Loco Truck Assemblies

[NSMoWandS]

I took this pic(cropped for thread) and noticed that the one wheel is new. I know that they turn wheels to bring them back to specs and you can only do it so many times before it needs to be replaced. My questions are... with just one new wheel and two old wheels... does the truck sit crooked? Or do the springs able to compensate for that new wheel? If the truck is rigid... how does it not put more pressure on the bearings?

[locotech]

The photo is of a GE steerable truck (I know... obvious).
However, regardless of the 3 axle truck type, the FRA rules regarding wheel diameter are the same.

First, Wheels are trued on a wheel machine that works kinda like a combination lathe and milling machine. This machine brings the wheels back to a standard profile.
Depending upon how out of profile the wheel is, more or less metal will be removed.

As for wheel diameter..........
The diameter of any two wheelsets in a 3 axle truck can not exceed 3/4" without the use of journal box shims.
If journal box shims are used to take up the diameter difference, then the diameter differential cannot exceed 1 1/4".

For diameter differences less than those stated, the springs do compensate but only to a point..

Now..... Time for truck and wheel class......

Trucks contain the wheel and traction motor combo sets and have a springing arrangement that allows the weight of the locomotive to be spread equally to all axles, even while traversing uneven track, allowing reasonably equal tractive effort to be maintained.

Lets take a good old EMD Flexi-Coil 3 axle truck (a rigid truck used through SD50s on Conrail and I think SD40 on SOU/NS).
The center plate on the locomotive frame fits into a round recess or "bowl" in the "H" shaped truck bolster.
This bolster sits in and atop of the truck frame with the arms of the H sitting on large coil springs that are in a recess in the frame.
This is the first part of your suspension.
The truck frame then sits upon the journal boxes on the ends of the axles.
Between the frame and journal boxes are another set of springs.
This makes it so that each end of each axle is individually sprung.

Now, when the weight of the locomotive (Say an SD40 at about 288,000lbs) sits on the two trucks, 1/2 of the weight is resting on each truck bolster (144,000lbs).
This weight is equally spread to four points on the truck frame, which then delivers 1/6 of this 144,000 to each journal box. (24,000lbs)

Now, if you had a wheelset that was considerably smaller than the other 2 in the truck in, lets say, the center position, the springs would have to un-compress further to make up the diameter difference.
This would result in less weight being carried by the smaller axle and it would have a tendency to slip.
The other two wheelsets would bear a heavier load which could possibly lead to bearing failure.
If the smaller wheel is on either side of the center, there would be a tendency to rock fore or back (depending on direction of travel) and cause slipping.
These conditions also create uneven truck wear.
For a smaller wheel we can use a shim to take up some diameter difference.

So... what is a shim???
A shim is a piece of metal that can be fitted atop the journal boxes of the smaller wheelset in order to shim the space between the box and spring to keep it properly exerting the  proper weight.
There is a limit to shim sizes and the maximum is 1/2", hence the 1 1/4" maximum wheel diameter difference with shims.

Steerable trucks generally do not use shims and AC locomotives do not account of their wheel control systems.
Of more interest is the EMD spec where the wheel diameters of their AC locomotives cannot exceed 1/4" between wheelsets due to the screwy EMD AC traction control system.

By the way, the FRA rules say nothing of a 4 wheel truck.
While not good mechanical practice to run badly mismatched wheels, there is no rule against it.
However, doing so results in very odd truck, bolster and center plate wear.

I hope this dissertation was successful at explaining things......

Kevin
Locotech

[NSMoWandS]

Thanks Kevin! That was great info... As my screen name implies... I am more a track and structure guy than a loco guy... Glad you have come aboard JREB!  Dan (NSMoWandS)

[Ptrainman]

Great reply Kevin. Nice to have an expert onboard.


Paul

[NSMoWandS]

Yes it is!

[rhotond]

The smaller diameter wheel set also has another problem.  For a given speed, the traction motor will turn faster than the others, unbalanciing the power of the motors.  For series traction motors (2 in series), the smaller wheel motor will have the same torque, but the motor will put out more power (more speed, same torque),  hence it may wear out sooner.  For AC units (like GE's)  where you have one inverter and one motor,  the computer can share the power more equally, but with EMD's  (one inverter and three motors) sharing is not possible and hence an inbalance in power output between the three ,  (and AC motors do not like to work in parallel.  It could get so bad that the smaller wheel diameter motor could be in brake while the other two are in power.  Hence the matching required of wheel diameters (especially on EMD AC products.

r

[locotech]

The FRA rules have more to do with weight distribution than torque but you are right.
The older SD units (SD40-2 and older) have traction motors in series parallel at start so there can be some unhappiness with mismatched wheels. Particularly as one axle will draw more current than the other, creating an imbalance electrically and causing the wheelslip system to pick up.
Wheel pairing varies but I usually see 1-4, 2-5, 3-6 pairings.
However, with the FRA mandated wheel diameter matching, this is not generally a problem.
On a 4-axle this is quite the possibility. And I have seen some horribly mismatched wheels on some shortline locomotives I have worked on!

The SD50 and later EMD DC units have the motors all in parallel.
The GE Dash-8 units and later also have motors in parallel.
B36-7 GE units were parallel, IIRC, but the B30-7 units I worked on were series-parallel at start.

The SD70MAC units have 1 inverter per truck and thus three motors in parallel with the inverter. Crappy system.
The GE, as stated, has an inverter per axle so they work pretty well.

More good conversation!
Thanks!

Kevin
locotech