Keywords: vibration-friction interaction, threaded fastener loosening, vibration testing, finite element modelling, finite element simulation, non-linearity.

The block-inclined plane is an elementary model of many mechanical elements, among them the threaded surfaces of a screw-nut and threaded fasteners [1,2]. Threaded fasteners have been extensively used in the aeronautical, automotive and electronic industry, in applications where material is sensitive to crack propagation and in those that require dismantling and re-assembly with ease. It is well known that when they are subjected to dynamic loads due to impact or vibration they can become loosened, resulting in failure of the functioning of structures or even in catastrophic accidents. The cost of manufacturing and maintenance efforts to overcome fastener loosening under the effects of dynamic loads is quite high.

The mechanism of interaction between friction and vibration has not been well understood, mainly because of a great many parameters involved and the non-linearity of the problem, in spite of research efforts over the last sixty years.

The rigid body block-incline model is satisfactory for design purposes but fails to describe the system's behaviour under shock and vibration loads. In the process of assembling and tightening, the threaded fasteners provide a positive holding force on the components, i.e. the mated surfaces of the threads are usually under some preloading. Loosening is supposed to take place when the friction forces that exist between mated surfaces are eliminated in the course of the interaction between friction and vibration. In order to understand this loosening effect on fasteners, the interaction between friction and vibration has to be studied. This paper investigates the dynamic behaviour of a block and an incline assemblage subject to harmonic excitation forces by both experiments and the finite element method (FEM). The assembly is a simple two-dimensional plane model of threaded surfaces in contact with friction under preloading. The assembly is first studied experimentally with preloading effected by magnetic forces due to small magnets buried underneath the block surface: it was found that besides the friction and the preloading, both the frequency of vibration and the amplitude of the acceleration influence the relative displacement of the block-incline interface and depending on the combination of amplitude, frequency and level of preloading one of three states can be observed: no relative movement of the block, movement of the block up the incline plane, and movement down the incline. The interaction between vibration and friction is then modelled by the FEM, using plane stress element and contact elements CONTAC48 of the ANSYS finite element software. The relative displacement between the block and incline as a function of time were obtained, from which the movement of the block relative to the incline can be monitored. The simulation results confirmed that under harmonic vibration the contact between the block and incline shows impactive clattering, which could result in no change of status, loosening or tightening as observed experimentally.

1

Green, W.G., "Theory of Machines", Blackie & Son Limited, London, 1962.

2

Shigley, J.E. and Mischke, C.R., "Mechanical Engineering Design", McGraw-Hill, London, 2003.

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