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THE RELATIONSHIP BETWEEN THE TIGHTENING TORQUE AND THE CLAMP FORCE OF SMALL INDUSTRIAL SCREW FASTENERS

Joseph O. Arumala, Robert McCulley, Emin Yilmaz

University of Maryland Eastern Shore/ /Filtronic Comtek Inc./ University of Maryland Eastern Shore

Abstract

Screw fasteners used in the assembly of microwave filters were subjected to tensile and torque tests to determine their yield strengths and the torque that can safely be applied during the assembly process. 2-56, 4-40, 6-32, and 8-32 screws with or without nylok and silver plate finish were tested in the project. Special holders were manufactured for the tensile tests. The torque tests utilized a rotary torque transducer attached to a display unit and a load cell washer. The results obtained show that the published yield and tensile strengths of the screws are quite conservative. The clamping forces generated in the screws were well below the yield strength of the screws. The nut factor in the torque formula decreased with the nylok and silver finish and thus increased the clamping force.

Introduction

This project began with the need to establish engineering torque values for various fasteners and to determine the associated clamp forces generated during the assembly process of microwave filters manufactured at Filtronic Comtek Inc. (see Figures 1 and 2). The filters designed at this facility utilize up to 400 screws per filter. The screws are in the range of 0-80 to 8-32 in size. In order to define proper torque values, the yield and ultimate tensile strengths of each fastener must be determined. Most fastener manufacturers will not give maximum torque recommendations due to liability issues but those that do, will not guarantee desired results due to the variation of applications. Applying the proper torque to fasteners in order to induce clamp force is essential in providing a reliable bolted assembly. There is a direct link between the torque applied and the resulting clamp force. Unfortunately there are many factors involved during the tightening process which must be understood in order to achieve a solid bolted joint design. The fastener engineer must know the strength of the fastener and how much stress (tension) can and will be induced during the torque process. Since tensile force in a

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McCulley, R., & Arumala, J., & Yilmaz, E. (2000, June), The Relationship Between The Tightening Torque And The Clamp Force Paper presented at 2000 Annual Conference, St. Louis, Missouri. 10.18260/1-2--8660