Factors affecting the tightening characteristics of bolts
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A study on the tightening characteristics of bolt-nut-washer assemblies was undertaken after a preliminary literature survey showed different opinions to exist on the effect of several commonly encountered factors on the torque-tension relationship.
The test program included tightening bolts up to the yield point under a planned series of combination of factors, and recording nut-tightening torque, bolt torque and tension.
Plating of the mating surfaces of bolt, nut and washer
Lubrication of the interfaces.
Among the subsidiary factors considered were the hardness and yield stress of the materials and the class of fit of the threads.
The tests were planned and analyzed with statistical techniques in order to obtain estimates of the effects of each factor and the more important interactions (singled out of the total number in a series of pilot tests) as well as of the errors to be considered for the tests of significance. The analysis was carried out with mathematical and graphical methods giving accurate results in an easily understandable form from a reasonably small number of experimental data. The isolation of the main effects and interactions was achieved with no loss of general applicability of the results, taking advantage of the peculiar properties of the factorial experimental design adopted.
A comparison between the torque wrench and the turn-of-nut method of bolt-tightening is made in terms of closeness of bolt tension control.
A simple bolt-testing machine was built for the tests. A motor-driven wrench with a strain-gage bridge measuring the input torque tightened the nut at low speed while a two-component strain-gage transducer measured bolt torque and tension. Provision was also made for measuring the angle of rotation of the nut and the torsion of the bolt's shank.
KeywordsTorque Yield Point Fluid Dynamics Pilot Test Graphical Method
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