GWR sandwich and double-framed locos

That'd be wasabi ....

 

I bet you wish you'd never started this, Tom.

 

Well, there was an interesting thread buried here once ...

Tom

 

Query: I've seen several references to steel, but surely GW locos from the earliest years predate the Bessemer process? Should we perhaps be considering the behaviour of wood sandwiched between plates of (wrought?) iron?

 

Maunsell'S sandwich business never did especially well, but the diversication into hot pies was a huge success.

Back to sandwich frames and baulk road, the whole thing is a bit of a puzzle to me. It's a reasonable presumption that designers didn't make frames any stronger and heavier than they thought they had to. So sandwich frames can't have been significantly weaker or more flexible than plate frames, or they just wouldn't have done the job without unsustainable levels of breakage. Presumably you can't just weld a patch on sandwich frames without undesirable consequences to the core - which, come to think of it, might be why they went out of favour.

 

For sure, but I don't think there's much difference when it comes to the concept.

 

I do wonder if you are onto something about materials availability.

What's the thickest slab of wrought iron that was readily available in the nineteenth century? As locos got larger, I wonder if creating a laminated frame was the only available solution to stronger frames if you couldn't increase the strength just by making them a from a single thicker slab? If the thickest piece of wrought iron you can get is, say, 1/2" and you want to make a frame 7/8" thick or whatever, your only options are double frames or a sandwich frame. Later usage of sandwich frames when steel was available would then be a mix of conservatism in design practice and the fact that a fundamental change in frame design would require considerable redesign work that could be largely avoided if you kept building more sandwich frame locos, just with steel rather than wrought iron plates?

Tom

 

Riddles took every designer's sandwiches and sold them in different areas to see which were most popular, then took his favourite elements from each and put them together to make new sandwiches. Except for a few cases he copied Ivatt's sandwiches, changed the spelling and made them as his own

 

Welding would have been very uncommon when sandwich frames were in common use. Patches on GWR sandwich frames were riveted on

Sent from my SM-A105FN using Tapatalk

 

I believe the sandwich frame was as JimC and Tom suggest, the solution to issues with materials. Wrought iron is made by hammering , and the more you hammer it, the better the impurities are stretched and drawn out, so effectively the stronger, for a given thickness, the wrought iron becomes. It also, obviously, gets thinner. You can't really have strong, thick wrought iron-the available steam hammers, the difficulty of heating through a really thick lump of iron etc meant that 1/4"-3/4" was the practical limit of what was possible.
If, like the web in an I beam, the wood sandwich was really there to hold the plates apart, and their relative location was ensured with bolts and/or rivets, an 'incompressible' filler material would increase the section and give a stiffer result than simply laminating two plates together without a gap.
As for welding wrought iron, with the hearth welding techniques available involve heating the whole area, and hammering, which would involve dismantling the item until you had something you could get on the hearth. I think oxy-coal gas welding came about 1900, prior to that it was a blacksmith job

 

The sandwich jokes have pretty much ceased to be amusing; almost all the good ones have been used. Please stop trying to show us how witty you are.

Noel

 

Yeah, they're a bit stale now...

 

The later Gooch broad gauge locos are recorded as having welded plate frames. Would this refer to.forge hammering thinner plates together to increase the thickness of the frames or to increase their depth at the horn blocks. On drawings of the Metropolitan class the frames do look quite shallow.

I don't think the sandwich frames of the 1887-9 period can be put down to traditional ways of doing things as most new locos had been built with plate frames for the preceding 20 years.

Perhaps it would be interesting to dismantle the replica North Star (in the regrettable absence of the original) to see exactly how the timber infill is arranged. When it was built some sandwich frame locos were still in use on the GWR.

 

In case anyone doesn't know what a samdwich frame loco looks like.

 

How much of it was a hangover from the "convertible" engines built at the time of broad-standard gauge transition?

 

Thanks Lester, that's an interesting point. I believe GWR sandwich frames and GWR plate frames were normally distinguishable because the sandwich frames had, like the image shown, much more of a 'skeleton' appearance. Below I've drawn sandwich frames on the left, plate frames on the right. I expect this reflects the different properties of the two forms of construction, but I'm not enough of an engineer to make suggestions as to why.




The convertibles were certainly responsible for some late implementations of outside frames, one example being the 2361class of not quite Dean Goods, but off the top of my head without checking I *think* they were all plate frames.

 

Does the relative "spindliness" of the earlier frames just represent the gradual increase of power of locomotives as time progressed, and therefore the need for progressively stronger frames?

I don't have figures for GWR locos, but take this sequence of goods 0-6-0s:

Cudworth Standard Goods (Built 1855 - 1875)
- 16" * 24" cylinders
- 120psi working pressure as built (later increased to 140psi)
- 10,800lbf Tractive Effort as built
- Weight (loco only): 27t 18cwt

Stirling O (Built 1878 - 1899)
- 18" * 26" cylinders
- 140psi working pressure as built (later increased to 160psi)
- 16,400lbf Tractive Effort as built
- Weight (loco only): 35t 9cwt

Wainwright C (built 1900 - 1908)
- 18.5" * 26" cylinders
- 160 psi working pressure
- 19,500lbf Tractive Effort
- Weight (loco only): 43t 16cwt

So over a period of about 50 years, tractive effort almost doubled, and loco weight went up by about 60% - that alone would require greater strength in the frames as the designs progressed.

Tom

 

The Egyptian State Railways used outside-framed engines on the Swindon model on all their locos (apart from a few experimental types) well into the 20th century. This was because the CME, Trevithick (grandson of) had come from the GWR.
I've written a short book of the subject which should be coming out later this year...

 

Tom, I imagine that was a factor as well, but the sandwich frames all seemed to have that appearance. Besides [grin] you should know better than to trust loco numbers for dates! The 136class above is an 1880s renewal of an earlier class, and of the "few parts reused" kind, and is actually a newer design than the Standard Goods on the R, but both those sketches are showing roughly turn of the century configuration of similar tractive effort.

 

Regarding the spindliness, or curly holey appearance of sandwich frames-it is very reminiscent of waggon/carriage construction. Waggons and carriages were almost entirely framed with wood, with a limited iron reinforcement, but the shaping to exploit the strength of timber, (or to avoid losing that strength) is mimicked in the 136 chassis.
Now if the timber filler were used end on, that shaping would have no effect at all- was it perhaps to avoid stress points in the wrought iron elements.
On the 388 drawing, the angled elements strengthening the horn guides would be very short lived in a thin wrought iron frame