Friction in threaded fasteners: Influence of materials and tooling
Threaded fasteners represent the most common type of machine element, with a unique function that facilitates ease of assembly and disassembly. This ease of disassembly allows machine parts to be reused, refurbished, and recycled. Easy as these components are to assemble, several factors must be con...
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Format: | Dissertation |
Sprache: | eng |
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Zusammenfassung: | Threaded fasteners represent the most common type of machine element, with a unique function that facilitates ease of assembly and disassembly. This ease of disassembly allows machine parts to be reused, refurbished, and recycled. Easy as these components are to assemble, several factors must be considered to achieve the desired clamp force and to utilize the fastener to its full load capacity. The research presented in the thesis compares different tightening strategies and assembly tools to show that the clamp force and it´s scatter are influenced by the variation in the coefficient of friction (CoF) to a much larger extent than by the accuracy of an assembly tool. The research therefore focus on understanding the frictional response in a threaded fastener joint during tightening.
A range of design and assembly factors are considered to identify how to increase reliability of the threaded fastener joints. These factors include tightening speed, coating, surface topography, fastener storage conditions, cutting fluid residue and joint material. A torque-controlled, two-step tightening method was mainly used in the studies as it is widely practiced across the production floor of the motor vehicle and general industries to tighten threaded fastener joints.
A state-of-the-art friction test rig (FTR) was built to quantify variations in the CoF in the thread and under-head contacts during tightening. Coatings and contact surfaces are also characterized using SEM, FIB, indenters, and optical microscopes to gain an insight to find the likely reasons behind CoF variation. Fasteners with different Zn-based coatings were tightened on plates with surface topographies similar to those found in the motor vehicle industry. The samples were not cleaned before the testing but used "as-received" from the supplier. The degree of damage to the joint surface and fastener thread from the tightening depends on the hardness of the coating. The hardest coating (Zn-Ni) remained relatively unchanged but gave twice as high CoF in the under-head contact compared to the softest coating (Zn-flake). The under-head friction often dominates the tightening process and may be significantly affected by the joint surface topography and the level of cleanliness.
In the automotive industry, many parts to be assembled are not thoroughly cleaned, increasing the risk of cutting fluid residue on the joint surface. Different types of cutting fluids were compared in a study with fasteners tightened |
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