Reverted Gear Train


A reverted gear train is very similar to a compound gear train. They are both used when there is only a small space between the input and output shafts and large changes in speed or power are needed.

There are two major differences between compound and reverted gear trains. First, the input and output shafts of a reverted train must be on the same axis (in a straight line with one another). Second, the distance between the centers of the two gears in each pair must be the same.
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Planetary or Epicyclic Gear Train Gear Train



Like a compound gear train, planetary trains are used when a large change in speed or power is needed across a small distance. There are four different ways that a planetary train can be hooked up.
A planetary gear train is a little more complex than other types of gear trains. In a planetary train at least one of the gears must revolve around another gear in the gear train. A planetary gear train is very much like our own solar system, and that's how it gets its name. In the solar system the planets revolve around the sun. Gravity holds them all together. In a planetary gear train the sun gear is at the center. A planet gear revolves around the sun gear. The system is held together by the planet carrier. In some planetary trains, more than one planet gear rotates around the sun gear. The system is then held together by an arm connecting the planet gears in combination with a ring gear.
The planetary gear set is the device that produces different gear ratios through the same set of gears. Any planetary gearset has three main components:
·         The sun gear
·         The planet gears  and the planet gears' carrier
·         The ring gear
Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset.
These four combinations and the resulting speed and power outputs are listed in Table
•         They have higher gear ratios.
•         They are popular for automatic transmissions in automobiles.
•         They are also used in bicycles for controlling power of pedaling automatically or manually.
•         They are also used for power train between internal combustion engine and an electric motor.





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Simple & Compound Gear Train


Simple Gear Train-

The simple gear train is used where there is a large distance to be covered between the input shaft and the output shaft. Each gear in a simple gear train is mounted on its own shaft.
When examining simple gear trains, it is necessaryto decide whether the output gear will turn faster, slower, or the same speed as the input gear. The circumference (distance around the outside edge) of these two gears will determine their relative speeds.
Suppose the input gear's circumference is larger than the output gear's circumference. The output gear will turn faster than the input gear. On the other hand, the input gear's circumference could be smaller than the output gear's circumference. In this case the output gear would turn more slowly than the input gear. If the input and output gears are exactly the same size, they will turn at the same speed.
In many simple gear trains there are several gears between the input gear and the output gear.
These middle gears are called idler gears. Idler gears do not affect the speed of the output gear.

Compound Gear Train-
In a compound gear train at least one of the shafts in the train must hold two gears.
Compound gear trains are used when large changes in speed or power output are needed and there is only a small space between the input and output shafts.
The number of shafts and direction of rotation of the input gear determine the direction of rotation of the output gear in a compound gear train. The train in Figure has two gears in between the input and output gears. These two gears are on one shaft. They rotate in the same direction and act like one gear. There are an odd number of gear shafts in this example. As a result, the input gear and output gear rotate in the same direction.
Since two pairs of gears are involved, their ratios are “compounded”, or multiplied together.
Example- The input gear, with 12 teeth, drives its mating gear on the counter-shaft, which has 24 teeth. This is a ratio of 2 to 1.
This ratio of DRIVEN over DRIVER at the Input - 2 to 1 - is then multiplied by the Output ratio, which has a DRIVEN to DRIVER ratio of 3 to 1.
This gives a gear ratio of 6 to 1 between the input and the output, resulting in a speed reduction and a corresponding increase in torque.


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Gear Train


A gear train is formed by mounting gears on 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; this provides a smooth transmission of rotation from one gear to the next.
•    A gear train is two or more gear working together by meshing their teeth and turning each other in a system to generate power and speed
•     It reduces speed and increases torque
•    Electric motors are used with the gear systems to reduce the speed and increase the torque

Types of gear train
§  Simple gear train
§  Compound gear train
§  Epicyclic gear train
§  Reverted gear train
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How does gear ratio affect Torque


First....What is torque?:

Torque is a twisting force- (it doesn't do any 'work' itself- it is simple an application of energy). 

Work (or 'stuff') happens, when torque is applied and movement occurs.
"Torque is a force that tends to rotate or turn things. You generate a torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut. 

English units of torque are pound-inches or pound-feet; the SI unit is the Newton-meter. Notice that the torque units contain a distance and a force. To calculate the torque, you just multiply the force by the distance from the center. In the case of lug nuts, if the wrench is a foot long, and you put 200 pounds of force on it, you are generating 200 pound-feet of torque. If you use a two-foot wrench, you only need to put 100 pounds of force on it to generate the same torque." 

In summary:
Torque equals Force multiplied by Distance

How does gear ratio affect Torque?
Simply put, torque at work (such as at a wheel) is your motor's torque times your gear ratio.
Motor Torque x gear ratio = torque at the wheel
Lets say we have a 10rmps motor that is capable of 5 oz Torque (we know this from our motor spec.)

Lets say we have 2 gears. Our input gear (attached to our motor) has 10 teeth Our output gear has 50 teeth

Our Gear ratio is 5:1

Motor Torque x gear ratio = torque at the wheel

5oz x 5:1 = 25 oz

What if our gear ratio were 1:3 ?

5oz x 1:3 = 1.6oz

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How does a gear ratio affect speed


The gear ratio tells us how fast one gear is rotating when compared to another.

If our input gear (10 teeth) is rotating at 5 rpms , and it is connected to our output gear (50 teeth), our output gear will rotate at 1 rpms. 

Why?
Our gear ratio is 50:10... or 5:1

If our small gear rotates 1x, our large gear only rotates 1/5. It takes 5 rotations of our small gear to = 1 rotation of our large gear. Thus our large gear is rotating at 1/5 the speed = 1rmp. 

What if our gear ratio where 1:3 ?
In this case our input gear is 3x larger as large as our output gear. 

If our input gear were rotating at 20rmps.... each rotation, would result in 3 rotations of our output gear. Our output would be 60rpms.
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Gear Ratio


The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear train.
In simple words, gear ratio defines the relationship between multiple gears.

Gear Ratio= Output gear # teeth / Input gear # teeth

For example, if our motor is attached to a gear with 60 teeth and this gear is then attached to a gear with 20 teeth that drives a wheel, our gear ratio is 60:20, or more accurately 3:1

If you do not want to count a gears teeth (or if they do not exist), gear ratio's can also be determined by measuring the distance between the center of each gear to the point of contact. 

For example, if our motor is attached to a gear with a 1" diameter and this gear is connected to a gear with a 2" diameter attached to a wheel, 

From the center to edge of our input gear is 0.5"
From the center to edge of our output gear is 1"
Our ratio is 1/0.5 or more accurately 2:1
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Use of Gear & Advantage of teeth on gear


Use of Gears-
  1. To reverse the direction of rotation
  2. To increase or decrease the speed of rotation
  3. To move rotational motion to a different axis
  4. To keep the rotation of two axis synchronized

Advantages of Teeth-
  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 gear ratio is controlled by the number of teeth even if the diameters are a bit off.
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Terminology of Spur Gear


  • Pitch surface : The surface of the imaginary rolling cylinder (cone, etc.) that the toothed gear may be considered to replace.
  • Pitch circle: A right section of the pitch surface.
  • Addendum circle: A circle bounding the ends of the teeth, in a right section of the gear.
  • Root (or dedendum) circle: The circle bounding the spaces between the teeth, in a right section of the gear.
  • Addendum: The radial distance between the pitch circle and the addendum circle.
  • Dedendum: The radial distance between the pitch circle and the root circle.
  • Clearance: The difference between the dedendum of one gear and the addendum of the mating gear.
  • Face of a tooth: That part of the tooth surface lying outside the pitch surface.
  • Flank of a tooth: The part of the tooth surface lying inside the pitch surface.
  • Circular thickness (also called the tooth thickness) : The thickness of the tooth measured on the pitch circle. It is the length of an arc and not the length of a straight line.
  • Tooth space: The distance between adjacent teeth measured on the pitch circle.
  • Backlash: The difference between the circle thickness of one gear and the tooth space of the mating gear.
Backlash =Space width – Tooth thickness
  • Circular pitch p: The width of a tooth and a space, measured on the pitch circle.
  • Diametral pitch P: The number of teeth of a gear per inch of its pitch diameter. A toothed gear must have an integral number of teeth. The circular pitch, therefore, equals the pitch circumference divided by the number of teeth. The diametral pitch is, by definition, the number of teeth divided by the pitch diameter.
  • Module m: Pitch diameter divided by number of teeth. The pitch diameter is usually specified in inches or millimeters; in the former case the module is the inverse of diametral pitch.
  • Fillet : The small radius that connects the profile of a tooth to the root circle.
  • Pinion: The smaller of any pair of mating gears. The larger of the pair is called simply the gear.
  • Velocity ratio: The ratio of the number of revolutions of the driving (or input) gear to the number of revolutions of the driven (or output) gear, in a unit of time.
  • Pitch point: The point of tangency of the pitch circles of a pair of mating gears.
  • Common tangent: The line tangent to the pitch circle at the pitch point.
  • Base circle : An imaginary circle used in involute gearing to generate the involutes that form the tooth profiles.
·         Line of Action or Pressure Line: The force, which the driving tooth exerts at point of contact of the two teeth. This line is also the common tangent at the point of contact of the mating gears and is known as the line of action or the pressure line. The component of the force along the common tangent at the p point is responsible for the power transmission.
The component of the force perpendicular to the common tangent through the pitch point produces the required thrust.
·         Pressure Angle or Angle of Obliquity (φ): The angle between pressure line and the common tangent to the pitch circles is known as the pressure angle or the angle of obliquity.
For more power ‘transmission and lesser pressure on the bearing pressure angle must be kept small. Standard pressure angles arc and 25°. Gears with 14.5° pressure angles have become almost obsolete.
·         Path of Contact or Contact Length: Locus of the point of contact between two mating teeth from the beginning of engagement to the end is known as the path of contact or the contact length. It is CD in the figure. Pitch point P is always one point on the path of contact. It can be subdivided as follows:
Path of Approach: Portion of the path of contact from the beginning of engagement to the pitch point, i.e. the length CP.
Path of Recess: Portion of the path of contact from the pitch point to the end of engagement i.e. length PD.
·         Arc of Contact: Locus of a point on the pitch circle from the beginning to the
end of engagement of two mating gears is known as the arc of contact in fig. 3.22, APB
or EPF is the arc of contact. It has also been divided into sub-portions.
Arc of Approach: It is the portion of the arc of contact from the beginning of engagement to the pitch point, i.e. length AP or EP.
Arc of Recess: Portion of the arc of contact from the pitch point to the end of engagement is the arc of recess i.e. length PB or PF.
·       Angle of Action (δ): It is the angle turned by a gear from the beginning of engagement to the end of engagement of a pair of teeth i.e. the angle turned by arcs of contact of respective gear wheels. Similarly, angle of approach (a) and angle of recess (β) can be defined.
S=a+ β
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Types of Gear


  1. Spur Gear
  2. Helical Gear
  3. Herringbone Gear
  4. Bevel Gear
  5. Worm Gear
  6. Rack and Pinion
  7. Internal and External Gear
  8. Face Gear
  9. Sprcokets
    1) Spur Gear-Parallel and co-planer shafts connected by gears are called spur gears. The arrangement is called spur gearing.

Spur gears have straight teeth and are parallel to the axis of the wheel. Spur gears are the most common type of gears. The advantages of spur gears are their simplicity in design, economy of manufacture and maintenance, and absence of end thrust. They impose only radial loads on the bearings.

Spur gears are known as slow speed gears. If noise is not a serious design problem, spur gears can be used at almost any speed.


2)     Helical Gear-Helical gears have their teeth inclined to the axis of the shafts in the form of a helix, hence the name helical gears.

These gears are usually thought of as high speed gears. Helical gears can take higher loads than similarly sized spur gears. The motion of helical gears is smoother and quieter than the motion of spur gears.

Single helical gears impose both radial loads and thrust loads on their bearings and so require the use of thrust bearings. The angle of the helix on both the gear and the must be same in magnitude but opposite in direction, i.e., a right hand pinion meshes with a left hand gear.

3)   Herringbone Gear - Herringbone gears resemble two helical gears that have been placed side by side. They are often referred to as "double helicals". In the double helical gears arrangement, the thrusts are counter-balanced. In such double helical gears there is no thrust loading on the bearings.

4)  Bevel/Miter Gear-Intersecting but coplanar shafts connected by gears are called bevel gears. This arrangement is known as bevel gearing. Straight bevel gears can be used on shafts at any angle, but right angle is the most common. Bevel Gears have conical blanks. The teeth of straight bevel gears are tapered in both thickness and tooth height. 

Spiral Bevel gears: In these Spiral Bevel gears, the teeth are oblique. Spiral Bevel gears are quieter and can take up more load as compared to straight bevel gears.

Zero Bevel gear: Zero Bevel gears are similar to straight bevel gears, but their teeth are curved lengthwise. These curved teeth of zero bevel gears are arranged in a manner that the effective spiral angle is zero.

5)      Worm Gear- Worm gears are used to transmit power at 90° and where high reductions are required. The axes of worm gears shafts cross in space. The shafts of worm gears lie in parallel planes and may be skewed at any angle between zero and a right angle.In worm gears, one gear has screw threads. Due to this, worm gears are quiet, vibration free and give a smooth output.Worm gears and worm gear shafts are almost invariably at right angles.

6)      Rack and Pinion- A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Torque can be converted to linear force by meshing a rack with a pinion: the pinion turns; the rack moves in a straight line. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s). Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack (infinite radius), and the tooth shapes for gears of particular actual radii then derived from that. The rack and pinion gear type is employed in a rack railway.

7)      Internal & External Gear- An external gear is one with the teeth formed on the outer surface of a cylinder or cone. Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or cone. For bevel gears, an internal gear is one with the pitch angle exceeding 90 degrees. Internal gears do not cause direction reversal.

8)      Face Gears- Face gears transmit power at (usually) right angles in a circular motion. Face gears are not very common in industrial application.

9)      Sprockets-Sprockets are used to run chains or belts. They are typically used in conveyor systems.

Gears may also be classified according to the position of axis of shaft:
a.Parallel
  1.Spur Gear
  2.Helical Gear
  3.Rack and Pinion
b. Intersecting
  Bevel Gear
c. Non-intersecting and Non-parallel
  worm and worm gears




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Gear Parameters


1)      Number of teeth
2)      Form of teeth
3)      Size of teeth
4)      Face width of teeth
5)      Style and dimension of gear blank
6)      Design of the hub of the gear
7)      Degree of precision required
8)      Means of attaching the gear to the shaft
9)      Means of locating the gear axially to the shaft
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