How to tell types of 3-axle trucks; “A1A” vs. “C”

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How to tell types of 3-axle trucks; “A1A” vs. “C”
Posted by JC UPTON on Tuesday, January 23, 2018 4:16 PM

What are the visual track-side differences between the”A1A” truck & the “C” truck (other than age of the loco)?

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from the Far East of the Sunset Route

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Posted by Overmod on Tuesday, January 23, 2018 4:58 PM

Not a hard-and-fast thing: the 'sure' spotting guide is to look underneath and see whether there is a traction motor or gearcase actually powering the center wheelset.

There is a reasonable guide to determining C trucks that *are* all motored, which is to look at the axle spacing: if this is irregular, all the axles will be powered.  There are some advantages to having nose-suspended motors and gearcases on the 'end' axles oriented inward (as they are on most of the passenger A-1-A trucks I'm familiar with).  If you want such a motor on the center axle, its effective length will add to the wheelbase between the two wheelsets that have 'motors between' but not the wheelbase between the other two: this gives you the familiar look of the 'trimount' trucks on things like PRR E-44s, and those early Baldwins with Commonwealth-style cast frames with unequal axle spacing.  You could make such a truck 'symmetrical' but then its overall wheelbase would be ridiculously long, and its materials cost needlessly higher.  The alternative, preferable in weight transfer if not yaw polar moment of inertia, is to have all three motors pointing 'the same way', usually with a motor exposed at the outer end rather than adjacent to the fuel tank or equipment at the center, which you can often see in a side view of the locomotive concerned.

Some of the modern A-1-A trucks have an interesting spotting feature: they have an arrangement to reduce load on the unpowered center axle to provide additional traction for AC-motor power at starting or slow speed on bad rail.  This shows as a cylinder and lever arrangement framing the center axle in an otherwise-innocuous-looking three-axle truck.

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Posted by M636C on Tuesday, January 23, 2018 5:05 PM

JC UPTON

What are the visual track-side differences between the”A1A” truck & the “C” truck (other than age of the loco)?

 
In general, this question was asked about early Alco road switchers which could be supplied with either truck arrangement.
 
In that case the spacing of the axles was the clue.
 
The space between the two inner axles on the C truck was greater than that on the A1A truck to allow space for the two motors between the axles.
 
I think this applied to quite different Baldwin truck designs as well.
 
Later truck designs with three motors had equally (or nearly so) axles. The EMD three axle Flexicoil and the later HTC truck had almost equal spacing, but all of these had three motors (except in some export applications)
 
For locomotives in the USA and Canada, equal axle spacing on older truck designs usually meant an A1A truck.
 
Most passenger cab locomotives with six axles had A1A trucks.
 
Peter
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Posted by M636C on Tuesday, January 23, 2018 5:20 PM

Having posted at the same time as Overmod, it is good that we've said mainly the same thing. I hadn't addressed the current GE AC locos, which as stated can be identified by two "brake cylinders" used for raising the axle at starting.

However:

 The alternative, preferable in weight transfer if not yaw polar moment of inertia, is to have all three motors pointing 'the same way', usually with a motor exposed at the outer end rather than adjacent to the fuel tank or equipment at the center, which you can often see in a side view of the locomotive concerned.

All  the designs with which I'm familiar, dating back to the MLW design of 1967, the EMD HTC and the GE "rollerblade" have the motors on the fuel tank side of the axle. I've never seen a loco with the motor ahead of the leading axle, apart from those with leading pony trucks on export designs (initially from GE) fo use in Aftrica.

Peter

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Posted by Overmod on Tuesday, January 23, 2018 5:31 PM

Aaugh, of course Peter is correct about the orientation of the motors. I got carried away without thinking.

You don't want the overhanging motor contributing to 'unsprung' lateral force and impact on the leading axle going into curves.  See also how EMD does two-AC-motor three-axle radial-steering trucks.  The ATSF 'one-spot twins' got similar treatment, first on the lead trucks and then (1940?) both ... but with the idler facing the nominal front of the unit on both ends.  (You can spot this even with equal axle spacing because the idler-end structure is visibly lighter...)

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Posted by cx500 on Tuesday, January 23, 2018 6:43 PM

Were there any North American freight locomotives built with A1A trucks after about 1960?  I can't immediately think of any. 

Recently of course we have seen the arrangement reappear, but that is to keep the cost of AC locomotives down by only having four motors.  Most branch lines needing light axle loadings have long since vanished (or sometimes been upgraded).

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Posted by MidlandMike on Tuesday, January 23, 2018 9:37 PM

EMD has a 2 motor 3 axel truck that has an outer idler axel.  The loco has a B1-1B wheel arrangement.  They could adapt existing C trucks without having to design a new A1A truck.

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Posted by Overmod on Wednesday, January 24, 2018 10:18 AM

MidlandMike
They could adapt existing C trucks without having to design a new A1A truck.

But then again, they could have simplified some of the parts commonalty and perhaps some of the radial steering rigging by demotoring the center axle instead of the outer one.  Part of the ongoing argument about increased wear from radial-steering trucks is, I think, addressed by EMD's decision to in essence provide an  unpowered leading 'steering' axle in either direction.  If my understanding is correct there have been operational 'issues' with these EMD four-motor versions that relate to the absence of a deloading system on the idler, so the perceived gain from the design had to outweigh that of optimizing TE and antislip performance under poor rail conditions.

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Posted by VOLKER LANDWEHR on Wednesday, January 24, 2018 11:08 AM

Overmod
I think, addressed by EMD's decision to in essence provide an  unpowered leading 'steering' axle in either direction.

If I understood correctly the SD70ACe-P4 has a B1-1B wheel arrangement with unpowered axle inside beside the tank not leading.
Regards, Volker

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Posted by Overmod on Wednesday, January 24, 2018 11:20 AM

Watch me get it wrong either way, old or new. 

Helps to actually study the references, and the reasons given for the engineering choices, in all these cases.

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Posted by JC UPTON on Wednesday, January 24, 2018 6:46 PM

VOLKER LANDWEHR
 

If I understood correctly the SD70ACe-P4 has a B1-1B wheel arrangement with unpowered axle inside beside the tank not leading.
Regards, Volker

 

 

Thanks, I did not think about the "B1" or "1B" option on a 3 axle truck.

 

Thanks to ALL for your great responses.  My initial question was along the line of more recent loco designs, which I had assumed to be (almost) exclusively of the "C" type

 

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Posted by MidlandMike on Wednesday, January 24, 2018 8:47 PM

My understanding is that the B1-1B arrangement was chosen because the axel closest to the center of the loco was the one with the "outboard" mounted motor.

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Posted by Overmod on Wednesday, January 24, 2018 10:58 PM

MidlandMike
My understanding is that the B1-1B arrangement was chosen because the axle closest to the center of the loco was the one with the "outboard" mounted motor.

Which of course was the argument I was making before I was wrong on all the older historical examples of TM 'handedness' ...

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