Overview SAR Rail Gauges Wheel Arrangement Classification Locomotive Frames Heat and Combustion

Technical - Locomotive Frames

The frame of a locomotive is the foundation to which the various parts are secured. Three types of frame were in use on the SAR, namely, the Plate Frame, the Bar Frame and the Cast Steel Frame.
 
Fig 1: The Plate Frame
 
The plate frame consists essentially of two parallel steel plates secured together by a buffer beam at each end, and by various frame stretchers.  Inside each buffer beam is a heavy casting known as the drag box, which carries the drawgear. Many plate frames are fitted near the rear end with a large casting called the bridle casting which in effect widens the frame out so that the lower portion of the firebox can fit inside the frame. Along each side of the frame are large slots, one opposite each coupled wheel, to which are secured the horns, in which the axleboxes for the coupled wheels are fitted. The cylinders and the smokebox saddle are bolted to the frame near the front end, and underneath the saddle is the top Bogie centre casting ­ the pivot on which the bogie can rotate.
 
Fig 2: The Bar Frame
 
The bar frame consists of two steel slabs usually from 3 to 4 inches thick, slotted out for lightness. As with the plate frame, a buffer beam and a drag Box casting are fitted at each end, but a bridle casting at the rear end is not often used, as it is possible to sweep a Bar Frame low enough to fit underneath the firebox. Several frame stretchers are fitted. The slots for the coupled wheel axleboxes exist on each side, as on the plate frame, but separate Horns are not necessary as the frame itself is thick enough to form an adequate bearing surface. The cylinders, smokebox saddle, top bogie centre, etc., are bolted to the frame in a manner similar to that used on a plate frame. On the SAR most modern locomotives, and some of the older ones, have bar frames, as there are certain advantages in this type of frame when applied to South African conditions. The most important of these are:

 - that the lower portion of the firebox is easily accessible for washing out and repair.
 - that bar frames are very rigid, thus largely eliminating any bending sideways on severely curved track.
 - that springs and compensating gear can be arranged in convenient and accessible positions.
 - that very few frame bolts and no rivets, which are likely to work loose, are necessary.

The two main disadvantages of the bar frame are its weakness vertically and its weakness near the front end, which can be very simply over­come by fitting deflection plates, commonly known as boiler barrel supports, between the boiler barrel and the top of the frame, and diagonal stays from the smokebox to the front end of the frame. In this way the boiler is used to stiffen the frame.
 
Fig 3: The Cast Steel Bed
 
The cast steel locomotive bed was based on a  modification of the bar frame, and was developed in the United States. In general design it is very similar to the bar frame, except that the buffer beams, drag box castings, frame stretchers and cylinders complete with steam chests and smokebox saddle are all cast in one piece with the two main Frame Members.
 
This reduces the number of bolts required to a very small quantity and has the further advantage, particularly on large locomotives, that the cylinders cannot become loose. There are certain other advantages over the normal bar frame, but as these are mainly finer points of design, they will not be detailed here.

Fig 4: The Hornstay
 
The gaps in the frame for the reception of the coupled wheel axleboxes are fitted with two separate wearing pieces. This is to avoid damage to the hornblocks in the case of a plate frame engine, or the frame itself in the case of a bar frame engine. One wearing piece is the horn shoe, which consists of a flat steel plate, with a small flange on each vertical edge, fitting tightly over one side of the horn, and bolted firmly to it. The other is the wedge which is a tapered steel block fitting snugly against the opposite side of the horn. By lifting the wedge, any reasonable amount of wear on the side faces of the axlebox may be taken up, which prevents the axlebox from knocking in the horns when the engine is running. The wedge is held to the horn by a securing bolt which passes through a vertical slot in the horn, thus allowing for adjustment of the wedge, and this adjustment is obtained by a long bolt passing through the horn stay, and locked by suitable nuts.
 
It will be seen from this sketch, that one side of the horn is tapered to receive the wedge, while the other side is vertical. The inner faces of the horn shoes and the wedges form parallel surfaces between which the axleboxes may slide up and down freely, as they are required to do by the irregularities on the upper surfaces of the rails. The horn gaps are weak spots in the frame, and in order to prevent bending and probably breaking of the frame, horn stays are bolted tightly across the bottom of the gap, after the wheels and boxes are in position. It is important that all horn stays be kept tight; otherwise cracking of the frame is almost bound to occur.