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Locomotive valve events

Discussion in 'Steam Traction' started by Richard Roper, Feb 20, 2021.

  1. Aberdare

    Aberdare Member

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    Tail rods on pistons.

    There are basically two types of tail rods on pistons.
    A. Tail rods that pass through a sealing gland in the front cylinder cover, similar to the arrangement in the rear cylinder cover.
    B. Tail rods that do not pass through a sealing gland in the front cylinder cover, but are kept steam tight by means of a long pressure tight cover over the extended rod.

    For type A the tail rod does indeed affect the surface area that the steam works against, in the same way that it does at the back of the cylinder.

    For type B the tail rod will either be hollow or will have some other passage to connect the space inside the long cover and the cylinder itself. This passage allows the steam inside the cylinder to also be acting on the end of the tail rod. Because of this the effective force on the piston is the same with or without the tail rod (ignoring any slight steam pressure drop along the passageway).

    Tail rods were avoided in most designs as they increase the mass (weight) of the reciprocating piston/piston rod/crosshead assembly and require additional balancing.

    Attached is a section of a S&D 7F drawing showing the cylinder and tail rod with cover (type B). In this design there is a passage to the end of the cover which can be seen in the photograph of the bearing bushes, steam passes through the ring of small holes. The large threaded hole is for the pressure lubrication fitting. On the 7F the pistons are made of steel and should not touch the cylinder bore, hence the use of tail rods to support the piston.

    For those who are wondering what the grey pipe is that connects both ends of the cylinder on the drawing, this is a bypass pipe with a valve in the middle. When coasting with steam shut off the valve opens and allows steam/air to pass directly from one end of the cylinder to the other, reducing the pumping action of the pistons.

    Andy.

    7F Tail rod drawing.jpg 2DF3CC63-6761-4C9A-BDBD-C39E61A4A893.jpeg
     
    Last edited: Feb 21, 2021
  2. Richard Roper

    Richard Roper Member

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    Yes, this has been my observation over the years too, having seen them in Haworth Yard, and also the components supplied for The Unknown Warrior.
    Out of interest, how rigid an arrangement was the LNER square boss? Did it have a tendency to become rounded off with the fore and aft movement of the eccentric and expansion link?

    Richard.
     
  3. LMS2968

    LMS2968 Part of the furniture

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    I haven't heard of that happening, but the square boss arrangement was originally chosen for the BR Standards, but later replaced by the LMS four studs and the drive slot type, although I can't remember the reason. It can be seen in early photos of the class.
    https://digitaltmuseum.se/021018098421/british-railways-br-brit-70000-britannia/media?slide=0
     
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  4. std tank

    std tank Member

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    Deleted
     
    Last edited: Feb 21, 2021
  5. Richard Roper

    Richard Roper Member

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  6. std tank

    std tank Member

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    The reason for the change of design in the return cranks was due to some fracturing taking place. All the Classes other than the Class 2s and the Duke, of course, were originally fitted with square boss return cranks until it was decided to modify the locos. The drawings for the modified return cranks were dated June 1954. Then, all new builds from that date were fitted with LMS style return cranks and others were modified on Works visits. Not all the locos were modified. It is interesting to note that the drawings for the original and modified versions were done at Doncaster Works Drawings Office, other than the original return cranks for the Class 3s, which were drawn up at Swindon.
     
  7. Steve

    Steve Resident of Nat Pres Friend

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    Tail rods reduce the available volume of the cylinder and make this the same as the piston rod side. Without the tail rod, the swept volume of the cylinder on the rear ( piston) side at any given position is always less than the front side. All else being equal, the mass of steam admitted will therefore be greater on the front side so the pressure at the moment of exhaust will be higher than on the front side. Perhaps not by much but more, nevertheless.
     
  8. Steve

    Steve Resident of Nat Pres Friend

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    NER Class H No.1310 (dating from 1891) has the rods from sister loco No. 1308. When 1310 had its major overhaul back in the 1990's, it was well worn with badly bent frames; so bad that the crank web was rubbing on the spring buckle on one side but when straightened, there was an inch of clearance at this point. The axleboxes and horns were overhauled and set up to be at the correct centres. When we came to fit the rods, which were not half brasses but had bushes, we discovered that the rods of 1308 were 1/8" shorter than they should be, according to the frame drawing. As a consequence, they are fitted with eccentric bushes. The loco, with its Joy valve gear, is decidedly off beat if pulled back more than one notch and I often wonder whether the driving axle is in the right place for the loco as built, as distinct from the correct place according to the drawing.
     
  9. Cosmo Bonsor

    Cosmo Bonsor Member

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    I’ve held off commenting to see what other knowledgeable people had say.

    I’m talking from my experience in the preservation era, I was born in ‘68!
    The locomotive will be set up as good as is reasonably practicable given the resources available.
    This mostly means the driving axle is where is supposed to be and that the cylinders are pointing the right way
    Contrary to at least one poster’s view, this can be done to a good level of accuracy, you will find many engineers on heritage lines are tool makers or highly skilled technicians, easily capable of working to very high standards. We have the benefit of using modern measuring methods unavailable 60 years ago.

    It is an interesting feature of steam engines that they wear in typical and predicable ways. For example crank pins wear oval and axle boxes usually end up tapered as well. A peculiarity of slide valves is a groove at the ends of the port face caused by the steam cutting its way along. Slide valves end up loose in the buckles, there is generous clearance to start off with 0.015” from memory.
    During rework, dimensions may change from existing, to account for wear for example or because the centre of an eccentric strap may end up slightly different. This not because of any defective workmanship but because it doesn’t matter when the set up takes this into account.

    Now you have your freshly overhauled loco with good valve events and start running up and down its home line or even the National Network.
    The wear happens, what do you end up with?
    Well if it’s a slide valve Stephenson’s one, the valve will get a bit sloppy in the buckle, it may even rock if the valve faces are vertical. There will be next to no wear in the motion parts themselves ie die block and links because they are glass hard. There may be some wear in the pins and the eccentric strap white metal. Your lifting link might get a bit of slop at a high mileage. That will result in reduced valve travel and lead because the two are linked. If there is any difference in wear tween the two sides you will hear an off beat.

    ‘Aha! I’ve got a shiny modern engine with outside Walshaerts and piston valves.’ you say smugly.
    Lucky you. Well the axle is probably where it should be because the shopping mileages were higher on more modern engines. There will be no wear on the roller bearing return crank and all the motion pins are grease lubricated. You might find some wear in the bronze die block, this could be different on each side. The piston valves won’t have moved because if they did you’d have more to worry about than an off beat engine. About the only problem the smarty pants owner will get is valve ring wear. The Drivers will be the first to spot this, usually one beat sounds a bit soft. A bit later the drain cocks jam open because bits of ring end up there. The steam and exhaust edges will be OK because the rings don’t wear much in the direction of travel.

    A small point of interest on some piston valve engines BR Stds I think (maybe it’s an LMS thing) is the front valve bore is 1/8” bigger than the back one so you can get the valves is easier. I doubt if you’d hear the difference though.

    Lastly next time you can get close enough to a SR engine you will see a raised dome shaped thing each side of the axle box. It has a centre punch mark in it. Called buttons, they are equal distance from the centre of the axle and at the reference ride height. We use these to machine the axle boxes correctly and set the loco up. I last did boxes quite some while ago. I was able to revise my evening class during each pass of the horizontal borer.

    Hope this is of interest.
     
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  10. std tank

    std tank Member

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    You make no mention of checking the distance between the centre of the cylinder and the centreline of the driving axle.
    Some locos had oil lubricated Walschaerts valve gear.
    BR Standard locos were also fitted with datum buttons.
     
  11. Cosmo Bonsor

    Cosmo Bonsor Member

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    I referred to it here," This mostly means the driving axle is where is supposed to be and that the cylinders are pointing the right way" IE through the centre line of the axle. Isn't some Swindon stuff offset though?
    I also failed to mention checking slide bar alignment straightness and clearance, ovality/ concentricity of bores, side clearance and sideways position of axle boxes, ring gaps, condition of valve heads, motion bracket alignment, frame twist, differences eccentrics and quite a lot else.
    I know that earlier locos had oil lubricated valve gear, I drive them.
    I left lots out because I'm a poor typist, but that wasn't your point was it?
     
  12. std tank

    std tank Member

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    Some LNER locos had offset centrelines.
     
  13. Eightpot

    Eightpot Well-Known Member Friend

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    Taking things further, and perhaps going off at a tangent, there is the question of heat affecting things. Peter Townsend in his book 'Top Shed' mentions the rough riding of the Peppercorn A1 Pacifics, one of which, 60136, was fitted with a shelter for taking indicator diagrams. It was found that heat absorbed by the frames from the outside cylinders actually moved the inside cylinder forward by about 1/16". Not a great amount, but it meant that the front port of that cylinder opened by 1/16" more and the back port that amount less.

    However, now transfer this thinking to the frames by the firebox of (say) a 4-6-0, what happens there? Assuming a 50 degree Fahrenheit temperature increase of the frames in the firebox region compared with the adjacent coupling rods, the frames will expand. According to my 1937 edition of 'Machinery's Handbook' steel expands by a factor of 0.00000636 per degree Fahrenheight. So, over a (say) 8' length, the frames will expand by 0.03", or nearly 1/32". As the cylinder/frame area affected is shorter than the firebox area, that area could possibly expand much more than 0.03".

    Either way this is considerably more that the 0.005" clearance in the coupling rod bushes that K. J. Cook refers to. Something has to give otherwise the loco would be 'bound up'. I believe that for the proposed (and now abandoned) project of the 5AT it was intended to have roller bearings fitted to both the axle boxes and coupling rods. With the minuscule clearances in such bearings the situation would be worse. Rubber coupling rods not a solution.

    Is it one of those situations like that of the Bumble Bee which scientifically cannot fly, but the said Bee, not knowing this, manages to do so anyway?
     
    Last edited: Feb 23, 2021
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  14. 30854

    30854 Part of the furniture

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    That was always a favourite of mine .... until it turned out the bees knew more than human scientists. It's now official, bees can fly! :)
     
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  15. Dunfanaghy Road

    Dunfanaghy Road Member

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    I believe that the Germans managed to equip their side rods and motion with roller bearings, as did the Yanks.
    Pat
     
  16. Eightpot

    Eightpot Well-Known Member Friend

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    Perfectly correct, but I think you will find that all locos so fitted did not have narrow fireboxes between the frames.
     
  17. Jimc

    Jimc Part of the furniture

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    Aerodynamic science is perfectly happy with how the bumblebee stays aloft. What aerodynamics told us is that the bumblebee can't fly like a conventional aeroplane, which it does not attempt to do.
     
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  18. 8126

    8126 Member

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    If you look at the Timken coupling rod drawing for an SAR 25NC, with all the above features except the narrow firebox, you'll find that they have 10 thou clearance on the nominal diameter in the vertical, and 30 thou horizontal, in one bore of each of the three rods (the Timken rods on the early 25s are staggered without knuckle joints, so each rod does actually sit on both crankpins). In short, the designers certainly knew they needed to give some extra clearance in that direction. They did have bearing trouble, but it was traced to the bearing specification; that rod configuration was perfectly successful on US classes. There are also roller bearings specified with deliberately large clearances for this sort of application, SKF C5 class bearings have minimum 0.46mm clearance in the diameter range 250-280mm.

    As for the bees, well, @Jimc has that covered. No serious aerodynamicist has ever sat there puzzling as to how the bumble bee can fly when their equations say it can't. What they may well have done is quietly ignored the fact that classical aircraft aerodynamics assume inviscid flow, even though the bumble bee's catastrophically low Reynolds number is telling them this absolutely cannot be the case, then demonstrated with mock puzzlement that said bee cannot fly, while actually knowing that a more viscous simplification of the Navier Stokes equations is required.
     
  19. Steve

    Steve Resident of Nat Pres Friend

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    Cooks 0.005" clearance was nothing special. The LMA standards give coupling rod bush tolerance between +0.003+ to +0.005" larger than the crankpin so it would be pretty normal.
     
  20. LesterBrown

    LesterBrown Member

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    I suppose the valve settings on Gooch's broad gauge Bogie class would have been interesting as they warmed up with the driving wheel frames fixed to the firebox and the cylinders fixed to the front end of the boiler which was the only connection between them.
     

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