Types of Flat Belt Drives

Types of Flat Belt Drives

                                       

The power from one pulley to another may be transmitted by any of the following types of belt drives:

1. Open belt drive. The open belt drive, as shown in Fig. 11.3, is used with shaftsarranged parallel and rotating in the same direction. In this case, the driver A pulls the belt from one side (i.e. lower side RQ) and delivers it to the other side (i.e. upper side LM). Thus the tension in the lower side belt will be more than that in the upper side belt.The lower side belt (because of more tension) is known as tight side whereas the upper side belt (because of less tension) is known as slack side, as shown in Fig.

2. Crossed or twist belt drive. The crossed or twist belt drive, as shown in Fig. 11.4, isused with shafts arranged parallel and rotating in the opposite directions. In this case, the driver pulls the belt from one side (i.e. RQ) and delivers it to the other side (i.e. LM). Thus the tension in the belt RQ will be more than that in the belt LM. The belt RQ(because of more tension) is known as tight side, whereas the belt LM (because of less tension) is known as slack side, as shown in Fig.

A little consideration will show that at a point where the belt crosses, it rubs against each other and there will be excessive wear and tear. In order to avoid this, the shafts should be placed at a maximum distance of 20 b, where b is the width of belt and the speed of the belt should be less than 15 m/s.

3. Quarter turn belt drive. The quarter turn belt drive also known as right angle beltdrive, as shown in Fig. (a), is used with shafts arranged at right angles and rotating in one definite direction. In order to prevent the belt from leaving the pulley, the width of the face of the pulley should be greater or equal to 1.4 b, where b is the width of belt. In case the pulleys cannot be arranged, as shown in Fig. (a), or when the reversible is desired, then a quarter turn belt drive with guide pulley, as shown in Fig. (b), may be used.

4. Belt drive with idler pulleys. A belt drive with an idler pulley, as shown in Fig. (a), isused with shafts arranged parallel and when an open belt drive cannot be used due to small angle of contact on the smaller pulley. This type of drive is provided to obtain high velocity ratio and when the required belt tension cannot be obtained by other means.

When it is desired to transmit motion from one shaft to several shafts, all arranged in parallel, a belt drive with many idler pulleys, as shown in Fig. (b), may be employed.

5. Compound belt drive. A compound belt drive, as shown in Fig., is used when power is transmitted from one shaft to another through a number of pulleys.

6. Stepped or cone pulley drive. A stepped or cone pulley drive, as shown in Fig, is usedfor changing the speed of the driven shaft while the main or driving shaft runs at constant speed. This is accomplished by shifting the belt from one part of the steps to the other.

7. Fast and loose pulley drive. A fast and loose pulley drive, as shown in Fig., is usedwhen the driven or machine shaft is to be started or stopped when ever desired without interfering with the driving shaft. A pulley which is keyed to the machine shaft is called fast pulley and runs at the same speed as that of machine shaft. A loose pulley runs freely over the machine shaft and is incapable of transmitting any power. When the driven shaft is required to be stopped, the belt is pushed on to the loose pulley by means of sliding bar having belt forks.

Governing of IC Engines

The process of providing any arrangement, which will keep the engine speed constant (according to the changing load conditions) is known as governing of I.C. engines. Though there are many methods for the governing of I.C. engines, yet the following are important :

1. Hit and miss governing. In this system of governing, whenever the engine starts running at higher speed (due to decreased load), some explosions are omitted or missed. This is done with the help of a centrifugal governor. This method of governing is widely used for I. C. engines of smaller capacity or gas engines.

2. Qualitative governing. In this system of governing, a control valve is fitted in the fuel delivery pipe, which controls the quantity of fuel to be mixed in the charge. The movement of control valve is regulated by the centrifugal governor through rack and pinion arrangement.

3. Quantitative governing. In this system ofgoverliing, the quality of charge (i.e. air-fuel ratio of the mixture) is kept constant. But the quantity of mixture supplied to the engine cylinder is varied by means of a throttle valve which is regulated by the centrifugal governor through rack and pinion arrangement.

4. Combination system of governing. In this system of governing, the qualitative and quantitative methods of governing are combined together.

Jigs vs Fixtures

                                                          

The terms “jig" and “fixture" are many times referred as the synonyms of each other, sometimes both the terms are used together as “jig fixture." Although, both jig and fixture are used in mass production process, functionally the two are different tools.

What is a Jig?

In simple terms, the jig is a tool that guides the cutting (or machining) tool. The most common type of jig is the drill jig, which guides the drill bit for creating holes at desired locations. Using drill jigs increases production rate drastically by eliminating the time spent using a square scriber, height gauge, centre punch, etc. The picture below shows the functionality of a simple drill jig:

Like drill jigs, welding jigs and woodworking jigs are also used in industry quite extensively. Woodworking jigs are useful for creating intricate wooden profiles.

What is a Fixture?

The fixture is a tool which holds the workpiece with the machine bed precisely at the desired location. The fixture also reduces the nonproductive loading, unloading, and fixing time of the workpiece. For example, you need to use a milling machine for giving a chamfer at the corner of rectangular work pieces. You can use a vice to hold it in the desired position, but in that case every new work piece will take lots of time for fixing it. On the other hand if you can make a milling fixture like the one shown below and bolt the fixture to the milling machine bed, then you need not waste much time for fixing the work pieces every time:

You just place the workpiece and it will automatically aligned to the required angle, and straight away you run the machining operation, no need to measure the angle, and no need to be worried about the accuracy.

Conclusion

Both the jigs and the fixtures are used to reduce the nonproductive time of any mass production process. The principle of location or the 3-2-1 principle, CAD tools (like ProE), and FEA tools (like ANSYS) are used for the design of the jigs as well the fixtures. The jig is used for guiding the cutting tool (like a drill bit), and for doing so, jigs have components like a bush, which comes in contact with the cutting tool. On the other hand, a fixture never comes in direct contact with the cutting tool. Fixtures assure the position and alignment of the work pieces for getting the required machining operation done.

What Is Thermal Equilibrium

What Is Thermal Equilibrium

When there are variations in temperature from point to point of an isolated system. the temperature at every point first changes with time. This rate of change decreases and eventually stops. When no further changes are observed, the system is said to be in thermal equilibrium.

Hydraulic Turbines

Hydraulic Turbines

A hydraulic turbine is a machine which converts the hydraulic energy into mechanical energy. The hydraulic turbines are also known as water turbines. Following two types of hydraulic turbines are important.

1.     Impulse turbine

2.     Reaction turbine

In an impulse turbine, the total energy at the inlet of a turbine is only kinetic energy. The pressure of water both at entering and leaving the vanes is atmospheric. It is used for high head of water. A Pelton wheel is a tangential flow impulse turbine.

In a reaction turbine, the total energy at the inlet of a turbine is kinetic energy as well as pressure energy. It is used for low head of water. The Francis and Kaplan turbines are inward flow and axial flow reaction turbines respectively.

Buoyancy and buoyant force

When a body is immersed wholly or partially in a liquid, it is lifted up by a force equal to the weight of liquid displaced by the body. This statement is known as Archimede's principle.

The tendency of a liquid to uplift an immersed body, because of the upward thrust of the liquid, is known as buoyancy. The force tending to lift up the body is called the force of buoyancy or buoyant force and it is equal to the weight of the liquid displaced. The point through which the buoyant force is supposed to act, is known as centre of buoyancy. It may be noted that

●      If the force of buoyancy is more than the weight of the liquid displaced, then the body will float.

●      If the force of buoyancy is less than the weight of the liquid displaced, then the body will sink down.

Different Types of Energies or Head of a Liquid in Motion

Different Types of Energies or Head of a Liquid in Motion. The following are the three types of energies or head of flowing liquids:

• 1. Potential energy or potential head. It is due to the position above some suitable datum line. It is denoted by z.
• 2. Kinetic energy or kinetic (or velocity) head. It is due to the velocity of flowing liquid. Its value is given by v2/ 2g, where v is the velocity of flow and g is the acceleration due to gravity.
• 3. Pressure energy or pressure head. It is due to the pressure of liquid. Its value is given by p/w, where p is the pressure in N / m2 and w is the weight density of the liquid in N / m3.

Note: The total energy or total head of a liquid particle in motion is given as follows :

Total energy, E = Potential energy + Kinetic energy + Pressure energy

and

• Total head, H = Potential head + Kinetic head + Pressure head