Most fasteners are right handed, so why do we need left-handed screws?
What is Screw Handedness?
The handedness of a screw refers to the direction in which the helical thread wraps around the screw shaft - right-hand threads run in a clockwise direction, whereas left-hand threads run in a anti-clockwise direction.
It may seem obvious that the ‘handedness’ of a screw originates from human physiology. The majority of the human race is right handed, and the right hand rotates (or more specifically ‘pronates’) in a clockwise direction, which makes right-handed screw installation ergonomically inclined towards the use of the right hand.
Are All Screws Right Handed?
The vast, vast majority of screws in use today feature a standardised, right-hand thread. So great is the bias against the left-hand thread, that the chance of encountering a left-hand thread in any regular fastening application is almost an impossibility. The prevalence of right-hand threads is the result of a range of factors, from the ergonomic considerations mentioned previously, to the simple fact that the manufacturing standards which dictate component manufacture favor a right-hand thread. Left-handed screws are rare, and almost only occur when they are deliberately chosen for special applications.
Why do Left-Handed Screws Exist?
Despite the origin of right-hand threads, it should come as no surprise that left-handed screws are not designed for left-handed humans. Fasteners are consumed at a staggering rate by the manufacturing industry, which relies on the universality and cross-compatibility of its interchangeable components. The creation of special screws for a small portion of the population would be counter-intuitive, and would cause no end of compatibility problems on production lines.
Sometimes, left-handed screws are implemented for the mechanical advantage of reverse threading. For example, some cars utilise left-handed threads on their left-hand wheels, which, unlike the wheels on the right, rotate anti-clockwise. This use of reverse-threaded fasteners prevents them from loosening gradually under the torque of the wheels. This particular use of left-handed screws is probably the most common, and is commonplace in products such as bicycle pedals, turnbuckles, or many types of machinery with a rotary blade.
Other times, left-handed threads are used to create deliberate incompatibility as a safety indicator. Gas lines, for instance, are often tapped with a left-handed thread. This safety feature distinguishes gas lines from water lines, and is the amateur plumber’s best defence against aquatic ovens and flammable faucets.
Occasionally, a left-handed thread can also be used in actuation to counteract the rotational forces of a right-hand screw in motion. An example of this would be on a piece of machinery with a sliding section such as a lathe or vice, which can use two identical lead screws of opposing handedness. When the two lead screws are made to rotate in opposite directions with a reverse idler gear, they transmit matching amounts of linear motion, but negate the residual rotational forces that would be a byproduct of a single screw design.