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# MSBTE_4thsem_TOM_Gear drives

20. Jan 2018
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### MSBTE_4thsem_TOM_Gear drives

1. TOPIC 4.3 POWER TRANSMISSION-GEAR DRIVES By, -Prof. Satish G Sonwane -Mechanical Engineering Department -DBACER (Formerly SVSSCOER), Wanadongri Nagpur
2. WHAT ARE GEARS?  Wikipedia says, A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part to transmit torque.  Geared devices can change the speed, torque, and direction of a power source.  Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine.
3. WHAT ARE GEAR DRIVES?  Mechanical systems that drive or run the other systems are called ‘Mechanical drives’.  Mechanical drives employing gears are called “Gear Drives”  These drives are called “POSITIVE” on account of absence of slip.
4. ADVANTAGES INCLUDE: 1. Transmit constant velocity ratio 2. Can transmit large power 3. High efficiency 4. Reliable drive 5. Compact DISADVANTAGES ARE: 1. Costlier 2. Error in mfg can induce noise and vibration 3. Requires continuous lubrication 4. Cannot transmit power over long distance
5. GEAR TERMINOLOGY
6. LAW OF GEARING  In order to have a constant angular velocity ratio for all positions of the wheels, the point P must be the fixed point (called pitch point) for the two wheels.  In other words, the common normal at the point of contact between a pair of teeth must always pass through the pitch point.  This is the fundamental condition which must be satisfied while designing the profiles for the teeth of gear wheels. It is also known as law of gearing.
7. CLASSIFICATION OF GEARS  Gears are classified on the basis of, position of axes of the shafts on which they are mounted  Parallel shafts Two parallel shafts are connected by, 1) Spur Gears 2) Helical Gears 3) Double helical gears OR Herringbone gears 4) Rack and Pinion
8. SPUR GEARS  Teeth is parallel to axis of rotation  Transmit power from one shaft to another parallel shaft  Used in Electric screwdriver, oscillating sprinkler, windup alarm clock, washing machine and clothes dryer
9. EXTERNAL AND INTERNAL SPUR GEAR…
10. HELICAL GEARS  The teeth on helical gears are cut at an angle to the face of the gear  This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears  If the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees
11. DOUBLE HELICAL OR HERRINGBONE GEARS GEARS  To avoid axial thrust, two helical gears of opposite hand can be mounted side by side, to cancel resulting thrust forces  Herringbone gears are mostly used on heavy machinery.
12. RACK AND PINION  Rack and pinion gears are used to convert rotation (From the pinion) into linear motion (of the rack)  A perfect example of this is the steering system on many cars
13. CLASSIFICATION OF GEARS  Intersecting Shafts Two non-parallel or Intersecting shafts are connected by, 1) Bevel Gears Bevel Gears can be Straight Bevel Gears or Spiral Bevel gears
14. BEVEL GEARS  Bevel gears are useful when the direction of a shaft's rotation needs to be changed  The teeth on bevel gears can be straight, spiral or hypoid  They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well  Applications include: locomotives, marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, railway track inspection machines, etc.
15. STRAIGHT AND SPIRAL BEVEL GEARS
16. CLASSIFICATION OF GEARS  Intersecting Shafts Two non-parallel or non-Intersecting shafts are connected by, 1) Skew Bevel gears or Spiral gears 2) Worm and Worm Wheel
17. WORM AND WORM WHEEL  Used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater  These drives are not reversible i.e. The worm can easily turn the gear, but the gear cannot turn the worm  Worm gears are used widely in material handling and transportation machinery, machine tools, automobiles etc
18. WORM AND WORM GEAR
19. GEAR TRAINS  Two or more gears are made to mesh each other to transmit power from one shaft to another. Such a combination is called a Gear train.  The nature of a train depends upon the velocity ratio required and the relative position of the axes of the shafts
20. TYPES OF GEAR TRAINS 1. Simple Gear Train 2. Compound Gear Train 3. Reverted Gear Train 4. Epicyclic OR Planetary Gear Train  In the first three types of gear trains, the axes of the shafts over which the gears are mounted are fixed relative to each other.  But in case of epicyclic gear trains, the axes of the shafts on which the gears are mounted may move relative to a fixed axis.
21. SIMPLE GEAR TRAIN  When the distance between the two shafts is small, the two gears 1 and 2 are made to mesh with each other to transmit motion from one shaft to the other  The teeth of this type can be spur, helical or herringbone.  Since the gear 1 drives the gear 2, therefore gear 1 is called the driver and the gear 2 is called the driven or follower.  It may be noted that the motion of the driven gear is opposite to the motion of driving gear.
22. ANALYSIS OF SIMPLE GEAR TRAIN  Let N1 = Speed of gear 1(or driver) in r.p.m., N2 = Speed of gear 2 (or driven or follower) in r.p.m., T1 = Number of teeth on gear 1, and T2 = Number of teeth on gear 2.
23. VELOCITY RATIO, TRAIN VALUE  The Speed ratio (or velocity ratio) of gear train is the ratio of the speed of the driver to the speed of the driven or follower  Ratio of speeds of any pair of gears in mesh is the inverse of their number of teeth, therefore,  It may be noted that ratio of the speed of the driven or follower to the speed of the driver is known as train value of the gear train. Mathematically,
24. FURTHER ANALYSIS
25. COMPOUND GEAR TRAIN  When there are more than one gear on a shaft, as shown in Fig., it is called a compound train of gear.
26. CONTINUED..  The advantage of a compound train over a simple gear train is that a much larger speed reduction from the first shaft to the last shaft can be obtained with small gears.  Usually for a speed reduction in excess of 7 to 1, a simple train is not used and a compound train or worm gearing is employed.
27. ANALYSIS OF COMPOUND GEAR TRAIN
28. REVERTED GEAR TRAIN  When the axes of the first gear (i.e. first driver) and the last gear (i.e. last driven or follower) are co- axial, then the gear train is known as reverted gear train as shown in Fig.
29. ANALYSIS OF REVERTED GEAR TRAIN  Let T1 = Number of teeth on gear 1, r1 = Pitch circle radius of gear 1, and N1 = Speed of gear 1 in r.p.m.  Similarly T2, T3, T4 = Number of teeth on respective gears, r2, r3, r4 = Pitch circle radii of respective gears, and N2, N3, N4 = Speed of respective gears in r.p.m.  Since the distance between the centres of the shafts of gears 1 and 2 as well as gears 3 and 4 is same, therefore, r1 + r2 = r3 + r4
30. CONTINUED…  Also, the circular pitch or module of all the gears is assumed to be same, therefore number of teeth on each gear is directly proportional to its circumference or radius. ∴ T1 + T2 = T3 + T4 or,
31. EPICYCLIC GEAR TRAIN  In an epicyclic gear train, the axes of the shafts, over which the gears are mounted, may move relative to a fixed axis.  A simple epicyclic gear train is shown in Fig., where a gear A and the arm C have a common axis at O1 about which they can rotate.  epi. means upon and cyclic means around.
32. CONTINUED…  The epicyclic gear trains are useful for transmitting high velocity ratios with gears of moderate size in a comparatively lesser space.  The epicyclic gear trains are used in the back gear of lathe, differential gears of the automobiles, hoists, pulley blocks, wrist watches etc.
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