Brake Tests

133 Piston Travel. 295. One of the factors, affecting brake rigging efficiency, which was given particular attention during these tests was the variations in piston travel with different cylinder pressures. Records were taken of the length of piston travel on a time basis using for this purpose a piston travel indicator which was a slack action indicator (Fig. 16), adapted for this special purpose. The indicator drum was driven at a constant speed, as before, while the pencil mechanism was arranged to move in pro- portion to the travel of the brake cylinder piston. Typical indicator diagrams from this device and curves derived from them are shown in Figs. 77 and 78. The piston travel time curves and the brake cylinder pressure time curves from the brake cylinder indicators were combined to form the brake cylinder pressure piston travel curves. 296. As a further study of the movement of the brake cylinder piston during the development of the brake cylinder pressure, diagrams were taken by means of a steam engine indicator screwed into the brake cylinder pressure head. The reducing motion of this indicator was connected to the piston cross head. Reproductions of typical indicator cards are shown in Fig. 79. 297. In Figs. 80 and 81 is shown the increase in running emergency piston travel over standing service for the different types of rigging tested and at various percentages of braking power. It will be noted that in these figures the increase in piston travel for the No. 1 and No. 3 clasp brake is less than for either the standard or the No. 2 clasp brake. The excessive piston travel on the No. 2 clasp brake was contributed to by the low hung brake shoes and is an element tending toward a reduced over-all efficiency of the brake. Locomotive and Tender Brake Rigging. 298. These tests as originally outlined did not contemplate any changes in the locomotive or tender brake rigging, but several features developed due to the large number of emergency applications, which were of sufficient importance to warrant changes in the rigging in order to insure the maximum efficiency in the operation of the brake. 299. The braking power standard for our tenders is 100 pet cent, of the light weight, based on fifty pounds brake cylinder pressure. The tender of locomotive 3394—K2s was used on all tests prior to test 622. Its tender was originally equipped with a rigging which gave but 70 per cent, braking power. This braking power was later increased to the standard amount by the application of a new rigging with a lever ratio sufficient to give a braking power of 100 per cent, of the light weight of the tender.