When the shock-absorbing member 50 of FIGS. 4(a) and (b) is used with such fluid channel 54, and when the impeller shaft 35 is screwed into the impeller boss 32 a, a fluid, such as air or grease, situated between the front end 35 b of the impeller shaft and the shock-absorbing member 50 may be forced through the fluid channel(s) 54.