What is the Best Screw Drive?
Phillips, Flat Head, Pozi and Torx. Why do we need so many drive types?
Fasteners are available in innumerable different shapes and sizes, and one of the most common permutations is drive type. An engineer's toolkit is incomplete without an abundance of screwdrivers and drive bits for all manner of different screw drives, so why is there not just one uniform type? Whether they were developed at different times, for different purposes, or just by different companies, there are numerous reasons for the myriad of fastener drive types on offer.
One advantage for the use of unusual drive types is the added security that an uncommon drive type can provide. A fastener that cannot be loosened without special tooling is inherently resistant to tampering, and Security Screws are specifically designed to be difficult to operate without the correct tooling. In total contrast with almost all other drive types, security drives are interesting in that their appeal lies in the fact that they are deliberately inconvenient to operate. Arguably, the more hard-to-find a security driver is, the more effective it is at its job.
Anyone who has ever put together flatpack furniture will be familiar with the weird and wonderful screw types which companies invent for their products. In many cases, unique fastener attributes can allow for innovations in product design and security, but in some cases, customers argue that standard components would probably have sufficed - This was famously seen when technology giant Apple launched its patented ‘pentalobe’ fasteners, which came under heavy criticism for being unnecessarily restrictive back in 2009.
Originally, with the introduction of electric power tools, the design of screws was developed to encourage cam-out (the disengagement or slippage of a driver from a fastener head) at high torque levels, this was because high-torque drivers could damage fasteners and housings when over-tightened, and disengagement from the driver was intentionally implemented as a safety feature. In applications where cam-out was advantageous, fasteners such as Phillips and Pozi drives were widely used, but as more sophisticated torque limiters were implemented into power tools and over tightening became less of an issue, high-torque socket drives such as hexagon (Allen) drives saw a resurgence. Shifting the torque limiting safety features from the screw drive to the power tool itself also allowed these drives to be further developed into even more effective torque transmitting heads, such as the Torx drive.
Drive Type Timeline
Pre-1600 - The Slotted Drive
The rudimentary slotted drive was the archetypal threaded fastener drive on some of the very first threaded screws. The slotted drive was a renaissance-era development which enhanced rotational leverage and simplified the installation process for fasteners. The first slotted drives were cut into the top of a simple nail with a saw or file - this simplistic design was effective and cheap to manufacture, and has remained in use for over 400 years.
1908 - The Robertson Drive
The Canadian square-shaped Robertson drive was invented in 1908, shortly after its inventor, P. L. Robertson, injured himself when his slotted screwdriver slipped from the head of a slotted screw. The goal of the Robertson drive was to improve upon the most dominant screw type of the time by preventing drive slippage, which was, and still is, a common complaint with slotted drives.
The Robertson drive was designed to give very strong engagement between the fastener and the driver, with a deep square socket drive which discouraged driver disengagement under stress; this allowed high levels of torque to be transmitted, especially when compared to a slotted drive. However, when the Robertson Drive was first introduced, torque limitation on tooling was non-existent, meaning that the Robertson drive had the potential to cause significant damage to tooling and fasteners by over-tightening.
1910 - The Hexagon Drive
The instantly recognisable hexagon drive, also known as the hex drive or the Allen drive, likely existed in an unofficial form several years before being patented by W. G. Allen in 1910, but became particularly widespread during the early 20th century. As with the earlier Robertson drive, hexagon drives had the potential to engage a little too well with a fastener, and could cause damage through over-tightening.
1935 - The Phillips Drive
The Phillips drive is the archetypal cruciform drive type, and is arguably the most well-known drive type in the world. The Phillips drive features a much shallower socket than Robertson or hexagon drives, which did allow the driver to disengage under excessive force.
The Phillips drive is regularly criticised for its tendency to cam-out at relatively low torque levels which can cause slipping or rounding of the screw head. Some suggest that this feature is by deliberate design - there is no official evidence to support this claim in the original designs, though this characteristic was suggested in subsequent patented design improvements.
1955 - The Pozi Drive
The Pozi drive, also known by the trade name ‘Pozidriv’, is an improvement upon the standard Phillips drive. It features a blunt tip, and four radial grooves for increased driver engagement. Pozi drives are, visually, very similar to Phillips drives, but can be most easily distinguished by the four shallow radial markings on their heads.
1967- The Torx Drive
As its name suggests, the Torx drive was designed to improve upon the torque transmission capabilities of previous socket styles. The hexalobular design of the Torx socket is an improvement upon the hexagon drive due to the addition of six deep ‘lobes’, which allow force to be transmitted at closer to a 90 degree angle in each corner, and greatly improves its ability to transmit rotational forces. The hexalobular design is also stronger and more resistant to rounding than a hexagon drive, making the Torx drive preferable on unhardened Stainless Steel Screws.
When the Torx drive was introduced in 1967, torque limiting features were relatively common in drills and tooling, mitigating the over-tightening pitfalls of previous socket designs. Torque transmission could now be safely improved, as torque levels could be accurately controlled and limited through the tooling itself. This development in tooling allowed torque maximising designs such as the Torx drive to thrive.
Interestingly, when the Torx drive was first introduced, it was considered tamper-proof by many users due to its unconventional shape, but the Torx drive proved so popular, and its use became so widespread, that its tamper-resistant properties were quickly dispersed.