How do six wheeled bogies take bends?

[ijmad]

This might be a slightly basic question for those of you who actually work on rolling stock but I've not been able to figure it out from Googling.

Looking at the Class 99, I see it will be built in Co-Co formation with bogies that have 6 wheels.

Now, I'm familiar with how train wheels are conic sections which enables them to roll at slightly different speeds while leaning around a bend. But with a six wheeled bogie, wouldn't a fixed middle axle be out of alignment with the rails on the way around a bend?

Is the simple answer that the middle axle is somewhat free to move laterally? Is this also the case on steam locomotives that often seem to have various sizes of wheels all the way down?

Answers much appreciated!

 

[leezer3]

Yes but no

There is a degree of lateral movement in both the bogie axle and for that matter the track itself. The wheelsets also shift in position on the railhead.
Axles can also have steering arms attached to alter thier angle based upon the position of the other wheels.

Any given piece of rolling stock (bogie or otherwise) will have what's called a minimum curve radius.
All of these factors combined add up to determine what this is.

Try the following PDF:

https://www.skf.com/binaries/pub12/Images/0901d1968019f48b-RTB-1-02-Bogie-designs_tcm_12-62732.pdf

(P38 or thereabouts for 3 wheelset loco bogies)

 

[pdeaves]

Some lateral movement, yes. (Some?) modern bogies have 'steering' outer axles, too, to help.

 

[ijmad]

Try the following PDF:

https://www.skf.com/binaries/pub12/Images/0901d1968019f48b-RTB-1-02-Bogie-designs_tcm_12-62732.pdf

(P38 or thereabouts for 3 wheelset loco bogies)

Thanks!

This diagram from that document shows exactly what I was asking and also seems to confirm what I thought, that there is axial/lateral play required.



Still interested to learn how power is transferred to these middle axles when they are displaced. Particularly in past times with steam locos.

 

[12LDA28C]

Thanks!

This diagram from that document shows exactly what I was asking and also seems to confirm what I thought, that there is axial/lateral play required.

View attachment 124232

Still interested to learn how power is transferred to these middle axles when they are displaced. Particularly in past times with steam locos.

Locos have been built with Co-Co wheel arrangements for many years, it's certainly not new to the Class 99s.

Given that traction motors are axle-hung, I assume they either move with the axle, or allow movement of the axle within the motor. I'm sure plenty of people here will know more.

 

[ijmad]

Apparently another option is to just have unflanged middle wheels, so they can slide around on the railhead. Potentially an unflanged wheel could lose contact altogether on tight radius curves, but that's unlikely to be a location where you need to get all the power down anyway.

 

[edwin_m]

Apparently another option is to just have unflanged middle wheels, so they can slide around on the railhead. Potentially an unflanged wheel could lose contact altogether on tight radius curves, but that's unlikely to be a location where you need to get all the power down anyway.

You wouldn't want the unflanged wheel to go off the side of the rail, otherwise the suspension would push it down and risk a derailment. I don't believe any recent locomotive or rolling stock has or them, but 2-10-0 steam locomotives did, and they are now banned from Network Rail due to incompatible pointwork being introduced some time after they disappeared from the main line. The curves on heavy rail are sufficiently gentle that the offset of a centre wheel is small enough to be taken up by a small amount of lateral play or steerable axles.

 

[snowball]

Apparently another option is to just have unflanged middle wheels, so they can slide around on the railhead. Potentially an unflanged wheel could lose contact altogether on tight radius curves, but that's unlikely to be a location where you need to get all the power down anyway.

Many decades ago I had a Hornby Dublo train set which I think adopted that solution.

 

[ijmad]

You wouldn't want the unflanged wheel to go off the side of the rail, otherwise the suspension would push it down and risk a derailment. I don't believe any recent locomotive or rolling stock has or them, but 2-10-0 steam locomotives did, and they are now banned from Network Rail due to incompatible pointwork being introduced some time after they disappeared from the main line. The curves on heavy rail are sufficiently gentle that the offset of a centre wheel is small enough to be taken up by a small amount of lateral play or steerable axles.

That makes sense as I was reading about how steam locomotives dealt with curves when this solution was mentioned. Thanks for the extra info!

 

[hwl]

EMD have had the option of steerable outer axles on their 3 axle bogies since the mid 1970s which reduces rolling / starting resistance on curves and also improves tractive effort.

In reality unlike the SKF diagram above the angle of attack on the leading axle on curves is the main issue on 3 axle bogies (the angle of attack on the mid and trailing axles is much lower and less problematic).

 

[XAM2175]

Is the simple answer that the middle axle is somewhat free to move laterally? Is this also the case on steam locomotives that often seem to have various sizes of wheels all the way down?

EMD have had the option of steerable outer axles on their 3 axle bogies since the mid 1970s which reduces rolling / starting resistance on curves and also improves tractive effort.

EMD's name for it is radial steering. It's reasonably common in the US and (IIRC) used here on Class 66s, and you might find a bit more info under that term.

This research paper has a few freely-viewable diagrams that illustrate the principle:

The effect of locomotive radial steering bogies on wheel and rail wear | Semantic Scholar

Since Electro-Motive Division's (EMD) 3-axle radial steering bogie for heavy haul freight locomotives was introduced in production in 1993, over 70 million miles of service have already been accumulated on this new design. By the end of 1996, over 800 AC and DC traction locomotives with this new...

www.semanticscholar.org

 

[ac6000cw]

EMD's name for it is radial steering. It's reasonably common in the US and (IIRC) used here on Class 66s, and you might find a bit more info under that term.

There's an interesting thread on the US Trains magazine forum on the development of the EMD radial-steering bogie ('truck' in the US) - https://cs.trains.com/trn/f/741/t/281717.aspx

Also an old EMD design paper about reducing weight transfer between axles in their HT-C bogie (50 years ago) - https://utahrails.net/loconotes/EMD_HTC-Truck_ASME_72-RT-3_March-1972.pdf