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Friday, 29 December 2017

GEAR TRAIN | TYPES OF GEAR TRAINS

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          We know that in every machine, motion is transmitted from one shaft to another. This is done by using Gears, Coupling etc., To transmit rotory motion a set of gears are used. "Gear train is a combination of gears which are used to transmit rotory motion from one shaft to another shaft". A gear train is constructed in a frame by mounting gears so that the teeth of gears passing away rom each other (Without slipping); It provides a smooth transition of rotation from one gear to another. 

--> A gear train is two or a lot of gears operating along by meshing their teeeth and turning each other to produce power and speed.
--> It increases Torque and reduces Speed and viceversa.

Types of gear train

§   Simple Gear Train
§   Compound Gear Train
§   Epicyclic Gear Train
§   Reverted Gear Train

Simple gear train :-
          By the name itself we are able to understand that it's a simple arrangement of gears. When there is just one gear is mounted on each shaft (or its own shaft) then this kind of gear train is termed as "Simple gear train".
         The relative speed of gear train is determined by the two gears circumference. It's necessary to come to a decision, the driven gear speed will be more, less or same as the drive gear before we tend to implement simple gear train. Simple gear trains are used to transmit motion from input shaft to output shaft wherever they're massive distance apart. When the gap between the input and output shaft is a smaller amount and up to the sum of centre distances of gears then the two gears 1 and 2 are made to mesh with one another to transmit power from input shaft to output shaft.
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          Some times, the gap between the input and output shaft is more than the sum of centre distances of gears then the motion is transmitted by using any number of intermediate gears, these intermediate gears are referred to "Idler Gears". Idlers gears speed dont have an effect on the speed of driven shaft. When even number of gears are present in gear train, the motion of driven gear is always opposite to the drive gear.
          The ratio of gear train is defined as, "it is the ratio of the angular velocity of  driver (Input gear) to the angular velocity of driven gear (Output gear)". If 'n' number of gears are present in a simple gear train then they are numbered from the driving gear to driven gear as 1, 2, 3 ...; their rotational speeds as N1, N2, N3.... and their number of teeth as T1, T2, T3 ... respectively. Then the gear ratio is represented as

Compound gear train :-
          Compound gear train is outlined as, when there is more than one gear (atleast 2) is rigidly mounted on same shaft then that kind of gear train is termed as Compound gear train. So that pairs have same speed (rpm).
         When there's a desire of huge changes in speed or power output and there is less space between input and output shafts then this kind of gear trains is employed. Both the direction of drive gear and number of shafts are using in gear train will determine the rotation direction of driven gear shaft.
Image Courtesy: Google
          Consider a compound gear train with 6 gears and 4 shafts as shown in Fig., Then the gear transmission ratio of compound gear train is

Speed ratio of meshing gears 1 & 2 is 
similarly speed ratios of (3 & 4) and (5 & 6 ) 
Where
                       N2 = N3 and N4 = N5           (Since they are on same shafts)             
Multipley all above equations,
We get
  
Planetary gear train :-
          Just like compound gears, planetary gears are used when there is a desire of huge changes in speed or power output across small distances.
          When compared to other gear trains the arrangement of compound gear train is more complex planetary gear train is similar to our planet system, that's the reason it got the name. In our solar system, sun present at the centre and planets revolve around the sun. Gravitational force holds them in their places.
          Basically in any planetary gear set consists three main components or gears they are: 
               .       The sun gear
               .       The planet gears     
               .       The ring gear
          The sun gear is present at the centre and the planet gears revolve around the sun gear. These gears are held together by ring gear. Based on system one or more than one planet gear rotates around the sun gear. Sometimes connecting arm is used in between planetary gears for holding them together.

Image Courtesy : Google
          Unlike ordinary gear trains, an epicyclic gear trains is used to define more than one input to obtain a specific output i.e., Each of these three gears can be the input, the output or can be held stationary. So analysis is a little harder and more natural. Compared to other gear trains, there are many gear ratios obtained from less space. 

These four combibations and the resulting speed and power outputs are listed in Table
Image Courtesy: Google

Reverted gear train :-
          A Reverted gear train is extremely just like a compound gear train. They're both used when there's a desire of huge changes in speed or power output but hte space for accommodating several gears is restricted.
Image Courtesy : Google
          A reverted gear train is also a compound gear train, having the input and output shafts on the same axis (Co-axial). If these two gears are mounted on the same shaft, one of them must be loosely mounted. And also the distance between the centres of the two gears in each pair must be the same. 
          The gear transmission ratio is calculated similarly like compound gear train.

Tuesday, 26 December 2017

GEAR TRAIN | WHAT IS MEANT BY GEAR TRAIN?

Image Coutesy : Google
          We know that in every machine, motion is transmitted from one shaft to another. This is done by using gears, couplings etc., To transmit rotary motion a set of gears are used. Gear train is a combination of gears which are used to transmit rotory motion from one shaft to another shaft. A gear train is constructed in a frame by mounting gears so that the teeth of the gears engage. Gear teeth are designed to prevent the pitch circles of gears passing away from each other (without slipping); It provides a smooth transition of rotation from one gear to another.

•    A gear train is two or a lot of gears operating along by meshing their teeth and turning each other to produce power and speed

•     It increases torque and reduces speed and viceversa

How does gear ratio affect Torque?

         While moving into this topic, first of all we have to recall some topics. They are Gear ratio , Speed ratio, torque etc.,

Image Courtesy : Google

Gear ratio :
          Gear ratio is found by "dividing the number of teeth in the output gear (gear being driven) by the number of teeth in the driving gear". In simple words, gear ratio defines the relationhip between the number of teeth of multiple gears.
                    Gear Ratio = Output gear #teeth/ Input Gear # teeth 
Speed Ratio:
         The ratio of gear train is defined as, "it is the ratio of the angular velocity of the driver (Input gear) to the angular velocity of driven gear (Output gear)", it is alo called speed ratio of gear train.
Gear ratio is directly proportional to the number of teeth of gear, so
Torque:
         "Torque is the force that tends to turn or rotate objects about its axis or fulcrum or pivot".
          "To calculate the torque, we just multiply the force by the perpendicular distance from an axis (Fulcrum or Pivot) to the line of action of force (moment arm)"
                                 Torque = Force multiplied by Distance
                                        τ = F X D
         From the above eqation we can say that the torque acting on the gear wheel 1 is the multiplication value of force (F1) acting on gear wheel 1 and the distance of application of force (Teeth end of gear) D1.
         When gear wheel 1 is rotating it transfer the force to gear wheel 2. so the force is same on both the gears F1 = F2.
                                       F1 = τ1/D1 = F2
The torque in gear 2 is τ2  = F2 X D2. 
                                       τ2  = (τ1/D1) X D2           (since F2 =  τ1/D1 F1 )

                                         Since D2 > D1
                                                     T2 > T1

Case 1: If Gear 1 and 2 will have same radii and same number of teeth then there will no gain in turning effect (Torque) nor speed. 
Image Courtesy : Google

Case 2: If Gear 1 has smaller radious and lesser number of teeth than Gear 2 then Gear 2 will gain Torque but looses number of rotation (rpm). 
         This is the principle used by cyclist by going up hill. When they go uphill they need to apply a turning effect in this case they change the bicycle chain from a gear having smaller radius to gear having having large radius with more number of teeth by this they loose speed but they gain torque, this will help them to go uphill.


By this we can say that Gear ratio will effect the Torque.   

how does gear ratio affect speed?

Image Courtesy : Google
          The Gear ratio tells us how briskly one gear is rotating compared to different when two or more gears are interlocked. In general, when we connect two gears of different dimensions, consider the drive gear (which is connected to motor) is larger than driven gear then the driven gear rotate more quickely, viceversa.

Determining gear ratio of gear train:
          To form a gear train atleast two gears must be enguaged with each other, the one which is attached to the motor drive is called drive gear, and the which is attached to the load shaft is called driven gear. To transmit power some times we use any number of gears between the driver and driven gears. These gears are called "Idler gear".
         To findout the gear ratio take two gears of different dimensions, count the number of teeth on both driver and driven gear manually or some times this information is present on gears itself.
          Let us say that, our drive gear (Input gear) have 20 teeth and driven gear (Output gear) having 50 teeth. divide the output gear teeth by drive gear to determine the Gear ratio. Based on teeth number we may get gear ratio as a decimal, a fraction or in ratio form. In this case, divide 50 teeth of driven by 20 teeth of driving gear, it gives us the gear ratio as 50/20 = 2.5 (or) 5/2 (or) 2.5:1.
         This shows that if our input gear (having 20 teeth) is to rotate two anf half rotations to get one complete revolution of driven gear.
Determining speed of gear train:
          If our input gear (having 20 teeth) is rotating at 100 rpm, and it is connected to our output gear (50 teeth), based on this we can say that our output gear will rotate at 40 rpm.
Why?
Our gear ratio is T2:T1 = 50:20 ..... Or 2.5:1
          If our smaller gear rotates 100 rpm, our large gear solely rotates 5/2. It takes 2.5 rotations of our small gear to 1 complete rotation of our large gear. So our large gear is rotating at 5/2 the speed i.e., 40 rpm. If the number of teeth of smaller gear increases the speed of bigger gear decreases and viceversa.

What if our gear ratio where 1:4?
          In the above case our input gear is 4x larger as our output gear. If our input gear is rotating at 20 rpm ... each rotation, would result in 4 rotations of our output gear. Thereby our output becomes 80 rpm. 

By this we can say that Gear Ratio will affect the Speed of a gear train.

Monday, 25 December 2017

GEAR RATIO | SPEED RATIO


Image Courtesy : Google
          The gear ratio is also necessarry to consider when selcting the gears for any system. The larger the gear ratio, the greater the difference in gear speed and torque. The ratio of gear train is defined as, "it is the ratio of the angular velocity of the driver (Input gear) to the angular velocity of driven gear (Output gear)", it is alo called speed ratio of gear train. The gear ratio can be computed directlly from the number of number of teeth of the various gears that engauge to form the gear train. 
          Gear ratio is found by "dividing the number of teeth in the output gear (gear being driven) by the number of teeth in the driving gear". In simple words, gear ratio defines the relationhip between the number of teeth of multiple gears.
                    Gear Ratio = Output gear #teeth/ Input Gear # teeth
Image Courtesy : Google
          For example, consider a motor is connected to a gear A with 70 number of teeth and this gear is then connected to anothe gear B with 40 number of teeth, that drives a wheel, then our gear ratio is 70:40 or more accurately 7:4.
          In the above example, if Gear A was driving the system, the gear ratio would be 7/4. If Gear B was driving the system the gear ratio would be 4/7.
          If you can't count the gear teeth (or if they do not exist), gear ratio's may also be calculated by measuring the distance between the center of every gear to the point of contact.
          For example, if a motor is connected to a gear with 1" (1 inch) diameter and this gear is connected to the a gear with 2" diameter connected 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 gear ratio is 1/0.5 or more accurately 2:1.
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Sunday, 24 December 2017

Terminology of spur gear

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Gear: Gear is the larger of two mating gears.
Pinion: A pinion is a smaller of the two mating gears.
Pitch surfaceAn imaginary surface passing through the teeth of one cogwheel, pinion, rack etc., so as to touch the corresponding surface in another, without sliding, when the two components are geared together.
Pitch circleIt is the imaginary circle that the mating gears roll without slipping. It is a right section of the gear's pitch surface.
Addendum circle: The addendum circle is an imaginary circle that bounding the tops of the teeth, in a right section of gear.
Root (or dedendum) circle: The Dedendum circle is an imaginary circle that bounding the bottom of spaces between the teeth, in a right section of gear.
Addendum: The Addendum is radial distance between the pitch circle and addendum circle. It gives the height of the gear teeth above the pitch circle.
Dedendum: The Dedendum is radial distance between the pitch circle and Dedendum circle. It gives the depth of the gear teeth below the pitch circle.
Clearance: The clearance is the amount of difference between the dedendum of one gear and the addendum of the mating gear.
Face of tooth: The working surface of gear tooth, between the pitch circle and addendum circle. 
Face width: The width of the tooth parallel to the gear axis.
Flank of tooth: The surface of gear tooth, between the pitch circle and dedendum circle. 
Tooth thickness (also called the circular thickness): The thickness of tooth measured along the length of an arc following pitch circle. It is not the length of the straight line, it is the length of an arc.
Pitch: It is the distance between a point on one tooth and the corresponding point on an adjacent tooth. It is a dimension measured along a line or curve in the transverse, normal, or axial direction.
Pitch Point: It is a point on the line of centers of two gears, at which the point of tangency of two mating gears occurs.
Backlash: The clearance between the circle thickness of one gear and the tooth space of mating gear.
                         Backlash = Space width - thickness of tooth
Circular pitch (p): Arc distance along a specified pitch circle corresponding to the width of tooth and space of mating gears.
Diametral pitch (Pd): It is the ratio of number of teeth of a gear per inch of its pitch diameter.
Image Courtesy : Google
Module (m): It is the ratio of pitch diameter to the number of teeth.
Image Courtesy : Google
Velocity ratio: The ratio of RPM of driving gear to the rpm of driven gear. 

Image Courtesy : Google

Image Courtesy : Google


Gear parameters

          In this article we will tell you some basic parameters of gears. Everyone must know about these parameters before selecting a gear or designing a gear.

1)      Number of teeth (Gear ratio)
2)     Form of Teeth
3)     Teeth Size
4)     Facewidth of Teeth
5)     Dimension and style of gear blank
6)     Module
7)     Degree of precision
8)     The Type of gearing (Means of attaching gear to the shaft)


Image Courtesy : Google

Types of gears

          Before we start choosing a gear, we need to know that we are employing a metric gear, which is having metric gear module, or inch gear, that uses pitch, to conform proper gear sizing and compatibility. The gear module can be found by using a simple formula, by dividing the pitch diameter, or diameter of the pitch circle, by the quantity of the teeth the gear has. It is vital to make sure that gears we are using have an equivalent module, otherwise they will not work along. While choosing a gear two other important gear features has to be keep in mind they are gear ratio and backlash.

Types:

1. Spur Gear     
2. Helical Gear
3. Double Helical gear or Herringbone Gear
4. Bevel Gear or Miter gear
5. Worm gear 
6. Rack & Pinion
7. Internal and External Gear
8. Sprockets

According to the position of axis of shaft gears are also classified as :

1. Parallel Axis
          a. Spur Gear
          b. Helical Gear
          c. Rack and Pinion
2. Intersecting axis
          a. Bevel Gears
3. Non-interssecting and non-parallel
          a. Worm Gears


Below we can discuss them indetail

1. Spur Gear:

          Simplest and commonest kind of gear is Spur Gear and these are employed in series for large speed reductions. As discussed previously these gears are used to connect shafts which are Parallel and Co-Planar. This sort of arrangement is termed as Spur gearing.
          The teeth of spur gears are straight and parallel to the axis of the gear wheel. The benifits of spur gears are their simplicity in design, economy of manufacture and maintenance, and absence of end thrust. They impose solely radial loads on the bearings.
Spur gear
 Image Credits : http://www.gurukrupaeng.com/images/spur-2-big.jpg
          Spur gears are referred to as slow speed gears. Spur gears mate only one tooth at a time, leading to high stress on the pairing teeth and noisy operation. If noise isn't a significant design drawback, spur gear can be used at almost any speed. A normal gear ratio range is 1:1 to 6:1.
          Spur gears are used to this reasult in Laundry machines, Mechanical Clocks, Blenders, Kitchen aid beaters, Construction equipment, Fuel pumps and mills.

2. Helical Gear:

          The leading edges of the gear teeth are not parallel to the axis of rotation, but are set at an angle in the form of a helix, hence these gears got the name Helical gears. The usual array of helix angle ranges from 15 to 30 degrees. Helical gears are used to transmit the power between non-intersecting shafts. When the gear system interact, they first contact at tips of the tooth and the contact progressively spreads as the gears rotate, till complete engagement of tools teeth obtain. Because of this gradual engagement helical gears operation is more smooth and quiet compared to spur gears.
Helical Gear   !!!   Image courtesy : Google
          These gears are usually considered high speed gears. Helical gears can take higher loads than similarly sized spur gears. Both radial loads and thrust loads are imposed by single helical gears on their bearings. Thrust bearings we need to employ to counteract these thrust loads and also maintain gear allignment. When we enguage gears, they have same helix angle, pitch and pressure angle but the angle of the helix on both the gears must be same in magnitude but opposite in direction, i.e., a right hand pinion meshes with a left hand gear. Helical gears can be used to adjust the rotation angle by 90 deg. When mounted on perpendicular shafts. Helical gears normal gear ratio range is 3:2 to 10:1.
Advantages:
  • Large contact area
  • Quietness in operation
  • Less vibration
  • Able to transmit large force.

3. Herringbone Gear:

          Herringbone gears also referred as "Double Helical Gears". Herringbone gears is a special variety of gear they looks like two helical gears placed side by side in opposite helix pattern. When we see from the top every groove looks like the letter "V". In herringbone gears there is no thrust loading on the bearings. In the double helical gears arrangement, the side thrust of on half is balanced by the counter half by this the thrusts are counter-balanced.
Herringbone Gear   !!!   Image Courtesy: Google 
4. Bevel Gear/Miter Gear:

          Bevel gears are used to connect intersecting but co-planar shafts. This arrangement is known as Bevel Gearing. Whenever we want to alter the direction of shaft rotation we use bevel gearing. Straight bevel gears are most common type. Straight bevel gears is 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 each thickness and tooth height. This slight curvature in teeth of straight bevel gears is called crowning.
Straight bevel gear    !!!    Image Courtesy: Google

Spiral Bevel Gears:

Spiral Bevel Gear
Image Courtesy : Google
          In spiral bevel gears the teeth are oblique. As we are well know about straight bevel gear, there is one problem similar to spur gear i.e., the mating gears teeth engagement is not be gradual and hence the operation of straight bevel gear is so noisy. Now solution of this problem will be to cut and prepare the teeth oblique (Curved) so that the mating gears teeth engagement will be gradual and higher tooth to tooth contact as compared to straight bevel gearing. Because of this reason spiral bevel gear will provide quieter operation and also take up more load as compared to straight bevel gearing.

Zero Bevel Gears:
Straight Bevel gear  !!!  Image Courtesy: Google
          Zero Bevel gears are just like straight bevel gears, but their teeth are curved lengthise. These curved teeth of zero bevel gears are arranged in a manner that the effective spiral angle is zero.

5. Worm Gear:

          These gears consists two elements called the worm screw (Driving element) and  worm wheel (Driven element). Worm gears are used to transmit power beteen two non-parallel and non-intersecting shafts (shafts at 90° generally) and where high reductions are required. Worm gears have reduction ratio 20:1 and even up to 300:1 or greater. The reduction ratio depends on two things they are number of starts of the worm and number of teeth on the worm gear. The shafts of worm gears lie in parallel planes and can be slashed at any angle between zero and a right angle. Worm and worm gear have sliding contact so they form a lower pair. In worm gears, one gear has screw threads. Due to this, working of worm gears is quiet, vibration free and also gives a smooth output.
Image Courtesy: Google
Advantages:
1. They operate silently and smoothly.
2. they are self-locking. (Worm can easily turn the gear, but the gear can't turn the worm. This will act as a break in conveyor when the motor is not working).
3. They occupy less space.
4. They have good meshing effectiveness.
5. High velocity ratio can be obtained in a single step.

Disadvantages:
1. Worm gear material are expensive.
2. High power losses and low transmission efficiency.
3. They produce a lot of heat.

Applications:
1. Gate control mechanism
2. Hoisting machine
3. Automobile stearing mechanism
4. Lifts
5. Conveyors

6. Rack and Pinion:

Image Courtesy: Google
          A rack is a flat, toothed bar or rod which is taken into account as a sector gear with an infenitely large radius of curvature. Torque can be converted to linear force by meshing a rack with a pinion: the pinion rotates; the rack moves in a straight line. Such a mechanism is employed in vehicles to alter the rotation of the steering wheel into the left-to-right motion of the tie rod(s). Rack also feature in the theory of gear geometry, where, as an example, the tooth shape of an interchangeable set of gears may be pecified for the rack (infinite radius) and the tooth shapes for gears of perticular actual radii then derived from that. The rack and pinion gear type is employed in a rack railway.
Rack and pinion -- stearing mechanism
Image Courtesy : Google

7. Internal & External Gears:

An Internal Gear is a cylindrical shaped gear, one with the teeth on the inner surface of cylinder (or Cone). Conversly an external gear is the one with the tooth fashioned on the outer surface of cylinder.
Image Courtesy: Google
    
8. Sprocket:

Sprocket is a profiled wheel with teeth, or cogs. Sprockets are used to run chains or belts. They are typically used in conveyor systems. Based on the transmission and reception sprockets are classified in many ways like Simplex Duplex, Half duplex, Full duplex etc.,
Image Courtesy : Google


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