A 3 car HST calls at Ealing Broadway

[big jim]

67s have special dispensation to run at 100 light engine when on Thunderbird duties

 

 

they can also run at 100mph light engine under normal circumstances on the chiltern mainline between marylebone and aynho jn as long as they are running under chilterns safety case

[david.hill64]

While the discussions about reduced speeds being applied for loco hauled coaching stock are correct, the same rules did not apply to HST's. I am not sure of the current position as I am no longer in a position to know.

 

HST power cars are not locomotives,which generally have lower brake force than coaching stock: therefore on a typical rake of loco plus coaches, much of the braking effort is provided by the coaches. When BR research ran test trains it was usually necessary for additional coaches to be added to the test rake to make up the brake force. Some disc braked mark 2 coaches that were previously used on the Glasgow-Edinburgh services were used for these purposes. I used to do the calculations to determine how many additional vehicles would be necessary.

 

HST power cars are different: they were designed to brake their own mass and not rely on the braking effort from the trailers, but with a slight complication. As originally built, the power cars had both tread brakes and disc brakes, with a notional 20% effort devoted to the tread brakes. I say notional as the friction coefficient of cast iron brake blocks increases considerably as speed reduces, so at low speeds the tread brakes were providing more brake effort. The complication comes that at speeds above 90mph the braking effort on the power cars was reduced so as to stop overheating of the brake discs and pads. At these speeds the trailer cars were providing more of the brake force Below 90mph the brake effort was increased on the power cars, which were then providing more brake effort than the trailer cars. So in the early days the braking distances of 7 car sets were different from those with 8 cars.

 

Even so the brake discs on the power cars continued to give problems so in the 1980's a change was made. The original design was a split disc which meant that if a disc needed to be replaced it could in theory be done without removing the wheel from the axle. The problem with split discs is that they need multiple attachments (bolts) to stop them flying off the wheel under the actions of the rotational forces. However, putting in lots of restraint to keep the discs on the wheel means that thermal expansion is constrained, so high forces are generated by the heat during braking: these high forces result in cracks which eventually if uncontrolled cause the disc to break up and get ejected. Initially this happened a lot with the trailer cars, so a different higher quality grade of cast iron was developed for use with the trailer cars. This was relatively successful. It was not so successful on the power cars so a trial was set up with brake discs of a continuous ring and a retaining mechanism that allowed the disc to expand and contract under the action of heat while still being retained on the wheel. At the same time the brake pad was changed to a conformable non-asbestos type and the two stage braking disabled. The brake force was set up so that the power cars braked their own weight over the full speed range. The tread brakes were still active so the actual total braking force is non-linear but still met the braking curve - the W125 curve - at all speeds.

 

When it came to braking trials, which were held on the western region - the WR operating department initially objected on the grounds that a pair of power cars were not allowed to run at 125mph on their own as they were locomotives and therefore subject to short train restrictions. The argument that they are not locos, but power cars that had design braking effort identical to coaches, won the day and tests were carried out using a pair of power cars running back to back at speeds of up to 125mph (and a bit!). The tests were successful and all of the fleet was modified. I was the responsible project engineer for the change and was on the tests and the acceleration was incredible!

 

In theory the braking effort of any combination of HST power cars with any number of trailer cars will be nominally identical with only differing train resistance and the usual variations in brake pad friction performance giving any difference in stopping distance from a given speed. So short train formations need not be speed restricted. But as I said above, I am no longer in a position to know what the current rules are. If restrictions are in force they are not there for any engineering reason.

 

I believe that Class 67s have disc brakes and therefore do not have the problems that cast-iron tread braked locos have.

 

Edit for comment on 7 and 8 car sets and 67 brake performance.

 

David

Edited October 20, 2014 by david.hill64

[big jim]

very interesting

 

as i put on a previous post the speed rules only apply to MK3 sleeper vehicles (and mk1-2 etc), there is no mention of MK3 day stock, also it says the rules do not apply to 'fixed formation' trains, is a HST classed as fixed formation for purposes of the rule book?

 

the old is a class 43 a loco question has been going on on another forum with much heated debate!!

 

regards class 67 they have disc AND clasp brakes fitted, as mick pointed out in scotland they had to fit them with different brake blocks the reason being the normal ones were wearing out too quickly on the highland lines, that resulted in them having to run at reduced speeds

 

when i learnt class 67 (with marcus37) we did a braking test, running light engine we were aproaching a green signal the driver hit the emergency stop plunger as we went over the magnet, we were doing 40 mph and managed to stop about 10M the right side of the signal!!

[The Stationmaster]

 

 

Even so the brake discs on the power cars continued to give problems so in the 1980's a change was made. The original design was a split disc which meant that if a disc needed to be replaced it could in theory be done without removing the wheel from the axle. The problem with split discs is that they need multiple attachments (bolts) to stop them flying off the wheel under the actions of the rotational forces. However, putting in lots of restraint to keep the discs on the wheel means that thermal expansion is constrained, so high forces are generated by the heat during braking: these high forces result in cracks which eventually if uncontrolled cause the disc to break up and get ejected. Initially this happened a lot with the trailer cars, so a different higher quality grade of cast iron was developed for use with the trailer cars. This was relatively successful. It was not so successful on the power cars so a trial was set up with brake discs of a continuous ring and a retaining mechanism that allowed the disc to expand and contract under the action of heat while still being retained on the wheel. At the same time the brake pad was changed to a conformable non-asbestos type and the two stage braking disabled. The brake force was set up so that the power cars braked their own weight over the full speed range. The tread brakes were still active so the actual total braking force is non-linear but still met the braking curve - the W125 curve - at all speeds.

 

When it came to braking trials, which were held on the western region - the WR operating department initially objected on the grounds that a pair of power cars were not allowed to run at 125mph on their own as they were locomotives and therefore subject to short train restrictions. The argument that they are not locos, but power cars that had design braking effort identical to coaches, won the day and tests were carried out using a pair of power cars running back to back at speeds of up to 125mph (and a bit!). The tests were successful and all of the fleet was modified. I was the responsible project engineer for the change and was on the tests and the acceleration was incredible!

 

In theory the braking effort of any combination of HST power cars with any number of trailer cars will be nominally identical with only differing train resistance and the usual variations in brake pad friction performance giving any difference in stopping distance from a given speed. So short train formations need not be speed restricted. But as I said above, I am no longer in a position to know what the current rules are. If restrictions are in force they are not there for any engineering reason.

 

I believe that Class 67s have disc brakes and therefore do not have the problems that cast-iron tread braked locos have.

 

Edit for comment on 7 and 8 car sets and 67 brake performance.

 

David

Regrettably it would seem not to be the case.  When we ran the 'Top Of The Pops' high speed 'record breaking' train to Bristol it was suggested that the formation be reduced to to 2_4 to give an ideal power:weight ratio at the sort of accommodation (for passengers) level we were looking for.  We accordingly sought technical advice from the Regional M&EE people and from Derby Technical Centre Train Performance people and we were very specifically told that brake performance would degrade if we ran with fewer than 5 trailer cars and that we could only run with 2+4 at a considerable expense in maximum speed if we wished/needed to maintain normal braking distances (I forget the exact figure but it was a very definite deal breaker in terms of overall time cost).

 

Subsequent inquiries about running with reduced formations gave us exactly the same answer every time - running with fewer than five trailers + 2 power cars created a penalty on braking distances which meant maximum permitted train speed (in relation to permitted line speed and hence to designed signal braking distances) had to be reduced.  For the Bristol run we even did a full check of signal spacing to see if we had any latitude but there was none although there were a number of places where it was quite safe to run up to the maximum speed permitted by the engine governors.

 

I am also aware from some ISA work I did in later years (in connection with the NMT HST) that running short formations below 2+5 does impact on braking distances and thus requires speed reductions.  In fact some of the project proposals involved running the train with Mk2 vehicles in the formation and I was suspicious of the braking data and asked for tests to be carried out - which as it happens proved my point (but that was in respect of Mk2; a short formation with Mk3s was not tested because the figures showed there would be a reduction  in braking effect).

 

So yes, you can run an HST down to 2+5 without problems or impact on braking distance, but if you go below 2+5 braking distances increase so speeds have to be reduced.

[david.hill64]

I think the Top of the Pops run was in 1984 before the brakes were changed.

 

With the original two stage braking then brake performance at higher speeds would definitely be impacted by short formations. As I said in my post the braking effort on the power cars was reduced significantly above 90mph (to about 6%g if memory serves correctly) and upped to about 12%g below 90.

 

 I know from being involved in the tests with two power cars running on their own with the new brakes that brake performance fully met the W125 curve at all speeds. Subsequently we did tests on a full rake when the new pad materials were introduced to the trailers and confirmed the results. The braking distances for a full set were not materially different than those running a pair of power cars on their own. Admittedly this is nearly 30 years ago and it is possible - though unlikely - that there might have been a reduction in power car braking performance since then

 

Most Mark 2's were cast iron block braked and would have been a liability in a set running at high speed. Don't forget that the standard braking rate for loco hauled vehicles at that time was only 7%g and the 9% rate was applied only to HST vehicles (and later IC225 sets) that were required to stop from 125mph in signal spacings that were designed for 100mph running. The ex Glasgow-Edinburgh push pull vehicles that were used by BR Research had modified distributors allowing the brake rates to be adjusted according to need. Any test results that were obtained with mark 2s are irrelevant when considering HST power car and trailer car performance.

 

There was an interesting problem first noticed on the western when the HST's were introduced. SIgnal spacings are reduced on rising gradients to take account of the shorter stopping distances. As the percentage effect of gradient was smaller with higher braking rates (if a gradient gave you a 1%g effective additional braking rate then the total service braking rate would be 8%g for 100mph stock or a 14% increase. For 125mph stock braked at 9%g the benefit was only 11%) there were some uphill locations where HSTs were limited to 120mph where normal line speed was 100mph and the expected HST limit would have been 125mph.

 

As the project engineer involved in the HST tests- and subsequently head of braking at BR Research - I am sure my information is correct for that time period. As I said in my post, I do not know what the current situation is, but I think it unlikely that the braking effort will have been reduced on the power cars.

[david.hill64]

This discussion inspired me to get the calculator out and do some approximate calculations.

 

Braking effort is provided by the combination of trains brakes and aerodynamic resistance (drag). To a first approximation we can ignore drag if we also ignore the effects of rotational inertia of the wheelsets and associated gearboxes and traction motors where provided.

 

The notional braking rate of 9%g for 125mph is actually an average rate from brake initiation to the train coming to stand. During the brake build up time the brake force builds up so that it then reaches the instantaneous rate which is about 9.6%g. However, to get a feeling for the problem we can ignore the effect of the free run time and use the average rate. I will also for simplicity assume that g is 10m/s/s so that 9%g = 0.9m/s/s

 

The key distance to achieve from 125mph is to stop in 1740m. This is the distance from the W125 brake curve. It allows a margin of 300m for variation in friction material performance and adhesion for a standard signalling distance from double yellow to red of 2 * 1020 = 2040m

 

Assuming linear deceleration, basic physics tells us that braking distance is ((initial speed squared) minus (final speed squared)) divided by 2 divided by deceleration rate.

 

So the key speed figures are 125mph = 55.8m/s and 90mph = 40.2m/s

 

Assume that an HST power car has a mass of 70tonnes and a trailer car a mass of 30tonnes.

 

In the post 1985(ish) set up with single stage brakes on the power cars set at 9%g retardation, the braking distance of any combination of power cars and trailer cars from 125ph is simply (55.8 * 55.8) / (2 * 0.9) = 1730m.

 

For the original set up with two stage brakes we need to consider the distance traveled from 125mph to 90mph and then the distance traveled from 90mph to rest.

 

For a 2+8 consist, the brake force from 125 to 90 is the sum of the brake force of the trailers plus the brake force of the power cars. Trailer brake force is proportional to 8 * 30 * 9. Power car brake force is proportional to 2 * 70 * 6. Total brake force is therefore 3000 units and deceleration will be 3000/train mass = 3000/380 = 7.9%g. 

 

From 90 to 0 using the same methodology gives the deceleration rate of 10.1%g,

 

The braking distance from 125 to 90 is then 947m and from 90 to rest is 800m or a total braking distance of 1747m. Given the assumptions in this simplified calculation that is close enough to 1740m not to matter.

 

Similar calculations give the stopping distance of a 2 + 7 set as 1752m; 2 + 5 as 1765m and 2 +4 as 1775m. A pair of power cars on their own would have been about 1920m.

 

When you are so close to the 1740m limit then you need to do the more detailed calculations. In particular the assumption that rotational inertia can be ignored is not valid for short formations including 2 power cars as the motors, gearboxes and wheels provide significant energy that has to be dissipated by the brakes. This extra energy contributes to brake fade where the friction coefficient between the pad and disc decreases with temperature. This will be worse on the power cars in short formations at high speed. I have also ignored the fact that in short formations the power car cast iron tread brakes will be less effective as they will run hotter: once the interface starts to melt friction falls considerably.

 

The Train Performance team's computer had all of these variables included and I can readily appreciate that as originally equipped a 2 + 5 set may have been just inside the curve while a 2 + 4 just outside.

[hexagon789]

On 20/10/2014 at 05:47, david.hill64 said:

While the discussions about reduced speeds being applied for loco hauled coaching stock are correct, the same rules did not apply to HST's. I am not sure of the current position as I am no longer in a position to know.

 

HST power cars are not locomotives,which generally have lower brake force than coaching stock: therefore on a typical rake of loco plus coaches, much of the braking effort is provided by the coaches. When BR research ran test trains it was usually necessary for additional coaches to be added to the test rake to make up the brake force. Some disc braked mark 2 coaches that were previously used on the Glasgow-Edinburgh services were used for these purposes. I used to do the calculations to determine how many additional vehicles would be necessary.

 

HST power cars are different: they were designed to brake their own mass and not rely on the braking effort from the trailers, but with a slight complication. As originally built, the power cars had both tread brakes and disc brakes, with a notional 20% effort devoted to the tread brakes. I say notional as the friction coefficient of cast iron brake blocks increases considerably as speed reduces, so at low speeds the tread brakes were providing more brake effort. The complication comes that at speeds above 90mph the braking effort on the power cars was reduced so as to stop overheating of the brake discs and pads. At these speeds the trailer cars were providing more of the brake force Below 90mph the brake effort was increased on the power cars, which were then providing more brake effort than the trailer cars. So in the early days the braking distances of 7 car sets were different from those with 8 cars.

 

Even so the brake discs on the power cars continued to give problems so in the 1980's a change was made. The original design was a split disc which meant that if a disc needed to be replaced it could in theory be done without removing the wheel from the axle. The problem with split discs is that they need multiple attachments (bolts) to stop them flying off the wheel under the actions of the rotational forces. However, putting in lots of restraint to keep the discs on the wheel means that thermal expansion is constrained, so high forces are generated by the heat during braking: these high forces result in cracks which eventually if uncontrolled cause the disc to break up and get ejected. Initially this happened a lot with the trailer cars, so a different higher quality grade of cast iron was developed for use with the trailer cars. This was relatively successful. It was not so successful on the power cars so a trial was set up with brake discs of a continuous ring and a retaining mechanism that allowed the disc to expand and contract under the action of heat while still being retained on the wheel. At the same time the brake pad was changed to a conformable non-asbestos type and the two stage braking disabled. The brake force was set up so that the power cars braked their own weight over the full speed range. The tread brakes were still active so the actual total braking force is non-linear but still met the braking curve - the W125 curve - at all speeds.

 

When it came to braking trials, which were held on the western region - the WR operating department initially objected on the grounds that a pair of power cars were not allowed to run at 125mph on their own as they were locomotives and therefore subject to short train restrictions. The argument that they are not locos, but power cars that had design braking effort identical to coaches, won the day and tests were carried out using a pair of power cars running back to back at speeds of up to 125mph (and a bit!). The tests were successful and all of the fleet was modified. I was the responsible project engineer for the change and was on the tests and the acceleration was incredible!

 

In theory the braking effort of any combination of HST power cars with any number of trailer cars will be nominally identical with only differing train resistance and the usual variations in brake pad friction performance giving any difference in stopping distance from a given speed. So short train formations need not be speed restricted. But as I said above, I am no longer in a position to know what the current rules are. If restrictions are in force they are not there for any engineering reason.

 

I believe that Class 67s have disc brakes and therefore do not have the problems that cast-iron tread braked locos have.

 

Edit for comment on 7 and 8 car sets and 67 brake performance.

 

David

Hi David, I wonder if you could confirm a couple of things about the original two-stage braking system if possible?

 

Or indeed if anyone could.

 

Did it definitely only apply to the power cars, that is to say the trailers always braked at the same rate?

 

Secondly what were the two braking rates 6%g above 90mph and 12%g under 90mph?

 

I ask as I recently found a source which seemed to suggest the trailers had a two-stage braking system but that same source made no mention of the power cars.

 

Cheers.

[big jim]

 

6 years since the last post, gotta be a record!

[hexagon789]

1 hour ago, big jim said:

 

6 years since the last post, gotta be a record!

Maybe, but that wasn't the point in posting I can assure you!

[big jim]

I’ve been waiting so long to use that meme! 

[david.hill64]

15 hours ago, hexagon789 said:

Hi David, I wonder if you could confirm a couple of things about the original two-stage braking system if possible?

 

Or indeed if anyone could.

 

Did it definitely only apply to the power cars, that is to say the trailers always braked at the same rate?

 

Secondly what were the two braking rates 6%g above 90mph and 12%g under 90mph?

 

I ask as I recently found a source which seemed to suggest the trailers had a two-stage braking system but that same source made no mention of the power cars.

 

Cheers.

Yes, only power cars and it was nominally 6%g above 90mph and 12%g below. I say nominally as the braking on the power cars was 80% disc brake and 20% block brake. While the brake force of the disc brakes was essentially independent of speed, that of the block brakes was very non-linear. So at low speeds - say below 20mph - the actual braking rate of the power cars would be above 20%. However, in a full rake such niceties could be ignored.

 

Trailer cars were only ever single stage braked at nominal 9%g. So for a standard length HST the overall brake rate worked out at about 9%g. Problems would occur with short sets. The Stationmaster has referred to special restrictions for the short formed 'Top of the pops' run. At speeds above 90mph a short set would have substandard brake performance - at low speeds below 90mph the brake rate would be higher than that of a full length set.

 

Various brake modifications were carried out. The original Ferodo brake pad material had a wonderful friction characteristic: stable at all speeds and temperatures giving a very consistent brake performance. But, as well as emitting the horrible smell during heavy brake applications (necessary if the driver had been in search of his '140 club' tie) the pads contained asbestos and were horrible to the discs causing hot spotting that led to cracking and, on occasions, ejection of the discs at speed. As an interim solution BR research developed a stronger form of cast iron, called compacted graphite, to replace the original flake graphite discs. Compacted graphite was more resistant to cracking than the flake iron and had a better thermal conductivity than spheroidal graphite iron. (Flake and spheroidal graphite irons were the most commonly used forms of cast iron).

 

The power car discs continued to have problems with an average life of about 12 months, so wheelset replacements were necessary between overhauls. This was costly. The pricing structure meant that the regions paid a fixed price for a replacement wheelset irrespective of what was wrong with the old one, so cost savings were made by doing disc replacements at depots. Although not an easy job, as the Lucas Girling discs were split (not continuous rings) it was possible.

 

A solution came with a change in disc and pad supplier. The German company BSI developed a continuous ring disc that was able to expand on its mountings to allow for the thermal expansion in braking. Allied to an asbestos-free brake pad produced by Becorit the discs were able to absorb the brake energy from 125mph at a constant 9% g rate, abandoning the two stage brake and giving a brake rate for HST's independent of train length. At about the same time the trailer cars were fitted with a UIC pad holder to replace the technically superior but patented Girling design and the Becorit pads used. This also required a change to the brake distributors as the friction coefficient of the Becorit pads, while conforming to the BR (and UIC) standard, was lower than that of the Ferodo pads.

 

I wasn't involved with later changes but Knorr Bremse also produced discs and I understand that the braking rate of the power cars was reduced again to extend brake life (presumably at the expense of brake rate).

[hexagon789]

7 hours ago, david.hill64 said:

Yes, only power cars and it was nominally 6%g above 90mph and 12%g below. I say nominally as the braking on the power cars was 80% disc brake and 20% block brake. While the brake force of the disc brakes was essentially independent of speed, that of the block brakes was very non-linear. So at low speeds - say below 20mph - the actual braking rate of the power cars would be above 20%. However, in a full rake such niceties could be ignored.

 

Trailer cars were only ever single stage braked at nominal 9%g. So for a standard length HST the overall brake rate worked out at about 9%g. Problems would occur with short sets. The Stationmaster has referred to special restrictions for the short formed 'Top of the pops' run. At speeds above 90mph a short set would have substandard brake performance - at low speeds below 90mph the brake rate would be higher than that of a full length set.

 

Various brake modifications were carried out. The original Ferodo brake pad material had a wonderful friction characteristic: stable at all speeds and temperatures giving a very consistent brake performance. But, as well as emitting the horrible smell during heavy brake applications (necessary if the driver had been in search of his '140 club' tie) the pads contained asbestos and were horrible to the discs causing hot spotting that led to cracking and, on occasions, ejection of the discs at speed. As an interim solution BR research developed a stronger form of cast iron, called compacted graphite, to replace the original flake graphite discs. Compacted graphite was more resistant to cracking than the flake iron and had a better thermal conductivity than spheroidal graphite iron. (Flake and spheroidal graphite irons were the most commonly used forms of cast iron).

 

The power car discs continued to have problems with an average life of about 12 months, so wheelset replacements were necessary between overhauls. This was costly. The pricing structure meant that the regions paid a fixed price for a replacement wheelset irrespective of what was wrong with the old one, so cost savings were made by doing disc replacements at depots. Although not an easy job, as the Lucas Girling discs were split (not continuous rings) it was possible.

 

A solution came with a change in disc and pad supplier. The German company BSI developed a continuous ring disc that was able to expand on its mountings to allow for the thermal expansion in braking. Allied to an asbestos-free brake pad produced by Becorit the discs were able to absorb the brake energy from 125mph at a constant 9% g rate, abandoning the two stage brake and giving a brake rate for HST's independent of train length. At about the same time the trailer cars were fitted with a UIC pad holder to replace the technically superior but patented Girling design and the Becorit pads used. This also required a change to the brake distributors as the friction coefficient of the Becorit pads, while conforming to the BR (and UIC) standard, was lower than that of the Ferodo pads.

 

I wasn't involved with later changes but Knorr Bremse also produced discs and I understand that the braking rate of the power cars was reduced again to extend brake life (presumably at the expense of brake rate).

Thanks for confirming that, much appreciated.

[Talltim]

On the subject of power to weight, as opposed to braking, I came across the US Brightline (for a while Virgin) operation which runs a kind of ersatz HST.

It has two 4000Hp Siemens Charger locos and four coaches. Unfortunately I’m yet to find the weight of the coaches, which will be greater than a Mk3

 

https://en.m.wikipedia.org/wiki/Brightline

Photo by PatrickHamiltonBrightline

Edited November 22, 2020 by Talltim

[Michael Hodgson]

What a hideous livery !