The HT-C (locomotive) truck was created in 1970 by Electro-Motive Diesel (EMD) as a component of their range of railway locomotives through to 1994. It was mounted under the experimental EMD SD45X, and applied to a majority of EMD Dash 2 (improved 40 series), and 50 series and 60 series six axle units built by EMD. It has two brake cylinders on the end axles when equipped with single shoe per wheel brakes, a shock absorber on the middle one and has an end transom on the fuel tank side to hold the #3 and #4 motors in line with the #2 and #5 motors, unlike the Flexicoil truck which these two face the #2 and #5 motor under the engine. The secondary suspension was changed to rubber compression springs from coil springs as on the Flexicoil truck.
The objective of these significant changes was lower weight shift between axles under power and braking to improve adhesion.
Externally, an HT-C truck appears quite similar to the earlier Flexicoil C truck, other than the end transom, but the Flexicoil truck's casting has two holes between axles while the HT-C has three.
This two-hole/three-hole difference is a consequence of differing internal coring in the frame casting, and is an identifying, although non-functional difference.
Yawing effect controversy
When Amtrak bought their EMD SDP40Fs from 1972 to 1974, they specified this truck to be fitted with hollow bolsters to mount the engine on the trucks with a lighter load rating to offset the weight of the secondary water tank that was mounted in front of the steam generators. This purportedly contributed to a yawing effect which was characteristic of the second SDP in the consist and the baggage car. The result was that the unit and the car pulled the outside rail out from under the train on high speed curves.
However, Santa Fe traded a larger number of lower-powered locomotives for the banned passenger SDP40s, on a horsepower-for-horsepower basis, replaced the hollow bolsters with conventional bolsters, removed the (above-frame) water tanks (replacing these with concrete ballast) and eliminated the (below-frame) fuel/water tank in favor of an all-fuel tank and it utilized these SDP40s in freight revenue service until their economic lifetime had been reached, after which they were stripped of usable service parts and the remainder of each were scrapped.
The Federal Railroad Administration and Electro-Motive jointly determined that the baggage car was to blame, not the HT-C truck. The HT-C truck went on to have a flawless record on nearly every six-axle 40 Series, 50 Series and 60 Series models.
This did not doom the truck as it did to the SDP40F. They went under almost every EMD six axle locomotive built from 1972 to 1994 except the Detroit Edison SD40s, Conrail SD40-2s and SD50s, Milwaukee Road SDL39s, EMD/BN/Siemens AG SD60MAC, and the EMD SD70 series.
- Logically, H (High-adhesion), T (Truck), C (Six-axle application); later versions would be called HTC-R, with R (Radial steering application), among others in this series; The earlier B (Four-axle application) and C (Six-axle application) trucks, variously "Blomberg B or M" (M=Modified B) or "Flexicoil B or C" could generally achieve factors of adhesion of about 0.25; The later high-adhesion trucks could achieve factors of adhesion of about 0.45, a significant improvement; after the HTCR (various submodels) came a lower cost "bolsterless", non-Radial design, the HTSC-2; in many cases, EMD will allow its customers to select which truck type they want.
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