The ornamental design for a shaftassembly for screens, as shown and described.
FIG. 1 is a perspective view of a shaft assembly for screens showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an assembled view for showing the shaft assembly for screens which is assembled between two screens, wherein the broken line portion is for illustrative purpose only and forms no part of the claimed design.