Hi Folks, I guess I should wade in on this subject. I no longer get GS's magazine so I didn't see the article. If Sheets claims different analyzers are getting different results I can't imagine which analyzers he's talking about. As Dave and others have said the electronics should all read the same. I've checked a number of different analyzers and always gotten consistent results.
To eliminate the effects of the clamp I test units with a 300rpm synchronous motor with a piece of shaft mounted like a windill. I've always gotten 299 or 300 with every unit I've tested. I do I final check on all my own analyzers with this motor. When I test a Cub Scout IV which reads to a tenth I've never gotten a reading below 299.9 or above 300.1. These readings are rare. I usually get 300.0. Clamping length and pressure of course have a effect. I ran some test years ago (see Tech Note ) where I ran the clamping pressure from virtually nothing to virtually destroying the shaft. A graphite shaft frequency changed only 2cpm, a steel shaft changes about 6 cpm. Clamping length is very critical since frequency varies inversely with the cube of the length of the shaft (actually the square root of the cube of length). So different clamping lengths effectively change the beam length of the shaft and consequently its frequency. There are other scond order factors that effect frequency such as the amount of the twang (say one inch vs two inches), the initial alignment of the shaft to the sensors and the number of repetitive twangs you've taken with the shaft. All these effects are pretty small and would never be noticed with a typical three digit meter. Actually I was quite surprised at the consistency of a shaft. If I made a consistent depth of twang (I had a little stop on the top of my bench under the tip weight) I could consistently repeat frequency readings to a tenth of a cpm. The only "funny" I ever saw was when I clamped steel shaft in a three jaw lathe chuck. I was getting very large frequency changes as added pressure turned the cross-sectional shape of the butt into a triangle. Didn't see the same efect with a graphite shaft. I guess because they're so much stronger in compression. Cheers, John K
