There seems to be some confusion as to what modification to the HTCR trucks was done. In my original design (patent 4,765,250), there is a link (known as the interaxle link, item 51 in the patent drawing) connecting the steering beams at the first and third axles to couple them to rotate in opposite directions. When the 1,000 unit SD70M order from UP was in the engineering phase, there was a big effort to find cost savings. We knew from the HTCR development testing in the late 80's that the force in the interaxle link was lower than originally forecast. At each end of the link there was a large (2" ID) spherical bearing that was oversized for the application. As a cost savings, the spherical bearings were downsized to 1.25" ID which proved to be to much of a load capacity reduction and those bearings wore significantly in about 5-7 years of service to render the link ineffective in providing any coupling of the axle motions. Since the worn bearings weren't detected until the locomotives had significant mileage and the trucks were not observed to have any operational or wheel wear issues, a test program where the link was totally removed from several locomotives was conducted for over a year. As no difference in performance or wheel wear was observed during the test period, the EMD recommendation was to remove the interaxle link at overhaul. That is what the UP stencilling denotes. A production change was made about the same time (2008) on new locomotives to remove the link and all HTCR trucks produced since then do not have the interaxle link. When I designed the fabricated frame HTCR-6 truck for the Tier 4 locomotives (see patent 9,434,395), not having to incorporate an interaxle link made the frame and steering beams much simpler. In summary, these are still radial steering trucks.
Back on topic of GE steerable trucks, GE's design to get stable running was to link the traction motors together in the lateral direction with structures that connect the adjacent nose links and roller support bearing housings at axles 1-2 and 2-3 so the axles would move laterally in unison (patent 5,746,135). Bellcranks linked to the bearing adapters at axles 1 and 3 allow for axle rotation and a very visible linkage on the outside of the truck frame connects the bellcranks to rotate in opposite directions and also is linked to lateral motion of the middle axle. This design does not use axle yaw dampers but relies on the constrained motions laterally and in yaw of the axles to avoid hunting instability. Wear at the connections of these linkages will lower the hunting threshold speed and may be what you experience but I'm not involved with those to be sure. I do know that speaking to shop folks at the CSX Huntington WV shop about 15 years ago that fairly early in their service life the steerable trucks required major work to replace worn out bushings. I don't what upgrades or improvements they may have made. The shop people also complained of the extra work involved with combo replacement on the steerable truck due to the structure connecting to the support bearing housing. These may be some of the reasons CSX stopped ordering steerable trucks on their latest orders.
The EMD HTCR truck family gets stable running by using axle yaw dampers the connect the ends of the steering beams to the truck frame to damp axle yaw motion along with truck yaw dampers between the truck frame and underframe. These yaw dampers have proven to be very reliable and are typically rebuilt at truck overhaul as they are not of welded construction but can be taken apart and rebuilt. Early production HTCR trucks did have a problem with a rubber pad between the axle bearing housing and truck frame that limits lateral axle motion; a redesigned pad solved that problem 20+ years ago.
Last edited by MEC407 on Fri Apr 19, 2019 3:53 pm, edited 1 time in total.
Reason: unnecessary quoting