• Contributing Design Factors in Tractive Effort

  • Discussion of General Electric locomotive technology. Current official information can be found here: www.getransportation.com.
Discussion of General Electric locomotive technology. Current official information can be found here: www.getransportation.com.

Moderators: MEC407, AMTK84

  by es80ac
Just out of idle curiosity that GE AC6000CW has a starting tractive effort at 188000 lbs while weighing at 423000 lbs, ES44AC has TE of 183000 lbs weighing at 432000 lbs, SD90MAC-H has TE of 200000 lbs weighing at only 415000 lbs.

What design factors contributes most to the maximum tractive efforts? Is it the traction motors or the truck design or software? Can we really trust the EMD SD90 has such a significant TE advantage over the GE counter parts while weighing less? I always thought GE being the electric component builder by pedigree has the better traction motors and electrical gears than EMD, why then does EMD seem to have an advantage in the starting tractive effort?
  by MEC407
I'm very skeptical of that EMD TE number. I suppose it's not impossible, but it seems improbable.
  by es80ac
MEC407 wrote:I'm very skeptical of that EMD TE number. I suppose it's not impossible, but it seems improbable.
Me too, especially with the EMD number being the perfectly rounded 200000 lbs:) Wonder if they got the measurement from the testing from a dyno car or more of a hypothetical figure.
  by bogieman
The 200klbs. tractive effort was a design requirement for the traction system validated during testing at Pueblo and in service. The SD80MAC/90MAC use a considerably larger traction motor than the SD70MAC and also bigger than the GE AC motor. The bigger motor required the development of the HTCR-II truck as it would not fit in the HTCR truck (the transoms were moved and the axle spacing altered to keep the wheelbase the same 164"). The larger diameter motor moved the armature further from the axle which leads to a more favorable gear ratio with a larger diameter wheel - minimum wheel diameter for the 80/90MAC is 41" versus 39" for the 70MAC; new wheels are 45" and 43" respectively.

EMD tractive effort rating is a combination of the capacity of the inverters and motors and the creep control system and has probability figured in for weather and rail conditions such that the rating is valid 95% of the time and is based on half worn wheel diameter. It also requires using EMD spec sand, some railroads use sand not meeting that requirement.

Testing of the SD60MAC's showed tractive effort exceeding EMD's expectations was feasible - those units developed over 190klbs. TE during testing which demonstrated 200klbs. was feasible with sufficient electrical/mechanical capacity in the traction system leading to the 80/90MAC design requirement.

  by es80ac
Thanks Dave for such outstanding insight, couldn't find such information anywhere else.

Do you know if the GE AC traction motors improved/altered between the AC4400CW models and the ES44AC/ET44AC models? Also, if the minimum/maximum wheel diameter difference seems to be 4 inches, does that imply the wheels really get that much worn in real usage? 4 inches seems like a huge amount of material for steel wheels.
  by bogieman
I believe GE has kept their AC traction motor the same physical size since it's debut, but as with any good design engineering company, they have probably made internal changes for both performance improvement and cost reduction.

Since at least the late 1930's, the standard freight locomotive wheel was nominally new 40" diameter, worn 37". Any less than 37" would create an FRA rail clearance violation where the gearcase is too close to the rail top - 2.5" is the minimum clearance for any equipment other than the rubber sand nozzles. Wheels have to be trued when either the flange wears to 1" in thickness (it starts around 1.25" for narrow flange wheels) or the flange height becomes 1.5" due to tread wear. On three axle non-steering trucks, wheels are generally trued for thin flange. Class 1 railroads all have underfloor truing machines to cut the wheels without removing them from the trucks. With the 40" wheels, they are typically trued 2-3 times during there lifetime. In the 1970's EMD started delivering locos to Southern Pacific with "fat 40" wheels that were new at 42" but wore to 37". This was simply done to extend the life of the wheels before they have to be pressed off the axle and replaced with new ones, an expensive, time-consuming process. When EMD and GE introduced AC traction, they both designed around 42" new, 39" worn wheels, which allow for a higher numerical gear ratio that more than offsets the increase in diameter effect on tractive effort - the AC motors can operate at higher rpm than the DC motors since they have more robust armatures and no commutator. A few years into production of AC motors, 43" wheels with the same 39" minimum were introduced to provide longer wheel life. Increasing the wheel diameter does have the negative effect of reducing the maximum tractive effort of the locomotive when the wheels are new, however, it's only about 2.4%.

Steering trucks like the GE Steerable and EMD Radial typically change the reason for truing from flange wear to tread wear which has the benefit of allowing more life. When the flanges are high but relatively thick, less material has to be removed from the tread to restore the flange profile as shown below in a figure taken from an EMD technical paper "Locomotive Radial Steering Bogie Experience in Heavy Haul Service" present by Curt Swenson at the 1999 International Heavy Haul Conference held in Moscow.
  by Jay Potter
The maximum tractive effort rating of GE AC-traction locomotives was initially 180,000 lbs, which was based on a software-imposed limit of 30,000 lbs for each of six traction motors. The purpose of that limitation was to prevent excessive mechanical and electrical stress. That per-motor maximum was later increased to 36,000 lbs, with an overriding per-unit limit of 200,000 lbs. The purpose of the per-unit limit was to reduce the risk of excessive in-train forces when operating two-unit consists. The per-motor maximum was ultimately increased to 40,000 lbs; and the overriding per-unit limit remains at 200,000 lbs. The purpose of the 4,000 lbs per motor increase is to maximize unit tractive effort in situations in which (1) rail conditions do not permit the unit to produce 200,000 lbs of tractive effort; but (2) some of its axles -- most frequently number six -- can maintain enough adhesion to produce more than 36,000 lbs of tractive effort.
  by Dan Robirds
In North American practice, the working limit for HT draft gear, couplers and knuckles is 360,000 pounds. The knuckle is actually designed to break first, and can be reasonably replaced by the train crew in the field. Otherwise the coupler, draft gear, or the car frame structure (typically back to the bolster) fails - not an easy fix.

So there is little reason to have a single unit over 180,000 pounds tractive effort, if the intent is to run a minimum of two units on the head end.

A rear DPU consist has their own limitations - a rear helper (or the road power while backing a heavy train) is generally limited to 250,000 pounds. Buff force in excess of this will often jackknife the train within a few cars of the power, especially if empty cars and/or curves are involved. If 12 AC axles are on the rear pushing, the DPU system has a CTE mode to limit their tractive effort (presumably to around 120,000 pounds each unit). If you need more power than that, the helper power has to be put mid train, or multiple helper sets used. To get 16,000 ton unit trains over the mountains here (NE Oregon), they typically have 2-3 ACs on the head end, 4-6 mid-train, and 1-2 on the rear.

Obviously if the engineer has too many axles of power on-line, they have to be careful how much power they use to stay within the 360,000 limit on the head end, and 610,000 pound (360,000+250,000) limit of any mid-train helpers. Same issue with dynamic braking effort.