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NEWS Continue to the category. Load more. One such fastener is the Rotabolt which measures bolt extension by the use of a central gauge pin which passes down a centrally drilled hole in the bolt. Underneath the head of the gauge pin, a rota is retained which is free to spin in a very accurately set gap.
The fastener stretches elastically, whereas the gauge pin does not move since it experiences no load. As tightening continues, the bolt will stretch sufficiently to eliminate the gap and prevent the rota from being able to be rotated. This is the indication that the bolt is correctly loaded. Another proprietary fastener uses a similar method. The HiBolt uses a pin located centrally down the bolt as does the Rotabolt except the pin is gripped by the slight contraction of the bolt diameter; the pin being locked when the correct preload is reached.
The use of load indicating washers is widespread in structural engineering. Such washers have small raised pips on their surface which plastically deform under load. The correct preload is achieved when a predetermined gap is present between the washer and the underhead of the bolt. This is measured using feeler gauges. Generally they are not used in mechanical engineering, but are, extensively, in civil engineering.
The extension which a bolt experiences can be measured either using a micrometer or by a more sophisticated means such as using ultrasonics. The extension can be related to preload either directly, by calibration, or indirectly, by calculation.
If ultrasonic measurement is used then the end of the bolt shank and the head may require surface grinding to give a good acoustic reflector. To assist the Engineer in overcoming the problems associated with the use of threaded fasteners and bolted joints, Bolt Science has developed a number of computer programs. These programs are designed to be easy to use so that an engineer without detailed knowledge in this field can solve problems related to this subject.
Methods of Tightening Threaded Fasteners We have a web site dedicated to training, have a look at www. Angle Controlled Tightening This method, also known as turn of the nut method, was introduced for manual assembly shortly after the second World War when a certain tightening angle was specified. The method has been applied for use with power wrenches, the bolt being tightened to a predetermined angle beyond the elastic range and results in a small variation in the preload due, in part, to the yield stress tolerance.
The main disadvantages of this method lie in the necessity for precise, and, if possible, experimental determination of the angle; also the fastener can only sustain a limited number of re-applications before it fails. Very accurate preloads can be achieved by this method by minimising the influence of friction and its scatter.
The method has its roots in a craftsman's "sense of feel" on the wrench which allowed him to detect the yield point of the fastener with reasonable precision. With the electronic equivalent of this method, a control system is used which is sensitive to the torque gradient of the bolt being tightened.
Rapid detection of the change in slope of this gradient indicates the yield point has been reached and stops the tightening process. This is achieved by incorporating sensors to read torque and angle during the tightening process. Since angle of rotation and torque are both measured by the control system, permissible values can be used to detect fasteners which lie outside their specification having too low a yield for example.
Tension Indicating Methods This category includes the use of special load indicating bolts, load indicating washers and the use of methods which determine the length change of the fastener.
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