Mechanical Gears Essay, Research Paper

I-How Does Gears Work?

You see gears in just about everything that has spinning parts. For example, car engines and transmissions contain lots of gears. Gears are generally used for one of four different reasons:

1- To reverse the direction of rotation.

2- To increase or decrease the speed of rotation.

3- To move rotational motions to a different axis.

4- keep the rotation of two axis synchronized.

Most of the gears you see in real life have teeth. The teeth have three advantages:

1- They prevent slippage between the gears. Therefore axles connected by gears are always synchronized exactly with one another.

2- They make it possible to determine exact gear ratios you just count the number of teeth in the two gears and divide. So if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is 3:1.

3- They make it so that slight imperfections in the actual diameter and circumference* of two gears don t matter. The number of teeth controls the gear ratio even if the diameters are a bit off.

*Circumference of a circle is equal to the diameter of the circle multiplied by Pi (Pi is equal to 3.14159 .).

To create large gear ratios, gears are often connected together in gear trains as shown here:

The right-hand (green) gear in the train is actually made in two parts, as shown. A small gear and a larger gear are connected together, one on top of the other. Gear trains often consist of multiple gears in the train, as shown in the following two figures:

In the case above, the green gear turns at a rate twice that of the red gear. The blue gear turns at twice the rate as the green gear. The yellow gear turns at twice the rate as the blue. The gear train shown below has a higher gear ratio:

In this train, the smaller gears are 1/5th the size of the larger gears. That means that if you connect the green gear to a motor spinning at 100 RPMs (Revolution Per Minute), the blue gear will turn at a rate of 500 RPMs and the yellow gear will turn at a rate of 2,500 RPMs. In the same way, you could attach a 2,500 RPM motor to the yellow gear to get 100 RPMs on the green gear. If you can see inside your power meter and it is of the older style with 5 mechanical dials, you will see that the 5 dials are connected to one another through a gear train like this, with the gears having a ratio of 10:1. Because the dials are directly connected to one another, they spin in opposite directions (you will see that the numbers are reversed on dials next to one another).

There are many other ways to use gears. For example, you can use conical gears to bend the axis of rotation in a gear train by 90 degrees. The most common place to find conical gears like this is in the differential of a rear-wheel-drive car. A differential bends the rotation of the engine 90 degrees to drive the rear wheels:

Another specialized gear train is called a planetary gear train. Planetary gears solve the following problem. Let s say you want a gear ratio of 6:1. One way to create that ratio is with the following three-gear train:

In this train, the red gear has three times the diameter of the yellow gear, and the blue gear has two times the diameter of the red gear (giving a 6:1 ratio). However, imagine that you want the axis of the output gear to be the same as that of the input gear. A common place to need this same-axis capability is in an electric screwdriver. In that case you can use planetary gear system, as shown here:

In this gear system, the yellow gear engages all three red gears simultaneously. They are all three attached to a plate (green), and they engage the inside of the blue gear instead of the outside. Because there are three red gears instead of one this gear train is extremely rugged. The output shaft is taken from the green plate, and the blue gear is held stationary.

Finally, imagine the following situation: you have two red gears that you want to keep synchronized, but they are some distance apart. You can place a big gear between then if you want them to have the same directions of rotation:

Or you can use two equal-sized gears if you want them to have opposite rotational direction:

However, in both of these cases the extra gears are likely to be heavy and you need to create axles from them. In these cases the common solution is to use either a chain or a toothed belt, as shown:

II-Gear Materials

Gears are commonly made of steal, cast iron, bronze, or phenolic resins. Recently nylon, Teflon, and sintered iron have been used successfully. The great variety of materials available provides the designer with the opportunity of obtaining the optimum material for any particular requirement, whatever it be high strength, a long wear life, quietness of operation, or high reliability.

In many applications, steel is the only satisfactory material because it combines both high strength and low cost. Gears are made of both plain-carbon and alloy steels, and there is no one best material. In many cases the choice will depend upon the relative success of the heat-treating department with the various steels. When the gear is to be quenched and tempered, steel with 40 to 60 points of carbon is used. If it is to be casehardened, one with 20 points or less of carbon is used. The core as well as the surface properties must always be considered.

Cast iron is a very useful gear material because it has such good wear resistance. It is easy to cast and machine and transmits less noise than steel.

Bronzes maybe used for gears when corrosion is a problem, and they are quite useful for reducing friction and wear when the sliding velocity is high, as in worm-gear applications. The AGMA (American Gear Manufacturing Association) lists five tin bronzes containing small percentages of nickel, lead, or zinc which are suitable gear materials. Their hardness varies from 70 to 85 Bhn.

Nonmetallic gears are mated with steel or cast-iron gears to obtain the greatest load-carrying capacity. To secure goodwear resistance, the metal gear should have a hardness of at least 300 Bhn. A nonmetallic will carry almost as much load as a good cast-iron or mild steel gear, even though the strength is much lower, because of the low modulus of elasticity. This low modulus permits the nonmetallic gear to absorb the effects of tooth errors so that a dynamic lad is not created. A nonmetallic gear also has the important advantage of operating well on marginal lubrication.

Thermosetting laminates are widely used for gears. They are made of steel materials composed of a fibrous or woven material, together with a resin binder, or a cast. Both nylon and Teflon, as gear materials, have given excellent results in service.

III-Heat Treatment

Heat treatment is the controlled heating and cooling of metal for the purpose of altering their properties and can perform this function without a concurrent change in product shape. Because both physical and mechanical properties can be altered by heat treatment, it is one of the most important and widely used manufacturing processes.

So in this project I will talk about the heat treatment of steel because it is used mostly in gears.

1-Processing Heat Treatment for Steel

A number of process heat-treating operations are classified under the general term of annealing. These maybe employed to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size, reduce segregation, or alter the electrical or magnetic properties of the material. By producing a certain desired structure, characteristics can be imparted that are favorable subsequent operations or applications. The material being treated and objectives of the treatment determine the temperature, cooling rate, and specific details of the process. In the process of full annealing, hypoeutectoid steels (less than 0.77% carbon) are heated to 50 to 100|F (30 to 60|C) above the temperature of the boundary between austenite and ferrite + austenite, held for sufficient time to convert the structure to homogeneous single-phase austenite of uniform composition and temperature, and then slowly cooled at a controlled rate to below the temperature of the eutectoid line. Cooling in usually done in the furnace by decreasing the temperature by 20 to 50|F (10 to 30|C) per hour to at least 50|F (30|C) below the eutectoid line. At this point the metal can be removed from the furnace and air cooled to room temperature. The resulting structure is one of coarse pearlite (widely spaced lamellae) with excess ferrite in amounts predicted by the phase diagram. In this condition, the steel is quite soft and ductile.

The procedure to full-anneal a hypereutectoid alloy (greater than 0.77% carbon) is basically the same, except that the original heating is only into the austenite plus cementite region. If the material were to be slow cooled from the allaustenite region, a continuous network of cementite in dispersed spheroidal form.

2-Surface Hardening of Steel

Many products require different properties at different locations. Quite frequently, this variation takes the form of a hard, wear-resistant surface coupled with a tough, fracture-resistant core. The methods developed to produce such properties can generally be classified into three basic groups: selective heating of surface, altered surface chemistry, and deposition of an additional surface layer.

In this project I will talk about the selective heating techniques. If a steel has a sufficient carbon attain the desired surface hardness, generally greater that 0.3%, the different properties can be obtained simply by varying the thermal histories of the various regions. Maximum hardness depends on the carbon content of the material, while the depth of that hardness depends on the depth of heating and the material s hardenability.

Flame hardening uses high-intensity oxyacetylene flame to raise the surface temperature high enough to reform austenite. This region in then water quenched and tempered to the desired level of toughness. Heat input is quite rapid and is concentrated on the surface. Slow heat transfer and short heating times leave the interior at low temperature and therefore free from any significant changes.

Laser-beam hardening has been used to produce hardened surfaces on a wide variety of geometries. An absorptive coating such as zinc or manganese phosphate is often applied to the steel to improve the efficiency of converting light energy into heat. The surface is then scanned with the laser, where beam size, beam intensity, and scanning speed (often as high as 100 in./min) have been selected to obtain the desired amount of heat input and depth of heating. It is possible that the heat can be effectively removed through transfer into the cool, underlying metal (autoquenching), but a water oil quench is often used.

IV-The Forming of Gear Teeth

There are larger number of ways of forming the teeth of gears, such as sand casting, shell molding, investment casting, permanent-mold casting, die casting, and centrifugal casting. Teeth can be formed by using the powder-metallurgy process; or, by using extrusion, a single bar of aluminum may be formed and then sliced into gears. Gears which carry large loads in comparison with their size are usually made of steel and are cut with either form cutters or generating cutters. In form cutting, the tooth space takes the exact shape of the cutter. In generating, a tool having a shape different from the tooth profile is moved relative to the gear blank so as to obtain the proper tooth shape. One of the most promising of the methods of forming teeth is called cold forming, or cold rolling, in which dies are rolled against steel blanks to form the teeth. The mechanical properties of the metal are greatly improved by the rolling process, and a high-quality generated profile is obtained at the same time.

Milling

Gear teeth may be cut with a form-milling cutter shaped to conform to the tooth space. With this method it is theoretically necessary to use a different cutter for each gear, because a gear having 25 teeth. Actually, the change in space is not too great, and it has been gound that eight cutters may be used to cut with reasonable accuracy any gear in the range of 12 teeth to a rack. A separate set of cutters is, of course, required for each pitch.

Shaping

Teeth may be generated with either a pinion cutter or a rack cutter, the pinion cutter reciprocates along the vertical axis and is slowly fed into the gear blank to the required depth. When the pitch circles are tangent, both the cutter and blank rotate slightly after each cutting stroke, since each tooth of the cutter is a cutting tool, the teeth are all cut after the blank has completed one revolution.

The sides of an inviolate rack tooth are straight. For this reason, a rack-generating tool provides an accurate method of cutting gears teeth. In operation, the cutter reciprocates and is first fed into the gear blank until the pitch circles are tangent. Then, after each cutting stroke, The gear blank and cutter roll slightly on their pitch. When the blank and cutter have rolled a distance equal to the circular pitch, the cutter is returned to the starting point, and the process is continued until all the teeth have been cut.

V-Teeth Finishing

Gears which run at high speeds and transmit large forces may be subjected to additional dynamic forces due to errors in tooth profiles. These errors may be diminished somewhat by finishing the tooth profiles. The teeth may be finished, after cutting, either by shaving or burnishing. Several shaving machines are available which cut off a minute amount of metal, bringing the accuracy of the tooth profile within profile within the limits of 8 micro millimeter. Burnishing, like shaving, is used with gears which have been cut but not heat-treated. Grinding and lapping are used for hardened gear teeth after heat treatment.

References

1- Joseph Edward Shigley, Mechanical Engineering Design.

2- E. Paul Degarmo J T. Black Ronald A. Kohser, Materials and Processes in Manufacturing.

3- http://www.howstuffworks.com