Towed locomotives

[redwingz]

Hi all,

I have a question concerning towing unpowered locomotives. This is often done when locomotives are added to a consist in order to move them between locations.  To save fuel, I assume the prime mover is switched off.  There is a direct connection of the traction motor's pinion gear to the axle's bull gear.  When towing an unpowered locomotive, the large bull gear ends up driving the smaller pinion gear which ends up turning the traction motor's armature.  Even when electrically isolated from the rest of the system (no current being input, no current being output, i.e., no electrical load or back EMF), you still have the weight of the armature being turned at a mechanical disadvantage due to the gear ratio (a large gear turning a much smaller gear).  When you multiply this "load" it by 4 or 6 traction motors, it seems to me that this is a tremendous amount of resistance to overcome.   Am I overlooking something here?  How is this condition dealt with as to not put unnecessary load on the powered locomotives doing the pulling?

 

Thanks,

Redwingz

 

[beaulieu]

redwingz

Hi all,

I have a question concerning towing unpowered locomotives. This is often done when locomotives are added to a consist in order to move them between locations.  To save fuel, I assume the prime mover is switched off.  There is a direct connection of the traction motor's pinion gear to the axle's bull gear.  When towing an unpowered locomotive, the large bull gear ends up driving the smaller pinion gear which ends up turning the traction motor's armature.  Even when electrically isolated from the rest of the system (no current being input, no current being output, i.e., no electrical load or back EMF), you still have the weight of the armature being turned at a mechanical disadvantage due to the gear ratio (a large gear turning a much smaller gear).  When you multiply this "load" it by 4 or 6 traction motors, it seems to me that this is a tremendous amount of resistance to overcome.   Am I overlooking something here?  How is this condition dealt with as to not put unnecessary load on the powered locomotives doing the pulling?

 

Thanks,

Redwingz

 

It isn't, the additional load isn't that great, as you point out you are only turning the armatures, even with the mechanical disadvantage. A mechanism to disconnect the pinion gear would introduce way too much additional costs and reliability issues to make the small savings worthwhile.

[wjstix]

My understanding is that a locomotive that is 'parked' in a yard, terminal etc. has to be tied down (i.e. hand brakes applied) to stop it from rolling away, which can happen even if on a very mild grade. So apparently an engine in idle or shut down will roll pretty well.

[Kootenay Central]

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[selector]

Kootenay Central

...The Engineer exceeded the tonnage rating, and had trouble boarding at any time....

Someone that portly would have to have his fireman mount at the same time on the other side of the cab to keep the engine from tipping.

-Crandell

[Kootenay Central]

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[wabash1]

redwingz

Hi all,

I have a question concerning towing unpowered locomotives. This is often done when locomotives are added to a consist in order to move them between locations.  To save fuel, I assume the prime mover is switched off.  There is a direct connection of the traction motor's pinion gear to the axle's bull gear.  When towing an unpowered locomotive, the large bull gear ends up driving the smaller pinion gear which ends up turning the traction motor's armature.  Even when electrically isolated from the rest of the system (no current being input, no current being output, i.e., no electrical load or back EMF), you still have the weight of the armature being turned at a mechanical disadvantage due to the gear ratio (a large gear turning a much smaller gear).  When you multiply this "load" it by 4 or 6 traction motors, it seems to me that this is a tremendous amount of resistance to overcome.   Am I overlooking something here?  How is this condition dealt with as to not put unnecessary load on the powered locomotives doing the pulling?

 

Thanks,

Redwingz

 

when engines are moved dead in consist they are figured into the tonnage of the train, a dash 8-9 or the evos and the sd 60-70 and soforth is rated at 200 tons and sd 40 s and soforth are at 150. these are the figures i use some think all should be 150, it dont matter to me what they use.  so if they have 4 dead dash 9s in my train i have 800 tons of dead engines added to the say 7900 tons of train for a real total of 8700.

[jrbernier]

  There is no real 'drag' issue when towing an unpowered locomotive.  The big issue is the maximum rotational speed of the traction motor.  A switch engine geared for a maximum speed of 55 mph is going to have the maximum rotational speed of the traction motor exceeded at 'track speed'.  Many times there will be a restriction when towing dead locomotive to make sure the windings in the traction motors do not fly apart.

Jim

[oltmannd]

A little bit of gear friction, but since there isn't any load, this is a very small force. There is a bit of windage from spinning the armature and some brush/commutator friction, but these are small, too. There is a lot of inertia to overcome spinning the armature up to speed, but this tends to act like stored energy rather than being wasted. The energy put in to spin it up to speed you get back when the train is coasting.

[wabash1]

jrbernier

  There is no real 'drag' issue when towing an unpowered locomotive.  The big issue is the maximum rotational speed of the traction motor.  A switch engine geared for a maximum speed of 55 mph is going to have the maximum rotational speed of the traction motor exceeded at 'track speed'.  Many times there will be a restriction when towing dead locomotive to make sure the windings in the traction motors do not fly apart.

Jim

tell me more about the restrictions on the engines dead in tow and the traction motors.

[redwingz]

oltmannd

A little bit of gear friction, but since there isn't any load, this is a very small force. There is a bit of windage from spinning the armature and some brush/commutator friction, but these are small, too. There is a lot of inertia to overcome spinning the armature up to speed, but this tends to act like stored energy rather than being wasted. The energy put in to spin it up to speed you get back when the train is coasting.

 

Thanks for the replies.  I see your point about once it is up to speed, like any object in motion, it tends to want to stay in motion, acting like a flywheel.  The windage from the spinning armature and the friction from the brushes are very small and can be ignored.  The mismatch of the gears and the shear weight of the locomotive are the biggest obstacles to overcome.  On the plus side for the towing locomotive is that getting up to speed is a relatively slow process.  If it wasn't, the wheels would be skidding on the rails during the initial acceleration period.

You can compare it to a friction motor in a toy car.  If you try to get the flywheel which is geared to the wheels spinning from a dead stop, you encounter a lot of resistance.  Once the flywheel is spinning fast enough, it keeps the car moving until the energy is dissipated.

redwingz 

[tleary01]

Jim’s points are right on.  The traction motor armature and gearing present very little resistance.  In fact a locomotive on virtually level track without brakes applied can be moved by nothing more than a strong gust of wind.  If there is the slightest of descending grade the locomotive will continue to move and accelerate.  Locomotives on a grade without sufficient hand brakes applied can start to move from the vibration of a passing train. 

 

When a locomotive is moved “dead” the traction motor power contactors are open and the traction motor armatures are open circuited, however if you were to connect a voltmeter to the armature cables (A and AA) it would indicate voltage.  This is due to the motor’s residual magnetism, but because there is no circuit, there is no power (volts and amperes) and therefore no mechanical resistance.

 

When I was manager of system electrical maintenance for Union Pacific I was sent to investigate an incident where the three traction motors in the front truck of an SD40-2 became un-wound while the locomotive was being moved dead in the consist after a road maintenance crew had re-railed it.  No serious damage was done to the truck and the DIC movement did not exceed a speed restriction of 45 mph.  What I found was the disconnected traction motor armature and field cables for each of the motors tied together with bailing wire and without the insulating rubber “boots” on the connector ends of the cables.  That made the traction motors self-exciting generators with only the motor acting to dissipate the energy.  The motor residual magnetism generated current through the motor fields which boosted the output of the armatures which in-turn boosted the field etc. etc…. until the armature current, heat, probably a few flashovers, plus rpm took their toll.  Bottom line: it is OK to tie up disconnected traction motor cables at the connection ends, but be sure that they are electrically insulated.