Bill Shields wrote: ↑Sat Jan 28, 2023 2:49 pm
In the States we usually put all the drivers under the boiler.
There were exceptions (Erie triplex):where we put drivers under the tender, but they had a habit of getting slippery as water and coal were consumed.
The Southern Railway (U.S.) made some interesting (but short-lived) experiments by putting the mechanism of obsolete locomotives under the tenders of newer locomotives, and feeding steam from the main boiler to the tender:
“Beginning in 1915, at least seven of the Southern engines -- 4535-4539 and 4576 (2-8-2s) -- were fitted with a "tractor" on the tender, essentially converting the tender into a 2-8-0 with 18 x 24" cylinders. In place of the usual two bogie trucks, the design placed the tender tank and bunker on a frame that held the cylinders, Four evenly spaced axles holding 50" (1,270 mm) wheels and a single axle at the front with 30" (762 mm) wheels. (The Erie and Virginian Triplexes used similar tender engines.) Thus equipped, the tender weighed 230,000 lb (104,326 kg( fully loaded and developed 23,100 lb (10,478 kg) starting tractive effort. (4561 received a 2-6-0 tender engine set up.)
Southern diagram (drawing number 25-F-79 dated 31 July 1917 shows the Consolidation layout with 18" x 24" cylinders. 4561's setup used the Mogul running gear originally delivered by Rogers as part of the 1889 locomotive (works number 4190).
The RME report described the result as a "duplex" engine and said the design was a response to congestion on the 68-mile (109.5 km), notoriously adversely graded, single-tracked Saluda grade in western North Carolina. (Note: this grade exceeded 5%). In service, the duplex could move 30% more tons per train than could its non-duplex duplicate. A moment's thought will show that as the boiler and grate weren't enlarged, this bonus had limited duration before the whole assembly ran out of puff.
This isn't to say the Southern oblivious to the demand side's problems. They reduced the Mike's cylinder diameters by an inch to 26" (660 mm). The fireboxes were fitted with brick arches and a feed water heater that used exhausted steam from the air compressor. The result, claimed RME, did not "overtax the Mikado boiler to any great extent."
Locobase 84 refers to a photograph of Ss class 2-10-2 5046 showing the tractor engine in a 2-6-2 wheel arrangement under the tender.
Another problem, which to a lesser extent also affected Beyer-Garratt locomotives, was that as the tender used up its water and coal, its factor of adhesion dropped considerably with greater slipping and loss of traction. It's also not clear to Locobase how steam admission was coordinated between the large cylinders forward and the smaller cylinders behind. As might be expected, the tender tractors were removed in a few years.”