Wheel/rail force measurements on high adhesion locomotives

Railway Age, June, 1993

The Association of American Railroads (AAR) has been involved in the assessment of a.c. traction technology since 1988. This research program led to the preparation of an A.c. Locomotive Performance Specification in 1992. This specification, recognizing the inherent ability of a.c. equipment to better utilize available adhesion, specified dispatchable adhesion levels of 45% at start and 35% continuous.

The AAR was involved in adhesion testing of the EMD freight prototype locomotives of their class SD60MAC in 1992. Recorded test results indicate that these units are capable of an "all weather" dispatch adhesion of 30% to 32%, at minimum continuous speed.

When the SD60MAC locomotives entered revenue service on the Burlington Northern in the fall of 1992, there was concern with regard to the long-term effect that high adhesion locomotives might have on longitudinal track forces and displacements,

Specifically, with fewer locomotives producing equivalent tractive forces, the force density (force per unit length) is higher in the track structure. Of particular interest was the level of longitudinal forces when ascending and descending steep grades, or areas of high tractive and braking forces.

Description of test

As a beginning point for a comprehensive research study into the effects of high adhesion, the AAR undertook a series of revenue service tests in cooperation with the BN, as well as the Santa Fe and Rio Grande/Southern Pacific railroads.

Test sites on ascending and descending grades were chosen on a major BN coal route over ATSF and SP trackage. The ascending grade site (high tractive effort operation) was located at Castle Rock, Colo., on a 1.3% grade in a three-degree curve as shown in Photo 1. The descending grade site (high dynamic braking effort operation) was located near Monument, Colo., on a 1.42% grade in a sixdegree curve, as shown in Photo 2.

Two track cribs were instrumented at each site, one in the spiral to curve and the other in the body of the curve. Each crib was gauged to record the following data: longitudinal rail force and displacement, vertical rail force and displacement, lateral rail force and lateral rail head and base displacements, and lateral tie displacement.

In the body of the curve, angle-of-attack measurements were made for each passing axle. The installed track instrumentation is shown in Photo 3. Data was recorded on a Hewlett Packard computer for "quick-look" review on site and for further analysis at the Transportation Test Center (TTC).

The test plan called for two test train configurations. The first was a 15,000-ton or greater coal train powered by three SD60MAC locomotives (a.c.). The second configuration was the equivalent coal train powered by the conventional locomotive consist (d.c.) of three SD40 units and one SD60 unit, or four SD40 units. Onboard train data for the conventional locomotives consisted of having an observer riding the train to manually record speed, throttle, loadmeter, brake usage, wheelslip occurrences, or sand usage through the test sites.

For the SD60MAC locomotive consist, the Electro-Motive Division research car was present to digitally record the previously mentioned data plus the consist drawbar force and individual axle tractive efforts on one unit.

The trains with d.c. locomotive consists were operated in the normal manner. This means that on the ascending grade, all locomotives were in throttle eight and operating at speeds from eight to 12 mph. On the descending grade, a train brake application was in effect with speed controlled by manipulation of the dynamic brake at around 22 to 25 mph.

The SD60MAC locomotives were operated in a similar manner for one scenario, but in a different manner in a second scenario on the ascending grade to obtain additional information. The second scenario included idling one of the three locomotives while operating the other two at lower speed (five to 10 mph) to obtain higher tractive forces per axle.

Test Results

Testing was conducted from Nov. 2-6, 1992, and included five a.c, locomotive trains and four d.c. locomotive trains. Tables 1 through 3 present a summary of the locomotive consist forces as they passed the two test sites.

Table 1 gives the d.c. locomotive data for both tractive effort and dynamic braking. Table 2 gives the a.c. locomotive data for tractive effort. Table 3 contains the a.c. locomotive data for dynamic braking.

Table 2 shows the higher tractive efforts obtained by the a.c. locomotives when one unit was off-line and the train slowed to between five and six mph.

Figure 1 presents a time history of the a.c. locomotive consist through the Castle Rock tractive effort test site at low speed, The plotted variables are consist drawbar force, average adhesion, and speed.

Figure 2 presents a similar time history for the a.c. locomotive consist through the dynamic brake test site at Monument. All consist drawbar data was fairly uniform based on the consistent total train weights day-to-day, and similar train speeds.