A Robertson screw, also known as a square screw[1] or Scrulox,[2] is a type of screw with a square-shaped socket in the screw head and a corresponding square protrusion on the tool. Both the tool and socket have a slight taper. Originally designed to enable the screws to be made using cold forming of the heads,[3]: 79–81  the taper has two other advantages which have helped popularize it: it makes inserting the tool easier, and helps keep the screw on the tool without the user having to hold it there.[3] The Robertson screw is specified as ANSI Type III Square Center.

Usage and design

Close-up of Robertson drivers

Robertson screws are commonplace in Canada, though they have been used in other countries[3]: 85–86  and are becoming much more common. As patents expired and awareness of their advantages have spread, Robertson fasteners have become popular in woodworking and general construction. Combination Robertson/Phillips/slotted fasteners are often used in the electrical trade, particularly for device and circuit breaker terminals, as well as clamp connectors.

Robertson screwdrivers are easy to use one-handed, because the tapered socket tends to retain the screw, even if it is shaken.[3]: 85–86  They also allow the use of angled screwdrivers and trim-head screws. The socket-headed Robertson screws are self-centering and reduce cam out. They also stop a power tool when set, and can be more easily removed if painted over, or old and rusty.[3]: 85–86  In industry, they speed up production and reduce product damage.[3]: 85–86 

History

US patent 161390, Allan Cummings, 1875, wood screw drives
Illustration from Robertson patent application
Advertisement: "A Study in Evolution"
Diagram of a Robertson screw head

Background

The internal-drive square socket for screws (as well as the corresponding triangular socket drive) had been conceived several decades before the invention of the Robertson screw and driver. An earlier patent covering both square-socket- and triangle-socket-drive wood screws, U.S. Patent 161,390, was issued to Allan Cummings of New York City on March 30, 1875. As with other drive types conceived and patented in the 1860s through 1890s, it was not manufactured widely during its patent lifespan due to the difficulty and expense of doing so.[3]: 79–81 

Invention

P. L. Robertson invented the Robertson screw and screwdriver in 1906 and received the Canadian patent in 1907 (CA103387, U.S. Patent CA103387A) and US patent 1911 (U.S. Patent 1,003,657) for a manufacturing machine. His breakthrough in 1908 was to design the socket's taper and proportions in such a combination that the heads could be easily and successfully cold formed,[3]: 79–81  making such screws a valuable commercial proposition. Today, cold forming (stamping in a die) is still their most common fabrication method, although rotary broaching is also common. Linear broaching, cutting corners into a drilled hole (similar to the action of a mortising machine for woodworking) has also been used over the decades.

Licensing

Screwdriver bits in different sizes for Robertson screws

When Henry Ford tried the Robertson screws, he found that they saved considerable time in Model T production. When Robertson refused to license the design, Ford realized that the supply of screws would not be guaranteed, and chose to limit their use to his Canadian division.[4][5][6]

Standard sizes

Square recess dimensions[7][8]
Color No Screw sizes Fraction Range
inmm
Orange #00#1, #20.05 1.3[9]
Yellow #0#3, #41⁄16 in +0.0696–0.071 1.77–1.80
Green #1#5, #6, #73⁄32 in -0.090–0.091 2.3–2.3
Red #2# 8, #9, #107⁄64 in +0.111–0.1126 2.82–2.86
Black #3#12, 1/41⁄8 in +0.1315–0.133 3.34–3.38
Brown #45/16, 3/83⁄16 in +0.1895–0.191 4.81–4.85

Combination screws for electrical applications

Many screws used in electrical applications (e.g., a typical NEMA 5-15R, breaker screws, and conduit screws) use a combination of a slotted/Phillips/Robertson screw head. A few tool manufacturers make bits to engage this screw head better than the traditional Phillips allowing for more torque before camout, e.g., the C1 & C2 from Klein[10] and the ECX #1 & #2 from Milwaukee[11].[12]

References

  1. ↑ Kelsey, John; Kirby, Ian J. (2004). Furniture Projects for the Deck and Lawn. ISBN 9781892836175. Retrieved 12 March 2012.
  2. ↑ Robertson Inc. "Robertson Inc. – The Original Robertson Fastening System". Robertson Inc. main site. Retrieved 28 September 2011.
  3. 1 2 3 4 5 6 7 8 Rybczynski, Witold (2000). One good turn : a natural history of the screwdriver and the screw. New York [u.a.]: Scribner. ISBN 0-684-86729-X.
  4. ↑ "Robertson Screws". Archived from the original on 8 October 2013. Retrieved 30 August 2021.
  5. ↑ "History of Screws and Screwdrivers". Archived from the original on 9 July 2012.
  6. ↑ Martindale, Barbara (1 March 1996). Caledonia: Along the Grand River. ISBN 978-0-920474-81-5.
  7. ↑ "McFeely's Square Drive Screws". Archived from the original on 22 November 2008. Retrieved 3 June 2010.
  8. ↑ "American Fastener". Archived from the original on 12 June 2010. Retrieved 3 June 2010.
  9. ↑ Wera Catalog, 2011/2012, P. 371
  10. ↑ "#1 Combination Tip Power Driver 1 Pk 5 - C115 | Klein Tools - for Professionals since 1857". 24 April 2022.
  11. ↑ "#1 ECX™ - 4" Screwdriver".
  12. ↑ "List of screw drives", Wikipedia, 8 April 2022, retrieved 20 April 2022
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