this is great. thanks a ton
upgees I have a few questions: 1. What is the safety hook for? 2a. is the vertical snubber the only kind of verticle damper between the axle housing and the frame? Or is there also a damper inside the coil spring? 2b. are there only vertical snubbers on once side of each truck? 4. do modern locomotives like the et44ac still have the same center pin bearing assembly (i think i've also seen it called a "traction pin")?
I have a few questions:
1. What is the safety hook for?
2a. is the vertical snubber the only kind of verticle damper between the axle housing and the frame? Or is there also a damper inside the coil spring?
2b. are there only vertical snubbers on once side of each truck?
4. do modern locomotives like the et44ac still have the same center pin bearing assembly (i think i've also seen it called a "traction pin")?
At Overmod's urging, I'll chime in here though his answer was pretty complete.
1. The safety hook on the GE Hi-Ad serves two functions - to both limit separation vertically in the case of derailment or when lifting the complete loco and to limit rotation of the truck. FRA requires an "anti-sluing device" with a specified strength to limit truck rotation to prevent the truck from rotating to the point where, in a derailment, it would roll under the locomotive as well as to keep the loco going mostly straight; the angle allowed is typically about 7 degrees either side of center and is sufficient for the minimum curve size the loco can negotiate. (The minimum curve coupled to a train is limited by coupler swing, shank length and type of car coupled to, not truck rotation.)
2a. On the Hi-Ad truck in addition to the vertical damping of the hydraulic dampers, there is some unpredictable friction damping created at the sliding interface of the pedestals controlling the longitudinal and lateral position of the journal bearing housing. There are no other dampers inside the coil springs.
2b. Both EMD and GE have placed primary dampers at many different locations over the years. Sometimes all on one side of the truck, sometimes at only both sides of the middle axle, sometimes at all axle ends. GE has also angled the dampers in some applications to get some lateral damping besides the vertical damping they normally only provide. It's based on testing and/or simulation to try to optimize the arrangement with the least cost.
4. Since the early 1990's, both builders have only provided bolsterless trucks for North American 3-axle locos; at EMD the HTRC series and the HTSC and at GE either the Hi-Ad or the Steerable trucks, which to the best of my knowledge are totally interchangeable. These trucks are all similar in that they use four rubber secondary springs that directly support the underframe without the bolster and centerbearing of previous truck designs like the SD Flexicoil and HTC at EMD and the Floating Bolster at GE. The exact center of rotation of the truck is indeterminate even though both builders use a center traction post that only carries tractive and braking forces and some lateral force. The EMD springs are round so have the same shear stiffness in all directions. The GE springs are oval shaped and have different stiffness depending on the direction of force. For the EMD designs, the center of rotation is nominally at the center of the four springs since the lowest rotational resistance occurs for rotation about that point, but because the truck has lateral freedom relative to the underframe (+/-1.75") and the linkage connecting the traction post to the truck frame is quite flexible, the point of truck rotation moves depending on the sum of all the forces in the horizontal plane. The same is essentially true of the GE trucks, but it differs due the angled position of the oval springs and rubber compression springs that are preloaded into the truck frame and carry the pivot connecting the traction post on the underframe which moves the nominal center to between the first and second axles.
Dave
Thanks for the detailed response! This forum is fantastic
Welcome to the forums! Your first few posts will be 'on moderation' and positive moderation can be a little lackadaisical sometimes. Keep posting and you'll be out of the Kalmbach doghouse in just a short time...
The 'safety hook' arrangement is to keep the truck from separating from the locomotive. You may notice that this is a 'centerless' truck, where the four 'bearers' carry the chassis. On EMD trucks, I believe only the torsion in the composite 'springs' (stacks of elastomer blocks and shear plates) provides truck-to-frame pivoting, but those are radial-steering. That is probably the same for the GE rollerblades. (Note that GE has its own sort-of-Rube-Goldberg radial steering arrangement, with the lever works on the outside; yo'll see a lot of these on CSX)
There is one hydraulic shock (acting as a snubber) for each axle; there really only needs to be one if it's capable of providing the necessary damping. (Be advised that a 'snubber' is often a tuned coil spring in a nest which breaks any resonance tendency -- the GG1s were built with these but they were later removed as unnecessary).
The traction pin, as on the Blue Tiger design, isn't a 'center pin' nor is it 'bearing' in the usual sense of a pin-guided truck (like an Adams truck on a steam locomotive). It was intended to facilitate zero-weight-transfer acceleration by providing a tractive link at axle-centerline level, so there is no tendency for the truck frame to 'cock' when the motors start experiencing torque moment through their (nose) suspension.
We have a suspension expert on the forums, Dave Goding, who I encourage to comment here even though his considerable experience was with EMD.
I've been looking at the following images:
https://trainiax.net/drawings/18-ge/et/r18-ge-et44ac-cn-1.GIF
https://trainiax.net/drawings/18-ge/et/18-ge-et44ac-cn-1.GIF
https://patentimages.storage.googleapis.com/ca/5b/10/609747d0f1082f/US20110203480A1-20110825-D00006.png
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