bottom of the chamber. Underneath the chamber and hammer is an anvil supported by a base. be tested are placed, one at a time, in this anvil prior to being fired. HOLDING FIXTURE The caps to It was important to know whether imprecise machining of the hammer used on the standard cap gun would cause any variation in the results. Therefore, several different hammers (see Figure 6) were used, and sound pressure level measurements were made at the six microphone positions shown in Figure 5. For these tests, the total mass of the hammer plus chamber was 1.25 kg. In all cases, the hammers were dropped 3.5 cm. corresponding to a momentum at impact of 1.0 kg m s-1. The average peak sound pressure levels (A-peak) and range of values recorded in these tests are given in Table 2. As can be seen from 4/ Although Type A roll caps were used in the previous tests on commercial cap guns, it was found, both from the data taken and through visual inspection, that there was a great deal of variability among rolls of the Type A cap and among caps on the same roll. For example, on a roll of Type A caps chosen at random, the diameter of the powder spots varied from 2.5 mm to 3.5 mm. On this same roll several powder spots were surrounded by stray nodules of gun powder. Both these factors can affect the sound level output of the caps. Since the purpose of the tests on the standard cap gun was to ascertain its variability, it was important to keep the variability of the caps to a minimum, and Type B caps met this requirement better than Type A. A second consideration of less importance is that the Type B caps misfired less often than the Type A, thus resulting in a considerable saving of time over the course of the tests. It should also be noted that Type B caps were tested in commercial cap pistols, and the A-peak levels recorded were similar to Type A caps. the data, the difference between the highest and lowest average peak levels was only 3 dB, and in most cases, the average data values were in the 156-157 dB range. These data indicate that it did not matter which hammer was used when conducting tests with the standard cap gun, and thus, that precise machining of the hammer is not a critical factor. Since all the tests that had been carried out on commercial caps and cap guns had been done with Type A roll caps, it was decided to test a commercial gun using Type B caps. From this information, it could then be determined if the data from the standard cap gun were representative of commercial products. The firing apparatus for commercial cap guns was set up and cap gun No. 1 loaded with Type B caps was attached to it. Measurements were taken only at the maximum position (position 5) of the test configuration shown in Figure 2. It should be noted that position 5 of Figure 2 is not necessarily equivalent to any of the positions used for testing the standard cap gun, although, superficially, it would be expected to correspond most closely to position 1 of Figure 5. The average Apeak sound pressure level found for cap gun 1 (position 5) was: 155 dB*. Comparing this to the results given in Table 2, it can be seen that this average value is within +3, -4 dB of the average values obtained for the various hammers. Thus, it can be concluded that (1) within the range examined, the hammer configuration does not affect the measurement results, and (2) the results produced are representative of commercial cap guns. Although it has been stated that the choice of hammer and its machining makes little difference in the results, it is recommended that the type (e) hammer (dome-shaped with 2.54 cm (1 in.) radius) be used because it is easy to make, and causes fewer misfires. In addition, the grooved hammer's would not be recommended at all. The grooves tend to become clogged with cap debris and have to be cleaned after every fifth shot. This cleaning process is more timeconsuming than actually running the tests. It was found that if the 1.25 kg hammer and chamber were dropped a distance less than 3.5 cm, the caps would not consistently explode. To determine if increasing the drop height affected the experimental results, hammer type (e) (dome-shaped with 2.54 cm radius) was placed on the standard cap gun, and the drop height was increased to 9.5 cm, corresponding to a momentum at impact of 1.7 kg m s-1. The mass of the hammer plus chamber was not changed. The averages and ranges of A-peak sound pressure levels for this test5/ are compared in Table 3 with the corresponding data (i.e., those for hammer e) from Table 2. *Detailed data on the average A-peak, A-duration, B-peak, and B-duration are given in Table C-11 of Appendix C. 5/ Detailed data on the average A-peak, A-duration, B-peak, and B-duration are given in Table C-12 of Appendix C. Figure 6. Table 2. Types of hammer used in testing the standard cap gun. The main draw- Average and range of A-peak sound pressure levels (based on ten shots) for the *Detailed data on the average A-peak, A-duration, B-peak, and B-duration are given in Tables C-5 through C-10 of Appendix C. The differences are seen to be small, and probably not statistically significant, in comparison with the spread in individual readings due mostly to the variation among individual caps. Thus, for the A-peak sound pressure level readings taken at hammer drop heights of 3.5 and 9.5 cm, it appears that the height from which the hammer is dropped does not significantly affect the data.6/ 3.3 Findings In summary, the use of different hammers or adjustment of the height from which the hammer dropped had no significant effect on the results of measurements made with the standard cap gun. Also, the standard cap gun produces results similar to those of a commercial cap gun. This should allay any concerns as to whether the standard apparatus is representative of the real-world situation. Therefore, on the basis of this information, the standard cap gun is recommended as a reliable means for detonating paper caps in a reproducible manner. The procedure for use of the standard cap gun is presented in the following section of this report. The standard cap gun shall consist of a hammer which strikes the cap, an anvil in which the cap is placed, and a mechanism by which the hammer may be dropped from a fixed height so as to accurately strike the cap in the anvil. Specifically, the cap gun shall meet the following specifications: 1. 2. 3. The steel hammer (see Figures 7 and D-2) shall be 0.95±.05 cm square and at least 0.6 cm thick. The steel anvil shall be of the general configuration shown in Figures 7 and D-2. The recessed The mechanism which is used to drop the hammer shall be such as to ensure that the hammer drops squarely, without rotating, into the receptacle in the anvil. The mechanism shall be such as to ensure that the hammer drops 3.5±0.5 cm before striking the anvil. 6/ It may seem that there is a contradiction between these findings and the much larger variations in peak sound pressure level which resulted from the use of different commercial cap guns. It is postulated that the variations observed for different commercial guns were due to one or more of a number of factors which affected acoustic radiation from the point of the explosion. These include: the design of the anvil in which the cap rests, the angle at which the hammer strikes relative to the anvil, and the extent to which the small mass of the hammer and the varying spring tension permit hammer recoil after the explosion. In the standard cap gun described here, anvil design is held constant, the hammer always hits squarely, and the hammer is too massive to recoil significantly under the force of the explosion. 7/ The standard cap gun is not, at this time, part of any Federal regulation. 4.2 Measurement Instrumentation The measurement system for the test shall include a microphone, a preamplifier, and an oscilloscope. (1) The measurement system shall have a free- the same horizontal plane, in line with the opening of the anvil, with a distance of 25 cm between the diaphragm of the microphone and the point of explosion (see the figure below). Let the hammer drop a height of 3.5 cm. Measure the resultant peak sound pressure levels and durations in accordance with the CHABA impulse-noise damage-risk criteria [3]. |