I am having a bit of trouble understanding the relationship between starting vs continuous tractive effort. It seems some locomotives have higher starting numbers and lower continuous numbers. others are just the opposite. I remember reading that the 2500 hp SD35 offered less continuous TE than the 2400 HP SD24. Is TE not a direct result of weight and overall HP? I also read somewhere some railroads fovored the SD38-2 for slow speed lugging ability. It seems to me that an SD40-2 would be much better. Why wouldn't railroads always choose to by the highest HP units they can.
This also makes me wonder about the relationship between 4 and 6 axle units of similar HP. Can a 3000 HP GP40-2 pull as much as a 3000 HP SD40-2? In a tug of war could an SD40-2 at 3000 equal a pair of 1500 HP SD7s. Seems to me the SD7s would win because they weigh more.
This has been discussed before.
Lets start with the last paragraph. At the same axle loading, an SD40-2 will pull 150% what a GP40-2 will. You are correct about one SD40 vs two SD7s.
Your first paragraph is pretty much a jumble, so lets start with the physics.
First, tractive effort is a function of weight on drivers and the factor of adhesion. Factor of adhesion for a DC locomotive is usually taken as 22-25% on dry jointed rail. Wet rail will knock adhesion down to about 18%, but sand will get you back to 22-25%. AC drive units are figured at 33% adhestioin due to their superior wheel creep controls, so they are rated at about 150% of a DC unit of the same weight on the drivers.
The other limit is the power limit of DC motors. Holding throttle constant, as speed decreases the amperage in each motor increases, and at some point the amperage becomes enough to burn out the isulation and destroy the motor. Each loco has an amp limit that corresponds to a minimum continuous speed at run 8.
These two limits are separate Starting TE could be higher than TE at minimum continuous speed, but MCS will be used to figure tonnage rating on a grade, which is what the operating department, and the locomotive engineer really cares about.
As to SD 38-40 the 38 will consume less fuel and does not have a turbo charger to maintain, but will have the same Tractive Effort as the 40, just at a lower MCS.
You can mess around with starting vs. MCS, but be aware of what lies behind the numbers and know that TE at MCS is what controlls tonnage ratings on ruling grades.
Also know that if units have differet speeds at MCS, then the entire consist must be rated at the highest MCS. That is the reason mountain railroads did not run an F7 or GP9 in a consist of three SD40's going over the hill, as that consist had a lower tonnage rating than the SD40s did by themselves.
Mac
oarb00It seems some locomotives have higher starting numbers and lower continuous numbers. others are just the opposite.
oarb00I remember reading that the 2500 hp SD35 offered less continuous TE than the 2400 HP SD24.
oarb00Can a 3000 HP GP40-2 pull as much as a 3000 HP SD40-2?
oarb00Seems to me the SD7s would win because they weigh more.
One thing to remember about "starting tractive effort" of a DC-motor diesel: nobody ever says what it is. For instance, 1975-era EMDs were limited to 1500 amps thru each motor. For a given gear ratio that means X pounds of tractive effort from each axle, assuming no slipping. So how much is X? Dunno, and no one else seems to know either.
So instead people just divide the unit's weight by four and call that "Starting Tractive Effort". That's easier, but doesn't mean much.
timzSo instead people just divide the unit's weight by four and call that "Starting Tractive Effort". That's easier, but doesn't mean much.
In the real world, starting tractive effort for a diesel is pretty meaningless. You wouldn't let a train out on the road that couldn't get over the ruling grade needing more than continuous TE.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
oltmannd timz So instead people just divide the unit's weight by four and call that "Starting Tractive Effort". That's easier, but doesn't mean much. In the real world, starting tractive effort for a diesel is pretty meaningless. You wouldn't let a train out on the road that couldn't get over the ruling grade needing more than continuous TE.
timz So instead people just divide the unit's weight by four and call that "Starting Tractive Effort". That's easier, but doesn't mean much.
My carrier rates it's tonnage rating based on moving tonnage over the ruling grade - not starting tonnage on the ruling grade. Trains that stop on the ruling grade will need a helper engine(s) to get the train started on the ruling grade.
Never too old to have a happy childhood!
oarb00I am having a bit of trouble understanding the relationship between starting vs continuous tractive effort. It seems some locomotives have higher starting numbers and lower continuous numbers. others are just the opposite.
Wikipedia has a section on Defining Tractive effort that discusses starting vs continuous.
But another thing to consider is how much torque a motor can generate while moving and when at a stand still without slipping. In general, it would depend on whether horsepower or torque increases or decreases with speed and where does it peak for a particular type of motor (e.g. DC/AC is one aspect).
greg - Philadelphia & Reading / Reading
oltmanndstarting tractive effort for a diesel is pretty meaningless.
oarb00 In a tug of war could an SD40-2 at 3000 equal a pair of 1500 HP SD7s. Seems to me the SD7s would win because they weigh more.
Be careful with this comparison as there were many SD7 and SD9 locomotives built with a light overall weight. Particularly those built for the Granger railroads like CGW, C&NW, MILW, and the Soo Line. In these cases two SD7 or SD9 locomotives would weigh less than one of their SD40-2 locomotives. Also the newer SD40-2 would benefit by having a more sophisticated wheelslip control system.
BaltACDTrains that stop on the ruling grade will need a helper engine(s) to get the train started
timz oltmannd starting tractive effort for a diesel is pretty meaningless. Lots of hills are short-- continuous TE and momentum will get extra tonnage up the hill as long as it doesn't get stopped. If it does happen to get stopped on the steepest grade, the engine's starting TE will be what matters (if you don't want to double, or back down to the bottom of the hill).
oltmannd starting tractive effort for a diesel is pretty meaningless.
Lots of hills are short-- continuous TE and momentum will get extra tonnage up the hill as long as it doesn't get stopped. If it does happen to get stopped on the steepest grade, the engine's starting TE will be what matters (if you don't want to double, or back down to the bottom of the hill).
Depends on how well engineered and constructed the line is.
Personal observation -
In the North, when railroads were constructed a lot of cuts and fills were used to maintain a steady grade profile - to a greater extent than not, these lines had sufficient financial backing to support the engineering and construction.
In the South, by contrast, with lesser financial backing and a more agrarian economic base, the tracks were basically laid on the contour of the land. Undulating track profiles, where with today's train sizes the train may be strung over multiple crests and valleys - a engineers nightmare.
timz BaltACD Trains that stop on the ruling grade will need a helper engine(s) to get the train started If UP ever starts running coal trains east from Fremont to the Blair bridge (instead of the long way via Omaha) they might be an example of that. Nowadays the ruling grade across Iowa is around 0.5%, but if they run via Blair they face a 1% grade about 1.8 miles long, preceded by near-level track and followed by downgrade. Suspect they can manage that with full tonnage if they don't get stopped, but...
BaltACD Trains that stop on the ruling grade will need a helper engine(s) to get the train started
If UP ever starts running coal trains east from Fremont to the Blair bridge (instead of the long way via Omaha) they might be an example of that. Nowadays the ruling grade across Iowa is around 0.5%, but if they run via Blair they face a 1% grade about 1.8 miles long, preceded by near-level track and followed by downgrade. Suspect they can manage that with full tonnage if they don't get stopped, but...
They do send coal trains the "short way" at times. Usually it's the shorter (and less heavy) trains because there is a lull in westbound traffic. Or if there is a blockage on the "long way" through Omaha.
Last week a wrong end draw bar brought things to a stop on the Iowa side south of Missouri Valley. I took 135 cars the "short way." I think we topped Arlington around 13 mph.
Unless there has been some failure, all the loaded trains run with a DP. Mine was no exception. Computer modelling the UP has done show that if a 135 car train was sent over the Blair Sub in conventional mode, the train would suffer a break in two because of stress on the couplers.
Jeff
BaltACDIn the North, when railroads were constructed a lot of cuts and fills were used to maintain a steady grade profile
As I recall, on the eastward PRR Allegheny climb the chart says the steepest half-mile is about 1.5%, but the grade doesn't average more than 1.1% over any two-mile stretch. PRR never felt the need to even out the grade on their heaviest route.
(Looking at the chart, it says the "ruling grade" eastward to Gallitzin is 1.5% but can't find any such on the eastward track-- just the expected 1% with lesser grades here and there.)
jeffhergertI took 135 cars the "short way." I think we topped Arlington around 13 mph.
timz BaltACD In the North, when railroads were constructed a lot of cuts and fills were used to maintain a steady grade profile "Steady" meaning "constant within 0.2% or so"; on most hills it apparently wasn't worth the expense to do better than that.
BaltACD In the North, when railroads were constructed a lot of cuts and fills were used to maintain a steady grade profile
"Steady" meaning "constant within 0.2% or so"; on most hills it apparently wasn't worth the expense to do better than that.
Steady meaning - when you go up, you go up - the rate of up may be different from section to section - but you aren't going down.
In undulating territory you may being going up for 3/8 of a mile and then down for 1/2 a mile and back up for a 1/4 mile and level for a mile and down for 1/8 of a mile - following the contours of the land.
timz jeffhergert I took 135 cars the "short way." I think we topped Arlington around 13 mph. With two units?
jeffhergert I took 135 cars the "short way." I think we topped Arlington around 13 mph.
With two units?
3 units in 2x1 configuration. Unless there has been a failure, it's rare to see only 2 units on a loaded coal train on the exCNW across Iowa anymore.
timz jeffhergert I took 135 cars the "short way." I think we topped Arlington around 13 mph.
beaulieu oarb00 In a tug of war could an SD40-2 at 3000 equal a pair of 1500 HP SD7s. Seems to me the SD7s would win because they weigh more. Be careful with this comparison as there were many SD7 and SD9 locomotives built with a light overall weight. Particularly those built for the Granger railroads like CGW, C&NW, MILW, and the Soo Line. In these cases two SD7 or SD9 locomotives would weigh less than one of their SD40-2 locomotives. Also the newer SD40-2 would benefit by having a more sophisticated wheelslip control system.
Lightweight SD7-9s still came in around 300,000 lbs, 50k per axle. SD40-2s could be as light as 368,000 lbs (Soo had some this light), up to BN coal service units over 400,000 lbs., close to 70k per axle.
The pair of SD7s should pull more than 1 SD40-2 at low speed. Around 25-30 mph, they should pull the train about the same, running into the horsepower limitations.
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