md gear report

7/27/2019 Md Gear Report

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1. INTRODUCTIONA round or cylindrical mechanical component with teeth, used to transmit power, this is the

definition of gear. Gears are designed to mesh with one another and can alter the speed, torque,

or direction of mechanical energy. Gears are used in many mechanical devices. If we think of all

the gears that are used in an automobile, the applications are numerous. The most obvious is thetransmission, where the torque and speed from the engine are modified by reducing the speed in

the lower gears, which correspondingly increases the torque. The differential further reduces this

speed to the wheels, thereby also increasing the torque. Other gear mechanisms include electric

seats, which used electric motors and gears to move the seats both fore and aft and up and down,

or electric windows, where the rotational speed of motors is significantly reduced with a

corresponding increase in torque, which then typically drives a rack and pinion gear system to

push the windows up or down. In the engine, gears may be used for opening and closing valves

and for many others purposes.

Think of other mechanisms that involve the use of gears. Gears can be very small, such as those

used in paper drives in computer printers. They can be very large, such as those that lower or

rotate bridges to allow ships to pass by. From all this we can begin to understand the multitude of

application in which gearing systems are used. Because gear systems either increase or decrease

rotational velocity, they have the inverse effect on the torque being transmitted. The principle of

gear systems is that the power transmitted through a set of gears is constant, with the exception

of minor frictional losses. The ratio of gear system results in a change in the output rotational

speed, and the torque changes by the inverse of that same ratio. The rotational velocity and

torque are a product of either the number of teeth or the radii of the gears. The force between

mating teeth is always equal and opposite, with the surface velocity of the mating gear systems

correspondingly identical


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2. TYPE OF GEARS

1) Spur Gear

Spur Gear connects parallel shafts, have involute teeth that are parallel to the shaft and

can have internal or external teeth. They cause no external thrust between gears. They are

inexpensive to manufacture. They give lower but satisfactory performance. They are used

when shaft rotates in the same plane. The main features of spur gears are dedendum,

addendum, flank, and fillet. Dedendum cylinder is a root from where teeth extend, it

extends to the tip called the addendum circle. Flank or the face contacts the meshing gear,

the most useful feature if the spur gears. The fillet in the root region is kinetically

irrelevant.


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2) Helical gears

Helical gears connect parallel shifts but the involute teeth are cut at an angle to the axis of

rotation. Two mating helical gears must have equal helix angle but opposite hand. They

run smoother and more quietly. They have higher load capacity, are more expensive to

manufacture and create axial thrust. Helical gears can be used to mesh two shafts that are

not parallel and can also be used in a crossed gear mesh connecting two perpendicular

shafts. They have longer and strong teeth. They can carry heavy load because of the

greater surface contact with the teeth. The efficiency is also reduced because of longer

surface contact. The gearing is quieter with less vibration.


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3) Herringbone Gears

They conduct power and motion between non-intersecting, parallel axis that may or may

not have center groove with each group making two opposite helices. The two helix angle

come together in the center of the gear face to form a 'V'. in these gears the end thrust

forces cancel themselves out. Its difficult to cut this type of gear but its made easier by

machining a groove in the face at the point of the apex of the 'V' creating a break in the

middle of the herringbone gear teeth. They do not have any separating groove between

the mirrored halves. Action is equal in force and friction on both gears and all bearings.

Herringbone gear also allow for the use of larger diameter shaft for the same volumetric

displacement and higher differential pressure capability. The most common application is


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in power transmission. They utilize curved teeth for efficient, high capacity power

transmission. This offers reduced pulsation due to which they are highly used for

extrusion and polymerization. Herringbone gears are mostly used on heavy machinery.

4) Angular Bevel Gears

The shafts are set at an angle other than 90 degrees. They are useful when the direction of

a shaft's rotation needs to be changed. Using gears of differing numbers of teeth can

change the speed of rotation. These gears permit minor adjustment of gears during

assembly and allow for some displacement due to deflection under operating loads

without concentrating the load on the end of the tooth. For reliable performance, Gears

must be pinned to shaft with a dowel or taper pin. The bevel gears find its application in

locomotives, marine applications, automobiles, printing presses, cooling towers, power

plants, steel plants, defence and also in railway track inspection machine. They are

important components on all current rotorcraft drive system.


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5) Planetary Gear

Epicyclic gearing or planetary gearing is a gearsystem consisting of one or more outer

gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears

are mounted on a movable arm or carrier which itself may rotate relative to the sun gear.

Epicyclic gearing systems also incorporate the use of an outer ring gear or annulus, which

meshes with the planet gears. Planetary gears (or epicyclic gears) are typically classified

as simple and compound planetary gears. Simple planetary gears have one sun, one ring,

one carrier, and one planet set. Compound planetary gears involve one or more of the

following three types of structures: meshed-planet (there are at least two more planets in

mesh with each other in each planet train), stepped-planet (there exists a shaft connection

between two planets in each planet train), and multi-stage structures (the system contains

two or more planet sets). Compared to simple planetary gears, compound planetary gears

have the advantages of larger reduction ratio, higher torque-to-weight ratio, and more

flexible configurations. The axes of all gears are usually parallel, but for specia l cases

like pencil sharpeners they can be placed at an angle, introducing elements ofbevel

gear(see below). Further, the sun, planet carrier and annulus axes are usually coaxial.
http://en.wikipedia.org/wiki/Gearhttp://en.wikipedia.org/wiki/Pencil_sharpenershttp://en.wikipedia.org/wiki/Bevel_gearhttp://en.wikipedia.org/wiki/Bevel_gearhttp://en.wikipedia.org/wiki/Coaxialhttp://en.wikipedia.org/wiki/Coaxialhttp://en.wikipedia.org/wiki/Bevel_gearhttp://en.wikipedia.org/wiki/Bevel_gearhttp://en.wikipedia.org/wiki/Pencil_sharpenershttp://en.wikipedia.org/wiki/Gear


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3. MECHANISM OF PLANETARY GEARIt is often necessary to design gear trains that will provide many gear ratios. Of necessity,

these trains must not be too large and bulky. The answer to this requirement is planetary

gearing.

Notice the arrangement of the components in the set. Because of the resemblance of the

planetary pinions to the planets of the universe circling around the sun, the set was given the

name planetary gear set. The center or sun gear can be either a spur gear or helical gear. It will

contain a through shaft so that it can act as either an input or output member.

Normally, three planetary pinions are in mesh with the sun gear at all times. Some sets will have

two and others four. They are mounted on and are free to rotate on individual shafts on the planet

carrier, which is a framework designed to hold the pinions in their respective positions. The

planet carrier can be rotated so that the pinions walk around the sun gear. The carrier also

contains a shaft so that it may act as an input or output member. The outer internal gear is in


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constant mesh with the planet pinions and is called the ring gear. It can also be an input or output

member. The principle on which the planetary gear set operates is based on driving one unit,

holding one unit, and taking the output from the free unit. If we place a brake band around the

ring gear, we can prevent it from turning. If the sun gear is driven under this condition, it will

cause the planet pinions to rotate. With the ring gear held from turning, the planet pinions will

have to walk around on the inside of the ring gear and the outside of the sun gear. In doing so,

the planet pinions will carry the planet carrier around with them. If the planet carrier is held so

that it cannot rotate and the sun gear is driven, the planet pinions will force the ring gear to turn.

If the planet carrier is held and the ring gear is driven, the planet pinions will force the sun gear

to turn. If the sun gear is held and the planet carrier is driven, the planet pinions will be forced to

rotate and they will drive the ring gear. Actual use of planetary gears in such things as automatic

transmissions, disk clutches, and brake bands control the holding and driving members. Usually,

the bands and clutches are controlled automatically.

Operating principlesTo fully understand the movement of each member of a planetary system, let's consider a few

basic operating principles of a planetary gearset. If the planetary carrier and the sun gear are

held together, the pinions cannot turn because they are locked by the sun gear. This will

cause the unit to turn as one unit. None of its parts will turn by themselves. This will give us

direct drive just as if we had a one-piece shaft. If the sun gear is held and the planetary carrier

is turned, then the ring gear will turn. The pinions will "walk" around the sun gear because

the sun gear will not move. The pinions turn as they walk around the sun gear and are in

mesh with the ring gear; therefore, the ring gear is pushed by the turning pinion. The ring

gear will turn in the same direction as the carrier. If the ring gear is held and the sun gear is

turned, then the carrier will turn. The pinions are in mesh with the sun gear, and, when the

sun gear is turned, the pinions will turn. The pinions are also in mesh with the ring gear. With

the ring gear held, the pinions therefore walk around the ring gear. This causes the carrier to

turn with the pinions. If the carrier is held and the sun gear is turned, then the ring gear turns

in reverse. Because the pinions are in mesh with the sun gear, when the sun gear is turned,

the pinions also turn. However, the pinions turn in the opposite direction of the sun gear. The

pinions are also in mesh with the ring gear and drive the ring gear because the carrier is held


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so that it cannot turn. When an external gear is driving an internal gear, the direction of

turning is the same. Therefore, the planet pinions turning opposite from the input rotate the

ring gear in reverse.

There are five basic rules of planetary gear operation:

If the planet carrier is used as the output, the set operates in reduction (slower speed,moretorque).

If the planet carrier is the input, the set operates in overdrive (more speed, less torque). If the planet carrier is held, the set operates in reverse. If any two parts are locked together, the set operates in direct drive. If no parts are locked together and if none are held, the set operates in neutralHere are a few more things you should also remember.

An input member receives power from a source such as an engine. An output member transmits power to the driving wheels of a vehicle. A stationary member is one that is held by a band or clutch so that it cannot turn. Locked members are held together.

Speed ranges of a planetary setUsing the first three of the above rules, we can get six speed ranges. Remember, when we reduce

speed, we increase torque, and when we increase speed, we reduce torque.

If the sun gear is held and the planet carrier is turned, the ring gear will turn faster thanthe carrier

(overdrive). If the sun gear is held and the ring gear is turned, the planet carrier will turn slower than

the ring gear (reduction).


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If the ring gear is held and the sun gear is turned, the planet carrier turns slower than thesun gear (reduction).

If the planet carrier is held and the ring gear is turned, the sun gear turns in reverse fasterthan the ring gear (overdrive and reverse).

If the ring gear is held and the planet carrier is turned, the sun gear turns faster than thecarrier (overdrive).

If the planet carrier is held and the sun gear is turned, the ring gear turns in reverseslower than the sun gear (reduction and reverse)

4) PLANETARY GEAR RATIODenote R, S, and P as the number of teeth on the gears.

R = Number of teeth in ring gear

S = Number of teeth in sun (middle) gear

P = Number of teeth in planet gears

The first constraint for a planetary gear to work out is that all teeth have the same pitch, or

tooth spacing. This ensures that the teeth mesh. The second constraint is:

R = 2 P + S

That is to say, the number of teeth in the ring gear is equal to the number of teeth in the

middle sun gear plus twice the number of teeth in the planet gears. In the gear at left, this

would be 30 = 2 9 + 12This can be made more clear by imagining "gears" that just roll

(no teeth), and imagine an even number of planet gears. From the illustration at left, you

can see that the diameters of the sun gear, plus two planet gears be must equal to the ring

gear size.

This can be made more clear by imagining "gears" that just roll (no teeth), and imagine an

even number of planet gears. From the illustration at left, you can see that the diameters of

the sun gear, plus two planet gears be must equal to the ring gear size. Now imagine we

take out one of the green planet wheels, and rearrange the remaining ones to be evenly


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spaced. Still the same size gear. Now imagine the wheels have teeth. The teeth would stick

out beyond the line of the wheel as much as they indent, so that the pitch line of the gears

would be the line around the gears. The geometry still works the same. If you go into the

gear generator and select "show pitch diameter", you can see how the pitch diameter is just

a circle that the teeth are centered over. The pitch diameter of a gear is just the diametrical

pitch times the number of teeth. The gear generator program tends to refer to tooth

spacing, but diametrical pitch is just tooth spacing divided by 2 PI. (divide by

6.28318)Here's another planetary gear set. The middle arrangement is removed.

If it is inserted, the planet gears have 12 teeth, the sun gear has 18 and the ring gear has 42

teeth.

So, applying

R = 2P + S

And we get

42 = 2 12 + 18


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5) REFERENCESi. http://woodgears.ca/gear/planetary.html

ii. http://en.wikipedia.org/wiki/Epicyclic_gearingiii. http://auto.howstuffworks.com/automatic-transmission2.htmiv. http://woodgears.ca/reader/walters/planetary.htmlv. http://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20pap

er_Eric&Jessie.pdf
http://woodgears.ca/gear/planetary.htmlhttp://en.wikipedia.org/wiki/Epicyclic_gearinghttp://en.wikipedia.org/wiki/Epicyclic_gearinghttp://auto.howstuffworks.com/automatic-transmission2.htmhttp://auto.howstuffworks.com/automatic-transmission2.htmhttp://woodgears.ca/reader/walters/planetary.htmlhttp://woodgears.ca/reader/walters/planetary.htmlhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://www.cs.berkeley.edu/~sequin/CS285/2011_REPORTS/CS285%20final%20paper_Eric&Jessie.pdfhttp://woodgears.ca/reader/walters/planetary.htmlhttp://auto.howstuffworks.com/automatic-transmission2.htmhttp://en.wikipedia.org/wiki/Epicyclic_gearinghttp://woodgears.ca/gear/planetary.html

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