Cutoff to brake steam engine

[Mim]

I've never driven a steam loco, so looking for some real world 1:1 scale experience. The situation:

 

A small steam locomotive used for shunting, or on an unfitted freight train. The only brakes are on the loco and on the brake van. Would the cut off control ever be used as a brake to slow the loco, as either light engine, or a train? Letting steam in to the cylinders against the direction of travel. Either in combination with conventional brakes, or alone? If not, why not? If it is, how much cutoff would be set?

 

Mim

[LMS2968]

That would be done only in emergencies. It imposes tremendous stresses on the working components and is not easily controllable. Having said that, it was said to be common practice on the LNWR, where the loco brakes were generally appalling.

[Gordon A]

The steam brake on a shunting loco would be the brake to use, quick to apply and quick to release.

Using the reverser would be a no no as LMS2968 explained.

The hand brake is only good for parking a static loco. To slow to apply and not enough force.

On the industrials I have driven you want at least 100lbs showing on the clock, preferable a bit more.

[Mim]

Thanks LMS2968 and Gordon A. That helps hugely. The reason for the question is that I am considering building a controller that goes some way to simulating steam loco controls, with throttle, cut off and brake, plus a dial to vary the amount of load on the engine, from light engine up the maximum it can pull. It would make controlling the train more involving and need some more skill to get it to do what you want. It would assume that you had a competent fireman to give you steam, but a built in simulated pressure gauge would show you up and the maximum speed would drop if you were not efficient in your driving.

 

Very early days yet. Just trying to think through the options and the control algorithms to use. There is a lot of experience of train simulation I might be able to use.

[Guy Rixon]

Reversing the engine for emergency braking was common in the 19th century. A lot of the accident-investigation reports read like "on seeing the obstruction, the driver reversed his engine but was unable to stop in time". (Note the selection effect: where the driver did manage to stop by reversing the engine there was no enquiry and we don't hear of the incident.)

[eastglosmog]

The coefficient of friction of steel on steel is higher when the point of contact is static than when it is sliding.  To get the highest braking force that the rail/wheel contact can provide, you need to keep the wheel rolling to maintain the static contact between wheel rim and rail.  If you reverse the engine (or even just lock the wheels), you will inevitably cause sliding between wheel and rail, thus reducing the available  friction force.

[LMS2968]

While there is much in what you say concerning the coefficients of rolling and sliding friction, and this is the theory behind modern ABS braking, you wouldn't necessarily lock the wheels or reverse them by this action. There is a report about one of the 8Fs sent to Persia and sent out on a heavily loaded test run over the mountains. The engine climbed alright, but speed picked up coming down the other side. The driver pulled her into back gear - 2% only - which stabilised the speed but didn't reduce it, but didn't lock the wheels either. I can't remember the actual figures but the train continued  for something over ten miles before it stopped. The engine was well and took the train on, but the crew allowed two hours for the cylinders to cool, having become very hot due to the excessive compression.

[Blandford1969]

3 hours ago, LMS2968 said:

While there is much in what you say concerning the coefficients of rolling and sliding friction, and this is the theory behind modern ABS braking, you wouldn't necessarily lock the wheels or reverse them by this action. There is a report about one of the 8Fs sent to Persia and sent out on a heavily loaded test run over the mountains. The engine climbed alright, but speed picked up coming down the other side. The driver pulled her into back gear - 2% only - which stabilised the speed but didn't reduce it, but didn't lock the wheels either. I can't remember the actual figures but the train continued  for something over ten miles before it stopped. The engine was well and took the train on, but the crew allowed two hours for the cylinders to cool, having become very hot due to the excessive compression.

Blimey at 2% unless going very quickly that would knock the frames about an awful lot. 

[jim.snowdon]

8 hours ago, Guy Rixon said:

Reversing the engine for emergency braking was common in the 19th century. A lot of the accident-investigation reports read like "on seeing the obstruction, the driver reversed his engine but was unable to stop in time". (Note the selection effect: where the driver did manage to stop by reversing the engine there was no enquiry and we don't hear of the incident.)

Bear in mind that when this measure of last resort was common, the ability of locomotives to haul trains rather exceeded their ability to stop them. Stopping was largely dependent upon hand brakes applied on the tender and various carriages in the train. Sanity didn't really start to reign until the general adoption of vacuum or air braking on every carriage. 

 

7 hours ago, eastglosmog said:

The coefficient of friction of steel on steel is higher when the point of contact is static than when it is sliding.  To get the highest braking force that the rail/wheel contact can provide, you need to keep the wheel rolling to maintain the static contact between wheel rim and rail.  If you reverse the engine (or even just lock the wheels), you will inevitably cause sliding between wheel and rail, thus reducing the available  friction force.

Actually, no. In a macro sense, the coefficient of sliding friction is lower than that for static friction, but at a detailed level, at low levels of slip, the coefficient of friction increases significantly before dropping off. Quite a few modern heavy haul locomotives exploit that effect, generally known as micro slip, to maximise the tractive effort that can be delivered to the rail head. Early examples were the Class 59s and the DB's E120 locomotives.

It is also behind why wheels don't suddenly slip, unless the tractive effort grossly exceeds the available grip; if it is just too much, the onset of slip is marked by a distinct shuddering (which a good driver can read in controlling his locomotive).

 

Jim 

[The Stationmaster]

14 hours ago, jim.snowdon said:

Bear in mind that when this measure of last resort was common, the ability of locomotives to haul trains rather exceeded their ability to stop them. Stopping was largely dependent upon hand brakes applied on the tender and various carriages in the train. Sanity didn't really start to reign until the general adoption of vacuum or air braking on every carriage. 

 

Actually, no. In a macro sense, the coefficient of sliding friction is lower than that for static friction, but at a detailed level, at low levels of slip, the coefficient of friction increases significantly before dropping off. Quite a few modern heavy haul locomotives exploit that effect, generally known as micro slip, to maximise the tractive effort that can be delivered to the rail head. Early examples were the Class 59s and the DB's E120 locomotives.

It is also behind why wheels don't suddenly slip, unless the tractive effort grossly exceeds the available grip; if it is just too much, the onset of slip is marked by a distinct shuddering (which a good driver can read in controlling his locomotive).

 

Jim 

You obviously haven't ridden on a Class 59 when the 'super creep' control cuts in - the nearest equivalent would, I think, be riding on angle grinder being used on some very tough material (and it sounds a bit like that too).

[johnb]

What a lovely descriptive description of what must be an interesting sensation on one's rear end!!!

[Gibbo675]

On 16/03/2019 at 12:26, LMS2968 said:

That would be done only in emergencies. It imposes tremendous stresses on the working components and is not easily controllable. Having said that, it was said to be common practice on the LNWR, where the loco brakes were generally appalling.

Hi 2968,

 

I stopped Tangemere after a wrong side vacuum brake failure by the use of compression. The reason I chose to do this over using the independent steam brake was that the steam brake only gives approximately 75% brake force of axle loading whereas using back gear and compression I managed to use the full axle loading of the wheels as a brake force, it pulled the train up without slipping and I only used the steam brake at below 5mph. I achieved control of the braking effect by pumping the regulator.

 

Depending upon the lead figures of the gear and also the type of gear employed some engines go quite well in back gear at speed. Walschearts has fixed lead and with Stepenson's the lead increases the nearer to mid gear it is set, therefore with Walschearts gear the reversed cut off will have to exceed the lead figure before any difference is noted. An 8F in 2% back gear will not be effectively working in compression and would suggest that 20% would be the likely minimum that would have any retarding effect. With Stephenson,s gear the lead and cut off exchange functions when placed in opposite gear to direction of travel which causes some locomotives to run more freely at speed at reduced reversed cutoffs than in the same figure cut off for the direction of travel up to about 5-10% depending upon type.

 

As for stresses induced into the connecting rods, crank pins and motion &c. I would suggest that the greatest problem is coasting at speed with the regulator firmly shut.

 

Gibbo.

[Titan]

21 minutes ago, Gibbo675 said:

Hi 2968,

 

As for stresses induced into the connecting rods, crank pins and motion &c. I would suggest that the greatest problem is coasting at speed with the regulator firmly shut.

 

 

Doesn't Tangemere have snifting valves? For those that are not aware, snifting valves automatically open when there is a vacuum in the cylinders, like for example coasting with the regulator shut. This would mean that the load should be minimal. 

 

I always thought reversing the engine was bad practice due to the potential sucking of smoke and soot from the smokebox into the cylinders, which would not do them any good at all.

[LMS2968]

I've looked in vain for the report. I thought it might have been in Alan Wilkinson's Stanier 8Fs at Work but no joy; it was probably in an article in one of the many 8F Society's magazines, so I doubt I'll find it.

 

But I definitely remember it and the figure of 2%, and wondered at the time if it might be a misprint as I couldn't see that reverse cut-off providing the necessary braking effort. By the way, not all the war-built 8Fs were fitted with decompression valves, as I just know someone will mention them! But I don't know if the engine in this incident was so fitted or not.

[Gibbo675]

21 hours ago, LMS2968 said:

I've looked in vain for the report. I thought it might have been in Alan Wilkinson's Stanier 8Fs at Work but no joy; it was probably in an article in one of the many 8F Society's magazines, so I doubt I'll find it.

 

But I definitely remember it and the figure of 2%, and wondered at the time if it might be a misprint as I couldn't see that reverse cut-off providing the necessary braking effort. By the way, not all the war-built 8Fs were fitted with decompression valves, as I just know someone will mention them! But I don't know if the engine in this incident was so fitted or not.

Hi 2968,

 

I think there is two ways to look at the 2% figure, either a misprint for what may be 20% or that it actually was 2% because that is where the crew got a comfortable ride when working down hill. Other than interest value I don't suppose it matters that much.

 

Gibbo.

[Gibbo675]

21 hours ago, Titan said:

 

Doesn't Tangemere have snifting valves? For those that are not aware, snifting valves automatically open when there is a vacuum in the cylinders, like for example coasting with the regulator shut. This would mean that the load should be minimal. 

 

I always thought reversing the engine was bad practice due to the potential sucking of smoke and soot from the smokebox into the cylinders, which would not do them any good at all.

Hi Titan,

 

No locomotive has snifting valves on its steam circuit but some, although not the Bulleid Pacifics, do have anti vacuum valves fitted. In my view anti vacuum valves are a complete waste of time as they are in no way large enough in area to admit enough air to stop the smoke box gasses being drawn into the valve chests or the cylinders at anything above walking pace, 2-3 mph.

 

Coasting at speed with the regulator shut will draw smoke box gasses and char into the valve chests and cylinders irrespective of cut off, although the longer the cut off the greater the effect.

 

Either full forward or full back gear at speed will certainly not do a locomotive any good in either direction with the regulator open, try putting a car into first gear or even reverse at 60 mph and see how that ends up. However if a locomotive is wound into gear opposite to direction of travel at speed with the regulator closed it will attempt to pump compressed air into the boiler, as you note, sucking the air in through the blast pipe, anti vacuum valves will not be of any help whatsoever in preventing this from happening at speed.

 

Gibbo.

[Blandford1969]

2 hours ago, Gibbo675 said:

Hi 2968,

 

I think there is two ways to look at the 2% figure, either a misprint for what may be 20% or that it actually was 2% because that is where the crew got a comfortable ride when working down hill. Other than interest value I don't suppose it matters that much.

 

Gibbo.

There is one thing though and that's I can't think of any screw reverser which could be so precisely put at 2%, 20% yes, Its certainly not possible with a lever as the positions are fixed by the notches.  

[Gibbo675]

13 minutes ago, Blandford1969 said:

There is one thing though and that's I can't think of any screw reverser which could be so precisely put at 2%, 20% yes, Its certainly not possible with a lever as the positions are fixed by the notches.  

Hi Blandford,

 

May I also add that some reverser scales were deliberately falsified to prevent damage to the locomotives due to grandmothers that do not require to be taught how to suck eggs.

 

Gibbo.

[Blandford1969]

49 minutes ago, Gibbo675 said:

Hi Blandford,

 

May I also add that some reverser scales were deliberately falsified to prevent damage to the locomotives due to grandmothers that do not require to be taught how to suck eggs.

 

Gibbo.

That would not be a surprise

[jim.snowdon]

4 hours ago, Gibbo675 said:

Hi Blandford,

 

May I also add that some reverser scales were deliberately falsified to prevent damage to the locomotives due to grandmothers that do not require to be taught how to suck eggs.

 

Gibbo.

Yes, there were instances, but none so far as I am aware where the reason was to prevent damage to the locomotive.

 

Jim

[The Johnster]

7 hours ago, Gibbo675 said:

Hi Titan,

 

No locomotive has snifting valves on its steam circuit but some, although not the Bulleid Pacifics, do have anti vacuum valves fitted. In my view anti vacuum valves are a complete waste of time as they are in no way large enough in area to admit enough air to stop the smoke box gasses being drawn into the valve chests or the cylinders at anything above walking pace, 2-3 mph.

 

Coasting at speed with the regulator shut will draw smoke box gasses and char into the valve chests and cylinders irrespective of cut off, although the longer the cut off the greater the effect.

 

Either full forward or full back gear at speed will certainly not do a locomotive any good in either direction with the regulator open, try putting a car into first gear or even reverse at 60 mph and see how that ends up. However if a locomotive is wound into gear opposite to direction of travel at speed with the regulator closed it will attempt to pump compressed air into the boiler, as you note, sucking the air in through the blast pipe, anti vacuum valves will not be of any help whatsoever in preventing this from happening at speed.

 

Gibbo.

Back in the late 80s, a bloke I used to see up the pub was a taxi driver (he didn't go the pub on duty) who was most miffed at the fact that his boss had given him a Lada to drive.  Taking matters into his own hands he decided to murder the car, and took it along Cardiff's Eastern Avenue bypass, not speed restricted in those days, late one night and, at over 70mph, 'accidentally' put the car into reverse and let the clutch out.  The Lada came to a fairly rapid standstill on the hard shoulder with some unpleasant smells and a bit of smoke and steam, and in total silence.  Convinced he'd successfully done the foul deed, he then proved the point by turning the ignition off, then back on again, and then turned the key for a start.

 

Brother Lada turned over 3 or 4 times, fired up, and roared into it's usual noisy and rather lumpy life as it nothing untoward had happened.  My friend's attitude to Ladas changed instantly; he had enormous respect for the quality of engineering that would allow such a thing to happen!  'Try doing that in a Ferrari', he'd say...

[Flying Pig]

11 hours ago, The Johnster said:

Taking matters into his own hands he decided to murder the car

 

In Soviet Russia, car murders you. And that's Comrade Lada if you please.

[The Johnster]

Agreed.  My dad was daft enough to own one in his dotage, because they were cheap and he hated spending money.  The best bit was the headlights, though the heater was pretty good if noisy (I suppose that's needed in Russian winters), and when he got to old to drive he offered it to me.  And I'd never done anything to deserve such treatment, honest...

[Gibbo675]

16 hours ago, The Johnster said:

Back in the late 80s, a bloke I used to see up the pub was a taxi driver (he didn't go the pub on duty) who was most miffed at the fact that his boss had given him a Lada to drive.  Taking matters into his own hands he decided to murder the car, and took it along Cardiff's Eastern Avenue bypass, not speed restricted in those days, late one night and, at over 70mph, 'accidentally' put the car into reverse and let the clutch out.  The Lada came to a fairly rapid standstill on the hard shoulder with some unpleasant smells and a bit of smoke and steam, and in total silence.  Convinced he'd successfully done the foul deed, he then proved the point by turning the ignition off, then back on again, and then turned the key for a start.

 

Brother Lada turned over 3 or 4 times, fired up, and roared into it's usual noisy and rather lumpy life as it nothing untoward had happened.  My friend's attitude to Ladas changed instantly; he had enormous respect for the quality of engineering that would allow such a thing to happen!  'Try doing that in a Ferrari', he'd say...

Hi Johnster,

 

If you think a Lada is tough I have experienced Kamaz trucks. In the late 1980'2 I once worked with two chaps from the Chelnyabinsk Order of Lenin Heavy Truck Production Facility.

 

Gibbo.

[Ruston]

On 16/03/2019 at 11:42, Mim said:

I've never driven a steam loco, so looking for some real world 1:1 scale experience. The situation:

 

A small steam locomotive used for shunting, or on an unfitted freight train. The only brakes are on the loco and on the brake van. Would the cut off control ever be used as a brake to slow the loco, as either light engine, or a train? Letting steam in to the cylinders against the direction of travel. Either in combination with conventional brakes, or alone? If not, why not? If it is, how much cutoff would be set?

 

Mim

I'm curious as to the type of railway, and period, that you are modelling to need to consider this. What wagons are you running that have no brakes?