ReBooting Steam (NW Mailing List)

NW Mailing List nw-mailing-list at nwhs.org
Tue Oct 9 16:46:55 EDT 2012


Jerry:

No, it has to do with the relative speed between a driving wheel and the
surface of the rail. It turns out that the wheel actually has to be VERY
slightly slipping relative to the rail surface to generate tractive effort.
Up to a certain percentage slip...and at those low levels of slip it's
referred to as "creep"...the amount of tractive effort that can be generated
increases with the amount of creep. Once you go over the peak, however,
you're in a "traditional" slip and the amount of tractive effort you can
generate falls off very rapidly.

Modern locomotives and MU cars with microprocessor controls can operate in
that positive slope "microcreep" region. That's why you've read of GE and
EMD AC-motored locomotives generating adhesion levels up in the 40+%
region. The height of the peak depends on a lot of factors, and of course dry
rail with sand improves things, while oily water reduces the peak big time.
Also, if some of the available adhesion has to scrub the running surface
of the wheel sideways, that reduces what can generate longitudinal forces,
which is why you've read about steerable-axle trucks.

Older locomotives and MU cars didn't have wheel speed sensing capability
that was precise enough to resolve speeds as finely and at low enough train
speeds to be able to operate in this region, so they just detected that slip
had gone over the peak, reduced tractive effort enough for the relative
slip to get back to - or near to - zero, and allowed reapplication, hoping
the natural rolling friction would stay below the peak. The first systems
that began to be able to use microcreep were the EMD radar systems. In fact,
the first technical papers that described the advantages of operating in
this microcreep region were written by EMD engineers. Shortly thereafter
microprocessors were introduced, and here we are today.

I hope this wasn't "more than you wanted to know". Its a very interesting
subject, and the secret of those prodigious tractive effort and adhesion
values today's state of the art locomotives generate (with the help of their
AC motors, of course).

Dave Phelps




In a message dated 10/9/2012 4:25:58 P.M. Eastern Daylight Time,
nw-mailing-list at nwhs.org writes:

For the uneducated... what is "microcreep"? Am I correct in assuming that
this is where cars are pulled through a flood loader at a set low speed?
--
jerry
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