Properties of Aluminium

Aluminium has a unique and unbeatable combination of properties that make it into a versatile, highly usable and attractive construction material.

Weight
Aluminium is light with a density one third that of steel, 2.700 kg/m3.

Strength
Aluminium is strong with a tensile strength of 70 to 700 MPa depending on the alloy and manufacturing process. Extrusions of the right alloy and design are as strong as structural steel.

Elasticity
The Young’s modulus for aluminium is a third that of steel (E = 70,000 MPa). This means that the moment of inertia has to be three times as great for an aluminium extrusion to achieve the same deflection as a steel profile.

Formability
Aluminium has a good formability, a characteristic that is used to the full in extruding. Aluminium can also be cast, drawn and milled.

Machining
Aluminium is very easy to machine. Ordinary machining equipment can be used such as saws and drills. Aluminium is also suitable for forming in both the hot and the cold condition.

Joining
Aluminium can be joined using all the normal methods available such as welding, soldering, adhesive bonding and riveting.

Corrosion resistance
A thin layer of oxide is formed in contact with air, which provides very good protection against corrosion even in corrosive environments. This layer can be further strengthened by surface treatments such as anodising or powder coating.

Conductivity
The thermal and electrical conductivities are very good even when compared with copper. Furthermore, an aluminium conductor has only half the weight of an equivalent copper conductor.

Linear expansion
Aluminium has a relatively high coefficient of linear expansion compared to other metals. This should be taken into account at the design stage to compensate for differences in expansion.

Non-toxic
Aluminium is not poisonous and is therefore highly suitable for the preparation and storage of food.

Reflectivity
Aluminium is a good reflector of both light and heat.

Ductility

Aluminium is ductile and has a low melting point and density. In a molten condition it can be processed in a number of ways. Its ductility allows products of aluminium to be basically formed close to the end of the product’s design.

Impermeable and Odourless

Aluminium foil, even when it is rolled to only 0.007 mm thickness, is still completely impermeable and lets neither light aroma nor taste substances out. Moreover, the metal itself is non-toxic and releases no aroma or taste substances which makes it ideal for packaging sensitive products such as food or pharmaceuticals.

Cement & Properties

What is concrete?

Concrete is not found in nature the way we would find aluminium, nickel or iron. Concrete is formed from combining water, a special cement and rock: PORTLAND CEMENT + H2O + ROCK = HARDENED CONCRETE + ENERGY(HEAT)
A common mistake people make is to use the words cement and concrete interchangably. It is important to remember that cement is only a component of concrete and concrete is the structural material. The cement used in concrete is not used as a building material because it would be too expensive and not as strong as concrete. So when you see a parking garage, a driveway, a sidewalk or a road remember it is made of concrete, not cement. And, by the way, that funny looking truck is a concrete mixer, not a cement mixer! But, if cement is not concrete, then what is it?
Cement is a general name for a material that binds other materials together. Yes, it is another name for glue. There are many materials which we would classify as cements and they are usually identified with certain uses, and can produce different types of "concrete". The type of cement used to make the riding surface of some of our roads (blacktop!) is called asphalt cement. It is a petroleum bi-product, and it binds rock into the road material we call asphaltic concrete.
Adding water to the dry cement starts a chemical reaction (hydration). While the mixture of cement, water, and rock is fluid, it can be poured into molds (called formwork) of arbitrary shape. This is a valuable property of concrete which allows us to build dams with the many different shapes which you saw in the history of dams. The compound gradually hardens into the desired final shape. The water/cement ratio (w/c) of the mixture has the most control over the final properties of the concrete. The water/cement ratio is the relative weight of the water to the cement in the mixture. The water/cement ratio is a design criterion for the engineer. Selection of a w/c ratio gives the engineer control over two opposing, yet desirable properties: strength and workability. A mixture with a high w/c will be more workable than a mixture with a low w/c: it will flow easier. But the less workable the mixture, the stronger the concrete will be. The engineer must decide what ratio will give the best result for the given situation. This is not an entirely free choice because the water/cement ratio needs to be about 0.25 to complete the hydration reaction. Typical values of w/c are between 0.35 and 0.40 because they give a good amount of workability without sacrificing a lot of strength.
	The other important component for strength is the aggregate, the rock that is being bound by the hardened cement. Aggregate is what makes the difference between hardened cement and the structual material, concrete. Aggregate increases the strength of concrete and is a fundamental economical factor because it takes up a large volume of the concrete and is much less expensive than an equivlant volume of cement. To make very strong concrete requires a low w/c and strong aggregate. There might be thousands or millions of tons of cement and aggregate in a large dam. Finding the aggregate for the dam, and transporting it and the cement to the dam site are important societal factors.

Properties of concrete

Concrete has relatively high compressive strength, but significantly lower tensile strength, and as such is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces is prone to creep.

Tests can be made to ensure the properties of concrete correspond to specifications for the application.The density of concrete varies, but is around 2,400 kg/m³ (150 pounds per cubic foot or 4,050 lb/yd³).lol= 0870312642}}</ref> As a result, without compensating, concrete would almost always fail from tensile stresses – even when loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced with materials that are strong in tension.

Reinforced concrete is the most common form of concrete. The reinforcement is often steel, rebar (mesh, spiral, bars and other forms). Structural fibers of various materials are available.

Concrete can also be prestressed (reducing tensile stress) using internal steel cables (tendons), allowing for beams or slabs with a longer span than is practical with reinforced concrete alone. Inspection of concrete structures can be non-destructive if carried out with equipment such as a Schmidt hammer, which is used to estimate concrete strength.The ultimate strength of concrete is influenced by the water-cementitious ratio (w/cm), the design constituents, and the mixing, placement and curing methods employed. All things being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than that with a higher ratio. The total quantity of cementitious materials (portland cement, slag cement, pozzolans) can affect strength, water demand, shrinkage, abrasion resistance and density. All concrete will crack independent of whether or not it has sufficient compressive strength. In fact, high Portland cement content mixtures can actually crack more readily due to increased hydration rate. As concrete transforms from its plastic state, hydrating to a solid, the material undergoes shrinkage. Plastic shrinkage cracks can occur soon after placement but if the evaporation rate is high they often can actually occur during finishing operations, for example in hot weather or a breezy day. In very high-strength concrete mixtures (greater than 70 MPa) the crushing strength of the aggregate can be a limiting factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the crushing strength of the aggregates is not so significant.

The internal forces in common shapes of structure, such as arches, vaults, columns and walls are predominantly compressive forces, with floors and pavements subjected to tensile forces. Compressive strength is widely used for specification requirement and quality control of concrete. The engineer knows his target tensile (flexural) requirements and will express these in terms of compressive strength.Wired.com reported on April 13, 2007 that a team from the University of Tehran, competing in a contest sponsored by the American Concrete Institute, demonstrated several blocks of concretes with abnormally high compressive strengths between 340 and 410 MPa (49,000 and 59,000 psi) at 28 days. The blocks appeared to use an aggregate of steel fibres and quartz – a mineral with a compressive strength of 1100 MPa, much higher than typical high-strength aggregates such as granite (100–140 MPa or 15,000–20,000 psi).

Reactive Powder Concrete, also known as Ultra-High Performance Concrete, can be even stronger, with strengths of up to 800 MPa (116,000 PSI). These are made by eliminating large aggregate completely, carefully controlling the size of the fine aggregates to ensure the best possible packing, and incorporating steel fibers (sometimes produced by grinding steel wool) into the matrix. Reactive Powder Concretes may also make use of silica fume as a fine aggregate. Commercial Reactive Powder Concretes are available in the 170–210 MPa (25,000–30,000 psi) strength range.

Gear

Gears are the most common means of transmitting power in mechanical engineering. There are tiny gears for devices like wrist watches and there are large gears that some of you might have noticed in the movie Titanic. Gears form vital elements of mechanisms in many machines such as vehicles, metal tooling machine tools, rolling mills, hoisting and transmitting machinery, marine engines, and the like. Toothed gears are used to change the speed, power, and direction between an input and output shaft. This site is all about Gears. Visit the pages linked below to know more about different types of gears:

Bevel Gear Reducer	Hypoid Gears
Bevel Gearbox	Gear Pump
Bicycle Gears	Gearbox
Fixed Gear Bicycle	Speed Reducers
Gear Coupling	Differential Gear System
Gear Cutting	Worm Gear Reducer
Gear Manufacturing	Worm Gear Speed Reducer
Gear Technology	Spur Gear
Helical Gears	Bevel Gears
Worm Gears	Gear Design
Gear Manufacturer	Gear Lube
Gear Ratio	Gear Wrench
Hydraulic Gear Pump	Planetary Gear
Gear Reducers	Plastic Worm Gears
Worm Gear Drives	Worm Gear Speed Reducers
Spider Gears	Gear Oil

Construction and Working of a Bevel Gear Reducer

One of the major applications of gears is gear reduction. The high power and low torque of the supply can be converted into high torque by coupling of a smaller input gear with a larger gear. There are different types of gear reducers depending on the types of gears coupled viz. worm gear reducer, spur gear reducer, bevel gear reducer etc.

Construction of a Bevel Gear Reducer
A bevel gear reducer consists of a small gear acting as a pinion coupled with another gear of a larger diameter at right angles to each other. The gears can be either of spur or helical type. Helical gears are preferred as they are less noisy due to gradual engagement. The material can be either plastic or metal depending on the application. Three gears can be used in case we need to keep the axis of rotation the same as that of input. The whole arrangement is sealed in a metal or plastic casing known as housing. The point of contact is lubricated with gear oil.

Working of a Bevel Gear Reducer
The diameter of the output being larger than that of the input gear, the torque of the system is increased at the expense of speed. Since the speed is decreased, this system is known as a reducer. The gears are at right angles to each other and hence the direction of the rotary motion is changed.

Advantages of a Bevel Gear Reducer
The efficiency of a bevel gear reducer is higher than that of a typical worm gear reducer. The bevel gears have high load capacity.

Applications of a Bevel Gear Reducer
Due to change in direction of the rotary motion the bevel gear reducer finds a variety of applications in industry and automotives. In industries it is used in turbines, pumps, cranks, etc. In automotives it is used in ordinary as well as limited slip differentials.

Hypoid Gears Are a Subtype of Bevel Gears

Gears are major part of any machine working on the principle of rotary motion. The motion can be produced either by a motor or engine and is called the input. The engine usually has a very high angular momentum but the torque is small. This torque is not enough for performing work under heavy load and hence cannot be used directly in industrial machines. To overcome this problem, coupled gears are used.

In coupled gear system, the input is connected through a driving shaft to a small gear called the pinion. The pinion is further coupled with a gear of larger diameter i.e. high gear ratio. Due to this form of coupling, the torque of the engine increases at the expense of angular momentum. A variety of gears can be used for this purpose viz. spur gears, helical gears, worm gears, etc.

However these gears have a limitation i.e. they can only be used in systems where the input and output shafts are in the same plane. To overcome this problem, a special type of gear called the hypoid gear is used.

The hypoid gears are a subtype of bevel gears. On observation the hypoid gear seems to be similar in appearance to the helical bevel gears. The main difference being that the planes of the input and the output gears are different. This allows for more efficient intermeshing of the pinion and driven gear. Since the contact of the teeth is gradual, the hypoid gear is silent in operation as compared to the spur gears.

These gears are usually used in industrial and automotive application and hence the material used is a metal like stainless steel. A major application of hypoid gears is in car differentials where the axes of engine and crown wheel are in different planes.