I was very fortunate to be assigned to the radial truck development team within the Truck Development Group in 1982 at EMD. I had spent the previous 9 years of my career there working on noise control projects culminating the introduction of exhaust silencers and quiet fans in response to the EPA noise regs that became effective the first of 1980. Mechanical/structural design was my expertise and I fit well into the team taking the design lead.
Radial steering trucks got a lot of attention in the rail industry globally in the 1970’s, particularly for freight cars. Scheffel in South Africa and List in the US were prominent designs with use in fleets with the Barber-Scheffel and Dresser DR-1 (subsequently AR-1 after ASF bought out Dresser) trucks, respectively. EMD recognized that radial steering trucks on locomotives could be very beneficial with advantages of:
- Reduced wheel flange and rail gauge face wear (locomotive trucks were projected to cause perhaps 20% of the rail wear based on numbers of wheels consumed),
- Improved adhesion in curves (conventional 3 axle trucks were known to lose about 10% of their adhesion in 10 degree curves),
-Reduce wheel squeal in curves.
In the late 1970’s EMD started seriously studying radial trucks beginning with literature study and the creation of math models to evaluate truck curving and adhesion performance. EMD had hired a brilliant mathmetical mind in Dr. Mostafa Rassaian and he and Karl Smith, who went on to become Direct of Engineering at EMD, worked on these math models on a part time basis until there was confidence that the models reasonably predicted truck curving performance based on EMD’s extensive test data from instrumented wheelsets. Karl moved on to lead other truck projects and in 1982 I joined with Mostafa to start developing truck concepts that had the freedom of axle movement to allow radial steering that were evaluated using the math models. We had about 10 design concepts schematically built that we
bogie_engineer
bogie_engineer
Apr 2020
Erik, When you examine how a truck goes thru curves, the trailing axle within each truck generally assumes close to a radial orientation to the curve and the axle(s) ahead of it have an angle of attack wherein the outer wheel is grinding against the flange. On a 3-axle truck due to the long wheelbase, the leading axle is at an angle of about 1.2 deg relative to radial line. So there is lateral slippage at the contact patch with the rail that takes away force that could be tangential to the rail where it is does the tractive work. Of course, that’s a simplification but the lead outer wheel always has the greatest “angle of attack” to the rail is trying to climb it.
Erik_Mag
bogie_engineer
Apr 2020
Dave,
Makes sense and thanks for the detailed explanation. I’d also imagine the angle of attack would also contribute to rail wear.
Model Railroader had an article on grades in issue from 1968, and the effect of curves on train resistance has intrigued me since. Specifically how much was due to the wheels slipping against each other and how much was drag from the forces pulling the wheels towards the center of the curve. Most of the texts on calculating train from years ago tend to punt on the “true cause”. Figure this is sort of the flip side of adhesion versus curvature.
Overmod
bogie_engineer
Apr 2020
I am watching this with considerable interest, and hope it turns into thread of the year…
Keep in mind that the dynamics are very different for powered wheels vs. trailing wheels being steered indirectly from carbody ‘guidance’ (including “Talgo-style” truck arrangements or Jacobs articulation).
I believe Mr. Goding has already discussed this in some respects; as I recall it was brought up in Wickens from ‘the opposite perspective’ regarding wheelset stability in unpowered trailing vehicles. I look forward to seeing a full discussion.
Erik_Mag
bogie_engineer
Apr 2020
I would expect that there are differences between powered and unpowered steered axles, with tractive forces being the opposite sign from drag forces as well as being MUCH larger magnitude when not braking.
Overmod
bogie_engineer
Apr 2020
There are also effects on the coned-tread-wheel and railhead interaction, and on the way the axles try to ‘steer’ the truck frame.
This is in part inherent in Mr. Goding’s description of how the trailing axle of a powered C truck is the only one that experiences ‘radial’ centering. You can easily model some of the force couples that the leading and center tread, fillet, and flange face experience when this is true on a pin-guided truck with three motors independently pulling (at what can inherently be three slightly different resultant speeds for a given common admitted voltage, but will not be different for an early EMD single-inverter-per-truck AC drive).
The role of the geometry involved is also important, both for the pivoted and semi-attached ‘centerless’ styles of truck rotation accommodation. As a comparison, look at the American Arch ‘articulated’ truck frame as seen on the early Woodard Super-Power engines. Here the rear wheelset (especially when fitted with a booster) was supposed to take up a radial position relative to the track – but the front articulation pivot location and the need for proper weight distribution make the subsequent location of the
leading wheelset geometrically improper for proper Bissel steering of a two-axle truck that is providing active guiding for the chassis. The ‘solution’ circa 1927 was to float that axle with minimal lateral ‘friction’ restoring force between it and the truck frame, but still with full springborne weight transfer to the equalization – this was done with a pair of hardened steel rollers acting on hardened bearing surfaces. Now, since this was an idler axle, there was no particular need to steer it radially, but it would ‘theoretically’ be possible to use Cartazzi-style curvature of the guide plates in the pedestals so the axle would ‘float’ in radial alignment with the effective forward pivot point of the articulated tr
