1. Simple Machine: Screws
Fast Fact
In 1967, one of the largest screws or bolts was manufactured in Scotland by the Penrith Engineering
Works. The length and diameter of these machines was about 24 feet and 4 feet, respectively. These
screws were made from a very strong element called iridium, and oil tankers were connected to
each other using such devices.
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A screw is one of the six simple machines, with the other five being the lever, wheel and axle, pulley,
inclined plane, and wedge. It is one of the most commonly used devices in the world. The screw has
an appearance of a thin metal stick with a turning head on top and grooves along its length.
Conventionally, it is defined as a mechanical device that has helical-inclined planes and is capable of
converting torque to linear force and vice versa.
The inclined planes that form the threads are wound entirely along the screw shaft, and hence, it
becomes easier to fix different objects, as driving it down in a rotational form requires less effort
than pushing a nail by application of effort in a vertical direction. Also, nails have the tendency of
bending when they are used, whereas a screw would never bend as force is evenly spread along the
entire shaft. These are a few reasons why using a screw is preferred over a nail, and why the latter
is being phased out slowly.
History
The initial design used was the Archimedes screw, which was developed from Archimedes himself. It
was mainly utilized to draw out water from wells, rivers, lakes, etc. This screw had a longer length
between its threading and was operated manually. Prominent uses of this device were seen when
water was drawn up to the Hanging Gardens of Babylon. The same design was also utilized in the
Mediterranean regions for many centuries for irrigation processes, and also to facilitate oil and wine
presses. The Archimedes Screw is used even today in numerous mechanical operations. It has a wide
application in the form of conveyor screw devices, especially used in the food processing industry.
Innovation of the modern screw that is used primarily as a fastener is associated with the
development of the lathe machine. In the period before modern devices were mass-produced, cotter
pins and pin bolts were widely used as fasteners. Actual development of the modern screw began
after Jesse Ramsden, a British instrument manufacturer, perfected a device called 'screw-cutting
2. lathe' in the 1700s. This lathe was further improved and patented in 1798 by David Wilkinson. In the
following decades, the importance of threaded fasteners and screws greatly increased in mechanics,
which led to the 'standardization of thread forms'. The process of standardization is still ongoing,
and has also led to the development of ISO metric screw threads and the Unified Thread Standard.
The 'turret lathe' and 'screw machine', which were developed in the 1840s and 1870s, respectively,
boosted the mass production of screws. Some important types of such devices that were developed
in the course of evolution of modern designs were the Robertson Screw (developed by P. L.
Robertson in 1908), and Hex Socket and Phillips-head Screw (developed by Henry F. Phillips).
Working Principle and Explanation
The helical-inclined planes present on the screw are referred to as threads or threading. The
structure, space, size, and material of which the screw is made up of determines its strength. This
strength is also referred to as the mechanical advantage. The turning head of the screw is a
provision or facility where the force from a screwdriver can be applied, and torque can be
generated. When force is applied on the screw head, it is transferred to the screw threads, which
further begin to advance in the downward direction. The exact opposite motion occurs when one
starts removing a screw from the material it is fixed to.
The distance covered by a screw in a straight line in one single rotation is called the lead, while the
measurement between two successive inclined planes is called the pitch. The Mechanical Advantage
(MA) of this simple machine is controlled by the degree of the lead; less distance covered means
more MA, and vice versa. Depending on the orientation of the screws, they are distinguished in two
types: right-handed (the helical threads move in a clockwise manner) and left-handed (the threads
move in an anti-clockwise manner). The former are most commonly used for numerous application,
while the latter are used only for specific purposes (for example, a left-side bicycle pedal and few
types of light bulbs). If the pitch is less, greater force is applied by the screw onto the material, in
which it is inserted. Also, if the incline of the threads increases, the input force also increases.
The ratio of the screw circumference to the distance covered by it at the time of every rotation is
called the Ideal Mechanical Advantage (IMA). This ratio is represented as:
IMA = Screw Head Circumference/Distance Covered by the Screw Shaft
If circumference of the head is 2??, where ? is the radius of the head surface, and ? is the distance
advanced by the screw on one rotation, the IMA can be represented as:
IMA = 2??/?
The Actual Mechanical Advantage (AMA) is the ratio of the force applied during turning of the screw
head to the force exerted by the device on the material, in which it is to be inserted. Thus, this type
of mechanical advantage can be represented as:
AMA = Input Force/Output Force
One amazing advantage of the screw over the nail is that it can get automatically locked in the
material, where it is inserted. This is caused due to the extra friction produced by the inclined
planes, when it rotates and gets fixed in the interior portion.
3. The following pictures illustrate the various types of screws that are used currently.
Applications in Modern Era
Though the latest designs were developed in a very short period of time, the ability of screws to
transfer torque has made it a very effective tool. It has also replaced rivets and fastening pins in the
mechanical industry due to its ease of operation and utility. This type of simple machine is used in
machinery, where power is transferred from one part to another.
A very good application of this device is regarding micrometer gauges, which are used in
mathematics and physics for precise measurement. Conveyor belts, propellers, drillers, helicopter
blades, etc., are a few prominent examples, where the use of screws can be seen in our everyday
lives. The concept of screws is also used in an architectural sense, like building spiral staircases; the
motion of a person climbing them is same as that of a normal screw.
This simple machine can handle about five times the holding strength, as compared to the same
strength of a nail. The screw is still undergoing many changes and is evolving continuously. Due to
its basic principle of physics, it is considered a member of the six simple machines, and has become
an indispensable part of modern mechanics.
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