(1) The influence of the tightening velocity on the friction coefficient of fastener μ was investigated. (2) The influence on μ of the lubrication state, the surface treatment and the material combination of bolts and nuts was examined from the viewpoints of the central tendency and the dispersion. (3) The screw-thread friction coefficient μS and the bearing-surface friction coefficient μW were measured separately. Both the mean value and the standard deviation of μ were calculated statistically with the data of μS and μW, and a comparison between the calculated and the experimental results was made. (4) Some coating materials to reduce the standard deviation of μ, which are called "μ-stabilizers", were tested, and it was found that they are useful for decreasing the variation among subgroups.
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Tomotsugu SAKAI (1978) studied this question.