Vacuum operated pull push gear

[snowy1051]

Hello, I am curious as to how the vacuum operated pull push gear works. I have added PP gear to three of my Jintys and a Coal tank including the extra vacuum bags. When adding these I wondered just how the system works. I can understand the braking application, similar I suspect to the valve in a guards brake van. But just how did the driver in the drive trailer work the regulator on the locomotive footplate? Also, I believe the Jintys were right hand drive and guess that the PP gear was added to the left side for space reasons, I note that the Coal Tanks with PP gear had their hardware on the right hand side,  Thank you for any info. 

[JDW]

That's an interesting question, and something I've always wondered too.  I'm not a steam person, so can't help with the answer, but am glad you asked!

[Titan]

I believe there was a shaft with universal joints under the buffer beam that was connected up.  I suspect in reality it did not give much precision, and was more of a device to tell the fireman what to do!

[The Johnster]

I think you are referring to the GW auto gear when you describe a shaft with universal joints.  This is the only one I know anything about but there were vacuum and air operated systems used on other railways (that's right, chaps, there were other railways besides the GW and some of them ran some interesting trains...).  The GW system was entirely mechanical, and the shafts and joints connected physically to an extension of the regulator lever which protruded beneath the cab of auto fitted locos.  The gear was carried to each end of the loco, and up to two trailers could be coupled to each end, so it was possible to have a 4 coach train with the loco sandwiched in the middle; this was common on the Plymouth area suburbans.

 

Play in the system made it inoperable with more than 2 coaches, and the fireman had more than his usual amount of work to attend to on the footplate, so experienced men were preferred for such work. As well as his usual duties, he had to be responsible for the reversing gear and the small ejector.  He was, of necessity, on his own on the loco for at least half the duty, and all of it if the loco was in a 'sandwich'.  The driver had a regulator lever in the trailer cab, which remotely mechanically operated the one on the footplate through the linkage, and a brake setter to apply the vacuum brake, along with a bell by which he could communicate with the fireman or the guard.  With 2 70 foot trailers, he could be more than 150 feet away from the fireman, possibly out of sight around a bend, who also had to sound the loco's whistle, so he also had a big warning bell on the front of the cab, operated by a foot pedal.

 

I could not state that GW or WR auto trains ever operated with the driver on loco simply propelling the stock to save time and bother when officialdom wasn't looking, so I won't state that.  I am not conclusively deducing that it never happened, either, where the view from the loco cab was not restricted by curvature...  It was a hangover from steam railmotor days, as were similar systems used by the absorbed and constituent companies with similar stock acquired at the Grouping and later converted to the GW standard system.  These coaches, common on Cardiff area suburban jobs, generally ran in their original sets, though.  It was 'handed', so a trailer could not be connected 'backwards'.

 

The LMS used a vacuum operated system which is what the OP is asking about, and the Southern used both air and vacuum systems.  AFAIK, which is not much beyond GW matters in this case, the limit of no more than 2 vehicles from the loco still applied, though I could not say for what reason.  The GW system was physically hard to operate, but presumably vacuum or air operation solved that problem.

 

The first multiple unit trains were electric, and used electric controls from the outset.  Dmus began to be developed in the 30s and made a major impact in the 50s and 60s, these used electric controls along with pneumatic operation of gearboxes.  Nowadays, everything's electrically controlled and long trains can be propelled at high speeds.

[Nearholmer]

I don’t know in detail, but the principle in air and vac is the same: different pressures either side of either a piston or a diaphragm (ambient air pressure vs higher than ambient; ambient vs lower than ambient). By controlling the pressure difference across a piston or diaphragm of fixed area, it’s position can be controlled.

 

Try putting a loose-fitting paper plug inside a straw - by gently sucking or blowing you can move it to any position within the straw; by sucking hard you can choke yourself.

 

One system needs a ‘blower’ (= compressor = air pump), the other a ‘sucker’ (= exhauster).

 

It is usually more difficult to obtain rapid, fine control of a vacuum, partly because a vacuum can’t be stored as compressed air can, so a compressed air system will usually be better, but an exhauster is a very simple thing to make and to use on a steam loco, so vac had its place for basic applications.

 

”The first multiple unit trains were electric, and used electric controls from the outset”

 

Not quite true. The first MU control used on electric trains (more like trams at that stage) was c1882, by Siemens & Halske, and it was mechanical, and fairly rubbish. It wasn’t until the Sprague system some 15 years later that true electrical MU control was perfected.

 

There is a thread on here somewhere about what MU control actually is, and it’s worth bearing in mind that a steam loco working an auto-train isn’t actually under MU control, although it is under (crude) remote control. If you had two locos working the auto-train, the two being simultaneously under remote control through the same system, then it would constitute MU control ........ the word ‘multiple’ is very important here.

 

Kevin

Edited May 3, 2019 by Nearholmer

[Compound2632]

Some photos of the Midland gear, if it helps (NRM Derby negatives, released under the Creative Commons Attribution-NonCommercial-ShareAlike (CC BY-NC-SA 3.0) licence):

 

 

 

DY2269 Motor Train engine 1632

 

 

DY2270 Motor Train engine 1632

 

 

DY2271 Motor Train regulator and brake.

 

S. Summerson, Midland Railway Locomotives Vol. 1 (Irwell Press, 2000) p. 105 describes the system but still not at the level of detail I think you want. There is a Derby drawing 08-7479 "Arrangement of vacuum controlled regulator" reproduced in F. James, D. Hunt and R. Essery, Midland Engines No. 1 (Wild Swan, 1999). This makes it clear that the valve on the side of the smokebox is in the line of the steam pipe, so adjusting its setting regulates the flow of steam to the cylinders. It doesn't do much for the streamlined flow of steam, introducing a double S-bend into the pipe, but for motor train operations I doubt that was a big deal. The lever mechanism for the valve is operated by a piston in the cylinder on the footplating. The pressure in this cylinder is controlled by the "regulator" valve in the driving compartment (and also the locomotive cab, I think) via the second vacuum pipe. The vacuum in this pipe was derived from the vacuum in the brake pipe via a "choke" valve - I assume this means a one-way valve to preserve the brake vacuum when air is admitted to the regulator vacuum pipe. The ordinary way of running was to have the engine's mechanical regulator wide open, with all control via the vacuum-driven regulator.

 

The whistle was controlled by a simple pull-wire that ran along the carriage roof - I think it can be seen hanging down inside the driving compartment. The thingy on the side of the chimney was a pulley bracket to guide the wire.

 

EDIT: on the Coal Tank, the cylinder is mounted higher up, as the sandbox built into the leading splasher gets in the way of mounting it on the footplate as on the Midland engines. The vacuum pipe to drive it can be seen coming up the side of the splasher.

 

Edited May 3, 2019 by Compound2632

[Nearholmer]

One thing that I have gleaned by a bit of googling, and which Compound confirms, is that the MR/LMS vacuum system employed a second regulator valve, in series with that  controlled by the cab regulator handle, which is a distinction from systems, which simply operated the cab handle by "a ghostly hand". This allowed the fireman to intervene and shut steam off in the event of wheel-spin, which I'm not sure he could do with the other systems (depends if there was a spring in the linkage, I guess).

 

It does all appear very simple, but I am slightly confused as to "which way round" it operated .......... does creating a vacuum open the regulator, or close it? 

 

The choke valve probably acted to control the rate at which vacuum could be created in the second vac pipe (a little hole effectively, as well as a one-way valve action), so that the action of a nice big ejector sucking away at the brake pipe couldn't overcome the "deliberate leak" created by the driver's regulator control valve in the cab of the coach.

 

My surmise is that, with the driver's valve closed, vacuum was relatively slowly created, through the choke-valve, the vacuum piston rose, driving up the rod and bell-crank, opening the regulator quite gently. Once desired speed was reached, the driver could then partially open his valve, creating leakage equal to the rate of evacuation through the choke-valve, causing the vacuum piston to remain static. Then, to slow down or stop, the driver could open his valve further, allowing more air in than the ejector could get out via the choke, allowing the vacuum piston to fall, ultimately closing the regulator.

 

If I've got this right, it is reasonably "right side" in its failure modes, because:

 

- a blocked choke valve will cause pressure in the regulator control vac pipe to slowly rise through leakages, ultimately closing the regulator, even if the driver keeps his valve closed;

 

- a fractured/leaky regulator control vac pipe will do the same, probably quicker; and, 

 

- the mass of that ruddy great rod and crank, and the vacuum piston itself, all pull down under the force of gravity, tending to close the regulator.

 

My guess would be that the down side of all this would be an inability to obtain snappy acceleration. The way to get that would be to have the system operate "the other way round", but that would introduce some scary failure modes ........ maybe it did work like that, and they relied on the fireman to close the loco cab regulator if the train appeared to be galloping away.

 

As an aside, I've discovered that the SECR also used a mechanical system on some trains, specifically the motor trains worked by P class locos, although the ex-LCDR motor trains used a compressed air system, which seems to have been the norm (logically so) for any railway that used air brakes.

 

Does the "manual for steam locomotive engine men" cover this? My copy is deeply buried. 

 

Can anyone confirm, correct, or contradict my surmise?

 

Kevin

 

PS: Notice the little chain to the left of the regulator valve in that photo of the driving compartment? I wonder if that was a primitive "dead mans handle", a chain that was meant to be clipped to the driver's belt, which pulled out a pin, opening the vac pipe to atmosphere, if he collapsed? Or, did it do the exact opposite, and pull a pin causing the regulator valve to be blanked-off, so that it didn't cause a leak and create trouble when the train was being driven in the other direction, from the loco, using the normal regulator handle and valve?

Edited May 3, 2019 by Nearholmer

[Compound2632]

The Midland Railway Study Centre holds a copy of the 1946 "Instructions respecting the working of rail motors and motor trains" (Item No. 77-11931) and Summerson reproduces the title page of the 1935 version; from snowy1051's photos, the Midland system of 1908 seems to have remained the LMS standard system..

 

Nearholmer, one thing I don't understand in your deduction of how the system worked is the suggestion that with the locomotive's mechanical regulator open, the vacuum-controlled regulator would effectively "leak" open. I would have thought there would have to be some positive action on the driver's part to start the train. 

 

I'm fairly sure the system didn't rely on the fireman working the mechanical regulator, though I don't think I've seen that in print. The Midland system is akin to DCC rather than DC!

 

 

[Nearholmer]

I hope I didn’t say that, because it wasn’t what I meant. I surmise that the driver would close his valve, allowing vacuum to be created, thereby opening the remotely controlled regulator valve. Leakage would, I think, soften or destroy the vacuum.

[Compound2632]

In the photo of the driving controls, both the regulator and brake valves are seen with the lever to the right. So presumably that's the "open" or "no vacuum" position: regulator closed, brakes on. It just seems counter-intuitive to move the lever anti-clockwise to start - but perhaps turning clockwise to increase something is just a modern thing. 

[Nearholmer]

There is no vacuum in the photo, as confirmed by the position of the needles in the dials, I think.

 

If you think about it, there is a logic in that ‘closed valve; vacuum up’ is the ‘go’ position for both brake and regulator.

 

Whether control valves were conventionally left open or closed in the quiescent, out of service state, I don’t know. As an electrical chap, I would want the system ‘deenergised’,  and any switches that can lead to energisation to be in the open position when quiescent, but Victorian mechanical people may have had different logic.

[Compound2632]

4 minutes ago, Nearholmer said:

There is no vacuum in the photo, as confirmed by the position of the needles in the dials, I think.

 

Indeed, I didn't notice that. So the control valves are open and need to be closed to start. No doubt the handbrake is also screwed on!

 

Edwardian (1908), though Victorian in the sense that that's when the designer would have had his training.

 

I hope snowy1051 is going to look in and thank us for our erudition, otherwise we're just talking amongst ourselves! 

Edited May 4, 2019 by Compound2632

[Nearholmer]

Ah, but it exercises my grey cells, which are in need of it.

[Compound2632]

... but hang on, modern vacuum gauges have the needle hard up against the clockwise stop when at atmospheric pressure; the needle goes anti-clockwise as the pressure decreases.

[Nearholmer]

Here is a railway one ..... but, you can find railway ones that read the other way, and two-scale ones where one needle moves clockwise and the other anti-clockwise.

 

In the photos, the control valves may already be closed, but no vacuum created because no exhauster is in action ....... hence my querying what state they conventionally left things in when quiescent ...... I simply don’t know whether the two valves are open or closed.

 

and, I’m still not totally convinced I’ve got the working principles overall right.

 

 

Edited May 4, 2019 by Nearholmer

[Compound2632]

The image available for download is not of the highest resolution but peering closely at DY2271 I think the middle gauge at least reads 0, 5, 10, 15, 20, 25, 30 clockwise; the unit being inches of mercury in a slightly convoluted way: atmospheric pressure should support a column of mercury approximately 30 inches high, so the scale indicates how many fewer inches would be supported by the pressure in the pipe. It's a case of "you know what I mean". In SI units, with rounding, the gauge would be labelled 100 kPa, 83 kPa, 67 kPa, 50 kPa... 

 

Anyway, confirms that the equipment in the photo is "up to air".

[Nearholmer]

IIRC (from forty years ago) vacuum braking systems were expected to sustain 21 inches, or maybe 22, and I seem remember reading that the GWR specified a very slightly ‘higher’ vacuum than everybody else.