I was driving a service with the 8F yesterday and noticed a difference in the loco brake behaviour compared to the Jubilee. Running with the small ejector on and the large ejector off, any tiny application of the combination brake resulted in nearly instant full application of the loco steam brake. Interestingly with the large ejector also running, the loco steam brake applied much more in proportion to the vacuum brake application. In both cases it was with an unfitted train so that can't be affecting it but driving with the large ejector running made the brakes operate as you would expect rather than the sudden on/off you get with only the small ejector. I assume this is probably a cause of the derail issues people are having as you would expect to run with the large ejector off. The effect is also there when running loco only. With the small ejector only, even with a slight reduction in the vacuum (to 20in) the steam brake applies suddenly to 100% In comparison the loco steam brake application of the Jubilee applies proportionally to the vacuum brake regardless of which ejectors are on. I expect both locos should operate the same and assume that the Jubilee is more what was intended. As a temporary work around, always open the large ejector before braking with the 8F.
Thats not a loco problem / bug, its the usual vacuum pipe behaviour. A long fitted train needs a while to create / destroy the vacuum. A shorter fitted train can create the vacum quicker. A loco only pulling an unfitted train you wont have any break / vacuum pipe, so its instant reaction & instant destruction of the vac. Tsw is on a good way. When people start to confuse animated physics with bugs, this is a sign of success of the franchaise.
That's great if that's the case but I'm not sure I have explained it well. Start with combination brake released, 21in vacuum, large ejector off, small ejector on. Any reduction in vacuum at all results (e.g. to 20in) in nearly instant 100% application of the loco steam brake. Alternatively with combination brake released, 21in vacuum, large ejector on small ejector on. An identical tiny reduction in vacuum (e.g. to 20in) results in a small application of the loco steam brake. A larger reduction results in a proportional steam brake application. The behaviour in the Jubilee is different where both scenarios described above behave like the second with a proportional application of the steam brake. Still feels like a bug, however it works both locos should work similarly surely?
On a steam loco, what actually powers the loco's brakes? Is the braking force derived from vacuum provided by the ejectors? If so, then a loss of steam pressure will lead to a loss of vacuum (ejectors become ineffective) and ultimately a loss of brakes.
You're thinking the wrong way around, train brakes need permanent air/vacuum to stay released, just like a truck (Air). If a train loses the complete air/vacuum, the brakes are automatically applied.
OK, but in the absence of vacuum, what force pushes the brake shoe against the wheel? Is it a strong spring?
The Steam locomotive brake shoes are usually applied mechanicly with a connected shaft mounted back to the brake cylinder. This "Vacuum cylinder" has a ball valve. I made few pics in tsw, the brakes is something dtg have modeled in detail. Recognice the piece ? Here a drawing where you can see it in detail: West Somerset made a video with a 30min explanation of the Automatic Vacuum brake, if you want to know about the technical principles in detail, i recommend to watch that. www.youtube.com/watch?v=kqV1JKuKAdY
Wow that's so kind of you to take the trouble to produce diagrams to explain the vacuum braking system. In conjunction with the excellent WSR video I now understand that the braking force is derived from a vacuum on one side of a piston coupled with a lesser vacuum on the other side which causes the piston to move and apply the brakes. So the brakes are kind of sucked on. It is a similar principle to air brakes, but with a vacuum instead of compressed air. Thank you so much meridian#2659.
Bit of an update, not sure what I was doing to cause what I was experiencing. But I can't recreate the effect. Thanks for the info on the vac brakes, very interesting
