Patent ID: 7107698

Claim:
A strengthened blade tape measure, comprising: a tape casing having a receiving cavity and a guider opening communicating with said receiving cavity; a retraction unit supported in said receiving cavity; and a ruler blade, having an inner end attached to said retraction unit and an outer end stopped at said guiding opening, adapted to slidably folded between a storage position and a measuring position, wherein at said storage position, the ruler blade is retracted to receive in said receiving cavity in a coil flattened configuration manner via the retraction unit, and at said measuring position, said outer end of said ruler blade is slidably pulled to extend said ruler blade in a concave-convex configuration out of said receiving cavity through said guider opening; wherein said ruler blade has two identical longitudinal central portions and two identical longitudinal side portions integrally extended from two sides of said central portions of said ruler blade respectively, wherein each of said side portions of said ruler blade has a curvature smaller than a curvature of each of said central portions of said ruler blade; wherein said ruler blade has a slope parameter C and a measuring length L extended from said guiding opening in a self-sustaining manner, wherein a L/C ratio is determined from a condition: L/C= 62+1200 T 1 wherein said slope parameter C is determined from a condition: C=2(S 1 t 2 +S 2 t 2 )/W, wherein W is a width of said ruler blade in said flattened configuration, S 1 is an arc length of each of said central portions, S 2 is an arc length of each of said side portions, t 1 is a slope of said central portion, t 2 is a slope of said side portion, and T 1 is a net thickness of said ruler blade.