Plate frames for 7.25 inch Garratt

[neathabbeyironworks]

I am in thecearly phases of building a Garratt in 7.25inch gauge. I have a few queries that I would value some advice on. The loco is a stanard gauge 0-4-0 0-4-0 Garratt William Francis- actually the first of class “Vivian”. The drawings that I have indicate that the plate frames come out at 3mm thickness. The bogies are heavily braced to withstand the forces generated by pistons cranks and wheels. There are horizontal stretchers and vertical stretchers including the buffer beam and a bufferless buffer beam at the other end. These are fixed to the plate frames with angle iron and rivets. Some of the stretchers have flanges formed by bending the plate through 90 degrees. The bogie plate frames are approximately 510mm long and 110mm top to bottom with approximately 60mm between frames. The boiler is supported on long narrow frames approximately 930mm by 80mm top to bottom tapering at the ends to accommodate the underlying bogies on the pivots. At each end is a box girder holding the pivot. The walls of this box girder 3mm thickness. There is a “platform” of 2mm sheet fixed at right angles to the boiler frame which might add some strength. There is a vertical stretcher incorporating support for the smokebox- boiler and a horizontal stretcher in front of the firebox.

So, this leads to my questions. Should I go up to 4mm thick plate for the frames? Even with 2mm steel sheet for stretchers I am anticipating trouble bending the steel through 90 degrees and achieving the correct dimensions between frames. I am considering fabricating this feature by MIG welding separate sheets/plates at 90 degrees. Is this feasible? Related to this I can see that to get the flanges parallel in 2 planes I will have to mill the surfaces flat, especially on the bogies where accuracy is paramount for valve gear and axles in horns. The box girders for boiler frame will have to be fabricated by MIG welding and the substantial end plates milled flat and true. There is a shaft running through this box girder to set the cut offs on the valve gear on the bogies. This raises the question of how to mill these faces at the end of a long box girder. If possible I could attach a FreeCAD image of the box girder with attached pivot if it would help. Because the drawings I have are limited, I am using FreCAD to scale the dimensions and work out how items fit together, this is quite apart from making patterns for such things as wheels, horn blocks and cylinders etc. This is going to be a long project.

[weary]

I have a 7 1/4" gauge steam locomotive with 3mm thick (double) frames. It is not an articulated locomotive and the (double) frames are all-but 1m long x 30mm high braced by two 1/2" thick horizontal frame stretchers each 100mm long, one slightly ahead of amidships & the other at the rear (drag 'plate') as well as the usual buffer-beams. My frames flex! By that I mean that they can be flexed by (strong!) hand pressure and flex in motion.

So ..... : as regards your "long narrow frames supporting the boiler" (i.e. the frames linking the two power bogies) I would suggest that plain 3mm thick steel plate would not be adequately rigid. I presume that the “platform” of 2mm sheet fixed at right angles to the boiler frame" are the running-plates & whilst they would add rigidity in the horizontal plane if firmly fixed I personally would not like to use them in that way.

I would suggest using the thickest plate that you can for these (vertical) frames; this could be milled to scale thickness along visible edges leaving the bulk as thick as possible. Provided you use hot rolled steel there should be no significant distortion issues with this approach. I appreciate that the firebox outer very visibly runs extremely close to the inside of the frames (illustrative pic here) so you may be limited as to what can be done in this area & may require some compromises here -> for example, your model live-steam boiler firebox could be narrower to fit inside the 'thick' frames with the cladding above the running-plates dropping into the milled recess so maintaining the 'thin frames' illusion, and a dummy firebox and if necessary a dummy grate outer below the side-frames similarly positioned.

Alternatively, or even in addition to the above, you could perhaps add a hidden (i.e. not visible from normal viewing angles) horizontal reinforcing rib welded inside and to the vertical frames where possible. Thus your frames would look like a 'T' on its' side, with the leg of the 'T' inside. Obviously, the thicker and 'wider' this reinforcing rib = the more rigid the whole assembly will be.

As regards your power-bogie frames; These sound so well braced from your description that I suspect 3mm thick plate will do the job. However once again I would suggest a similar approach to the longer 'boiler supporting' frames using thickest material possible etc. Rigidity will make life so much easier in the long-term! Note that Ken Swan used 3/16" (4.75mm) thick plate for 'Bridget' frames & I think that you will find that this thickness of plate is commonly used in 7 1/4" frames.

Turning to your frame-stretchers I have had a number of similar stretchers (like an elongated 'H' or 'adjoined L's when seen in plan) to make for my loco. I made them from three pieces silver-soldered together. I would not try to 'bend' stretchers to make the ends, even with heat; fabrication would be my preference as it is neater and more accurate. I prefer silver-soldering to welding for this approach as these 'small' (in my case) stretchers were visible under normal viewing and I wanted them to look neat. Fix parts together with small sacrificial screws to make soldering/welding easier. Make the ends (i.e. bolting faces to vertical frame members) over-thick (and even over-sized in all dimensions) and mill to final dimension(s) to ensure accuracy of final width (etc.). I also made a large 'U' shaped stretcher (analogous to your 'box-stretcher') by a similar silver-soldering method. To avoid milling-dramas in this last case I made all the parts accurately (erring toward overlength perhaps) and screwed it together with small sacrificial screws before silver-soldering - it was as 'close as dammit' when soldered-up and simply required rubbing of the ends on abrasive paper laid on a flat surface to bring to exact size (or as close as I could manage to measure with my equipment).

An alternative to my 'accurate component measurement & assembly' method would be to make the 'box-stretchers' over-length with over-thick ends and mill the ends flat & to length by mounting on the lathe cross-slide & using the lathe as a horizontal mill (à la LBSC method before mills were common in home-workshops).

You could of course weld rather than silver-solder if that is your preference.

Just a few ideas 'off the top' for you (and others?!) to 'mull-over'.

Regards & .....
......Best of luck!

Phil

[ettingtonliam]

Full size locos had quite substantial running plates, often 3/8" thick with 4 or 5" angle edges on both sides, rivetted or bolted to the main frames and these were known tpo add substaially to the horizontal stiffness of the frames. In BR days, locos with the running plates attached to the boiler had much more robust horizontal stretchers to compensate.

[neathabbeyironworks]

Thank you both for your informative replies. 4mm plate it is going to be. On the bogies this will make no difference to the outward appearance and there is nothing that can be impeded, between the frames, the hornblocks will need adjustment. As regards the boiler frame, the distance between frames is significantly larger than non articulated locos so proportionately the change in firebox width is much less. Cladding thickness can be adjusted. It is not as if I am dealing with a GWR Belpaire boiler with a waist at the bottom.