Robertson Drive (Square Socket)
Direct Answer
The Robertson drive (Square socket / SCRULOX, standardized under ANSI Type III / CSA B35.4) is a 4-sided tapered square recess drive invented in Canada by P.L. Robertson in 1908. Known for its distinct tapered geometry that locks the screw onto the driver bit for one-handed operation, it is the dominant construction and electrical fastener across Canada.
Source: CSA B35.4 / ANSI/ASME B18.6.3 (Type III Square Recess)
Specs
| Standard Specifications | CSA B35.4 / ANSI Type III Square Recess / ASME B18.6.3 |
| Drive Geometry | 4-sided square recess with a slight interior conical taper (approx. 3° taper) |
| Self-Holding 'Stick-Fit' | Yes (Taper friction-locks screw onto driver bit for horizontal/overhead 1-handed driving) |
| Cam-Out Tendency | Near Zero (Straight square drive walls resist climbing) |
| Color-Coded Sizing System | #00 Orange, #0 Yellow, #1 Green, #2 Red (dominant wood/deck size), #3 Black, #4 Brown |
| Primary Geographic Usage | Universal in Canada (electrical/framing/decking) / Pocket-hole woodworking globally (Kreg) |
The Tapered 'Stick-Fit' Advantage
Robertson's key breakthrough was adding a subtle 3-degree taper to the square socket. When a Robertson driver bit is pressed into the screw, the tapers engage with tight friction ('stick-fit'). A tradesperson can place a screw on the end of a magnetic or non-magnetic bit and drive it overhead or into deep wall cavities one-handed without the screw tipping over.
Historical Henry Ford Dispute
In the 1910s, Henry Ford used Robertson screws on Model T production in Canada, cutting assembly time by over two hours per car. Ford wanted exclusive licensing rights to manufacture the screws in the US; Robertson refused to surrender control of his patent, prompting US automakers to later adopt Phillips screws instead.