-=> Quoting Alec Cameron to Gregory Procter <=-
 AC> Hi Gregory
 AC> On (15 May 97) Gregory Procter wrote to Alec Cameron...
 GP> I can't let this one go by!!
 GP> The pulling power of a locomotive is a function of the weight of the
 GP> locomotive
 GP> drivers on the rails (assuming the gearing is not arranged for ultimate
 GP> speed)
 AC> And is compromised by a few other things- like the fact that with
 AC> coupled drive wheels, a slip on greasy or leafy rails leads to almost
 AC> total loss of traction.
These factors are exactly the same for steam, Diesel, electric and etc.
Diesels and elecs having independent drive to
 AC> each axle, some may grip while others slip and a decent drawbar pull is
 AC> maintained by the loco as a whole.
 Once the limit of traction is reached, if one axle slips, the load will go 
n
 to the otheraxles and they will slip to. (I'm assuming that we are talking
 about a loco at maximum load here) Coupled axles should slip later than
 individually driven axles, however steam locos give 4,6 or 8 power pulses 
er
 revolution against a near constant turning force for a Diesel. This is good 
for
 breaking the stiction of starting a train but worse for maximum TE.
Then there is the beaut feature of
 AC> positive creep whereby even more traction is obtained by deliberate,
 AC> micro slippage possible thanks to microprocessors.
This works the opposite way around from the way you have written it!
The amount of friction between wheel and rail reduces drastically when 
lipage
occurs, the electronics takes the motors to the point of slippage and then 
drops
back the current and then advances it again. The cycle repeats continuously, 
but
the point of it is to keep the wheels immediately below the slipping point 
or
the highest percentage of the time. (same thing I know:-)
 GP> The horsepower rating is more indicative of the maximum speed that the
 GP> locomotive can maintain with its rated load than the size of the load
 GP> itself.
 AC> However few locos are allowed to run steadily at optimum speed. In the
 AC> real world it is stop, start, retard, accelerate. Steamers do not take
 AC> so kindly to this and that is why for locos of comparable HP, the
 AC> steamer achieves lower average speeds over a duty cycle, than do the
 AC> diesels/ elecs.
 We're comparing apples and pears here. A 1997 steam locomotive could be
 designed with individual axle drive on two axle bogies, with positive creep,
 MUing, single driver, modern ashpan and grate  operation etc. all the
 advantages that the Diesel and electric have.
The design of steam locomotives really stopped around 1930, the few new 
designs
were just "more of the same" but simplified to the limit, while Diesels were
vastly more complex and required more specialised servicing.
 What we would have in a 1997 steamer is a multi fuel-loco able to use
 indiginous fuel supplies without the expense of electrification.
 Whether this would ofset the extra cost of the boiler maintainance and the 
fuel
 and water haulage is a matter of conjecture. I'm sure it will never happen 
s
 it would require an complete set of facilities.
 GP> The British tended to keep steam from electrified lines because of the 
low
 GP> mounted overhead wires and the perceived danger of flash-over.
 AC> I wonder if there ever WAS a flashover. Many's the time I have looked
 AC> at the blast of steam and unburned carbon from an engine at low speed
 AC> and wondered how the HV insulator above being so blasted, could
 AC> possibly survive.
 There was an instance reported in our paper a week or so ago:
 A reveller on his way home had a leak over the overbridge parapet and 
achieved
 a flashover! He survived but with severe burns apparently!
 AC> GP> Steam locos could often start heavier trains than
 AC> diesels or electrics GP> (comparing equivalent total weight on drive
 AC> wheels) because diesels and GP> electrics are limited in the amount of
 AC> current they can pass through their GP> axle motors from starting. A
 AC> small steam shunter might start twice the GP> weight
 GP> that an equivalent diesel electric shunter could.
 AC> For that to happen, would earn the elec designer a thump for not doing
 AC> his job right. Every NSW diesel/ elec loco I have known of was limited
 AC> at start from rest, by the wheel slip threshold not the armature
 AC> current.
The newest NZR loco is 30 years old, but all the graphs of of current  (or 
E)
vs Velocity I have (BR, NZ) show a curve from 0 speed/high current to max 
speed/
low current with a cut off at maximum current rating, otherwise the current
would head towards infinity at 0mph.
Maybe the cases you know of, were misapplication where
 AC> passenger locos were having to slog in freight or switching service. It
 AC> is common for different gearing to be used so that freight locos motors
 AC> get to spin faster at low road speeds, assuring quicker acceleration
 AC> and enhance the motor shaft cooling fan output.
 AC> The 3780HP one I bragged about, had a bit of my own [trivial] design
 AC> work in it. For long distance coal traffic on a mountains route, these
 AC> were provided with two ducted blower motors in the body. I designed the
 AC> rivet pitching on the drivers' doors!
Wow! What were the design parameters for the ... :-)
I haven't designed locos over 3.5" gauge.
 AC> No I did not forget turbine power. Nor accumulator battery drive. Nor
 AC> Kitson Still. Nor Atmospheric Railways. Interesting experiments,
 AC> successful in some very small scale applications but catch on they did
 AC> not!!
 No, yes, no, no-ish. The accumulator electric certainly did catch on for
 industrial use, and in Europe for branchline passenger service. The last
 of the German battery railcars was withdrawn within the last 5 years, the 
first
 of the series beginning 95 years before! I hear talk of atmospheric railways
 again, now that there is an alternative to leather and lard seals with rats.
 It was shown on the Australian "Beyond 2000" TV program in use, so
 unfortunately I can't go back and find the reference.
 GP> Ok, I vote for steam :-)
 AC> I love ugly ducklings too! Cheers......ALEC
 OK, I still like historical steam and I would like to see a 1997 steam loco!
 (then I'd probably vote electric)
Bye til next instalment,
Greg.
  
... Catch the Blue Wave!
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