How to Lock-Up Brakes?

[PaulMac]

Forgive the newbie question, but how is it possible to 'lock-up' (ie overcharge) carriage air brakes? I've heard stories of exasperated freight drivers having to walk the entire length of their train to release air from the brake cylinders of every carriage.

From what I understand of air brakes - admittedly not a whole lot - the brake pipe won't go higher than 500 kPa. I know that with the No 4 W valve, which runs air straight from the main reservoir to the locomotive brakes, it's possible to end up with over 500 kPa in the locomotive brake cylinders. But how is this possible with brake pipe air running via an equalising reservoir? What is the unfortunate combination of applications with a 26L that could leave you with overcharged brake cylinders?

Many thanks for any information.

 

[GW43125]

Were they definitely talking about air brakes? In vacuum brake days different locos would create varying vacuum pressures, the brakes would have to be released if the loco that took over wasn't creating as much vacuum.

With air brakes, a brake overcharge is often done as a matter of course when a new loco is attached, charging the train pipe over the usual 5bar, towards 5.5bar. This then leaks back out over about ten minutes and equalises the brakes.

 

[PaulMac]

Thanks for the reply.

It's definitely in the context of air brakes - particularly the 26L valve, which is ubiquitous in Australia and the North America. As I mentioned, it's an unusual occurrence caused by a poor choice of brake applications and charges.

At the end of it, you end up with a higher pressure in the brake cylinders than in the brake pipe. This means that you can't release the brakes via the brake pipe and must do it manually carriage-by-carriage.

What I can't quite grasp is how it happens, given the mechanics of the triple valve. Hopefully there's an obliging driver out there.

 

[43183]

On the air braked trains I've dealt with, there's a control reservoir connected to the distributor. The control reservoir gets charged up to the same pressure as the brake pipe (5 bar) when the brakes are fully released, then when the brake pipe is dropped to apply the brakes, the brake cylinder pressure applied is the difference between the control reservoir (theoretically still 5 bar) and whatever the brake pipe has reduced to.

The problem comes when the control reservoir manages to get slightly more than 5 bar in it, so you can't raise the brake pipe high enough to overcome the control pressure and fully release the brakes. There's generally a lever connected to the distributor so you can manually release some of the pressure from the control reservoir and get it back down to 5 bar

 

[TractiveEffort]

It can be an issue with top and tailed trailers or wagons, i.e. HST. If one power car BCU is charging the brake pipe slightly higher than the other, when changing ends at a terminus, if you are going from the high charging p/ car to the lower charging one, then you'll get brake drag (or a visible indication on the brake cylinder pressure gauges, that they aren't dropping to zero) due to incomplete release.