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| Real Long Read On NYCPropulsion/Braking (333083) | |||
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Real Long Read On NYCPropulsion/Braking |
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Posted by chicagopcclcars on Thu Nov 2 20:32:01 2006 With the threads on speed, man did I find this read from the NYC Transit Modelers Yahoo Group interesting. I'm passing it on for your consideration.David Harrison [NYC_TRANSIT_MODELERS_GROUP] R-44 rough ride - warning a long read! This is a long dissertation on second generation subway car braking and propulsion which came about originally as an explanation as to why R-44 cars brake so roughly. It evolved into a whole project and I apologize to anyone who is bored to death by it. If you only want the short of it skip to the second to last paragraph. Beginning in the 80's, Car Equipment Department engineering decided for various reasons centered mostly around financing, to change the way subway cars brake. This began a chain of events that would change the subway system forever. The traditional friction brake supplemented by dynamic was flipped: dynamic would now be the sole force in service braking. As planned following exhaustive testing, the air brake system would apply brake cylinder pressure to the wheels. As current built to the desired level, the air would be vented off to allow the dynamic to function and reduce show wear. A small amount of air would keep the shoes against the wheels to reduce reaction time as the dynamic faded at low speed. It helped in avoiding a rubber band effect as one braking force ended and other began; thus a smooth transition. Another benefit was that the brake shoes would warm and make a smoother stop. This was called inshot. It had other benefits as well: later on with composition shoes, the warmed up shoes were even more important. If an emergency application occurred while under service braking conditions, it prevented a period of total braking loss while the dynamic was stopped due to the emergency contactor being open. Keep in mind it takes a moment for air pressure to travel the dozens of feet from the operating unit to each truck and then charge all the brake cylinders. With a properly functioning inshot valve, air was already present in the cylinders and the time reduced. All that would need to occur is an increase of pressure. Shortened braking distances are the result. Inshot had other purposes as well. Dynamic braking has a minimum setting that is felt well below that of the pneumatic service brake system. By using inshot, when a motorman pulled the brake handle back the first felt effort was barely a perceptible inshot pressure. Then gradually supplemented by the dynamic. Each car is equipped with a load sensing device which is to allow the brake and propulsion systems to provide identical acceleration and deceleration rates irrespective of car loading. Above certain speeds and or car loading, the inshot would allow additional air pressure to assist the dynamic braking to maintain the exact rate of deceleration as called for up to the maximum rate. On a pre overhaul SMEE car with cast iron brake shoes this could be as high as 110 PSI. The first series of GOH cars arrived with inshot valves installed. Some the reasoning behind removing them was cost effectiveness. While brake shoe wear was a concern, the actual wear caused by the use of an inshot valve was inconsequential. Dynamic braking and pneumatic control systems must operate as an integrated package in multiple arrangements up to 11 strong. As we all know this does not always happen. Frequent issues with locked axles and sliding wheels led to an epidemic of flat spots in the mid to late 80's as composition shoes began to replace the old cast iron ones en masse. It was well known that the ceramic composition required less force to obtain the same effort as doe's cast iron. The replacement J relay valves did not have a sufficient brake cylinder pressure to load weigh pressure ratio to keep wheels from sliding under many situations. Motormen fanning the brakes and trains of mixed consists did not help any. Training and better train consists helped but did not end the problem. Flat wheels are not as much a nuisance as they are dangerous. From the Authority's point of view they are incredibly expensive. Each inch of flat spot requires a ¼ inch of wheel rim to be removed to make the wheel smooth again. Only two inches are available to be trimmed before the entire axle must be replaced on the truck. Expensive when you have a 6000 car fleet! There was also a rise in noise complaints. If a car had older shoes on it, they had a tendency to glaze over on the tread surface which caused a terrible squealing sound. In a tunnel this would be unbearable. Usually this would be the fault of a dead motor; no dynamic would cause a car to use only its air brakes. However the engineers chose to pile the blame on the inshot valves. The 10 PSI in there was not enough to cause the squeal in most cases. So there: the inshot valves were removed and covered over with a blank plate. But the story doesn't end. Flat wheels didn't stop. In their next attempt to reduce the flat wheel problem cheaply (replacing 6000 J relay valves simply cost too much now) the dynamic brake which now supplied ALL of the service brake, none of the safety features and none of the comfort was TUNED DOWN. Scary huh? It gets better… Many motormen, especially as more and more GOH cars came into service began to shut off the dynamic brake and rely solely on the pneumatic brake. It was commonly felt that you could get a smoother and faster stop this way. Management never really caught on but the flat wheel problem hadn't abated either. Since the issue with excessive brake cylinder pressures for the replacement brake shoes had not been properly address how could it? The size of the pistons in the brake cylinders was reduced. This would not change the air pressures used but would reduce the actual force felt upon the tread of the wheels. Now dynamic was actually a better brake. Shutting off the dynamic was lengthening stops, and as previously explained loosing dynamic while under braking meant the train would be coasting free for as long as a full second. At 30 MPH a train would travel an additional 100 feet or more. More about this in a moment. Motormen returned to braking the traditional way and it seemed the flat wheel problem was being managed. Signal systems are complex equations of time, distance and speed. The formula for keeping trains separated safe distances is based on what need to be UNCHANGING data. As we all know from high school math, if you change one part of the equation the entire problem needs to be re computed. The distance between signals, known as blocks, are the length of track of a predetermined distance the use of which is governed by block signals. This distance is based on what is calculated to be the maximum possible speed of a train in the given area and the train's full service braking distance from that speed. Plus a safety margin. In some areas the distance is small because the train's speed is thought to be reduced by preceding signals. This is easily understood from the storm door window next time you ride the subway. Watch as the signals along an express track are far apart then as you get closer to a station they get closer together. If a train was stopped in the station they would time down and allow a preceding train to creep in closer. This is called station time as opposed to grade time which is as we all should know is for curves and hills and though integrated with the block signal system is not dependent upon it. If you alter this equation, by changing any one of the parameters you instantly make the signal system WORTHLESS. Faster trains, or longer braking distances, or both would mean a recipe for disaster. No joke, but this happened one fine day on the Williamsburg Bridge. Initial reports by a sensational media laid the blame on the poor motorman who passed away at his controls. But as in any accident it takes a chain of errors to overcome the fail safes built into modern transportation systems. To overcome the additional weight dropped into the GOH cars they were supplied with an extra 60 horsepower. This had almost no effect on top speed. The gearing was the same. Acceleration is predetermined and cannot be altered because of the need to MU the cars. Many perceived a speed increase but this was really because the newly returned cars had fewer dead motors and other propulsion problems. Far more common and far less understood by the rail fan community is that DC rheostatic propulsion suffers from defects which can not be readily diagnosed without test equipment and at speed. A car can take power and accelerate to only series, or to parallel without its field shunt coils having been phased in to the circuit and slow a train's top speed without affecting its initial acceleration. A perception of normal operation unless a long straight track is encountered is the usual. More on this in a minute. Since the period following the Second World War, the subway system has been owed wholly by the city of New York. With municipal operation came long overdue improvements to the older, formerly privately held segments of the system. The arrival of faster and better braking subway cars like the R-10 and R-12/14 cars meant that the older BMT and IRT's new signals would have to take that in to account. The IND had been well designed and needed almost no changes to its then new signals. The planned top speed of the R-10 was 55 MPH. The GOH cars performed at the same level. Going downhill it was possible to obtain slightly higher speeds but this was in only a very few places in the system and was rare. Even then the signal system was sufficient. When the accident on the Williamsburg happened, the signal system performed as well as it could within the limitations placed upon it by the changes to the braking system, not the propulsion system. Excessive speed was not an issue as the speed attained was what was normal and expected. When the train passed the stop signal and tripped on the stop arm, its brakes went into emergency and applied at their full yet reduced from design parameters pressures. Of course this lengthened stopping distances: unfortunately the next block was occupied and people were hurt. Many items made their way into news accounts and crew room or rail fan discussions. Much of it was wrong. Stop arms misadjusted, suicide attempts, train going too fast, etc. However if you want the full account and the findings of the NTSB, you have to read their report which is where I got much of the above information. Among the recommendations of the NTSB was that the TA repair the brake systems to the design parameters. Using a test set of R-42's NTSB found trains which were stopping nearly 100 feet longer than their design specifications. Other recommendations were that the trains be equipped with speedometers and their speed controlled like those of the cab signaled railroads. Since the NSTB findings are non binding the Authority chose the cheapest way out. All subway cars having been equipped at GOH with energy savings devices meant that it could be as simple as unscrewing and taping a wire at the next inspection. A zero cost modification. Forever thereafter DC subway cars would operate three points on their master controller cam shaft short of full power. The field shunt coil was removed from the circuit this way and the cars would operate at a maximum of about 45 MPH. More importantly they would accelerate through the final stages of start up slower and for all practical purposes usually operate at about 35 MPH. The TA looked like they actually did something, and save a fortune on power consumption. Probably enough over time to pay for all those speedometers. Of course under the right conditions a train might even meet 50 MPH. Down a hill going into certain river tubes, out in the Rockaways. Some car classes are of course better than others at this. We all know which. Commonly known is that PATH operates a similar fleet of equipment. Due to FRA regulations the Port Authority is denied to opportunity to tinker with their own trains the way the TA can. PATH cars have their field shunt coils de active in full parallel like the TA and yet can attain 60 MPH or so because of gearing. This is for somewhat the same reasons: the signal system cannot deal with cars that will go 75 MPH which is what they will do if allowed to use the fourth point on the motorman's controller. TA controllers have only three; the method of controlling a "fourth" point was the light switch energy conservation control mounted on the wall. Cheap way typical of the Transit Authority instead of buying all new motorman's controllers. PATH bought them that way originally instead. I am told the plan was to improve the railroad beyond Journal Square for the high speed and thus fourth point. Apparently this never happened. The difference here is the brakes on PA 1-4 cars is the same as delivered. RT2 WABCO brakes with inshot and proper brake cylinder pressure. PATH brakes work great in my experience and of several of my friends. We're among the fortunate few who have operated trains in both rail systems. At full service brake the cylinder pressure is correct for the weight and dynamic functions smoothly. I am told by one fella who was around before GOH and now works at PATH that these cars feel and run like R-46's did once. Of course without the P-wire, he meant the performance. They are pretty fast and the brakes are easy and forgiving. Which brings me to my original premise: why DO R-44's operate so rough? Now that we understand how inshot works and what happens when it's not there, let's talk about controlling dynamic brake. The Straight Air Pipe (SAP) is among other things primarily a control signal. It's pressure as it is used nowadays is rather anachronistic. It's a substitute for an electrical signal since what it controls is an electric bake. Pressure changes are sensed and the signal converted to a request for dynamic braking by the propulsion system. Since it was at first designed as an air brake with a dynamic to assist rather than the other way around, the brake valve or in the case of the R-44 the Westcode controller is designed to flow air with electrical attachments included. Included are a cam shaft operated by the handle with a number of electrical switches which are adjustable. It's specified to close the switch to the #5 trainline wire at about 12 PSI on the SAP gauge. Just after the snow brake detent on ME-42B and ME-43 brake valves or in the case again of the R-44 just after the minimum brake detent on the single handle. The number five trainline is the wire that engages dynamic brake and as soon as it's energized dynamic begins at it's minimum value for the load of the car. If the switch is adjusted correctly the dynamic will come on when there is a small amount of brake cylinder pressure present. Without the inshot valve the air will vent out through the dynamic brake lockout valve which keeps both air and dynamic form occurring simultaneously. This happens typically smoothly and unnoticed by most. If you listen carefully you can hear the puff of air as it vents just a quick moment after the motorman pulls the brake handle. Inshot was only a part of this process inasmuch as the venting of pressure when the lockout magnet energized was not total as it is today but to the reduced pressure setting of the inshot valve. In cars where the cam switch is misadjusted the dynamic will actually come on with ZERO pressure in the SAP and since dynamic comes on suddenly with no gradual buildup of pressure like air does it has a very hard application. The R-44 being heavy compared to other cars has an initial dynamic setting accordingly higher. Poorly placed switches, removal of the inshot valve and less than perfect Westinghouse propulsion system not designed for the brakes it now carries all contribute to the poor ride R-44's give. I will not miss them. You know which cars I miss most of all? My Cadillac of subway cars, the R-46. As rebuilt these cars finally became the star performer and comfortable ride they were originally intended to be. Had they been delivered this way would think of them the way we think of R-36's or R-42's and not lump them in with the R-44. Thank you for reading all this. I know it was long. I look foreword to all of your responses. Erik _ |
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