Knurls on Push and Turn Lids

Abstract

This study investigated the effects of knurling on exertable torque on “childproof” prescription lids; that is, where a “push and turn” action is required. Twelve lids with the same height (12.7 mm) were used. Six knurling designs were evaluated for two lid diameters (32 mm and 54 mm): a smooth lid, a lid with two levers, and lids with 7–8 knurls/cm, 4 knurls/cm, 2 knurls/cm and 1 knurl/cm. The 35 male and 13 female subjects pushed and turned the 12 lids in a clockwise direction against a torque meter. The effects of gender, size, knurl design, and the interaction of gender and size were significant. Males exerted 42% more torque than females. Subjects exerted 200% more torque on the 54 mm lids than the 32 mm lids. The interaction of size and gender showed that increasing lid diameter was more advantageous for males than females. The lid with two levers provided 54% more torque than the smooth lid for the 32 mm lid but only 21% more for the 54 mm lid. The knurl designs gave 4–15% more torque than the smooth lid.