-=> Quoting Alec Cameron to Gregory Procter <=-
 AC> On (19 Jun 97) Gregory Procter wrote to Alec Cameron...
 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)
 AC> Isn't the gearing ALWAYS arranged for ultimate speed? hence freight
 AC> locos geared different from express passenger, and suburban multiple
 AC> untis geared differently again.
I was refering to the potential speed for the locomotive only, not locomotive
and train. The record holding TGV (515Km/hr or whatever) gearing was a
compromise that went only as far as replacement gears which would fit the
standard gear housing and oversize wheels. It's ultimate speed would be 
greater.
 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.
 AC> Not really. Steam locos' drive wheels are always coupled but the
 AC> others, hardly ever.
 Would you like a list of all the Diesels and Electrics built with rod drive?
 And a list of steam locos without coupling rods?
 Both lists would be quite long!
 AC>
 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.
 AC> One diesel or elec loco axle slipping does not inevitably result in
 AC> its buddies letting go. The first axle to slip has most often been the
 AC> lead axle because due to weight transfer at high load, there is less
 AC> weight on the lead axle than there is on the back ones. Some crazy elec
 AC> loco designs used to have a heavy weight in the cab that was rolled up
 AC> close to the driver cab, to overcome this weight transfer. At the end
 AC> of the line, the loco returning with the rear cab now at front, would
 AC> have this weght cranked up to the other end ie the "front" of the
 AC> returning train loco.
 Sounds like "state of the art 1847".
 AC> Some "smart" control systems apply less power to the lead axle, while
 AC> loading up those further back. The smartest, just regulate the slip so
 AC> that the drive power to each motor is different from all its buddies,
 AC> and the drive power ramps down and up rhythmically at the onset and
 AC> recovery from slip. And the extremely smart locos provide constant
 AC> positive slip at each wheel so as to win adhesion considerably above
 AC> the conventional "25%" figure.
 You could do the same thing with a 1997 steam locomotive also.
 AC> A serious loss of adhesion for a steam loco, may be the "shimmying"
 AC> effect as the loco sways left, right, left with the reaction to the
 AC> horizontal forces of the piston rods. This must be a real dance routine
 AC> for Beyer Garretts and other locos having two asynchronous engines
 AC> beneath the boiler. At one stage the pistons moving in synchronism
 AC> [jazz waltz] and minutes later the front piston flying backward while
 AC> its partner on the same side is flying forward [fox trot].
We started this discussion with me postulating a modern steam engine with
individual axle drive, without coupling or connecting rods to the wheels.
A Diesel or electric with coupling rods is not perfect at high speeds.
 AC> (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.
 AC> The "pulses" of a steamer are bad news slip- wise because once slip
 AC> begins it tends to get worse as much more steam [energy] does work and
 AC> the delivered power increases greatly without a commeasurate increase
 AC> in torque and TE. But in an electric motor, a small increase in rpm
 AC> yields a great reduction in torque- and power too: the slip tends to be
 AC> a constant steady speed, self limited.
 AC> The low powered slipping of an elec motor, is often un- noticed by the
 AC> loco driver. The racket made by a slipping steamer, is conspicuous to
 AC> 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:-)
 AC> Sorry, you are not up to date. There are many large locos today where
 AC> positive slip is maintained for long periods. What you have
 AC> [correctly] described is now obsolete technology.
I admit to not being up to date on the latest technology, but I have a 
assing
knowledge of the laws of nature, and even a little knowledge of physics. I
assume that the latest technology hasn't managed to by-pass the laws of 
physics?
I do know that once the wheel slips, the coefficient of friction drops
drastically, perhaps by as much as 50% for steel on steel. The system you 
alk
about can only slip for a very small percentage of the time or the train 
ould
grind to a halt!
I live on a mountain
 AC> crest, the up line is steep, I have watched the locos start from rest
 AC> and these function as described by the makers. The effect is that a
 AC> modern Bo-Bo loco will handle a train almost as effectively as a middle
 AC> aged Co-Co of the same engine rating.
 Yes, I was aware of that.
The old Co'Co' you mention would have to run about 20-25% below the 
theoretical
maximum tractive effort to maintain it's footing. The Bo'Bo' with controlled
slip and controlled weight transfer could manage with 2-5% below maximumum. 
The
result is of course, equal tractive effort.
 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.
 AC> You might like to consider, why this hasn't been done.
That's easy! To be competitive, the steamer would have to be proven and be 
sold
in sufficient numbers to be near the price of the big Diesel loco builders.
It's like electric cars, GM has them for sale, based on petrol engined cars, 
but
the price is 10-15 times that of the petrol car, even though the electric is
simpler.
 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.
 AC> These graphs are usually [and necessarilyl prepared to display the
 AC> overall limits as determined by traction motor, generator, control
 AC> resistors, switchgear, power diodes, cooling fans and any assumptions
 AC> about redundancy, and engine, plus the control characteristics designed
 AC> in to the generator field control and the engine throttle. It isn't
 AC> that easy to point the finger at the traction motors as being the
 AC> principal limitation.
True, but it is the best that can be done with all those fiddly bits in
combination :-)
And I get the same curve while graphing the current/rpm curve of a universal
motor.
  
... Catch the Blue Wave!
--- FMail 1.02
---------------