5at Train

The 5AT Group - Steaming Ahead with Advanced Technology

Enhancing Performance -- Improving Reliability -- Reducing Costs -- Controlling Emissions

Questions about the 5AT Technical Specification

The following questions were put David Wardale by Chris Newman in 2002, just after he became involved in the project, when he was largely ignorant about the 5AT concept and about Modern Steam in general. Anyone who is new to the subject may therefore find them instructive.



Newman's questions are shown in red; Wardale's answers in blue.

Item numbers relate to the clause items in the 5AT Technical Specification.

2.1. The locomotive excluding the tender shall be based on the British Railways Standard Class 5MT 4-6-0 design of 1951, being a type and size of locomotive most suited for the intended duty of main line leisure train service(1).. A characteristic of this type is the deep narrow firebox, which is most suited to high combustion rates, as noted by Chapelon (La Locomotive a Vapeur, English Edition, p. 36-7). In particular, in order to facilitate certification by Railtrack and give the widest route availability, it shall conform to this design in respect of:

As a matter of interest, if the locomotive is to be an oil-burner, then does the firebox shape make much difference? [Past locos built primarily as oil burners tended to have deep narrow fireboxes, as this one. The firebox shape will affect burner design, number of burners and burner positioning.]

3.1. Welded plate frames shall be used, with the main longitudinals spaced as far apart as possible to allow the maximum width of firebox.

This is something I've never properly understood - what is the benefit of plate-frames over "bar-frames"? My understanding is that bar-frames were preferred in the USA because they coped better with poor track - from which I deduce that they probably flexed more and were less susceptible to cracking. If so, then what are the benefits of plate frames and the dis-benefits of bar-frames? I.e. why use plate frames and not bar frames for this loco? [Plate frames must be used because the bottom of the deep firebox lies between the frame longitudinals and the much wider bar frames would unduly restrict firebox width. Bar frames are actually much stiffer than conventional plate frames, which is better (frames must definitely not flex) but our plate frame will be so braced as to be as stiff as a bar frame (see Chapelon H-Section frame Fig 37 p.100).]

3.3. Very robust and very securely fitted horn stays shall be used.

Someone once told me that one good feature of the Bulleid Pacifics was the postioning of the horn-guides so that their centres and the frame centres coincided - i.e. vertical forces were concentric between axle-boxes and frame. Presumably this could only be achieved by moving the frame plates "inwards" towards the longitudinal centre-line of the locomotive (reducing space between the frames). Was this a correct observation [Yes], and was/is there any significant benefit from this practice? [Not significant, and it cannot be done on our loco as this would restrict firebox width].

5.2. The piston stroke shall be 800 mm and the cylinder bore diameter 450 mm.

800 mm seems a very long stroke, and 450mm diameter seems relatively small (compared to 482mm of the original Standard 5s). I guess I have the old idea stuck in my mind about long strokes and high piston speeds which are probably long-outdated. Nevertheless, I would have thought that a larger diameter piston with shorter stroke would be a preferable arrangement where space permits. Am I misguided in my thinking? Long stroke small diameter cylinders have a number of advantages, such as lower clearance volume, and lower heat transfer losses. The objection to them was poor "breathing" and balancing difficulties. Both of these can be overcome.

5.16. The crossheads shall be of the lightest possible design. Each crosshead shall have a single white-metalled shoe, which may be of aluminium alloy, sliding in an enclosed 3-bar type slidebar. The gudgeon pins shall be fitted with roller bearings, which may be either in the crosshead (e.g. Timken type split crosshead with taper roller bearings) or in the connecting rod (e.g. Chinese QJ design with needle roller bearings). The union link shall be pivoted from an extension of the gudgeon pin.

What does "split taper roller bearing type" imply? Does it mean that the small end of the connecting rod is to be fitted with taper roller bearings? [The bearing is not split. The crosshead is in two halves (i.e. split) each of which has a taper roller bearing. Each half clamps the piston rod and crosshead slipper which runs in the slidebar. It is similar to Chapelon Fig 35 p.96.]

5.20. The driving and coupled wheel centres shall be of the Scullin type. The rim section should be the absolute maximum allowed by the axle load.

I don't know what a Scullin type wheel centres are [See Chapelon Fig 41 p.103] - but I presume they are something akin to the Bullied's style. Is it correct that they give more uniform support to the tyre than spoked wheels [Yes], or is it simply that they are lighter [Yes] and/or easier to build? Either way, why did BR standards use spoked wheels? Just conservatism? [Yes - patterns existed]

5.20.1. The shrink fit of the driving and coupled wheel tyres to the wheel centres must guarantee that the tyres will not loosen during severe braking, in which case they may be shrunk on the wheel centres without retaining rings. Is this unusual? [Standard practice in USA]The tyre profile shall be of Railtrack's standard Heumann type, if possible altered at the outer part of the tread to improve adhesion. Is this a function of material selection [No]or rim profile [Yes]? The driving and coupled wheel tyres are to be shrunk on without retaining rings.

5.21. The rotating masses on each wheel, including those of the eccentric crank and eccentric rod, shall be dynamically balanced on the same wheel. A percentage of the reciprocating masses on each side of the locomotive, which percentage is to be determined by calculation and which may be zero, is to be dynamically balanced, the total balance being shared equally amongst all the driving and coupled wheels. The individual wheel, individual axle, and total dynamic augments shall not exceed those of the BR Class 5MT at the same speed, and it shall be the aim to make these values at the locomotive's maximum permitted speed (8,5 Hz at 180 km/h) not greater than those allowed by the 1928 Bridge Stress Committee at 5 Hz.

I've never fully understood locomotive balancing. Probably too complex to explain in simple terms …. but what constitutes the reciprocating masses on each side of the locomotive? [Pistons and rings; piston rods and tail rods; crosshead; union link; bottom of combination lever; front part of connecting rod.]

Presumably, pistons, valves, connecting rod and other rods, which combine to generate both horizontal[Yesand vertical [No] out-of-balance forces. And presumably it [is]possible to design rotating balance weights on the wheels to counteract all these forces. [Yes, but these then generate vertical out of balance forces] Is hammer blow anything to do with these unbalanced forces, or is that something quite different? [It is the vertical component of the force due to the balance weights in the wheels for balancing the reciprocating masses]

5.22. The driving and coupled axles, which may be hollow, shall run in sealed grease-lubricated spherical roller bearings mounted in split cannon type axleboxes.

What are split-cannon type axleboxes?[See Chapelon Fig 51 p.112. The axleboxes are split in two halves, top and bottom, and bolted together around the roller bearings. They are like a tube, hence the term "Cannon".]

5.24. The underhung driving and coupled axle springs on each side of the locomotive shall be compensated, giving, together with the bogie, 3-point suspension of the sprung masses of the engine as a whole.

Is this normal practice - to support the locomotive (effectively) on a three-point arrangement? [Yes - everywhere except British Practice.] I've recently started to take an interest in locomotive suspensions - after I stuck my head under some of the exhibits in the York Railway Museum a couple of years ago, when I was amazed to discover the large GNR Atlantic fitted with a combination of leaf and coil springs (coils on the rear drivers and leafs on the front if my memory and photo are correct)! Even so, I wasn't aware that compensation levers were used till I saw them on a SY loco here in China a month or two ago. I guess such things are rather more visible on Chinese locos than on UK ones, but I'm still wondering if spring compensation was routinely used on UK locos [No]. Doesn't compensation automatically correct variations in wheel load? [Yes] Wouldn't it have avoided the old routine of wheel load checks and spring adjustments? [Yes - no means of adjustment is provided on a compensated system]

5.25. The Walschaerts valve gear shall be designed to give the required valve events, and shall be as light as possible. The minimum full gear cut-off shall be 72%. Is this an exact requirement or a minimum requirement? [Minimum] The eccentric rods shall be fitted with sealed grease lubricated spherical roller bearings and the other joints throughout the gear may be fitted with sealed grease lubricated needle roller bearings. The connection between the radius rods and combination levers may be arranged for variable lead (D & R G W system). Never heard of this - sounds interesting! Is the variable lead driver-controlled or automatically adjusted? [Automatic adjustment] The expansion links shall be of the box type, and the dieblock-link rubbing surfaces shall be mechanically lubricated with oil fed via flexible hoses to the radius rods. The manual screw reverser shall be power assisted if possible.

Has no-one come up with an improved design of expansion-link and die-block that overcomes the lubrication and wear problems? If these have to be continuously oil-fed from hoses fixed to the radius rods, isn't it going to be near-impossible to stop oil spatter over driving wheel rims etc?[Best I know of were SAR mechanical lubricated ones. Some oil spatter will occur but my design will minimize it.]

7.3. Clasp brakes, with brake blocks acting on the tyres, shall be fitted to all axles, space permitting.(24) The line of action of all braking forces should pass through the axle centre line. DR type double brake blocks may be used.

What does "clasp brakes" imply? [Brake blocks pressing against tyres on both sides of axle] Is it similar to the arrangement of brakes on the Bullied Pacifics with brakes shoes clamping against opposite sides of the wheel? [Yes - this is it] This always seemed to me to be a rather simple and effective way of applying the braking forces to the wheels and it surprises me that it wasn't more widely used. [Complications with brake rigging]

7.5. For slip control as well as braking there shall be an independent engine brake acting on the driving and coupled wheels only.

Does this imply a separate brake actuator[Separate brake valve only] - but actuating on the same brake-shoes as the main brake? [Yes - was standard on latter-day SAR locomotives] Incidentally, why not use disk brakes on the driving axles? [It is a possibility]

9.1. It is intended that the locomotive shall require lubricating only by servicing staff, i.e. that locomotive crews shall have no lubrication duties.
Great idea - why was it not implemented before?[You tell me!] Surely this doesn't require 21st century technical innovation? [No - it was implemented in the USA]

9.5. Soft grease is to be used for all rubbing surfaces not fitted with 'dry' materials which see little movement, such as the boiler expansion slides. It shall also be used for any valve gear pin joints not fitted with roller bearings.

What valve-gear pin-joints are not likely to be fitted with roller bearings? [Will only be decided at detailed design stage] I thought I read somewhere (Red Devil?) that non-rotating pin joints were subject to higher wear than rotating pin joints [No]

9.6. For mechanically lubricated items, an appropriate oil check valve is to be placed in the oil pipe as close as possible to the item concerned, and drip trays shall be fitted where necessary to prevent spent oil contaminating the tyres and rails.

Where will drip-trays drain to? [Anywhere clear of the tyres and rail heads]

10.5. The cab front shall be inclined vertically and horizontally (similar to the Milwaukee F7 Class 4-6-4's) and the cab roof shall be extended forwards to provide a shield over the cab front windows.

What does this imply - cab front inclined vertically and horizontally? What is the reasoning behind it? [See Chapelon Fig 183 p.275 for example. Gives better streamlining and larger cab front window]

10.6. The cab shall be heated by radiator panels taking steam or (flashed) boiler water.

What is flashed boiler water? [Boiler water which is allowed to escape. As soon as it reaches a lower pressure area it "flashes" (changes) to steam]

12.10. The length of the tender body shall be approximately 9 metres. This will give an overall length of engine and tender wheelbase of approximately 18,9 metres and an overall length over buffers of approximately 22,1 metres, slightly less than that of the largest former British steam locomotives. The maximum allowable length for the available turning facilities is to be confirmed.

Any provisions needed for water softening? [Yes]

14.1. The locomotive shall have a smooth outline to minimize its rolling resistance. Partial streamlining of the front of the locomotive, similar to that of the DB 10 Class 4-6-2's, and including a streamlined cover at the front of the smokebox, hinged for access to the smokebox door, shall be used, not only to decrease frontal drag but also to improve exhaust lifting, the whole front of the locomotive being designed to maximize exhaust lifting and hence guarantee good forward visibility. The smokebox streamlined cover may also act as smokebox thermal insulation and may provide the yellow warning panel required for high-speed running. The sides of the locomotive and tender shall be as smooth and free from projecting fittings as possible.

I'd like to see the front streamlining being easily removable [No. It will be hinged to provide access to the smokebox only], and the loco capable of running without it, so that it can be seen for what it is - a development of an ordinary 1950's design incorporating common sense ideas and better engineering. [No. It is not just that. It is a state of the art design based on the size and format of the Class 5 only. It is not merely a development of it. The streamlined front has two important functions: lowering frontal drag and improving exhaust lifting. It is not just a gimmick, the question of exhaust lifting at speed being very important if high speed running is to be allowed]

14.2. Witte type exhaust deflectors shall be fitted.

Will smoke deflectors [They are exhaust deflectors. Do not call them smoke deflectors!] have much effect with a bullet nose? [Both are necessary for creating a synergistic effect] I would have thought that a flat or sloping front would have made them work more effectively.[No] Anyway, will there be much smoke to deflect if it's burning oil? [What about steam on a cold/damp day?]

16.6 Consideration may be given to BFB or Boxpok driving and coupled wheel centres.

I guess BFB are Bullied Firth Brown wheels if my memory is correct. [Yes] How do they differ from the Scullin type? [See Chapelon Fig 41 & 42 p.103]

16.7 Single spring wedges may be preferred to the Franklin double spring type.

Why? (Out of curiosity) [It is argued that the double spring is unnecessary. I have not made up my mind about it yet]

16.8 Consideration shall be given to an outside-frame leading bogie, for reasons of rolling stability and axle bearing accessibility.

I've often wondered why loco bogie wheels are almost always fitted with inside bearings, [usually clearance with the cylinders] because, one of the more endearing features of the GWR Kings was the outside bearings on their front bogie wheels! I would have thought that outside bearings were more accessible for lubrication and maintenance, besides anything else. Nevertheless, appearances are important to a lot of people, and the loco will be better received if its appearance is kept as traditional as practicable! [It must be designed according to functional necessity]

16.9 Disk brakes may be considered for non-powered axles.

Why not powered axles? [Because (1) the axles of the coupled wheels are highly stressed and I am wary of mounting anything on them that gives rise to stress concentrations due to localized extra stiffness (as for example, eccentrics did), and (2) because it has been proven that the scoring action of brake blocks on tyres improves adhesion]

(Omitted from latest version of the specification) A rail washer, using boiler water, may be fitted to clear sand from the rails behind the trailing coupled wheels during normal (non-emergency brake) sanding.

Why is this an option? I thought it was a "good idea" and one that is easy to provide. [Yes, but it is not so necessary with short passenger trains as with long freight trains.}