[Genma Saotome]

Jarad, I dropped the question on to the STMFC and the replies so far are 70PSI for the WWI era, 80PSI at some unknown date after that -- all the way to the UP MP merger in the mid 1980's (it was the MP who was using 80PSI) and apparently most everyone is at 90PSI today.

So for me, focused on 1940-56, it is either 70 or 80PSI. I'll wait to see if anyone knows when the change from 70 to 80 might have occurred.

Something else I was told was the extra pressure does not result in higher braking but instead provides a higher safety factor in simply having more air on hand, albeit with longer re-pressure times.

Thia does raise a question in my mind... why put this parameter into .wags at all? Would in not be determined entirely by what the air compressor in the locomotives could produce?

[jared2982]

The charging pressure is determined by the feed valve setting of the locomotive not the compressor. The compressor should be producing 25-50psi more than what the brakes system is charged to. A typical road unit today produces somewhere between 125 and 140 psi. An equalizing reservoir set at 90 on an engine will charge the brake system to 90 psi on all the cars in the consist. I have never done a test in OR to determine if this holds true in the sim.

The comment about the initial charging pressure of the system not affecting braking force I am have to argue with a bit. On a system charged to 70psi a full service reduction will produce 50lbs of pressure in the brake cylinder. An emergency application of the brakes dumps the emergency reservoir into the mix and the pressure equalizes at 60psi between the emergency and auxiliary reservoirs, and the brake cylinder. A system charged to 80psi has 57psi in the brake cylinder at full service and 69 psi in emergency. A 90psi charge is 64 at full service and 77 at emergency. All of these assuming the system is fully charged. So looking at the numbers the more pressure you have in the brake cylinder the greater the braking force. While the braking equipment has not changed much in 100 years the weight of the equipment and the train length has and I believe that this is the reason for the increase in the charging pressure of the brake system.

[Coonskin]

Yup, Jared is right.

Don't know what you youngin's call it, but I was taught that a train line that was not fully charged was a "soft pipe". Example: If you make, say, an 8 pound set, on a train of empties, and they squat too much... you can kick off... give the pipe a bit of time (talking just a few seconds here, for sure you don't want a full release), and then draw off again, just a bit more, say 9-10 pounds. The result: Seeing as the line was not fully recharged, the reset will be "softer" (even though more PSI at the gauge) because less braking force air is getting to the brake cylinders. Thus, (if done correctly) joy can be had because now your train will glide merrily down the grade because you won't have quite the braking force you had when you pulled off your air with the fully charged pipe.

[Genma Saotome]

jared2982, on 07 May 2018 - 04:53 PM, said:

While the braking equipment has not changed much in 100 years the weight of the equipment and the train length has and I believe that this is the reason for the increase in the charging pressure of the brake system.

I suppose the question then is whether the brake linkages need the extra pressure to set to obtain an ideal force... certainly not for an empty but for a very heavy load... yeah, I can see that (I'm assuming empty/load switches are pervasive).

I do think the overall mass and speed of modern trains must be a factor here... much different than the old days, especially speed. Keeping in mind that force = mass*velocity^2 faster speeds do make a BIG difference in what needs to be overcome in order to slow down. Might the higher pressure give a greater amount of train safety by virtue of simply not running out of useful air pressure as soon as if it were 10 or 20PSI lower? It's still a dangerous business.

[jared2982]

Genma Saotome, on 07 May 2018 - 05:50 PM, said:

Might the higher pressure give a greater amount of train safety by virtue of simply not running out of useful air pressure as soon as if it were 10 or 20PSI lower? It's still a dangerous business.

I think the big advantage in the greater system pressure is the increased brake cylinder pressure and thus increased brake force. At 90 psi you have 26lbs of Air you can draw off air you can draw off and that’s it. This full service reduction causes the auxiliary reservoir and brake cylinder to equalize at 64psi. This is also the amount of air left in the brake pipe. Even if you draw the brake pipe down another 10lbs you will get absolutely no more pressure in the brake cylinder. With a 70psi brake pipe a full service reduction is 20 psi with 50 psi in the brake cylinder. Between the 70 and 90 psi you only get an extra 6lbs to play with, which isn’t much, but the increased pressure at the brake cylinder from 50 to 64 psi is where the benefit comes in. This is not to say that the extra air isn’t useful especially when cycle braking using the method Andre described above. We use that method quite a bit coming down the south side of Rich Mountain with grain loads. Usually a minimum isn’t enough and 9-10 is to much. Of course this is used in conjunction with dynamic brakes usually on the head end and the rear.

[R H Steele]

Coonskin, on 07 May 2018 - 05:38 PM, said:

Yup, Jared is right.

Don't know what you youngin's call it, but I was taught that a train line that was not fully charged was a "soft pipe". Example: If you make, say, an 8 pound set, on a train of empties, and they squat too much... you can kick off... give the pipe a bit of time (talking just a few seconds here, for sure you don't want a full release), and then draw off again, just a bit more, say 9-10 pounds. The result: Seeing as the line was not fully recharged, the reset will be "softer" (even though more PSI at the gauge) because less braking force air is getting to the brake cylinders. Thus, (if done correctly) joy can be had because now your train will glide merrily down the grade because you won't have quite the braking force you had when you pulled off your air with the fully charged pipe.

This type of behavior is not modeled in OR, correct.? Anyone. That would be a real nice addition if it was.

[atsf37l]

Yep RH, I can do pretty much what Andre's talking about coming down a long hill in OR. :bigboss:

[jared2982]

Herb's right, OR will allow you to cycle brake down the hill. There are still a couple of features OR does not have right. When you set a minimum with a system charged to 90 psi you should get 10-12 psi in the brake cylinder. OR uses the triple valve ratio from the eng file to compute this so instead of 10-12lbs with a 6lbs reduction you get 15lbs in the brake cylinder. I know everyone is thinking, what difference does 3lbs make.... alot with 100 + cars. OR also uses emergency reservoir air to recharge the brake system from a minimum reduction. This is also not correct. Emergency reservoir air should not be used to assist in recharging the system until you have made at least a 10lbs reduction.

[atsf37l]

Yep that has always been a problem in both MSTS and ORTS - too much pressure in the cylinders on a minimum set. It's too darn quick. You end up cycling for a simple station stop. All this would be so easy if we were actually working an E6 or 24RL brake handle. Pushbuttons really don't get it. To quote Lt. Commander Montgomery Scott: "Ah, a keyboard. How quaint." And you're working the brakes with the wrong hand! :bigboss:

[dajones]

jared2982, on 10 May 2018 - 12:31 AM, said:

When you set a minimum with a system charged to 90 psi you should get 10-12 psi in the brake cylinder.

I can't find anything in New York Air Brake or Westinghouse documentation that would cause this. A six PSI set should give you more than 15 PSI in the brake cylinder, because the quick service and regular service features will work together. The quick service feature will move air from the brake pipe to the brake cylinder until the quick service limit is reached (7 to 12 PSI according to the ABD code of test document). And air will move from the aux res. to the brake cylinder until the valve moves into the lap position, and that won't happen until the aux res. drops by six PSI. The end result will be more than 15 PSI in the brake cylinder. If the aux res. wasn't fully charged or if the brake cylinder leaks, the quick service feature will cause the brake cylinder to have at least 7 to 12 PSI.

Frame mounted brake cylinders need 4 to 5 PSI to move the brake shoes in contact with the wheel, and no brake force is generated until the brake cylinder pressure exceeds that. I don't know if OR takes that into account.

Doug