Mixing diesel with different gear ratios

First things first: unlike most of our models, in real life diesels use spur gears to connect the traction motors on each axle to its wheels.   The ratio is usually about 4:1.  All of the gears lie in the same plane, and the wheels can turn the motor.  So a faster loco can always "push" a slower one without bucking, etc.

As I understand it, this gear ratio, the number of traction motors, and the design of the traction motors determines at what speed the loco can safely maintain its maximum rated tractive effort.

Example 1:  A four-axle freight diesel geared for 65 mph might achieve a peak 50,000 lbs of tractive effort at 13 mph for a maximum of 5 minutes (which would hopefully be enough time to accelerate the train to more than 13 mph.)

Example 2: A passenger diesel geared for 90 mph might generate 36,000 lbs of tractive effort at 18 mph, again for a max of 5 minutes "short time rating."

So if the two were MU'd together and struggled with a heavy train at speeds below 18 mph for more than 5 minutes, the crew would be at risk of overheating the traction motors on the passenger unit. 

I think some of the later computer-controlled diesels automatically "derate" themselves or even shut down rather than burn up their motors.  However, when the weaker engine quits, this would cause the train to stall and it couldn't move again until the motors cooled off.  That's a long day on the railroad!

Some brands of diesels, like Baldwin, had a reputation for very durable traction motors that could withstand abuse.  But a good trainmaster wouldn't pair an ex-passenger loco and a freight loco on a long distance assignment, because the passenger unit might overheat in mountainous terrain, or the freight unit could not be used to its full hauling capacity.  Now all of the above applies to real-life diesels with DC traction motors as was typical before the 1990s. 

Newer locos meant for heavy hauling have AC traction motors.  These motors are extremely powerful and controlled in a different way.  So they might be able to maintain their full tractive effort indefinitely at very slow speeds for loading coal, yet still achieve top speeds of 75 mph, fast enough for the hottest container freights.  I've heard some engineers say they are difficult to modulate at low speed.  So Lionel isn't the only one that struggled with the "Odyssey Lurch!"

There are a lot of folks on this Forum who are professional railroaders with seat time in a real diesel locomotive.  Hopefully one of them will respond and correct me if I'm wrong!

If locos of different gear ratios are mixed, the consist has to be limited to to the top speed of the slowest loco.  To tow a loco at a speed higher than it is designed for risks having the armature windings thrown out of their slots.  If that happens, not only has it wrecked the traction motor, it may lock up the wheels.  If a loco has a wheel set locked up on the mainline, major surgery with a cutting torch may be required to get it in the clear.

Thank you for the replies so far.  I should have made it clear that I do know the basic mechanics of the diesel-electric locomotive.

What I believe is confusing me is this.  As the engineer in the lead unit notches out the throttle the train will accelerate.  However, the units with passenger gearing will try to go faster than the ones with freight gearing.  What keep them from bucking against each other without damaging the traction motors.

Or, when a freight diesel is assigned to help a passenger unit (train) would they have to use two crews to independently control the units and not m/u them?

Stuart

@Stuart posted:

Thank you for the replies so far.  I should have made it clear that I do know the basic mechanics of the diesel-electric locomotive.

What I believe is confusing me is this.  As the engineer in the lead unit notches out the throttle the train will accelerate.  However, the units with passenger gearing will try to go faster than the ones with freight gearing.  What keep them from bucking against each other without damaging the traction motors.

Or, when a freight diesel is assigned to help a passenger unit (train) would they have to use two crews to independently control the units and not m/u them?

Stuart

Allow me to explain, for those who wonder what this is all about, that  DC Diesel electric locomotives produce high traction motor amperage at low speed, and -- even if the throttle remains in Run-8 -- amperage decreases as speed increases.  The reverse is true when speed decreases.  Optional gear ratios move this entire speed range higher or lower on the particular locomotive, and horsepower also affects the rated minimum continuous speed.  The Engineer must not exceed the maximum speed of the locomotive with the lowest gear ratio, and not drop below the speed of the locomotive with the highest minimum continuous speed .

In reply to each question:

• On the head end consist, all units will be pulling hard as the train accelerates.  Horsepower, not gear ratio, will determine how hard each locomotive is working at speed unless all units have identical horsepower but different gear ratios.  So, even if the gear ratios are identical, as the speed increases, the pulling power of the respective units in the consist fades on the lower horsepower units first.  Even if the lowest horsepower unit is the controlling unit, the drawbars between all of the units will be stretched.  No locomotive unit will shove another in that scenario.
• Nope.  No bucking due to either gear ratio or horsepower.  Each unit will work according to how much horsepower it has and at what speed it is unable to pull as hard as higher horsepower units in the consist, but all will pull at any speed.  A GP38 in leading position might only be able to make 200 amps at 70 MPH, while the SD45's behind it are putting out 400 amps, but the consist remains stretched.  If a helper consist is positioned within the train, pushing some cars while pulling others, all locomotive drawbars will be stretched.  If a helper is entrained at the rear, all locomotive drawbars will be compressed.
• No.  The units will run fine when m-ued.  You see this all the time when an Amtrak unit has trouble and the railroad finds an SD40-2 to couple onto the front.  In that case, the gear ratio makes a difference, but only as to the maximum speed allowed on the lowest geared unit, and the rated minimum continuous speed of the highest geared unit.  Mixing AC and DC units in the same consist is something for a different discussion, as AC units do not have a minimum continuous speed rating.  If it's a true helper situation, where the helper is going to cut off at the crest of a grade, then you would call a helper crew and the helper would not m-u to the road consist for practical reasons related to the delay required for extra locomotive air brake tests and making and breaking connections both electrical and air, as well as setting out and securing the helper in an auxiliary track.

@Stuart posted:

Thank you for the replies so far.  I should have made it clear that I do know the basic mechanics of the diesel-electric locomotive.

What I believe is confusing me is this.  As the engineer in the lead unit notches out the throttle the train will accelerate.  However, the units with passenger gearing will try to go faster than the ones with freight gearing.

Actually no, such is not the case. The whole purpose of different gear ratios is to limit the maximum RPM of the DC armature. Thus, a passenger gear ratio of say 52:25 (the highest speed ration in EMD's catalogue, good for over 112 MPH), would have the traction motor armature turning quite slowly at say 15 to 20 MPH, regardless of assigned to a passenger train or freight train. However, the "down-side" of passenger service gear ratios is, the higher minimum continuous speed, which will cause main generator damage if the unit is operated at slow freight train drag speeds, say below 15 MPH, too long. The EMD standard freight unit gear ratio was 62:15, providing a minimum continuous speed of around 15 MPH, with a maximum horsepower of 65 MPH (the maximum traction motor armature speed would have been about 72 MPH). Conversely, that passenger unit gear ratio of 52:25 would have a minimum continuous speed of something above 25 MPH (maybe even 30 MPH), with the maximum horsepower speed of something above 112 MPH). 

  What keep them from bucking against each other without damaging the traction motors.

There is no "bucking against each other" with diesel electric locomotives in MU consists. Just as there is no "bucking against each other" when a 1500 HP GP7 is MU'ed in a consist with a 3000 HP SD40-2.

Or, when a freight diesel is assigned to help a passenger unit (train) would they have to use two crews to independently control the units and not m/u them?

That would depend on how the freight unit "helper" was added, i.e. simply added to the front as a true "helper", then yes there would be two Engineers. However, if the freight unit was actually added and MU'ed to the front of the passenger unit/units, then the passenger train Engineer would simply move into the cab of the leading freight unit and operate the consist from there.

Stuart

Tom and Hot Water, thanks for the great answers, I just learned something new today, I found your answers very informative and I had no clue that was how they worked.

Good questions to ask and you got two very good answers from guys who know their business.

Super job from Tom and Jack, even on old man can appreciate and education.....