Abstract: Abstract The value of weft insertion force during the insertion process of an air-jet loom is related to the relative velocity between the weft and airflow,the density of the airflow,the diameter of the weft,the insertion length and the friction coefficient of the yarn. The friction coefficient of the yarn is a variable under different relative velocities.As the velocity and desity of air-flow in mai nozzle are hard to measure,in order to deduce the relatation,a simulation of air-flow velocity and density and a measurement of the insertion force are made in this paper.Results after the analysis of all the data indicate that the friction coefficients of weft yarns decreased with the increasing of relative velocity in a certain range of pressure.By fitting the coefficients to the relative velocity,the values of coefficients under any relative velocity can be deduced.The results have directive significant to the accurate calculation of weft insertion force and the deduction of weft flight speed.

Key words: air-jet loom, weft insertion, relative velocity, friction coefficient