EXAMPLES Liquid Lubricants: Mineral Oil, Petroleum Oil,
Vegetable Oil etc. Semi Solid Lubricants: Petroleum jellies Solid
Lubricants: Graphite, Molybdenum Disulphide etc.
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Mechanical stability: Mechanical stability determination is
useful for judging the lubricant under conditions of high pressure.
This is determined by the four balls extreme pressure lubricant
test. In the equipment the three lower balls remain stationary
while the fourth upper ball rotates in the presence of the
lubricant under test. If the lubricant is satisfactory under the
given load, the ball bearings after the test comes out clean.
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However, when the load is progressively increased, and if
liberated heat welds the balls, then lubricant is said to have
failed completely. Therefore this test enables one to determine the
maximum load that can be carried out safely by a lubricant. A good
lubricating oil should posses high mechanical stability.
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Explosives An explosive is a substance or a mixture, which when
subjected to thermal or mechanical shock, gets very rapidly
oxidized exothermically into products of greatly increased volume
with sudden release of potential energy. In explosive reactions the
products get heated up to very high temperature and exert very high
temperature on surroundings which can be used for constructive or
destructive purposes. When an explosion occurs in a confined space,
the high pressure conditions developed within the system can be
instrumental to shatter the confining walls. The quantity of power
realizable from given weight(or volume) of an explosive is called
power to weight(or volume) ratio-which is quite small in the case
of gases but large in solids or liquids when used as explosives.
Explosives are mainly used for defense purposes, for blasting mines
for obtaining ores, drilling holes in mountains for constructing
tunnels, quarrying purposes, excavating earth(for dams), seismic
prospecting etc.
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Classification of Explosive Explosives may be in the form of
solids TNT(trinitrotoluene), liquids or gases (H 2 and O 2 ). They
may exist as a single component (TNT), a mixture of two or more
substances none of which is an explosives by itself (gun powder), a
mixture of one or more explosives compounds (TNT + NH 4 NO 3 ) or
an explosives compound mixed with a non- explosive material
(dynamite). A variety of inorganic or organic compounds can serve
as explosives. They are generally in the form of nitrates,
chlorates, perchlorates, peroxides or compounds containing nitro
(-NO), nitroso (-NO), azide (-N 3 ) or fulminate (-ONC) groups. All
known explosives can be categorized under three heads: Atomic,
mechanical and chemical explosives. Besides this Explosives are
broadly classified into three broad groups.
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Primary Explosives or Detonators These are highly sensitive
explosives which can explode under slightest shock or blow, by
ignition, and have to be very carefully handled. They are used in
comparative small quantities in blasting caps and cartridges. Lead
Azide (PbN 6 ) It is low cost, excellent initiating action and
stable in storage. It reacts with brass and caps loaded with it are
made of aluminum. Mercury Fulminate (Hg(CNO) 2 ) More sensitive as
well as more expensive lead azide but it is slightly toxic.
Tetracene (C 2 H 7 N 7 O) it is low initiating primary explosive.
It is mainly used as detonator. Diazodinitro phenol (DDNP) it is
quite sensitive and has high brisance value. It is used in blasting
caps.
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High or Secondary Explosives This type is quite insensitive to
mechanical shock as well as to flame i.e. they do not explode on
ignition. They explode with great violence when initiated with an
aid of detonators. They possess higher energy contents then primary
explosives and are stable. Secondary high explosives are mainly
Nitro compounds, Nitrate and Nitramines used as such or with an
initiators or as mixtures. Trinitrotoluene(TNT), Ammonium
nitrate(AN), RDX, Picric acid, Dinitrotoluene,
Pentaerythritotetranitrate(PETN), ethylene dinitramine etc. come
under this category of explosives which are generally used in the
main body of projectiles. Primary and secondary high explosives are
generally referred to as high explosives.
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High explosives are sub-divided into 1. Military high
explosives, 2. Blasting or industrial purpose explosives. Examples:
GTN: Glyceryl trinitrate PETN: Pentaerythetol tetranitrate TNT:
Trinitrotoluene
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Ammonium nitrate: It is half as powerful as TNT (2,4,6
trinitrotoluene) and employed in making binary explosives. It is
dangerous to store near any inflammable material. It cannot be used
in contact with brass, since it produces a detonator tetrammino
cupric nitrate. TNT: It is used in shell-firing and under-water
explosives. It can be loaded in containers because of its low
melting point (81C). Because of its (1) non-hygroscopic nature and
(2) inertness to metals, TNT is used as safe explosive in the
manufacture, storage and transportation. RDX or cyclonite
(cyclotrimethylene trinitroamine): It is a powerful high explosive.
It is more sensitive and less toxic than TNT. It is used both in
military and industrial purpose explosive. Picric acid (or
trinitrophenol): It is replaced largely by TNT since it forms
shock-sensitive picrates with metals. Explosives, based on their
state of aggregation are classified as solid (e.g.,TNT), liquid
(e.g., nitroglycerine) and gaseous (e.g., mixture of oxygen and
acetylene).
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Precautions During Storage of Explosives Following precautions
should be taken while storing the explosives 1. All wiring and
electrical fittings should be properly insulated and checked
regularly. 2. Jerks or drops of explosive should not be allowed to
take place. 3. Detonators and explosives should be stored
separately. 4. Different explosives should be stored in separate
boxes. 5. Fire or smoking should be strictly prohibited within the
radius of 50 m from the explosive store (magazine). 6. Only
authorized persons with magazine shoes should be allowed to the
explosive store. 7. The explosive store should not be within 500m
from any working kiln or furnace. 8. Lighting conductors should be
provided to safeguard the magazine.
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Blasting Fuses Safety Fuse Safety Fuse consists of a small
diameter core of black powder enclosed in water- proof fabrics. It
has an approximate burning speed of 30-40 seconds per foot. While
blasting, a sufficient length of the fuse is used so as to allow
sufficient time for the shot firer to reach a point of safety
(analogous to the wick present in fire crackers). Detonating Fuse
Detonating Fuse consists of a charge of high velocity explosives
(6000 meters per second) e.g. TNT contained in a small-diameter
bent tube. The line of fuse is in contact with the charge
throughout its length and this is responsible for instantaneous
detonation. These are mainly used for exploding charges of
explosives in deep-holes.
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Liquid Propellant They have several advantages over solid
propellants. They are more versatile and possess high specific
impulse. But the engine is more delicate and is less suitable for
rough handling than the engine for solid propellant. Liquid
propellants may be mono-propellants or bi- propellants.
Mono-propellants: They require single storage tank and one fuel
injection and control system. The fuel as well as oxidizer are in
the same solution. For example hydrogen peroxide, nitromethane,
hydrazine, ethylene oxide, 21.4% methanol and 78.6% hydrogen
peroxide is highly reactive, metal oxides catalyse the
decomposition, hence storage tanks must be made of special
materials.
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Bi-propellants: Bi-propellants are widely used. Liquid oxygen
(LOX), hydrogen peroxide, ozone, fuming nitric acid, liquid
fluorine are the common oxidizers used in bi-propellants. In bi-
propellants, liquid fuel and oxidizers are kept separately and
injected separately into the combustion chamber. Ethyl alcohol and
25% water is a good fuel. Addition of water reduces flame
temperature and molecular mass of combustion gases, which
compensates for reduction in performance. Liquid oxygen is safe,
non-toxic and good oxidant, but it has to be stored under pressure
in insulated containers. Ozone though a powerful oxidant but
explodes at high concentration. Liquid fluorine is toxic,
corrosive, volatile, very reactive, but a good oxidant. It is
difficult to store and handle.
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