Mechanisms and Simple Machines

Mechanism: the fundamental physical or chemical processes involved in or responsible for an action, reaction or other natural phenomenon.

Machine: an assemblage of parts that transmit forces, motion and energy in a predetermined manner.

Simple Machine: any of various elementary mechanisms having the elements of which all machines are composed. Included in this category are the lever, wheel and axle, pulley, inclined plane, wedge and the screw.

The word mechanism has many meanings. In kinematics, a mechanism is a means of transmitting, controlling, or constraining relative movement (Hunt 78). Movements which are electrically, magnetically, pneumatically operated are excluded from the concept of mechanism. The central theme for mechanisms is rigid bodies connected together by joints.

A machine is a combination of rigid or resistant bodies, formed and connected do that they move with definite relative motions and transmit force from the source of power to the resistance to be overcome. A machine has two functions: transmitting definite relative motion and transmitting force. These functions require strength and rigidity to transmit the forces.

The term mechanism is applied to the combination of geometrical bodies which constitute a machine or part of a machine. A mechanism may therefore be defined as a combination of rigid or resistant bodies, formed and connected so that they move with definite relative motions with respect to one another (Ham et al. 58).

Although a truly rigid body does not exist, many engineering components are rigid because their deformations and distortions are negligible in comparison with their relative movements.

The similarity between machines and mechanisms is that

* they are both combinations of rigid bodies
* the relative motion among the rigid bodies are definite.

The difference between machine and mechanism is that machines transform energy to do work, while mechanisms so not necessarily perform this function. The term machinery generally means machines and mechanisms. Figure 2-1 shows a picture of the main part of a diesel engine. The mechanism of its cylinder-link-crank parts is a slider-crank mechanism, as shown

Basic Engine Parts

The core of the engine is the cylinder, with the piston moving up and down inside the cylinder. The engine described above has one cylinder. That is typical of most lawn mowers, but most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer), as shown in the following figures.







Spark plug
The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can occur. The spark must happen at just the right moment for things to work properly.

Valves
The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

Piston
A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston rings
Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

* They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.
* They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.

Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly.

Connecting rod
The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates.

Crankshaft
The crankshaft turns the piston's up and down motion into circular motion just like a crank on a jack-in-the-box does.

Sump
The sump surrounds the crankshaft. It contains some amount of oil, which collects in the bottom of the sump (the oil pan).

HOW CAR ENGINES WORKS

Internal Combustion

The ­principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine!

Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in Figure 1. They are:

• Intake stroke
• Compression stroke
• Combustion stroke
• Exhaust stroke

1. The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure)
2. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)
3. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure)
4. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe. (Part 4 of the figure)

Now the engine is ready for the next cycle, so it intakes another charge of air and gas.

Notice that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear (straight line). In an engine the linear motion of the pistons is converted into rotational motion by the crankshaft. The rotational motion is nice because we plan to turn (rotate) the car's wheels with it anyway.

Now let's look at all the parts that work together to make this happen, starting with the cylinde

GEAR AND ITS PROFILES..

Gears

Gears are machine elements that transmit motion by means of successively engaging teeth. The gear teeth act like small LEVERS

Gear Classification

Gears may be classified according to the relative position of the axes of revolution. The axes may be

1. parallel,
2. intersecting,
3. neither parallel nor intersecting.

Here is a brief list of the common forms. We will discus

s each in more detail later.

• Gears for connecting parallel shafts
• Gears for connecting intersecting shafts
• Neither parallel nor intersecting shafts

Gears for connecting parallel shafts