I’m reading and hearing some talk about “steerable wheels” on locomotives. What are “steerable wheels” and how do they work? What locomotives are equipped with them? Is this specific to a manufacture?
Look under a newer EMD with Rockwell steerable trucks/ radial trucks like certain SD-70’s that were first tried in the mid-1980’s under certain EMD SD60’s. DOFASCO makes (made?) something similar. The wheel axis cants radially as the carbody leans during application of centripital force in a curve.
Physics book. Physics book. I know it’s here somewhere…[:D]
Where does the engineer sit to steer the train?[(-D][(-D][(-D][swg]
The steerable wheels is a bit of a misnomer, in reality there are steerable trucks. In the US the best known is EMD’s Radial steer HTCR and HTCR II trucks. This design was used on all SD70, SD75, SD80, and SD90 locomotives. Newer SD70M-2 and SD70ACe have the non-radial HTSC truck as standard, with the HTCR II as an option. On a normal “C” truck the three axles are always parallel, on curves this forces the front and rear axles to skid around the curve, the sharper the curve the greater the skid. This causes extra wheel and rail wear, and also extra stress on the rail fasteners (it wants to straighten out the curve). With the Steerable Truck the front and rear axles are diagonaly linked, and their mounting allows them to get out of tram (parallel). The diagonal linkage means that the three axles can all be “Radial” to the curve and do not need to skid around the curve. EMD’s HTCR design is patented, and when it became popular GE was forced to design their own steerable truck. The first design was unsuccessful, it was tested on three early UP(C&NW ordered) AC4400CWs. GE’s second design is workable but more complicated than EMD’s, also the time before overhaul is less, because of this, CP after buying all their GE’s with the design once it became available, has gone back to the non-steerable Hi-Ad truck for their ES44ACs. EMD tested a two axle steerable “B” truck under a ATSF GP50 but never put the design into production, and that ATSF GP50 now has standard Blomberg trucks. In Europe where “C” trucks are less common, SLM designed a steerable “B” truck for their “Lok 2000” family of Electric locomotives. These were sold to the SBB, BLS, NSB, and VR
I haven’t been able to find any reference to an HTSC truck except for EMD exported engines to India from the late 90s. What does this truck look like? all the ACes I’ve seen have what look like HTCR II trucks.
Ah, the EMD page has learned me good. Still, there’s no real reference, just a mention of that truck. Looks like the redheaded stepchild of a Flexcoil/HT-C and a HTCR.
I’m going to have to look closer at the Aces I see now.
HTSC Truck under BNSF SD70ACe
HTCR II under UP SD70ACe
Look at how the axle is connected to the truck frame, the two heavy damper mounts on the truck side frame is the easy visual way to spot the HTCR II.
Don’t have the reference to the technical paper handy, but the wheels in a normal truck frame don’t skid. There is some amount of “viscoelastic creep”; there is also some give in the journal box mounting that allows the wheels to self-steer in response to the cone taper on the wheels. If there is not enough give, the story is that you will be up on the flanges.
This radial direction business requires a little explanation – the axles want to be aligned so that the axles point in the direction of the center of the curve circle hence “radial direction.”
The advantage of axle steering is that a normal truck has a restoring force that resists wheelsets taking up the radial direction while a steering truck allows them to take the radial direction with no constant resisting force.
The interesting thing is that while unguided single axles “hunt” back and forth and are unstable, paired axles in a truck are stable up to a “critical speed” because one axle resists the turning tendency of the other. The reason for this critical speed is that while the distance over which a wheelset hunts is so many feet depending on the taper, the rate at which it hunts goes up as you go more feet per second, and when the rate exceeds a critical value, the damping forces are not enough to prevent from hitting the flanges. A 1:20 taper (freight car) has a lower critical speed than a 1:40 taper (passenger car) although a 1:40 taper takes more wheel maintenance on account of wheel wear, and the rail profile fits into the effective wheel taper hence the need for rail griding to maintain rail profile on high speed track.
You would think that a long wheelbase would make for a high speed truck, but if the wheelbase is too long, you have to put more give into the journal box guides so you are not up on the flanges which results in a lower critical speed – the reported troubles with the two-axle GM Aerotrain car having a ride that went hay
Along slightly different lines, curved track has a longer distance on the outer rail than it has on the inner rail. How do the axles account for the differences in distance travelled by each wheel?
This type of wheelset dates it’s origins to 1901 in Europe with the development of six wheel tram (trolley) trucks which were introduced in 1889. The end wheelset was guided by the middle one when inside of a curve. In Munich alone some 290 motor cars and 225 trailers were so equipped.
The wheels are tapered, giving the outside wheel a larger diameter than the inside wheel.
The way the taper works is the the inner end of the wheel is of bigger diameter than the outer end. When a wheelset slides over to the left, the left wheel is riding on a bigger diameter while the right wheel is riding on a smaller diameter. This turns the wheel to the right, and if the wheelset keeps going right, now the right wheel will be up on the bigger part of the taper and the left wheel on the smaller.
A single pair of wheels on a solid axle with such a taper will “hunt” – follow a wavy line down the rail line. The spacing of each wave in feet depends on the wheel diameter and taper while the frequency of the wave depends on how many feet hence how many waves you are covering per second.
A pair of axle is connected into a two-axle truck, they will suppress each other’s hunting tendency because each axle will be in a different part of the wave cycle. A truck will run OK until you hit the critical speed where the wave frequency is such that you will get constructive interference of the wave pattern of the two axles and then bad things happen. A shallower taper (1:40) has twice the wavelength and hence double the critical frequency of a steeper taper (1:20), but a steeper taper may be needed for freight cars to go around sharp bends in yards and in industrial track.
The effective taper depends on the combination of both the wheel profile and the railhead profile. Unfortunately, both these tapers wear down in the direction of reducing the critical speed, hence the need for wheel grinding and also rail grinding. There is also an art to the profile of flanges to get either an abrupt or a a fillet with a more gradual flange contact.