On (19 Jun 97) Gregory Procter wrote to Alec Cameron...
 -=> Quoting Alec Cameron to Gregory Procter <=-
 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
 GP> ultimate
 GP> speed)
Isn't the gearing ALWAYS arranged for ultimate speed? hence freight locos
geared different from express passenger, and suburban multiple untis geared
differently again.
 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.
 GP> These factors are exactly the same for steam, Diesel, electric and etc.
Not really. Steam locos' drive wheels are always coupled but the others, 
hardly
ever.
     
 GP> Diesels and elecs having independent drive to
 AC> each axle, some may grip while others slip and a decent drawbar pull
 GP> is
 AC> maintained by the loco as a whole.
 GP>  Once the limit of traction is reached, if one axle slips, the load will
 GP> go on
 GP>  to the otheraxles and they will slip to.
One diesel or elec loco axle slipping does not inevitably result in its
buddies letting go. The first axle to slip has most often been the lead axle
because due to weight transfer at high load, there is less weight on the lead
axle than there is on the back ones. Some crazy elec loco designs used to 
ave
a heavy weight in the cab that was rolled up close to the driver cab, to
overcome this weight transfer. At the end of the line, the loco returning 
ith
the rear cab now at front, would have this weght cranked up to the other end
ie the "front" of the returning train loco.
Some "smart" control systems apply less power to the lead axle, while loading
up those further back. The smartest, just regulate the slip so that the drive
power to each motor is different from all its buddies, and the drive power
ramps down and up rhythmically at the onset and recovery from slip. And the
extremely smart locos provide constant positive slip at each wheel so as to
win adhesion considerably above the conventional "25%" figure.
A serious loss of adhesion for a steam loco, may be the "shimmying" effect as
the loco sways left, right, left with the reaction to the horizontal forces 
f
the piston rods. This must be a real dance routine for Beyer Garretts and
other locos having two asynchronous engines beneath the boiler. At one stage
the pistons moving in synchronism [jazz waltz] and minutes later the front
piston flying backward while its partner on the same side is flying forward
[fox trot].
     
 (I'm assuming that we are
 GP> talking
 GP>  about a loco at maximum load here) Coupled axles should slip later than
 GP>  individually driven axles, however steam locos give 4,6 or 8 power 
pulses
 GP> per
 GP>  revolution against a near constant turning force for a Diesel. This is
 GP> good for
 GP>  breaking the stiction of starting a train but worse for maximum TE.
The "pulses" of a steamer are bad news slip- wise because once slip begins it
tends to get worse as much more steam [energy] does work and the delivered
power increases greatly without a commeasurate increase in torque and TE. But
in an electric motor, a small increase in rpm yields a great reduction in
torque- and power too: the slip tends to be a constant steady speed, self
limited.
The low powered slipping of an elec motor, is often un- noticed by the loco
driver. The racket made by a slipping steamer, is conspicuous to say the
least!
 GP> 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.
 GP> This works the opposite way around from the way you have written it!
 GP> The amount of friction between wheel and rail reduces drastically when
 GP> slipage
 GP> occurs, the electronics takes the motors to the point of slippage and 
then
 GP> drops
 GP> back the current and then advances it again. The cycle repeats
 GP> continuously, but
 GP> the point of it is to keep the wheels immediately below the slipping 
point
 GP> for
 GP> the highest percentage of the time. (same thing I know:-)
Sorry, you are not up to date. There are many large locos today where
positive slip is maintained for long periods. What you have [correctly]
described is now obsolete technology. I live on a mountain crest, the up line
is steep, I have watched the locos start from rest and these function as
described by the makers. The effect is that a modern Bo-Bo loco will handle a
train almost as effectively as a middle aged Co-Co of the same engine rating.
 GP> The horsepower rating is more indicative of the maximum speed that
 GP> the
 GP> locomotive can maintain with its rated load than the size of the load
 GP> itself.
Yes. Some yard shunters of only 600- 1000HP with body weight added, may move
mountains up the hump in the yard.
 GP>  We're comparing apples and pears here. A 1997 steam locomotive could be
 GP>  designed with individual axle drive on two axle bogies, with positive
 GP> creep,
 GP>  MUing, single driver, modern ashpan and grate  operation etc. all the
 GP>  advantages that the Diesel and electric have.
You might like to consider, why this hasn't been done.
 GP>  There was an instance reported in our paper a week or so ago:
 GP>  A reveller on his way home had a leak over the overbridge parapet and
 GP> achieved
 GP>  a flashover! He survived but with severe burns apparently!
I guess the Earth Moved for him. AC/DC?
 GP> The newest NZR loco is 30 years old, but all the graphs of of current  
(or
 GP> TE)
 GP> vs Velocity I have (BR, NZ) show a curve from 0 speed/high current to 
ax
 GP> speed/
 GP> low current with a cut off at maximum current rating, otherwise the
 GP> current
 GP> would head towards infinity at 0mph.
These graphs are usually [and necessarilyl prepared to display the overall
limits as determined by traction motor, generator, control resistors,
switchgear, power diodes, cooling fans and any assumptions about redundancy,
and engine, plus the control characteristics designed in to the generator
field control and the engine throttle. It isn't that easy to point the finger
at the traction motors as being the principal limitation.
 AC> rivet pitching on the drivers' doors!
 GP> Wow! What were the design parameters for the ... :-)
 GP> I haven't designed locos over 3.5" gauge.
The design parameters were 1. an Australian Standard, I forget the name and 
.
ANZR [Australian and NZ Railways] codes for uniformity in some design 
ters.
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