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Merchant Navy poor adhesion


Alister_G
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Why was it that the Merchant Navy class and modified merchant Navy class had such poor adhesion?

 

What was it about the design that made this such a problem?

 

I've just been watching some video on youTube of locos manouvering on Nine Elms shed, and there's footage of a loco "French Line CGT" moving off the turntable. When the driver tries to brake, even at low speed, the main driving wheels lock up and the loco just keeps on going.

 

Obviously the track must be quite slippery, but given the loco is light engine and moving slowly, I would have thought that stopping the main driving wheels turning would stop the engine very quickly.

 

Then there is further footage of another merchant Navy setting off from the shed - again light engine, and it is spinning the drivers as it sets off.

 

I don't think I've ever seen another class of engine which was so sensitive.

 

Interested in any thoughts?

 

Al.

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I've seen 'Gordon' on the Severn Valley behaving similarly with no more than a bit of dew on the rail on an otherwise fine and dry sunny summer morning.  Pacifics of all sorts were notorious for poor adhesion on starting, as the adhesive weight on the drivers is relieved by the trailing wheels that allow the loco to have a wide firebox, and this may well account for the sliding, though any loco will pick up and slide on a greasy rail.  On top of all that, maybe our drivers are a bit heavy handed; they weren't all as good as some of them thought they were...

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In their original forms, the Bulleid Pacifics were prone to leaking oil, which did nothing for adhesion either accelerating or braking. In rebuilt form they were better, but still had a lot of power for their weight, and, as The Johnster just said, as Pacifics, they still had the limitations that applied to that wheel arrangement in general.

There is an archival film on YouTube somewhere with steam entering and leaving Waterloo, and in one clip, a rebuilt Light Pacific running light engine sails gracefully past the camera in reverse with the driving wheels locked solid! If I can find this footage again I'll edit this post to add it in.

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All Pacifics, as The Johnster says, were light on their feet. They had a lot of power but limited adhesive weight, and when pulling strongly some of this would be removed from the trailing coupled axle and transferred to the Bissel truck, reducing it further.

 

Almost all engines will slip, even light engine, if the regulator is opened too fat too quickly. Rail condition also plays a part: slippery surface, curvature and unevenness, Crossing pointwork also transfers weight to different axles and can initiate a slip.

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7 hours ago, SRman said:

In their original forms, the Bulleid Pacifics were prone to leaking oil, which did nothing for adhesion either accelerating or braking. In rebuilt form they were better, but still had a lot of power for their weight, and, as The Johnster just said, as Pacifics, they still had the limitations that applied to that wheel arrangement in general.

There is an archival film on YouTube somewhere with steam entering and leaving Waterloo, and in one clip, a rebuilt Light Pacific running light engine sails gracefully past the camera in reverse with the driving wheels locked solid! If I can find this footage again I'll edit this post to add it in.

 

I think this is spot on. Given many Southern lines had weight limits on the low side, Bulleid had to keep the weight down, so leading to relatively poor adhesion.

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5 hours ago, Corbs said:

@Gibbo675 may be able to shed more light on this as he’s had a lot of experience with Light Pacifics

Hi Corbs,

 

I have had a good read of the comments posted so far and some are quite usual in that they display a lack of understanding of 1. basic physics 2. how locomotives actually operate and 3. the detail differences within various types of locomotive design and modes of operation that such comments totally ignore. This is before we even mention ham fisted drivers, poorly set up or maintained locomotives, tyre wear and track defects.

 

Myth and legend is a mighty force and efforts to dispel it are generally fruitless due to the gusto to which myth and legend progenitors apply yet more myth and legend to any answers to questions raised. Why spoil the truth with a good myth and legend !

 

There will now a barrage of abuse toward an man that not only rebuilt Tangmere after a so called Bulleid expert (myth and legend twerp) had cocked it up in the first place leading to a catastrophic failure to its valve gear. You may wish to note that after I had rebuilt it Network Rail upgraded it from a class 7P to a class 8P.

 

I might also add that 34067 didn't very often slip when I was driving it if that tells you any thing.

 

Gibbo.

Edited by Gibbo675
I have changed on word, "they" for "some"
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Thanks for all the replies so far, and thanks Gibbo for your thoughts.

 

It's not so much the wheel spin that surprised me about that clip, as you say heavy-handed driving can cause that on any loco, but it was more the fact that a loco with no stock coupled  couldn't stop itself at slow speed even with the main driving wheels locked.

 

I would have thought that 90-odd tons of weight on the wheels would have ensured decent contact on the rail-head, I can only assume that that bit of track was exceptionally greasy.

 

Al.

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For what it's worth, a modern unit, well ours are all 20+ years old now, will slip and slide quite easily on a damp rail. 

All wheels braked with over 10 tons on each axle, even at very slow speed they can and do slide surprisingly easily.

A few years ago I was working back from Matlock on one of the illumination evenings. It had been fine going up half an hour or so earlier but, cautious approach notwithstanding, we nearly overran at least one station and came to a shuddering halt with all wheels locked.

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2 hours ago, Gibbo675 said:

Hi Corbs,

 

I have had a good read of the comments posted so far and they are quite usual in that they display a lack of understanding of 1. basic physics 2. how locomotives actually operate and 3. the detail differences within various types of locomotive design and modes of operation that such comments totally ignore. This is before we even mention ham fisted drivers, poorly set up or maintained locomotives, tyre wear and track defects.

 

Myth and legend is a mighty force and efforts to dispel it are generally fruitless due to the gusto to which myth and legend progenitors apply yet more myth and legend to any answers to questions raised. Why spoil the truth with a good myth and legend !

 

There will now a barrage of abuse toward an man that not only rebuilt Tangmere after a so called Bulleid expert (myth and legend twerp) had cocked it up in the first place leading to a catastrophic failure to its valve gear. You may wish to note that after I had rebuilt it Network Rail upgraded it from a class 7P to a class 8P.

 

I might also add that 34067 didn't very often slip when I was driving it if that tells you any thing.

 

Gibbo.

Well, that was a pretty generalised put down of everyone who replied but completely failed to come up with anything of substance. Please don't generalise that no-one else knows anything about the subject: I and no doubt others on here know more than you give credit. Personally, I hold an Honours Degree in Mechanical Engineering and taught the subject for fourteen years. I have also been involved in the preservation, maintenance, repair and operation of steam locomotives since 1969. If you want to talk of coefficients of friction, factors of adhesion, power versus torque and all the rest, please do so, but don't just give us unsupported opinions like this.

 

Slipping can be caused by many factors, and while poor maintenance (incorrect spring adjustment, binding axleboxes, stiff regulator valve, etc.), indifferent track, including surface contamination, poor alignment in the vertical plane, and poor driving technique can contribute to slipping, as a general rule they are one-offs and apply to all types of loco. Pacifics are generally more prone to slipping than a 4-6-0, but I'm not sure the Bulleids are any worse than any other designers'.

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It used to be rather entertaining watching 'Merchants', in particular, trying to get away and get a grip of their train on the Up Fast at Basingstoke where the rail was a bit greasy plus the track geometry didn't help.  They tended (and these were the rebuilds) to be worse than the light pacifics from what I regularly saw there but you'd even see a Standard 5 slip occasionally.

 

But it was - as was well known - a common problem with pacifics.  Brits were notorious for it when trying to get away on the Down Main at Reading with loads which 'Castles' always managed to get moving with no fuss at all although I can't ever remember seeing a Brit slip at Liverpool Street (maybe the loads were lighter than on the Western?).

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20 minutes ago, LMS2968 said:

Well, that was a pretty generalised put down of everyone who replied but completely failed to come up with anything of substance. Please don't generalise that no-one else knows anything about the subject: I and no doubt others on here know more than you give credit. Personally, I hold an Honours Degree in Mechanical Engineering and taught the subject for fourteen years. I have also been involved in the preservation, maintenance, repair and operation of steam locomotives since 1969. If you want to talk of coefficients of friction, factors of adhesion, power versus torque and all the rest, please do so, but don't just give us unsupported opinions like this.

 

Slipping can be caused by many factors, and while poor maintenance (incorrect spring adjustment, binding axleboxes, stiff regulator valve, etc.), indifferent track, including surface contamination, poor alignment in the vertical plane, and poor driving technique can contribute to slipping, as a general rule they are one-offs and apply to all types of loco. Pacifics are generally more prone to slipping than a 4-6-0, but I'm not sure the Bulleids are any worse than any other designers'.

My dear LMS 2968,

 

May I ask you a question:

 

Have you, at any time, put forward any of what may be termed "myth and legend" ?

 

The reason I ask is that for if you have not done so then you for one, along with all the others that have not done so, are not included in my put down of  "myth and legend" and only of "myth and legend".

 

Do please see what I have written for what it says and not what you wish to be offended by.

 

Gibbo.

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1 hour ago, Alister_G said:

Thanks for all the replies so far, and thanks Gibbo for your thoughts.

 

It's not so much the wheel spin that surprised me about that clip, as you say heavy-handed driving can cause that on any loco, but it was more the fact that a loco with no stock coupled  couldn't stop itself at slow speed even with the main driving wheels locked.

 

I would have thought that 90-odd tons of weight on the wheels would have ensured decent contact on the rail-head, I can only assume that that bit of track was exceptionally greasy.

 

Al.

Hi Al,

 

My thoughts so far have been generally tongue in cheek as a way to weed out all the "Bulleid Bashers" (a lot of which have never had anything to do with them other that look at photographs, no doubt there will be some that bite upon the bait including what I have just written !), I was going to wait until all the sillier comments had been made and then comment sensibly to explain the silliness away.

 

But what the hell its Sunday and I wrote what I wrote.

 

I shall write about what I know through my experience of taking a Barry wreak to a working main line locomotive in good time when a few more comments are available to be challenged should that they be nonsense with good reason and experienced example.

 

Gibbo.

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36 minutes ago, Gibbo675 said:

I shall write about what I know through my experience of taking a Barry wreak to a working main line locomotive in good time when a few more comments are available to be challenged should that they be nonsense with good reason and experienced example.

 

Gibbo.

I look forward to reading this, and that wasn't meant sarcastically; it should be an interesting read, having done similar myself although certainly not alone.

 

The Bulleids are not locos with which I have great experience: being born and raised in Liverpool, they were hardly classes I came across very often. But I'm not a 'Bulleid basher'; they did their jobs probably as well as any other similar class. As to adhesion, the two which graced SVR metals did seem to have problems leaving Highley northbound. Perhaps all the SVR's drivers are ham fisted?

 

If you think a comment made on here is wrong you have every right to challenge it and provide the evidence to prove your point. You might well be right. But belittling other people's opinions as ' myth and legend' doesn't win friends, or put them into a receptive frame of mind when they read on.

 

To be honest, I don't really want to continue this personal backbiting by either of us, it isn't what the thread or this forum is about. But tell us why you believe what was said was 'myth and legend'; you might win some converts.

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3 hours ago, Talltim said:

You have to wonder why more effort wasn’t given to designing steam locos with all wheels powered. Obviously that wouldn’t help with braking, were the non-driving wheels brakes?

There have been various attempts at this besides Leader.  A loco for fast running needs to have carrying wheels to ride acceptably, safely, and without destroying the permanent way, but these and the axles and frames associated with them are dead weight that contribute nothing to traction and in fact reduce it on the driving wheels by taking weight that should bear down on the drivers.  A pacific has a big boiler to supply the very large amounts of steam required for heavy express passenger work, and this needs a big, wide, firebox to produce the steam quickly, that cannot be fitted between the rear driving wheels; it can't be a long thin firebox as the fireman can't reach the front of it, so carrying wheels in the form of a pony truck beneath the cab take the weight of that end of the loco, assisted by the front of the tender.  The way this is set up has a direct bearing on the tractive weight available to the driving wheels and the ride of the loco, and the set up is not always perfect...

 

'Booster' engines, smaller steam engines acting on the trailing wheels, were tried on some GNR and LNER locos, and IIRC the NER had a go with some Atlantics as well.  Back in the 1850s, the GN's CME Archibald Sturrock designed a heavy goods 0-6-0 with a steam tender, an extension of the same principle.  The boosters were intended to assist with starting and steam to them was shut off once the loco was under way and gripping properly.  Success was elusive, and the idea never seemed to have been thought worth pursuing or developing.  I suspect it's one of those things that sound like a good idea but introduces more difficulty than it solves in practice.  The crews complained about the Sturrock steam tender locos that they were being asked to fire and drive 2 locos; they may have had a point!

 

Pacifics are in some ways an extension in principle of atlantics, also designed for fast running with a big boiler and wide firebox.  But the 4 coupled nature of atlantics sometimes made them better grippers than bigger Pacifics, and able to reliably start heavier trains, because although a pacific has an extra pair of wheels to power the train with, the adhesive weight bearing down on (which is the thing that prevents it from slipping) is less than on an atlantic.

 

Braking is in some ways a reversal of some of the principles outlined (very crudely, I'm no engineer) above.  It may seem counter intuitive, but a greater adhesive weight on the braked wheels enables more effective braking and less chance of the wheels 'picking up' and sliding.  Once sliding starts, braking is very ineffective and the brake have to be fully released to get the wheels turning again, and another attempt at braking made.  Sliding is Very Bad, as if it is unchecked it will wear a flat on the wheel's tyre which will damage the track and require the loco to be taken out of service for reprofiling on a wheel lathe, not to mention being a very poor and ineffective way of stopping a loco.  Similarly, the reversal of the loco in an attempt to stop in an emergency often seen in Westerns is dramatic effect for the camera; the loco can stop much more quickly with steam shut off and the brakes properly applied.

 

Braking performance is better with a train attached, as the train's brakes can assist, which is why light engines are speed restricted to 70mph so as to be able to pull up in the prescribed braking distances between signals.

 

All this has a bearing on model railway practice, as the ballasting of model locos to improve pickup and haulage addresses a very similar problem and works in much the same way!

 

On some locos, the non-driving wheels were braked, and on the GW, the loco's brakes were operated by the vacuum brake as well as the loco's steam brake, read straight air brake for diesel or electric traction.

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21 minutes ago, LMS2968 said:

I look forward to reading this, and that wasn't meant sarcastically; it should be an interesting read, having done similar myself although certainly not alone.

 

The Bulleids are not locos with which I have great experience: being born and raised in Liverpool, they were hardly classes I came across very often. But I'm not a 'Bulleid basher'; they did their jobs probably as well as any other similar class. As to adhesion, the two which graced SVR metals did seem to have problems leaving Highley northbound. Perhaps all the SVR's drivers are ham fisted?

 

If you think a comment made on here is wrong you have every right to challenge it and provide the evidence to prove your point. You might well be right. But belittling other people's opinions as ' myth and legend' doesn't win friends, or put them into a receptive frame of mind when they read on.

 

To be honest, I don't really want to continue this personal backbiting by either of us, it isn't what the thread or this forum is about. But tell us why you believe what was said was 'myth and legend'; you might win some converts.

Hi LMS,

 

Thank you for your kind reply.

 

I shall in time gladly reply to comments that do not make any real sense or are that are just plain wrong in good time. I do not and shall not offer my experience as a belittlement to anyone at all, more that people may actually learn by way of my experience.

 

One thing I can tell you for sure bye way of bitter experience is that, "when they are good they are very, very good and when they are bad they are horrid!", to quote the well know nursery rhyme.

 

Gibbo.

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5 hours ago, Alister_G said:

 

This is the video I was watching.

 

The sequence I'm talking about occurs near the end, at around 43:40, but the whole thing is worth a watch.

 

 

 

Al.

Thanks for sharing this as it also includes a tour travelling from Middleton Junction to Oldham which is close to where I was brought up. Only ever seen 08's on freight to Chadderton Coal yard in my time and the line is long lifted.

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2 hours ago, The Stationmaster said:

It used to be rather entertaining watching 'Merchants', in particular, trying to get away and get a grip of their train on the Up Fast at Basingstoke where the rail was a bit greasy plus the track geometry didn't help.  They tended (and these were the rebuilds) to be worse than the light pacifics from what I regularly saw there but you'd even see a Standard 5 slip occasionally.

 

But it was - as was well known - a common problem with pacifics.  Brits were notorious for it when trying to get away on the Down Main at Reading with loads which 'Castles' always managed to get moving with no fuss at all although I can't ever remember seeing a Brit slip at Liverpool Street (maybe the loads were lighter than on the Western?).

 

Stratford was the first shed allocated Brits and considering the motive power they’d had to deal with before 1951,hardly surprising that they learned to love them quickly and responsively.But yes,Canton’s could slip impressively.They looked good though......

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Canton Brits could take several minutes to clear Cardiff General’s platform 2 and were not much better getting away from Newport; Castles with similar loads just sat down on their rear drivers and set off with little fuss.  This is fundamentally the difference between a good 2 cylinder pacific and one of the best 4 cylinder 4-6-0s of all time; one is not comparing like for like!

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Some of the NYC Hudsons had powered trailing bogies to try and improve performance but it doesn't seem to have been particularly successful. Did any other designs do that?

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