty widely.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
The 'simplest' way is to make up the appropriate circumferential clamps on crosshead and rear gland, then 'break' the connection between piston rod and crosshead, roll the engine back or advance the piston, and work the (seamless) part over the rod. Inspection when needed is done (with the engine side on back dead center) by disengaging first one end of the bellows and compressing it back, then doing the other end.
Yes, the assumption is that there will be no need to remove the piston and rod (as would likely be needed to resurface or replace elements in the gland) or break the crosshead-to-rod joint again until major servicing. (As an aside, the roller bearing between the forward rod eye and the crosshead may or may not be accessible without removing or disassembling the crosshead, and this is likely to be the 'first' component that would require physical R&R for checking, as it may not be amenable to being constructed as a grease-lubricated package bearing.)
Remember that there are three sets of criteria here: lowest net cost with historical labor; future net cost for 'new steam'; and lowest operating field cost for preserved or special-interest excursion locomotives. For the latter category it may well be worth spending several million on special purpose-built support vehicles if practical operation on freight-critical main lines is the 'prime criterion' ... as it is very likely to be, in my opinion. To an extent the use of various kinds of 'nonprototypical' (but often era-appropriate) technology to achieve greater confidence and redundant operability can be justified. I look at the devices we're discussing in this thread as being far more relevant to category #2, where "historical appearance" can take something of a back seat to much higher in-service reliability.
I appreciate the intersting responses above, and while I used the cab forwards as an example, am aware of other examples. However, what I specifically asked was whether the cab forwards actually suffered the need for more maintenance and repair due to this configuration when compared with similar arrangements used conventionally. Did they spend more time in the shop than frontways lcomotives of simmilar type? Did their piston rods and related exposed mechanicals actually experience higher wear levels and problems?
RME ... I am in favor of fairings that exclude dust and grit from contact with critical running-gear components. That isn't nearly as simple an actual design exercise as it might appear. Most 'streamlined' locomotives had considerable vacuum effect underneath that actually sucked dust into areas from which it ought to have been excluded...
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Very good and important point, RME, one that I failed to consider. Either the fairing would have to be almost entirely sealed (good luck there, Bud!), or filtered air fed into the compartment to maintain a positive pressure...again...good luck. Mebbe fairings aren't the way to go after all.
I hope our OP can get an answer to his question; is it necessarily so that rearward on rodded engines one would have to conduct more maintenance because of contaminants, erosion, corrosion, increased friction...etc? And would forward facing cylinders have a tougher time as a result of their placement?
EnzoampsHowever, what I specifically asked was whether the cab forwards actually suffered the need for more maintenance and repair due to this configuration when compared with similar arrangements used conventionally. Did they spend more time in the shop than frontways lcomotives of simmilar type? Did their piston rods and related exposed mechanicals actually experience higher wear levels and problems?
Problem is that the operating conditions for SP's "forward" articulateds (the renowned AC-9s) were very different from most of the cab-forward operations. I don't know if anyone at CSRM can point you at sources of detail maintenance records that might let you put this specific cost in perspective.
The conventional sources I have read gloss over the issue a bit; they just say operating costs for the cab-forward classes were not materially greater -- than what, isn't particularly well called-out. I suspect you'll have to be very careful in extracting the right metrics out of general cost and expense data to determine anything meaningful (regretfully, I doubt this can be done at this late date with any kind of reliably measurable statistical significance) but I for one would be interested to learn more 'either way'.
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