Fastening tools with floating magnet sleeves

A fastener driving tool, comprising:

• a shaft with a first working region disposed at a first end of the shaft and configured to drive a fastener, a second portion disposed proximate a second end of the shaft and configured to couple the shaft to a power tool, and a reduced diameter torsion zone disposed between the second portion and the first working region;
• and
• a floating sleeve having a radially inwardly projecting retention member and a magnet at a front end of the sleeve, the floating sleeve receivable over the shaft with the retention mechanism received in the torsion zone such that the floating sleeve may move axially between a forward position in which the magnet is proximate the working region and able to engage a fastener being driven by the working region and a rearward position in which the magnet is retracted from the working region,
• wherein a length (L) and radius (c) of the torsion zone have been determined based on an equation
• ϕ
• =
• T
• ⁡
• (
• 2
• ⁢
• ⁢
• L
• π
• ·
• c
• 4
• )
• ⁢
• 1
• G
• where φ is an angle of twist, T is a torque input, and G is a shear modulus of the shaft such that the length (L) and the radius (c) of the torsion zone have been optimized so that approximately half of breakages occur in the working region and approximately half of breakages occur in the torsion zone.