Raising or lowering an axle to improve traction

[daveklepper]

YES

[aegrotatio]

Are we raising an unpowered axle then?  Sorry if I seem to have an oblique understanding of adhesion.

[bogie_engineer]

There is no air bag involved, in fact there isn't an air bag on any modern US freight loco truck.  I don't know what you see but there are two extra air cylinders (they are using standard air brake cylinders) that connect to a lever tied to a rod with a bellcrank connected to a chain that puts an upward force on the journal bearing housing.  The cylinders and lever are all that is visible on the side of the truck but the GE patents on the system show how it works.  If you search the US patent office files you can find them.

[Mr Fixit]

The problem of adhesion in A1A-A1A locomotives was first encountered back in the day of the A1A equipped "Covered Wagon" first generation passenger diesels, where four traction motors was sufficient power to move the unit at passenger train speeds and the unpowered axle was used for weight distribution. Railroads operating such units encountered adhesion issues on steep grades particularly in wet rail situations [rain or snow] and so worked with the locomotive builders to modify the bogie's to provide greater adhesion by reducing the load distributed via the centre unpowered axle.

Adhesion of steel wheel on steel rail is a formula of the total weight of the locomotive and the percentage of that weight applied to the rail via the powered [or driven axles in the case of steam] versus the percentage applied via unpowered axles. The exact formulas and percentages are the stuff engineers get excited over due to all the maths and rail conditions so I wont go into that.

As for the ES44C4, the following quote is from Wikipedia and may help explain how it works;

ES44C4

The ES44C4 (Evolution Series, 4400 HP, AC traction, 4 traction motors) is the newest model in the series. It is similar to the ES44AC, except that it has four traction motors instead of six. The center axle of each truck is unpowered, giving an A1A-A1A wheel arrangement. BNSF is the launch customer for this model, ordering 25 units numbered 6600-6624.^[3]

A unique feature of these units is that there is a variable traction control system in their computer systems. One of the differences between an ES44AC and an ES44C4 are the air cylinders and linkages on the truck side-frames of the ES44C4; these are part of the traction control system. Every time a variable in grade, traction, or wheel slip occurs, the computer will adjust the pressure in these cylinders to maintain sufficient adhesion, by varying the weight on the drive axles.

End quote.

In some of the close up photos you can see a heavy duty rubber airbag and this works in the same way as the air suspension bags on modern trucks and trailers to raise one or more axles clear of the road surface when the truck or trailer is empty to provide better road handling and reduce tire wear from scrubbing and skidding. The air is sucked from the suspension bag and atmospheric pressure the forces the bag to compress. At sea level this is 14 pounds/square inch or 10 Tonnes /square metre so the wheels and axles retract up to the chassis rails.

The only difference is that the ES44C4 does not lift the centre axle clear but reduces the amount of load distributed to that axle by altering the air pressure in the suspension bag to increase the adhesion of the powered axles all of which is monitored by the units computer system.

I hope this helps you to understand the modern system.

Mr Fixit

[Cajon_92]

The middle axle on BNSF's ES44C4s can be lifted while moving, which spreads the weight out over the other two axles. The original idea was to lash these up & run them between LA & Chicago on the Transcon, the modern version of Santa Fe's Dash 8 40BWs. From what I understand, they were successful (although can be found working every where, on all types of trains) & BNSF now owns 53 units, 6600-6652 with more on the way in 2011 (or so I've read). Below are a few pictures of BNSF 6613s front truck & middle axle.

http://www.rrpicturearchives.net/showPicture.aspx?id=1908899

http://www.rrpicturearchives.net/showPicture.aspx?id=1908897

http://www.rrpicturearchives.net/showPicture.aspx?id=1908898

Cajon

[edbenton]

IIRC from what I have read and heard from teh crews.  The center axles can be moved to put 15% of the weight that they carry onto the outer axles but they never leave the rail.  How this is done is with Air cylinders and bellcranks.  However just moving that much weight is something to see. 

[timz]

Just to clarify: the BNSF A1A-A1As don't actually raise the axle, do they? The wheels are always in contact with the rail?

But they do reduce the weight on the center axle. How much?

[edbenton]

The Only ones I know that can do that is the NEw GE EVO AC44C4.  The Only Road that has them is the BNSF.  They get the Pulling power of a DC GE44EVO but the Reliablity of an AC Traction motor System.  Crews from what my scanner is saying they like them and they run them on about anything there is from General Merchandise to the Hottest Pigs.  Also the Axle can only be lifted at speeds below 15 MPH.

[aegrotatio]

I honestly forget where I read this so forgive me.  Whatever became of the locomotives that raise an unpowered axle or lower a powered axle to improve traction?  The adhesion of locomotives and their immense weight has always fascinated me and this innovative solution sounded good but I wonder how it worked out in practice?  Was there a scientific comparison of raising unpowered vs. lowering powered axles or did it just consist of raising the unpowered axle to put more force down on the powered axles?

Thanks for the info.