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Class 100 Gloucester RC&W 2-car DMUs


Strength Testing

In order to prove that the design satisfied the British Transport Commission strength requirements it was decided to test the coach body shell, using electrical resistance strain gauges to determine stress levels at selected points. For this purpose a shell was taken out of the production line and set up in such a way as to represent the actual conditions of body support which existed in service, and a special testing rig was erected for the purpose of applying a compression load to the side buffers.

With reference to the stress analysis of the coach 200 positions around the structure were selected for stress measurement. For applied vertical loads, theoretical analysis indicated that the most severely stressed regions would be in the solebar adjacent to the body bolster, the body bolster itself, the quarter, cantrails and solebars framing one of the doors between bogie centres. Additional gauges were located across the floor and roof midway between bogie centres, and at the door openings mentioned above. Gauges were also applied to the quarter immediately above the bolster, since it was desired to ascertain the force distribution in this region of the coach.

Under an end buffing load, the most highly stressed parts indicated by theory were the solebars, the tubular longitudinals immediately behind the side buffers and those supporting the centre couplers, and the horizontal beam between the lower solebar tubes immediately behind the headstock. As it was also desired to study the behaviour of the corrugated floor under buffing load, gauges were located on this floor in such a way as to ascertain both the stress distribution across the floor, and the load diffusion into the floor from the butted tubular longitudinals.

The vertical load was applied by iron bars laid in the coach to represent a distributed passenger load. The end buffing load was applied by means of a hydraulic ram at the driving end of the coach, acting on an equalising beam which in turn acted on the side buffers. Another beam on rollers was held against the buffers at the non-driving end of the coach by means of a total of four circular-section tie bars running the length of the coach and reacting the ram thrust of the driving end. A series of strain gauges was attached to these tie bars to measure the force exerted by the ram, which was also checked by pressure gauge.

The test programme included the following loadings:

(a) A uniformly distributed load of 17 1/2 tons, 15 tons of which was applied and removed a minimum of five times during the course of the test programme.

(b) An end buffing load of 80 tons which was applied and removed a minimum of five times during the course of the tests, a uniformly distributed load of 2 1/2 tons remaining in the coach as a settling vertical load the whole time.

(c) A combination of the above, i.e., a total uniformly distributed load of 17 1/2 tons together with an 80-ton buffing load. This combination was applied once.

In order to ensure absolute reliability of the test results, it was necessary to work the structure for a number of cycles by applying and removing the maximum vertical and buffing loads several times. After such working of the structure the strain readings were not only linear (except for a very few lightly loaded points) but were consistent with each repeat of test. For each case (a) and (b) above, the test proper was carried out three times, a complete set of strain readings being taken at various load increments during each test run.

In a general way the tests showed that the attention paid to the joints of pillars to solebars and cantrail, together with the use of butted tubular pillars, was justified, since the vertical deflection of the coach at the solebars, midway between bogie centres, under a load of 15 tons, was only 0.13 in., and the inward movement of the body sides at the waist rail of the bolster quarter, door quarter, and single pillar at the coach centre line was 0.0312 in., 0.0548 in., and 0.101 in. respectively. These figures combined to show that the applied loads were reacted by the whole depth of the body side without distortion of the coach cross sections. Further, the stresses recorded in the pillars did not show any sudden change of stress level around the bend at the solebar.

The gauges located on the corrugated floor and the buffing gear support structure showed that the butted tubular longitudinals immediately behind the buffers diffused the end load into the whole width of the floor very efficiently. By the centre-line of the bolster the compressive stress level approached a more or less uniform value over the whole width of floor.

The applications of the combined vertical load and 80-ton buffing load caused the coach to contract elastically 0.46 inch over its length and no part of the structure suffered any permanent set.

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