A Robertson screw, also known as a square screw or Scrulox, is a type of

screw A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...

with a

square In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90-degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length adj ...

-shaped socket in the screw head and a corresponding square protrusion on the tool. Both the tool and socket have a slight

taper Taper may refer to: * Part of an object in the shape of a cone (conical) * Taper (transmission line), a transmission line gradually increasing or decreasing in size * Fishing rod taper, a measure of the flexibility of a fishing rod * Conically ta ...

. Originally designed to enable the screws to be made using

cold forming In metallurgy, cold forming or cold working is any metalworking process in which metal is shaped below its recrystallization temperature, usually at the ambient temperature. Such processes are contrasted with hot working techniques like hot roll ...

of the heads, 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. The Robertson screw is specified as ANSI Type III Square Center.

Usage and design

Robertson screws are commonplace in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, though they have been used in other countries and are becoming much more common. As patents expired and awareness of their advantages have spread, Robertson fasteners have become popular in

woodworking Woodworking is the skill of making items from wood, and includes cabinet making (cabinetry and furniture), wood carving, woodworking joints, joinery, carpentry, and woodturning. History Along with Rock (geology), stone, clay and animal parts, ...

and general construction. Combination Robertson/ Phillips drives 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. They also allow the use of angled screwdrivers and trim-head screws. The socket-headed Robertson screws are self-centering and reduce

cam out Cam out (also cam-out or camming out) is a process by which a screwdriver slips out of the head of a screw being driven once the torque required to turn the screw exceeds a certain amount. Repeatedly camming out damages the screw, and possibly also ...

. They also stop a power tool when set, and can be more easily removed if painted over, or old and rusty. In industry, they speed up production and reduce product damage.

History

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, , 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.

Invention

P. L. Robertson Peter Lymburner Robertson (December 10, 1879 – September 28, 1951) was a Canadian inventor, industrialist, salesman, and philanthropist who popularized the square-socket drive for screws, often called the Robertson drive. Although a square-socke ...

invented the Robertson screw and screwdriver in 1906 and received the Canadian

patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...

in 1907
CA103387
) and US patent 1911 () 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, making such screws a valuable commercial proposition. Today, cold forming (stamping in a die) is still their most common fabrication method, although

rotary broaching Broaching is a machining process that uses a toothed tool, called a broach, to remove material. There are two main types of broaching: ''linear'' and ''rotary''. In linear broaching, which is the more common process, the broach is run linearly ...

is also common. Linear broaching, cutting corners into a drilled hole (similar to the action of a

mortising machine A mortiser or morticer is a specialized woodworking machine used to cut square or rectangular holes in a piece of lumber (timber), such as a mortise in a mortise and tenon joint. Square chisel mortiser The square chisel mortiser (also called '' ...

for woodworking) has also been used over the decades.

Licensing

When

Henry Ford Henry Ford (July 30, 1863 – April 7, 1947) was an American industrialist, business magnate, founder of the Ford Motor Company, and chief developer of the assembly line technique of mass production. By creating the first automobile that mi ...

tried the Robertson screws, he found that they saved considerable time in

Model T The Ford Model T is an automobile that was produced by Ford Motor Company from October 1, 1908, to May 26, 1927. It is generally regarded as the first affordable automobile, which made car travel available to middle-class Americans. The relati ...

production. When Robertson refused to

license A license (or licence) is an official permission or permit to do, use, or own something (as well as the document of that permission or permit). A license is granted by a party (licensor) to another party (licensee) as an element of an agreeme ...

the design, Ford realized that the supply of screws would not be guaranteed, and chose to limit their use to his Canadian division.

Nüvo drive system

A new variation of the Robertson drive is the Nüvo drive system. Its screws are compatible with Robertson drive tools, but have rounded lobes that, when used with Nüvo drivers, "dramatically reduce wobbling and stripping out, enabling single-handed operation".

Combination Screws for Electrical Applications

Many screw heads used in electrical applications (e.g., a typical NEMA 5-15R, breaker screws, and conduit screws) use a combination of a slotted/Phillips/Robertson bit. 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 and the ECX #1 & #2 from Milwaukee.

References

{{Reflist Mechanical standards Screws