ReBooting Steam (NW Mailing List)

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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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