Phil Hom wrote:This was EMD's first testing of s steerable truck. A pair of three motor flexicoil truck was modified by removing one set of wheels from each truck and connecting them together. BN took part of the test under field condition (real world testing).
Only one unit was ever tested.
I was the engineer who was in charge of testing this truck in 1984. I posted the following in March, 2007 at Trainorders.com in answer to a question:
> Can anyone help me out with some information about
> Burlington Northern SDP45 #6599 after it was
> rebuilt with a B-B rear radial truck?
There seems to be a misconception in the railfan community that the HT-BB four axle articulated truck is a radial truck. It is just two rigid two axle trucks connected by a span bolster for vertical load transfer, a low mounted traction linkage to minimize weight shift, and a patented interlink between the two 2-axle trucks that improves the leading axles angles of attack to the rail. It can't be called a radial truck because all axles can never be geometrically radial to a curve.
It was developed ahead of the HTCR series of trucks in 1983-84 at EMD. BN's CMO, Thompson (not sure that was his name) was interested in the project because of the potential for high tractive effort. It actually began when EMD was bidding on South African 11E electric locos in conjunction with ASEA. With 8 axles on 3'-6" gauge track, it was a solution to use EMD design traction motors instead of 6 much larger ASEA motors. In the end, the ASEA motors were used in a GSI zero weight shift truck. Scheffel of SAR, a radial truck designer himself, was not a fan of the HT-BB truck since it was not a true radial truck and dismissed it from consideration.
The BN donated the 6599 for the conversion which was done in EMD's high bay in LaGrange, as loco production was way down at the time. The test trucks were made from two SD Flexicoil trucks where the single motor end was cut off, leaving a wheelbase of 81.5" on each sub truck. I don't recall the spacing between axles 5 and 6 but it was quite small, about 54-56" if I had to guess. Transoms were added made from plate and a cross link added. Holes were cut in the side frames between axles for the shafts that the traction linkage bellcranks pivot about. The bellcrank pivot bearing was a standard Class F journal bearing. A solid bolster was cast at Rockwell (Atchison) as a one-off using a simple pine pattern. Regular SD Flexicoil secondary and primary coils were used. The traction linkage was only about 10" above the rail and had spherical bearings at each end to allow movement but the lower the better for weight shift. Side posts were welded into the underframe after the underframe was lengthened about 4 feet, using the end of a trade-in loco. The center bearing stayed in the original location. A smaller fuel tank without water tank was added to make room for the truck extra length. I think that came from a trade-in unit as well but I don't recall the size. The center axle of the leading SD truck had it's traction motor removed for use in the HT-BB truck and cabling was modified to suit. Air duct openings also had to be reworked to match the motor locations.
The truck was tested in September, 1984 at Stampede Pass in Washington state over a two week period. EMD's instrumented wheels were applied at the BN Interbay shop the week prior to the testing. EMDs test car, I think it was the ET840, was connected and EMD braking units were used. Stampede Pass was closed to traffic at that time but reopened especially for this test. Downed trees had to be removed from the line to make it passible. The consist tied up each night at a yard in Auburn.
Testing showed wheel lateral loads better than the SD and less than expected from a single two axle truck. The unit road well with all that suspension travel of the Flexcoil springs. The truck just never found an application to justify the expense to develop it for production as well as the added cost of motors and wheels, not to mention a new alternator to handle 8 motors in parallel. "
This locomotive was simply modified to test the truck and no intention of producing a 7 axle loco ever existed - if used in production there would have been two HT-BB trucks. But the truck was a little too late - the HTCR radial truck development was in progress and AC traction was seriously under study, as well as a larger DC motor for North American use. Five years earlier, it might have had supporters in the right places within EMD.