Abstract

In modern, high capacity circular knitting machines, the small and fragile latch needles are often a performance-limiting machine element (Schuler, 1980). For these machines, the operating circumferential speed may exceed 1.5 m/sec. Due to the kinematics of the process, the cam driven needles are exposed to periodic excitation with frequency proportional to the speed. Increase in rotational speed gives rise to needle head fatigue breakages unless special design measures are undertaken. Frequencies up to 15 kHz have been observed and up to 60 kHz may be expected in the vibration spectrum. To understand what particular features of needle design may be responsible for their longevity, several techniques were developed to apply Finite Element Analysis software to estimating the fatigue life under a non-harmonic periodic loading.

The known FEA packages handle dynamics of a system with such loading as a general non-stationary problem, whereas much more efficient solution can be constructed by combining the analytical solution for a one DOF system under recurring impulses (5-functions of amplitude A) at the moments 0, T, 2T, …, and the natural modes of the system (with the driving point fixed) provided by FEA. As applied to the system in question, from the broad frequency range of the excitation forces and displacements, only frequencies close to those providing maxima to the transfer functions from the driving point to the head of the needle were selected. These frequencies are referred to further on as the response frequencies. Then the time history simulating polyharmonic stresses in the dangerous area was generated and processed according to the chosen fatigue criteria (a corrected linear hypothesis of damage summation, see below). Thus for any given node of the needle a point on the S-N diagram was obtained.

A wide spread desktop package, ALGOR, was selected as a Linear Stress Analysis solver. The described procedure is built on top of this package and allows design engineers to make judgements as to what design is more advantageous for needle longevity.

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