Two Stage Braking - Blue Pullman - HST - and UIC "Rapid" braking.

[darwins]

The driver's instructions for the Blue Pullman describe an EP brake system that provides a higher brake force (higher brake cylinder pressure) when speed rises above 40 mph and until speed falls below 30 mph.
At lower speeds the brake cylinder pressure would be the 45-50 psi normally obtained from the air brake distributor.
What pressure was used at higher speeds? The main reservoir pipe was charged to 110-125 psi, but I doubt that the BC pressure was allowed to rise that high.
Does anyone happen to know the brake forces or brake percentages of the vehicles on the Blue Pullman train?

HST power cars seem to be the reverse of other two stage braking, with lower brake force (34%) at higher speeds (above 145km/h) and higher brake force (50%) below that speed.
The instructions for the prototype describe the same two stage braking on Mk 3 carriages.
Can anyone provide further information about this or give the brake cylinder pressures used? (And brake % for the carriages?)

Similar question for European stock with two stage braking. "R" or rapid braking on stock with cast iron brake shoes required higher BC pressure and brake force to be used on stock with cast iron brake blocks.
Further information on actual brake cylinder pressures would be welcomed. [Brake weights given on UIC stock give an indication of stopping distance in G, P or R modes, but do not relate directly to brake % as used historically in UK.]

 

[Fragezeichnen]

Typically 3.8 bar for rapid braking on coaches, and 8 bar on locomotives(although this would only be used if the electrical dynamic brake failed), instead of 2 bar for tread brakes and 3 bar for disc brakes. This is active somewhere above 50km/h depending on the individual type. On locomotives the driver is provided with a positive indication that rapid braking is armed, since without it the train's emergency braking distance would be reduced and it may be restricted to a lower speed.

 

[GC class B1]

The driver's instructions for the Blue Pullman describe an EP brake system that provides a higher brake force (higher brake cylinder pressure) when speed rises above 40 mph and until speed falls below 30 mph.
At lower speeds the brake cylinder pressure would be the 45-50 psi normally obtained from the air brake distributor.
What pressure was used at higher speeds? The main reservoir pipe was charged to 110-125 psi, but I doubt that the BC pressure was allowed to rise that high.
Does anyone happen to know the brake forces or brake percentages of the vehicles on the Blue Pullman train?

HST power cars seem to be the reverse of other two stage braking, with lower brake force (34%) at higher speeds (above 145km/h) and higher brake force (50%) below that speed.
The instructions for the prototype describe the same two stage braking on Mk 3 carriages.
Can anyone provide further information about this or give the brake cylinder pressures used? (And brake % for the carriages?)

Similar question for European stock with two stage braking. "R" or rapid braking on stock with cast iron brake shoes required higher BC pressure and brake force to be used on stock with cast iron brake blocks.
Further information on actual brake cylinder pressures would be welcomed. [Brake weights given on UIC stock give an indication of stopping distance in G, P or R modes, but do not relate directly to brake % as used historically in UK.]

Some information that may be helpful. Cast iron brake blocks have a lower coefficient of friction than composition brake blocks or disc brake pads. Composition materials (brake blocks and pads) have a fairly consistent coefficient across the speed range whereas cast iron has a much higher coefficient of friction at lower speeds. I don’t know about the Blue Pullman units but if they had cast iron brake blocks, which seems likely, then they would have needed a higher brake cylinder pressure at higher speeds to provide a reasonably consistent deceleration force across the speed range. A very high deceleration force resulting from the higher coefficient of friction at low speeds could result in overheated wheels and the risk of moved tyres and/or wheel flats so should be avoided.
If I remember correctly Mk3 coaches for HSTs were originally designed with two stage braking with a lower brake cylinder pressure at higher speeds but this was decided to be unnecessary. I don’t know why this decision was made or any details but a possible reason is that as Mk 3 coaches were the first production build high speed vehicles with disc brakes they may have been designed to be consistent with tread braked coaches with cast iron brake blocks. These tread braked vehicles have a higher deceleration rate at lower speeds. Signalling stopping distances are designed around the characteristics of cast iron tread braked vehicles with shorter stopping distances relative to speed at lower speeds (below 40 MPH).
Maximum brake cylinder pressure for tread braked coaches is 3.8 bar (55 psi). Disc braked vehicles typically have lower brake cylinder pressures.

 

[darwins]

Looking up a 1960 WR document on stopping distances that I found in the NRM, I have found an answer to one of my questions.

Diesel Pullman Multiple Unit (air brakes).
Power Cars both bogies 41/41.
Aux. Power Cars motor bogies 48/48.
Aux. Power Cars trailer bogies 70/50.
All other Cars both bogies 70/50.

The pressures first given apply to the high speed braking conditions and the second to the normal.

(Numbers of course would be in psi)

 

[hexagon789]

The driver's instructions for the Blue Pullman describe an EP brake system that provides a higher brake force (higher brake cylinder pressure) when speed rises above 40 mph and until speed falls below 30 mph.
At lower speeds the brake cylinder pressure would be the 45-50 psi normally obtained from the air brake distributor.
What pressure was used at higher speeds? The main reservoir pipe was charged to 110-125 psi, but I doubt that the BC pressure was allowed to rise that high.
Does anyone happen to know the brake forces or brake percentages of the vehicles on the Blue Pullman train?

HST power cars seem to be the reverse of other two stage braking, with lower brake force (34%) at higher speeds (above 145km/h) and higher brake force (50%) below that speed.
The instructions for the prototype describe the same two stage braking on Mk 3 carriages.
Can anyone provide further information about this or give the brake cylinder pressures used? (And brake % for the carriages?)

Similar question for European stock with two stage braking. "R" or rapid braking on stock with cast iron brake shoes required higher BC pressure and brake force to be used on stock with cast iron brake blocks.
Further information on actual brake cylinder pressures would be welcomed. [Brake weights given on UIC stock give an indication of stopping distance in G, P or R modes, but do not relate directly to brake % as used historically in UK.]

HSTs only had two-stage braking on the power cars and only until about 1985 when the power car brake disc mountings and type of pads were changed, so the original reason for the two-stage braking was no longer there hence it was disabled and the brake rate increased to the same above 90mph as below.

Looking up a 1960 WR document on stopping distances that I found in the NRM, I have found an answer to one of my questions.

Diesel Pullman Multiple Unit (air brakes).
Power Cars both bogies 41/41.
Aux. Power Cars motor bogies 48/48.
Aux. Power Cars trailer bogies 70/50.
All other Cars both bogies 70/50.

The pressures first given apply to the high speed braking conditions and the second to the normal.

(Numbers of course would be in psi)

The Robertson book on the Blue Pullmans, goes into some detail on this - the braking pressures were modified, from memory more than once but I can check the book tonight, as the original settings didn't give the desired braking results in the real world that had been calculated beforehand. I can't recall if the figures you quote there were the original or modified figures.

If I remember correctly Mk3 coaches for HSTs were originally designed with two stage braking with a lower brake cylinder pressure at higher speeds but this was decided to be unnecessary. I don’t know why this decision was made or any details but a possible reason is that as Mk 3 coaches were the first production build high speed vehicles with disc brakes they may have been designed to be consistent with tread braked coaches with cast iron brake blocks. These tread braked vehicles have a higher deceleration rate at lower speeds.

Disc brakes were tested in the late-60s on a specifically marshalled set of modified Mk2As used on the Talisman on the ECML, this was ostensibly to prove disc brake performance in real world conditions at regular 100mph running.

Additionally, the modified Mk2Z (with a few 2A vehicles thrown in) used on the Edinburgh-Glasgow push-pull service with Class 27s from 1971-80, had two-stage air disc brakes with Girling WSP. On these the brake cylinder pressure was reduced above 60mph, the deceleration rates being approximately 12%g below 60mph and 6%g above 60mph not unsimilar to the way the two-stage system worked on the HST power cars until disabled in the mid-80s.

 

[Taunton]

Additionally, the modified Mk2Z (with a few 2A vehicles thrown in) used on the Edinburgh-Glasgow push-pull service with Class 27s from 1971-80, had two-stage air disc brakes with Girling WSP. On these the brake cylinder pressure was reduced above 60mph, the deceleration rates being approximately 12%g below 60mph and 6%g above 60mph not unsimilar to the way the two-stage system worked on the HST power cars until disabled in the mid-80s.

I recall the Class 27 Push-Pull well, and had some notably rapid stops with them, including one spectacular arrival with late braking from high speed at Haymarket which I really felt was going to be an overrun, but with jerking and hissing (presumably from the wheel slip protection valves) just managed to pull up at the east end of the platform. The brake re-installation was done on these dedicated vehicles at St Rollox works.