-=> Quoting Reggie Arford to Gregory Procter <=-
 AC> Then there is the beaut feature of
 AC> positive creep whereby even more traction is obtained by
 AC> deliberate, micro slippage possible thanks to microprocessors.
 GP> This works the opposite way around from the way you have written
 GP> it! The amount of friction between wheel and rail reduces
 GP> drastically when slipage occurs, the electronics takes the motors
 GP> to the point of slippage and then drops back the current and then
 GP> advances it again. The cycle repeats continuously, but the point
 GP> of it is to keep the wheels immediately below the slipping point
 GP>  for the highest percentage of the time. (same thing I know:-)
 RA> Umm..., no. I was told on a plant trip to EMD that they achieve
 RA> maximim tractive effort at 125% speed; that is, the wheels turn 1.25
 RA> times as fast as the rails go by. They call it "controlled creep".
I checked up with my friendly Railways development engineer (retired)
We're all right! The New Zealand (and Aussie) situation is slightly different 
to
the US as our axle loadings are much lower (20 T as against 35T). The rails 
are
harder and the tyres softer as we don't have the same problem of hard frosts 
to
crack rails.
Yes, there is controlled creep (you knew, but I didn't) the purpose of which 
is
to cut through the crud on the rail-head to get steel to steel contact. The
maximum used on NZR is 7% My development engineer was rather horrified at the
concept of 25% controlled creep for New Zealand conditions, visions of red 
ot
wheels and steel spawlling off the railheads!
The 7% is also a part of the system of cycling that I mentioned, the original
systems cut back the amperage when slipping was detected and then advanced it
again. As the cut back had to be an appreciable amount, the average maximum 
E
was about 95% of the theoretical maximum. By increasing the cut-back speed 
point
to 107% of rail speed, the 95% TE was raised closer to 100%
 AC> However few locos are allowed to run steadily at optimum speed.
 AC> In the real world it is stop, start, retard, accelerate. Steamers
 AC> do not take so kindly to this and that is why for locos of
 AC> comparable HP, the steamer achieves lower average speeds over
 AC> a duty cycle, than do the diesels/ elecs.
 RA> You are misapplying the term "comparable horsepower". Remember,
 RA> steam locos do not have "a" rated horsepower as a Diesel does;
 RA> they cannot. As any motor turns faster, its power increases (up
 RA> to a point). To rate a Diesel, the Diesel motor is reved up to
 RA> its maximum, and rated there. For a steam loco, the "motor" is
 RA> the drive axles themselves. Any horsepower rating is thus good
 RA> for one speed only; and the "rated horsepower" is actually the
 RA> maximum obtainable at the best speed. At all other speeds, it
 RA> must be less; so the "comparable" steam loco will always be
 RA> underpowered relative to the Diesel whose maximum is (almost)
 RA> always available.
Hmm!
A steam boiler can be rated for a maximum continuous output, but it can also
deliver whatever the cylinders etc can consume for a short period. That is
probably why the internal combustion engine took 50-60 years to replace the
steam engine!
Certainly in NZ the steam engines achieved slower average speeds than the
Diesels that replaced them. But what are we comparing? a 1920s steam engine
with heavier trains than it was designed for, compared with a Diesel designed
for the load and a bit more in hand for future requirements. In addition the
tracks are heavier and straightened. It's not surprising the Diesel comes out
ahead!
 RA> On the other side of the coin, if you consider comparable
 RA> tractive effort, the steam engine will be overstrong relative to
 RA> the Diesel. Available steam TE is mantained higher as you
 RA> accelerate relative to the Diesel, again due to the math.
 RA> As an expansion engine (as opposed to a combustion engine), the
 RA> reciprocating steam "motor" increases its efficiency at lower
 RA> power output. (This is done not by throttling, but by adjusting
 RA> the valve timing to use less steam, and expand it more.)
 RA> Therefore, at the usual utilization rate of 70% of full power, a
 RA> steam engine /of proper size/ will be effective and efficient.
This made sense when manpower was cheap and plentiful, now it's more 
fficient
to have large numbers unemployed and spend millions to provide the same work
with low maintainance, low manpower machines
I'll get back to your final paragraph!
Greg.P.
  
... Catch the Blue Wave!
--- FMail 1.02
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