aperture 14 in the cone has a diameter equal to or slightly smaller than the external diameter of the trailing tubing 35 permanently attached m the needle hub 33 as shown in fig4 from cross section 4 4 in fig3 if the largest trailing diameter of the needle hub 33 is equal all around the hub will become circumferentially wedge impacted 41 within the hollow cone by a force proportional to area of surfaces making contact and proportional to the retractive force exerted to pull a solid cylinder into a hollow cone of smaller diameter because the hubs of needles or luer loks for holding needles are usually rigid compared with semi rigid plastics used to make needle shields the surface areas wedge impacted 41 will depend on differential elasticity as well as retractive force as shown in fig5 also from cross section 4 4 in fig3 if the largest trailing diameter of the needle hub 33 is not equal all around as is the case with respect to large but soft rectangular and multi angular needle hubs commonly used in blood banks and plasma collection centers one can expect the softer elastomeric hub material to compress with forced retraction into a relatively rigid cone and thus increase surface area wedged in mutual contact 41 as depicted by contact line thickness in fig4 5 as shown in fig6 the hollow conical shield 11 when applied over trailing tubing 35 resides significantly behind the iv access needle 32 and its hub 33 how far behind depends on trailing attachments to the tubing in the case of standard phlebotomy equipment blood plasma collecting and dialysis assemblies trailing attachments usually vary from 12 72 34 behind the needle hub 33 this distance leaves ample room to use the leading needle hub and.