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As I mentioned in a previous post we currently have a Duratap sieving machine. We calibrated it and it runs with 265 rpm and 147 taps per minute. Strictly speaking the rpm is too slow and is neither compliant with SAE J444 nor with ASTM B214. The manufacturer tested a different sheave, but didn't manage to get the machine compliant.

So, to ensure that we are strictly compliant with NADCAP regulations, we bought a Tyler RX-29 Ro-tap. This gives us the opportunity to compare test results of these two machines. We tested it with ASR 170 per AMS 2431/1 specifications (100 g; 10 min) and this gave the following results:
Although the differences are not large, it can be concluded that the Duratap is actually a bit more efficient. That is, the shot better flows through the sieves: in the Duratap all larger sieves (30-40) have less shot on them, while the smallest sieve (45) has more shot on it.

The difference can be explained by the machine layout. The duratap's tapping arm is both longer and heavier. This arm is punched upwards by a shaft. On both machines this arm is placed at 95 m from the tapping arm hinge and both shafts have the same upward deflection. However due to the different tapping arm lengths the maximum tapping arm height above the sieve stack cover is only 30 mm, while on the Duratap this is 45 mm. Combined with the heavier tapping arm this results in a higher tapping force on the Duratap. This confirms our expectation that tapping force and revolution amplitude are important characteristics as well.

Ironically, we can now conclude that with the Tyler Rotap we are fully compliant with J444, but the sieving results are actually slightly worse.

What does this mean for the actual shot peening process? With the Tyler a shot mix has to consist of slightly more small particles to be acceptable, because otherwise too much particles will be captured on sieve 30 and 35. A mix with more small particles will give slightly lower intensity when other peening parameters are kept constant.
In this particular example the cummelative weight on sieve 35 has to be less than 50 g. The cummelative weight on sieve 35 as measured with the Tyler is approx 15% higher than when measured with the Duratap. Extrapolating, when a mix is just within limits as measured on the Tyler it will consist of approx 15% less big particles than a mix that is just withing limits as measured on the duratap. This may have a significant effect on the peening intensity!