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Presentation on theme: "FACULTY OF ENGUNEERING AND TECHNOLOGY"— Presentation transcript:


2 GEARS Power transmission is an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to another device. A gear is a component within a transmission system that transmits or transforms mechanical energy to match the requirement of an application, as the operating point of motors is generally at higher speeds than the application requires. It can also be defined as: A wheel with teeth along its rim

3 Gear Ratio

4 Types of Gears “According to the position of axes of the shafts”
Parallel Spur Gears Helical Gears Herringbone Gears B. Perpendicular Axis C. Planetary Intersecting Bevel Gears Non Intersecting Worm Gears Planetary Gears

5 SPUR GEAR 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

6 External and Internal spur Gear…

7 Helical Gears “drive shaft and driven shaft are aligned in parallel”
The teeth on helical gears are cut at an angle to the face of the gear The teeth of helical gears are generally designed for infinite service life. This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears

8 Evolvent Tooth Advantages:
• Not sensitive to deviations in the distance between axes. • smooth movement transmission, i.e. low-vibration running. • Ease of manufacture, which leads to low costs. Evolvent Tooth Disadvantages: • In the case of external gearing, convex edge parts run against convex edge parts, which limits the load capacity. • When using a small number of teeth, the teeth are undercut due to the manufacturing process.

9 Meshing of two teeth with Evolvent tooth profile
One tooth each of the driving wheel and the driven wheel contact each other at a point on the mesh line. Pure rolling of the teeth only takes place at the point when the pitch circles (dw1 and dw2) form an intersecting point with the mesh line.

10 Herringbone Gears “Double Helical Gears”
To avoid axial thrust, two helical gears of opposite hand can be mounted side by side, to cancel resulting thrust forces. Axial Thrust is a force that is generated in an axial direction which is along the shaft. Herringbone gears are mostly used on heavy machinery.

11 Planetary Gears “suited for highly dynamic drives”
With their weight advantage, planetary gearboxes are also well established for extremely high output torque and for mobile applications. The maximum ratio that can be achieved in a single stage is approximately i = 12. Larger ratios are then achieved by adding further downstream stages. Alongside the increased load capacity, it is also reduce noise emissions.

12 Bevel gears “converting torque between intersecting axes ”
Useful when the direction of a shaft's rotation needs to be changed . Used to transmit and convert torque and speed between axes that intersect and cross one another. Usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The design is much more complex.

13 slightly poorer efficiency with this type of tooth system.
Bevel gear sets typically have a maximum ratio of i = 6, that’s why bevel gear are used as pre-stage for planetary or helical gearboxes. Advantage: higher load capacity. Disadvantage: slightly poorer efficiency with this type of tooth system. Types: Straight Spiral Examples: locomotives, marine applications, automobiles, power plants, steel plants,etc

14 Worm Gears “globoid wheel is used with a cylindrical worm”
the opposite configuration with a globoid worm is expensive so it is only used for high performance gearboxes. Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm.  This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place. Worm gears are used widely in material handling and transportation machinery, machine tools, automobiles etc.

15 The efficiency of a worm gearbox is based on the ratio and drops sharply as the ratio increases.
Depending on the ratio, the start-up efficiency can be some 20 to 30% below the efficiency in operation, due to the lack of a lubricating film between the worm and the wheel. Worm gearboxes can implement ratios up to 50 or even more. Adding rips on the gearbox housing offer a bigger surface area to improve heat dissipation.

16 If the efficiency is below 50%, which is possible with high ratios, self-locking kicks in when reversing the direction of force. This occurs when the worm lead angle ( γ ) equals tan-1 (μ). (µ is the friction coefficient) Self-locking is that the gear does not allow the interchangeability between the driving and the driven gear. Most of the worm gear trains used in industry are of the self-locking type.

17 Combining gearboxes with motors
These geared motors can be created by connecting the motor to the gearbox using a coupling. The pinion z1 of the first gearbox stage is connected to the shaft of the motor via a suitable shaft/hub connection. This eliminates the need for bearing mounting the input shaft of the gearbox. Geared motors cannot only be formed together with standard three-phase AC motors, but also with servo motors This then creates geared servo motors.

18 The various gearbox types can be combined with different motor types.

19 Definition OF SPUR GEARS

20 Definitions Pitch surface: The surface of the imaginary rolling cylinder 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 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. Flank of a tooth: The part of the tooth surface lying inside the pitch surface.

21 Manufacturers must take the following criteria into account
Tooth root strength: If the permissible loads are exceeded, the teeth tend to break off at their base. The tension that occurs at the root of the teeth primarily depends on the length of the tooth and shape of the tooth in the root area. Tooth flank load capacity: If the maximum tolerable pressure of tooth flanks in mesh is exceeded, parts of the tooth flank break off, leaving behind recesses that resemble pitting.

22 Lubricant Scuffing load capacity and wear load capacity:
Scuffing of the tooth system describes situations when tooth flanks are briefly and repeatedly welded together and then separated again as a result of the lubricant film failing. Wear occurs in the form of abrasion on the tooth flanks when slip takes place with mixed or dry friction. Lubricant The oil dissipates the high temperatures in the mesh area. Allows heat exchange with the gearbox housing.

23 Power loss The figure clearly shows that the transferable power of a gearbox rises more sharply as gearbox size increases. With high output torque and high output speeds, the steady state temperature can rise above the permissible temperature range. Once power output exceeds 50kW gearboxes are generally fitted with an active cooling system to dissipate the power loss.

24 GEAR TRAINS Types of Gear Trains:
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 Types of Gear Trains: Simple gear train Compound gear train Planetary gear train

25 Simple Gear Train The most common of the gear train is the gear pair connecting parallel shafts. The teeth of this type can be spur, helical or herringbone. Only one gear may rotate about a single axis

26 Simple Gear Train Compound Gear Train N3
In this arrangement we want both gears(yellow and blue to rotate in the same direction. Without the red gear: Vr= N3/N1 With the red gear: Vr= (N2/N1)*(N3/N2) = N3/N1 So, the size of the red gear is not important since it is just there to reverse the direction of rotation (it doesn`t change the ratio). N2 N1 Compound Gear Train For large velocities, compound arrangement is preferred. In this arrangement to avoid using large gear sizes to achieve certain ratio(large gearboxes introduce higher power loss) gears 2 and 3 are connected to the same shaft and then 3 is coupled with 4 . Gear 4 doesn`t have to be very large to achieve the ratio,

27 Planetary Gear Train Advantages Applications
In this gear system, the yellow gear (the sun) engages all three red gears (the planets) simultaneously. All three are attached to a plate (the planet carrier), and they engage the inside of the blue gear (the ring) instead of the outside. Advantages Planetary gear sets can produce different gear ratios depending on which gear you use as the input, which gear you use as the output, and which one you hold still. They have higher gear ratios. Applications Automatic transmissions in automobiles. Used in bicycles for controlling power of pedaling automatically or manually.

28 Gear train example: For the set of gears shown below, find output speed, output torque, and horsepower for both input and output conditions and overall velocity ratio. Solution: Vr = (N2/N1)(N4/N3) = (60/20)(60/20) = 9 n4= n1/Vr = 3600/9 = 400 rpm T4 = T1Vr = 200 * 9 = 1800 in-lb hpin= T*n/63000 = 200*3600/63000= 11.4 hpout= 1800*400/63000 = 11.4

29 Selecting Gear Drives Gears can be selected, rated, installed, and maintained by most users through common standards and practices developed by the American Gear Manufacturers Association(AGMA). The major selection factors include: shaft orientation, speed ratio, design style, nature of load, service factor, environment, mounting position, ratio, lubrication, and installation.

30 “ Input to output shaft position”
Shaft Orientation “ Input to output shaft position” Orientation Types: parallel Shafts Shafts at right angles with intersecting axes Shafts at right angles with nonintersecting axes Skewed Shafts Speed Ratio “The ratio of input speed to that of the output” determining if a single high-ratio stage is sufficient or if multistage gearing is required. Geared systems can be driven at constant or varying speeds, depending on the requirements of the application.

31 3. Nature of load “determining the nature of the load is important for long life and reliable service.” Load considerations: Maximum horsepower. Drive inertia. Overhung load(the radial load on the output shaft extension produced by a gear). Speed limit of the gear. 4. Environment “ The type of gear selected must compensate for many unwanted environments ” Dust may contaminate the lubricant. Heat may accelerate lubricant breakdown and lower gear capacity by lowering material properties and distorting the gear. Wide temp. variation can cause improper lubrication, thereby shortening the life of the unit. Moisture infiltration may accelerate lubricant breakdown and wear of teeth.

32 5. Gear Rating Design Style
There are two types of ratings for a geared unit : Mechanical rating is based on the strength of the gears, shafts, load pressure, or the resistance of the gears to pitting or abrasion. Thermal rating specifies the power that can be transmitted without exceeding a specified limit above the operating temp. In high power units additional cooling methods are used: Air cooling the housing. Circulating water around the unit. Using a separate oil sump for greater heat dissipation. Using cooler mounted inside the gear housing. Design Style ̒ ̒ In any design shafts should not introduce external load to the gear ̕ ̕ Enclosed speed reducer with oil lubrication is the preferred design. Grease-lubricated open gears can be used in relatively clean environments.

33 If the gearbox type only permits a solid output shaft, a coupling is needed to connect the drive shaft to the application. A gearbox design with a hollow shaft allows it to be integrated into the machine`s drive shaft directly. (Hollow shaft gearboxes are significantly more expensive.)

34 7-Lubrication “Failure to supply lubricant to the bearings and gears will result in their damage ” The type of lubricating system should be chosen carefully  If the unit is operating where temperatures vary widely, oil viscosity should be changed to suit the conditions. For low temperature operation the oil should have a pour-point  lower than that of the extreme minimum temperature encountered. (pour-point is the lowest temperature at which it will flow under prescribed conditions). Lubrication systems: -    Splash systems in which one of the gears dips into an oil path and transfers the lubricant to the contacting teeth as it rotates. -     Forced-spray lubrication reduces oil churning in which lubricant is pumped to the gear train, then it`s returned to the reservoir to be recirculated. ( Churning is an undesirable friction between fluids due to continuous excitation of the lubricant)

35 Losses in gears Power losses in gear systems are associated primarily with tooth friction and lubrication churning losses.    Churning losses are relatively independent of the nature of the gears and the gear ratios - they are primarily related to the peripheral speed of the gears passing through the fluid.  Churning losses are difficult to calculate and estimated based on experience are often used in initial gear design.   The frictional losses are related to the gear design, the velocity ratio, the pressure angle, gear size, and the coefficient of friction.

36 Summary Speed of mating gears is inversely proportional to the number of teeth. Mating gears should have the same diametral pitch( The ratio of the teeth to the pitch diameter) for a reliable operation. Good gear design should take care of power, speed, life and material properties. A number of gear manufacturing methods are available such as: Type Normal  Ratio Range Efficiency  Range Spur 1:1 to 6:1 98-99% Helical 1:1 to 10:1 Double Helical 1:1 to 15:1 Bevel 1:1 to 4:1 Worm 5:1 to 75:1 20-98%


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