Did this improve performance? Streamlined King + Castle

I find it interesting, now I've done some research, that they're selecting this locomotive class as the one to best Mallards record. It doesn't appear to me as a layman to be the best choice.

Surely you want best adhesion possible and more coupled wheels for speed?

Does make me feel like the Pacific wheelbase has produced the fastest locomotives traditionally for a reason.

 

The thinking behind the divided drive on the Pennsylvania duplex locomotives was initially to reduce the massive thrusts on the crankpins.
Crankpin failures had become a problem in the US in the 1930s, with the ever larger locomotives then being produced.
Once the drive was divided, this led to lighter weight rods being used, that subsequently reduced hammer blow on the track.
Also, steam was not throttled passing through the ports of large piston valves, although the T1 s were built with both piston and poppet valves.
This allowed large ports to flow to smaller diametre short stroke pistons, reducing piston speeds.
Considering that some US passenger locomotives were achieving very high mileage figures, piston and valve maintenance was a factor to be taken into account.
Unfortunately, the slipping problem was never fully considered or resolved.

 

I know there will be people out there who'll say that this is bo**ocks, but it's true: coupling rods add substantially to the rolling resistance of a locomotive. Robert Stephenson knew this when he designed and built 'Rocket'; Frank Webb knew it when he built his three-cylinder compounds without side rods; and it explains why the single lasted as long as it did in the UK. Jim Markland in one of his books about Bolton men tells of the crew delivering a 4F to Horwich works; the loco had sheared a crankpin (which the class was prone to do) so was running as a 2-2-2. They were amazed at how far it would run with steam off, and speculated the results of running it on a local with two or three coaches.

The problem isn't friction between the rods and crank pins; it's more subtle than that. The rods ensure that all wheels are rotating at the same rpm. The snag is that the wheels are of very slightly different diameters because the turning process wasn't (isn't) perfect: there are tolerances built in, and although all wheels are nominally the same size, there are fractions of an inch differences. This means that, since the loco is making a linear speed, each wheel wants to rotate at a slightly different rpm, and that induces increased resistance in the vehicle as a whole. The same thing happens on a curve where the wheel on the outside wants to turn faster than that on the inside. It can't, and a train's rolling resistance is increased quite a lot on a severe curve.

I would also say that the need for adhesion declines as speed rises, although it never disappears completely. A loco's tractive effort (the TE actually delivered to the rails, not the Nominative TE in the Ian Allan ABC) begins to fall as soon as it starts to move, although the decline is slow at first, but getting exponentially larger as speed rises. Power is defined as Force (TE) x Velocity (speed) so although the TE is declining the power rises as the speed is increasing faster than the fall off in TE. There comes a point however where the two values cross. This is the peak power output, after which power begins to decline. At high speeds therefore the actual TE being delivered is relatively small and usually far less than the maximum that the adhesion can deal with. It isn't a rule, though, and slipping at speed isn't unknown, but is less of a problem than it is when getting the train on the move in the first place.

 

I'm certainly not going to disagree with the technical side of this, it was arguably part of the reason for the great success of the Schools. However, I would guess that a design which starts well and can still deploy a lot of its power in poor adhesion conditions contributes as much to successfully keeping to the timetable as anything else, look at Chapelon going from very successful 4-6-2s to downright outrageous 4-8-0s

My favourite of the American streamlined steam classes has to be the N&W J class 4-8-4. There's a claim (I know not how well verified) of one member of the class reaching 110mph hauling 950 tons on level track when on loan to the Pennsylvania, which isn't hanging about on 5'10" drivers. Pretty much the same rpm as Mallard's record, only with two enormous cylinders and a much less enlightened exhaust system (and unlike the T1, not destroying its own valves in the process).

To my eyes at least, also the best looking of the American streamliners. This or an A4 .... tough one.

 

I think it was to avoid the problems of excessive thrust on a single crankpin.

 

The T1s were fine machines, video here: which includes footage of a slip.

 

Would it be fair to say it's very similar to the styling of the NSW no.3801? That being a Pacific of course and not a Berkshire (?)

A very handsome machine indeed.

 

In motion, what a machine.

 

What controls did they have? Did they have a single regulator, or two? What about reversing gear - presumably power-operated (compressed air)? Was their one set or could the two engines be linked up separately?

Tom

 

Pedant mode on.
If it's simple expansion it can't be a Mallet. A Mallett is compound expansion.
Pedant mode off.

 

Correct. It's not a Berkshire. A Berkshire is a 2-8-4, the J is a 4-8-4, known as a Northern is some quarters.

 

Even with the four cylinders, the two sets of drivers could have been coupled, just by one more coupling rod each side. It's not obvious what disadvantage that would have had.

 

Not even if the rear cylinders were set further out, taking advantage of the wide US loading gauge?

Edit, but I suppose it would be difficult to fix the cylinders to the frame in such a way as to leave space for a coupling rod in between.

Further edit: how about having the rear cylinders behind the drivers? You'd need to allow enough length between the rearmost driver and the firebox, but wouldn't that have been another possibility?

 

I think it was the need for such a rod to go though the cylinders that was the big challenge [grin]. Presumably a solution would be inside couple rods as per PLM 151A. http://www.douglas-self.com/MUSEUM/LOCOLOCO/duplex/duplex.htm#plm

I wonder what the effect of the fire was as the cylinders went in and out of sync.