2 Axle trucks versus 3 axle trucks

[Peaks1985]

I have a question that some of you may be able to answer. Switchers generally have 2 axle trucks and mainline locomotives 3 axle trucks. What's the difference? Why not just use the larger trucks on the switching lines? When I was in Flagstaff, AZ the two BNSF "switchers" (really just Dash-7 I think) with only 2 axle trucks. It seems to be a waste to purchase these locomotives for the light duty.

Another comment, trucks often have axles which can be lifted up from the road (for example dump trucks which use 2 axles sometimes, and when the load is too much put the 3rd axle down), why not design a truck like that for a locomotive? Any help would be appreciated.

[Komachi]

I think the answer has something to do with weight distribution on the axles... the weight of the locomotive is less per axle on a six axle locomotive than on a four axle locomotive. Then again, there's also more "oomph" (for lack of a better term) with six traction motors as opposed to four (atlhough, with the A-I-A-type truck, there were two traction motors per truck with the center axle being an "idler...").

However...

I think your answers will better be had in the General Discussion: Locomotives, Rolling Stock and Equipment Forum.


So, in the imortal words of Jackie Gleason, "And away we go!"

[FarmallBob]

Like Komachi mentions, "per axle" rail loading limits are a prime reason for the existence of 6-axle locomotives. However 6 vs 4 axles provide several other advantages:

- By using 6 powered axles, individual traction motors can be smaller. Example: A (4) axle, 4,200 HP locomotive on requires (4) 1,050 HP traction motors. However using (6) axles the same locomotive will require (6) 700 HP motors. Smaller motors are lighter. And lighter motors = less unsprung weight per axle = better ride/easier on the rail at high speed.

- By properly sizing traction motors and gearing, 6 axle locomotives can apply full engine power to the rail at lower speeds without overloading the motors than can a 4 axle unit. Ie. a 6 axle unit will have greater maximum tractive effort - albeit at lower speed. This is helpful when starting/accelerating a heavy trailing load.

- In the event of a failure of a single traction motor out on the road, a 4 axle unit loses 25% of it’s pulling power; a 6 axle will lose only 17%.

- Modern 3 axle trucks are radial design – ie. they are “steerable”. This permits all 3 axles to remain exactly tangent to the rail while negotiating curves. This lessens flange and rail wear; it also reduces fuel consumption slightly. It likely also enhances tractive effort a bit on curves.

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With regard lifting one axle on a 3 axle locomotive when not needed, consider the following:

- A highway truck’s gross weight can vary by 3:1 or even even 4:1 loaded vs empty. Being able to lower additional axles/tires onto the road when fully loaded offers a real advantage.

However a locomotive’s weight is essentially constant. (About all that changes is the weight of the fuel between a full and an empty tank). So unlike a dump truck a locomotive experiences no significant weight change “loaded” vs “empty”.

2 - The mechanics of such a wheel lifting arrangement in a locomotive truck would be cumbersome. Complicated dual-mode load equalizers, suspension springs, brake rigging, etc would be required.

3 – A 3 axle truck with a liftable axle would of necessity be of a conventional rigid frame design. It could not incorporate the “steering” feature of a 3 axle radial truck and thus enjoy a radial truck’s operating advantages.

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Bottom line there's simply no technical nor economic reason to provide liftable axles on 6 axle locomotives!

[EDM5970]

I don't think anyone ever put smaller motors on a six axle unit than it's four axle sibling would be equipped with, at least not here in the US. An Alco C-430 has four GE 752 motors, a C-630 has six of the same motors. Maybe a later version, but the same basic motor. They didn't put 752s under a C-430 and, say, six 726s or 731s under a C-630. It just doesn't work that way-

[Allen Hazen]

EDM5970--
Your "at least not here in the US" (two posts back) may be an important proviso. Railroads outside North America tend to be much more constrained in permissible axle-loadings (= weight per axle). I don't know of an example where otherwise similar 4 and 6 axle units for the same customer have different traction motors, but it wouldn't surprise me. Certainly both EMD and GE, when supplying locomotives to non-North American, users have used a variety of traction motors even on standard gauge. Recent examples: both the British "Class 66)" (EMD) and the Australia "NR" (GE, built under license by Goninan in Oz) have smaller and lighter traction motors than North American EMD and GE units.

[Allen Hazen]

Peaks and FarmallBob--
Well, the "lifting axle" has made it to locomotives! Though not, perhaps, completely lifting the un-needed axle from the rail.
GE's ES44C4 (so far only 25 units built, for BNSF to service test) is an A1A-A1A unit with AC traction motors, billed as capable of equalling an ES44DC in general service. It has an arrangement of air cylinders allowing the weight on the non-motored trucks to be varied as needed.
FarmallBob's comment about incompatibility of this feature with radial trucks is perhaps right. The ES44C4's trucks are not radial. The majority of North American railroads for the moment seem to favor non-radial trucks: performance advantages perhaps being outweighed by maintenance disadvantages.

[CN Sparky]

Most of the rails around here wouldn't be able to handle the torture of 6-axle power on yard duties. Poor track would get all torn up... mainline rail is much beefier.

[v8interceptor]

I have a question that some of you may be able to answer. Switchers generally have 2 axle trucks and mainline locomotives 3 axle trucks. What's the difference? Why not just use the larger trucks on the switching lines? When I was in Flagstaff, AZ the two BNSF "switchers" (really just Dash-7 I think) with only 2 axle trucks. It seems to be a waste to purchase these locomotives for the light duty.

Another comment, trucks often have axles which can be lifted up from the road (for example dump trucks which use 2 axles sometimes, and when the load is too much put the 3rd axle down), why not design a truck like that for a locomotive? Any help would be appreciated.

The BNSF Dash 7's (that must have been awhile back as BNSF has retired all of their Dash 7s) were originally purchased as mainline freight power with the added capability of handling local freight and switching,they weren't just bought to work branches...

[litz]

Most of the rails around here wouldn't be able to handle the torture of 6-axle power on yard duties. Poor track would get all torn up... mainline rail is much beefier.

There's a practical aspect too ... our track on the Ga Northeastern (which includes the Blue Ridge Scenic) has many curves that are too tight for a 3-axle truck.

Therefore we don't allow either cars (freight or passenger) or locomotives unless they are 2-axles per truck. And realize, this is on a 10mph freight, 15mph passenger railroad!

As a comparison, the Great Smokey Mountain RR -- which is on very similar L&N track -- has 3-axle passenger cars, and they have to slow down considerably (almost walking speed) on several turns, to ensure the cars negotiate the turns successfully.

- litz

[tecocraftuk]

I've always been intrigued by the differences in axle configurations on trucks, especially in the context of locomotives. From what I've gathered, two-axle trucks are generally used for switchers because they're lighter and more suitable for the sharp curves and frequent stops involved in yard work. On the other hand, mainline locomotives with three-axle trucks are designed for hauling heavier loads over longer distances.

The idea of a locomotive with a liftable axle sounds fascinating, though! It reminds me of those heavy-duty dump trucks where they can lift an axle to manage load and maneuverability.