12.6.1 Overview
This method of preload control is long established and is included here for completeness. It has a number of disadvantages that are outlined below.
12.6.2 Method of Application and Practical Considerations
A micrometer is used to measure the fastener extension by taking readings before and after tightening. To make accurate measurements, it is necessary to provide gauging surfaces at each end of the fastener. This is usually done by grinding the end faces square or by providing a conical recess into which a steel ball can be located.
The method needs access to both ends of the fastener and sufficient clearance to accommodate a caliper type micrometer.
The method also needs the application of both analytical and practical judgment, and most of the disadvantages cited for the angle controlled tightening method apply. Tension cannot be rechecked without slackening the nut and retightening it unless a log is kept of all fastener dimensions.
The level of accuracy to which the preload can be controlled, assuming a uniform precision in measurement, is primarily a function of the fastener length.