Frame design and construction
Locomotive design

Bartram, J.H. Locomotive frames (a comparison of types).  J. Instn Loco. Engrs., 1926, 16, 299-308. Disc.: 309-12. 7 diagrs. (Paper No. 193)
Presented in Manchester on 4 December 1925 chaired by Kidd
Frame evolution. The first locomotive had no frame, the boiler providing the attachment for the cylinders and wheels. Springs and brake gear were unknown, but independent frames were soon introduced and springs and brakes added. Built-up frames, of wood with wrought iron flitch plates, thin plates riveted together, cast iron frames, welded bar frames, forged, slab and double frames were used and gradually developed into the modern frame. An early defect in the design of the locomotive was the rigid connection of the frames to the smokebox and firebox, and it was also a common practice to couple the drawbar to the rear of the firebox. This system proved a permanent source of boiler trouble that was only remedied by making the frame self-contained and resting the boiler upon it so that it could expand freely. The slab plate frame, introduced by Beyer Peacock at an early stage in the history of the locomotive, provides an example which may be described as midway between  later British and American practice. The frames were of wrought iron, the hornblocks being forged solid with frame plates, thus entirely avoiding trouble with loose bolts or rivets and’giving a solid support to the axleboxes. American engineers adopted the bar frame which had been introduced by Stephenson in 1825, and subsequently developed
Plate frames
advantages:
very rigid vertically.
certain amount of lateral elasticity; gives slightly  on curves at speed
strong buffer beams and attachments.
stays, gussets and brackets easily attached.
deep firebox and efficient ashpan.
large coupled wheels can bc used under firebox.
good support to firebox, well above the grate.
Disadvantages:
axlebox guides and spring brackets offset
large number of bolts and rivets liable to work loose
cylinder bolts have to resist large shearing force.
Firebox inaccessible below frame for washing out.
Large number of cross stays necessary.
Bar frame
advantages
solid backing to axlebox guides and cheap and easily renewable shoes.
simple disposition of springs and compensating gear
f'ew bolts and no rivets to work loose.
cylinders easily fixed.
firebox accessible for washing out.
few cross stays required.
sections can bc proportioned to meet the varying stresses in different parts of the frame.
disdvantages
excessive lateral rigidity.
lack of vertical rigidity.
weak at front end at buffer beam and behind cylinders where bogics are used
boiler centre must be very high to allow of ample depth of firebox at tubeplate
ashpan narrow and inefficicnt.
very heavy.
large coupled wheels cannot he used under the wide firebox.
Conclusion. the bar frame was particiularly adaptable to American conditions of ample loading gauge and large fireboxes and grate area, while the plate type was most suitable for British requirements, except for special designs such as the engine units of the Garratt locomotive. Plate frames are the best for side tank engines, but this type is rarely used in America. .Any change from plate to bar frames, or vice versa, could not bc economically carried out. It was rather remarkable that on those railways abroad where British-built plate-framed locomotives had to compete. against American bar-framed engines, the fuel consumption was always in favour of the plate frame, which permits the use of a decp firebox and efficient ashpan.

Cox, E.S. and Johansen, F.C. Locomotive frames.  J. Instn Loco. Engrs., 1948, 38, 81-115. Disc. 115-96. 43 digrs. Bibliog. (Paper No. 473)
Paper was mainly concerned with plate frames. William M. Sheeehan (written communication 148-58), Vice President, General Steel Castings Corp., Pennsylvania, wrote at length of the advantages to be gained from the cast steel locomotive bed. 6000 had been manufactured for the USA and Canada, and they were also in service in Mexico, Australia, France, USSR, Turkey and the Union of South Africa.

Reed, Brian. 150 years of Brtish steam locomotives. 1975. pp. 65-6
Large rolled iron plates transformed main frame design, strength and manufacture principally because numerous bolted and rivetted connections along the side plates could be eliminated and no working could occur between individual frame and horn plates. In this aspect the plate frame was ahead of the bar frame, for not until the casting of a whole side frame in one piece in the 20th Century could a main bolted joint at a highly stressed location be eliminated from bar frame construction. By the 1860s single rolled iron plates to suit 2-4-0s and 0-6-0s were available though not in general use, but by then in the larger works the old type of sectional frame was being welded up under 10cwt steam hammers. Advantage was sometimes taken of the sectional method to enlarge the area round driving and coupled axleboxes to get the box central in the frames, and even, as in the Met tanks at a width of 5in, to act as the axlebox guide thrust face. In some designs this thickened section was used to change the distance between frames fore and aft of it to get an extra two or three inches firebox width or to suit cylinder spacing. By the time of these developments frame conditions themselves had eased through the elimination of the frame-firebox tie-in and the substitution of expansion brackets, so that despite greater locomotive size and power the frames from the 1860s were a far more rigid and better maintenance proposition than those of the 1840s and 1850s.
With the increase in size of rolled plates, progressive steps had to be made in machine tools that could handle them, but probably not until the Smith & Beecroft machine introduced 1858-9 was there a frame slotter of accuracy able to take more than one set of frame plates. For long years thereafter the method of frame contouring remained awkward. From the rectangular plate the contour was shaped roughly by punching or drilling overlapping holes round a template, annealing and straightening to remove any strains caused by punching, and then putting six to 10 plates together on a slotter for the final machining, after which the plater and his mate with large hammers gave a final straightening to individual plates laid on large cast iron slabs.
Steel plates without any thickening over iron permitted higher piston loads to be absorbed and greater weight supported, but in the 20th Century with larger 4-4-2, 4-6-0 and 4-6-2 locomotives the almost standard 1in was thickened whenever weight permitted to 11/8in and even 1¼:in; so small an addition as 1/16in was appreciated by some designers. Cross staying was the weak point, though alleviated by the use of steel castings for inside motion plates. Some of these castings, as on GCR 4-4-2, 4-6-0, 2-8-0 and 2-6-4T classes, were used to give great support at the location where the frame plates were set in at the front to clear the side movement of guiding wheels.
Only in the twilight of steam were horizontal or racking stays adopted to any extent. They were difficult to apply with inside cylinders or inside motion, and an advantage of outside-cylinders with outside valves and motion was always the possible stronger frame structure – if designers were so minded. They were not always so minded, because long decades of inside cylinders and motion and flabby frames brought designers to a self-defensive postulate that frames ought to be made deliberately with a little lateral flexibility. So frames became the weak point in large 20th Century multi-cylinder locomotives such as the GWR four-cylinder types and the LMS Royal Scots. By 1939 not one of the 79 Gresley 180lb and 220lb three-cylinder Pacifies of 1922-34 was still running with its original frames and rate of Royal Scot frame cracks had more than quadrupled in six years.
Not until the Bulleid Pacifies of 1941 did a designer show how ample horizontal bracing could be provided with an inside cylinder, crank throw and motion. The later BR two-cylinder 4-6-2s and 2-10-0s with clear space between the frames also had full racking stays. In all three classes just men- tioned was revived the old Beyer-Met practice of the frame centred above the axleboxes; and the BR types also followed the practice evolved on the LMS of link-and-pin cross tie-rods between the frames at the horns, and manganese steel liners for boxes and horns that greatly reduced the wear.
Towards the end of steam the composition of the usual 26/30-ton mild steel for frames was adjusted to suit oxy-acetylene flame cutting, for that process reduced considerably the time taken in preparation and machining. Then steel suitable for welding was introduced with small quantities of copper, chromium and manganese, and ultimate strengths up to 35-40 tons/sq in. By this means many bolted and rivetted connections could be obviated and the whole frame structure made up as one piece, an idea foreshadowed in England in 1869 when Webb had proposed that the frames, cross stays and back carriages are cast in steel with the necessary horn blocks enclosed and fixed in one piece'. Undeveloped foundry and machining techniques at that time prevented practical application.