Gear Trains: Types & Gear Ratio

A gear train is a mechanical system formed by mounting gearson a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next.

Types of gear train are given below:

• Simple gear train
• Compound gear train
• Reverted gear train
• Epicyclic gear train

Simple Gear Train:
 

If there is only one gear mounted on each shaft, the gear train is known as simple gear train. that is, each shaft has only one gear.

Usually when two gears mate, they rotate opposite to each other. When we use three gears each mounted on separate shaft, the direction of rotation of the last gear will be same as that of the direction of rotation of first gear. Suppose the number of gears are increased to four the direction of rotation of first and last gear will be opposite to each other. A typical simple gear train is shown in the figure 1.
 

So from above we can conclude that in a simple gear train, if the no of gears is odd then the direction of rotation of first and last gear will be the same, it it is even, direction of rotation will be opposite. 

The speed ratio is given by, (N1/N2) = -(T2/T1), where, N1 is the speed of the driver, N2 is the speed of driven gear, T1 is the no of teeth on driver gear and T2 is the no of teeth on driven gear.

Compound gear train:

If there is more than one gear on a shaft, the gear train is said to be compound gear train. Here speed of rotation of the gears mounted on a shaft will the same and also the direction. By using this arrangement, the power will be transmitted to the shaft which is placed not in the row. The system is compact here. Also we can reduce the size of the gears using this arrangement. This is illustrated in the Figure 2.

 

Reverted gear train:

 

If the axes of driver shaft and driven shaft is co-axial, then the gear train is termed as reverted gear train. This is kind of compound gear train. This is illustrated in the figure 3. So reverted gear train may be called as a compund gear train but all compound gear train can not be a reverted gear train. This reverted technology is very much useful when the power is to be transmitted within the less space. 

 

Epicyclic gear train:

 

This is important gear train compared the said above. In this case, One of the gear is rotating over and around another gear. Epi means over, Cyclic means around. There is an arm connecting such two gears. This gear train finds great application in various field. This is illustrated in the figure 4.

Modes of Heat Transfer – Conduction, Convection & Radiation

Heat is a form of energy which transfers between bodies which are kept under thermal interactions. When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs. In this article, we are going to deal with the different modes of heat transfer. Heat transfer occurs basically in three modes:

1. Conduction
2. Convection and
3. Radiation

CONDUCTION:

 
Conduction is the mode of heat transfer occurs from one part of a substance to another part of within the substance itself or with another substance which is placed in physical contact. In conduction, there is no noticeable movement of molecules. You might be think that then how this heat transfer occurs? The heat transfer occurs here by the two mechanisms happen.

By the transfer of free electrons. (Good conductors like metals have a plenty of free electrons to make conductive heat transfer. The atoms and molecules having energy will pass those energy they have with their adjacent atoms or molecules by means of lattice vibrations.

Now we can think how this conduction occurs in gases and liquids. In the cases of gases, the molecules having energy in the form of kinetic energy and during their random movements, they exchange their momentum and energy by colliding with others. By doing so, the first molecule loses the energy while the second one gains it. This is how energy is transferred in the case of gases.

In the case of liquids also, the working is similar to that of gases. Here, the only difference is that, the molecules in liquids are more closely packed and hence inter molecular forces came into action in the case of liquids.

Fourier Law of Conduction:

Q = -kAdT/dx
Where:  Q is the heat flow rate by conduction
              K is the thermal conductivity of the material
              A is the cross sectional area normal to direction of heat flow and
              dT/dx is the temperature gradient of the section.

CONVECTION:

Conductive heat transfer occurs within a fluid itself and it is carried out by transfer of one fraction of the fluid to the remaining portion. Hence unlike conduction, transfer of molecules occurs during convection. Since movement of particles constitutes convection, it is the macro form of heat transfer. Also convection is only [possible in fluids where the particles can moved easily and the rate of convective heat transfer depends on the rate of flow to a great extend. Convection can be of two types:

Natural convection: In this type of convection, the movement of particles which constitutes convection occurs by the variation in densities of the fluids. As we already know, as temperature increases, the density decreases and this variation in density will force the fluid to move through the volume. This cause convection to occur.

Forced Convection: The difference between natural convection and forced convection is that in forced convection, a work is done to make movement in the fluid. This is done using a pump or blower.

Newton’s Low Of Cooling:

Q = hA(Ts-T∞)
Where:  Ts is the surface temperature
              T∞ is the fluid temperature
              h is the heat transfer coefficient

RADIATION:

Radiation is the third mode of heat transfer. This mode of heat transfer didn’t require any medium to occur. Every matter having a temperature above absolute zero will emit energy in the form of electromagnetic waves and called radiation. It is the same way the energy of the Sun reach us. The key features about radiation are it do not require any medium and also laws of reflection is applicable for radiation.

Stefan- Boltzman Law:

Q = A∑Ts⁴
Where:  Ts is the absolute temperature of surface
              ∑ is the proportionality constant.

Generating Electricity from a Bicycle Dynamo

The proposed mini-electricity generator project is very simple to build and can be used by students as a school project, or just for hobbyists. The set up can be used to charge a battery with electricity produced from wind power.A dynamo is a type of alternator commonly associated with bicycles for generating electricity that is used for lighting a small head lamp. The unit eliminates the need of a battery and provides an easy alternative for illuminating a lamp whenever the bicycle is in motion.

A dynamo is a type of alternator commonly associated with bicycles for generating electricity that is used for lighting a small head lamp. The unit eliminates the need of a battery and provides an easy alternative for illuminating a lamp whenever the bicycle is in motion.

A dynamo is a pretty interesting little generator which starts generating pure electricity the moment its wheel is rotated. Basically, it works on the fundamental principles of electromagnetism where current is induced in coils of copper wire under the influence of a rotating magnetic flux, generated by alternate shifting of the magnets North and south poles.

Before we move on to the actual project, interested enthusiasts may try building a homemade dynamo with the help of the explanation provided in the following section.

How to Make a Homemade Dynamo

You will need the following materials:

• A flat iron bar = six inches long, half mm in thickness,
• Super enamelled copper wire = 28 to 30 SWG, 25 meters approximately,
• Small magnet bar = square in shape, 1.5 square inch, half mm in thickness.
• Suitable spindle, clamp, wheel mechanism set-up as discussed in the text and in the diagram.
• Torch Bulb = 3 Volts

Procedure:

Bend the iron bar in “U” shape with dimensions as shown in the diagram.
Cover the horizontal portion of the “U” with a reasonably thick paper former or insulate it with some kind of PVC tape.
 

Wind the copper wire neatly and gently over the above-dressed section of the “U” channel, through uniform overlapping steps, until you have at least 6 inches of wire ends left for external connections.
 

Take the magnet and fix it (by glueing or some other suitable method) over a central metal rod and arrange the mechanism just as directed in the diagram.
 

Connect the coil ends to a small 3-volt torch bulb.
Now it’s just a matter of rotating the central rod/magnet assembly as fast as the mechanism permits.
 

If the winding and the mechanism specifications are perfectly optimised, it will instantly produce a nice glow over the filament of the bulb.
 

Your homemade DIY dynamo is ready.
 

However, the above make cannot be even close to a readymade dynamo as far as efficiency is concerned, so for our next main project, we would want to procure a good quality readymade bicycle dynamo.

Iron Carbon Phase Diagram

The iron carbon phase diagram shown in Fig 1 actually shows two diagrams i) the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop specially in the low temperature and low carbon range hence the metastable diagram is of more interest.

iron carbon phase diagram
Many of the basic features of this irpn carbon system also influence the behavior of alloy steels. For example, the phases available in the simple binary Fe-C system are also available in the alloy steels, but it is essential to examine the effects of the alloying elements on the formation and properties of these phases. The iron-carbon diagram provides a solid base on which to build the knowledge of both plain carbon and alloy steels.

There are some important metallurgical phases and micro constituents in thr iron carbon system. At the low-carbon end is the ferrite (?-iron) and austenite (?-iron). Ferrite can at most dissolve 0.028 wt% C at 727 deg C and austenite (?-iron) can dissolve 2.11 wt% C at 1148 deg C. At the carbon-rich side there is cementite (Fe3C).

iron carbon phase diagram
Between the single-phase fields are found regions with mixtures of two phases, such as ferrite & cementite, austenite & cementite, and ferrite & austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid & austenite, liquid & cementite, and liquid & ferrite. In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names that facilitate the understanding of the diagram.

Main micro-structures of iron and steels in equilibrium are
 
1. Austenite or ?-iron phase – Austenite is a high temperature phase and has a Face Centred Cubic (FCC) structure (which is a close packed structure). ?-iron is having good strength and toughness but it is unstable below 723 deg C.

2. Ferrite or ?-iron phase – It is relatively soft low temperature phase and is a stable equilibrium phase. Ferrite is a common constituent in steels and has a Body Centred Cubic (BCC) structure (which is less densely packed than FCC). ?-iron is soft , ductile and has low strength and good toughness.

3. Cementite – It is Fe3C or iron carbide. It is intermediate compound of Fe and C. It has a complex orthorhombic structure and is a metastable phase. It is hard, brittle and has low tensile strength, good compression strength and low toughness

4. Pearlite is the ferrite-cementite phase mixture. It has a characteristic appearance and can be treated as a micro structural entity or micro constituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates (“spheroidizes” or “coarsens”) into cementite particles dispersed with a ferrite matrix after extended holding below 723 deg C. It is a eutectoid and has BCC structure. It is a partially soluble solution of Fe and C. It has high strength and low toughness.

Bourdon Tube Pressure Gauge Diagram