woksyop power point 2

8/8/2019 Woksyop Power Point 2

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Introduction to Rotary motion past to presentRotary motion is the most common type of motion for a shaft or anaxle. One way in which an engineer uses rotary motion is bytransmitting it from one shaft to another when the shafts areparallel. This can be done by using pulleys and belts. A pulley is a

wheel which may or may not have a grooved rim.The figure below shows a stacked vee pulleys and vee belts oftenused in car engines.


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The main function of pulleys and belt systems are to transmit

motion and torque from an engine to a machine. Various types

of pulleys and belts are used on different machines. Machines

used in the home, such as sewing machines, washing machines,

spin dryers and vacuum cleaners. The picture below shows a flatbelt and flat pulley used to transmit motion from an old heat

engine.


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From flat pulleys to toothed pulleys

In machines where a positive drive is essential and no slip between beltand pulleys can be accepted, a toothed belt and pulley is used. Toothedbelts are mainly used for timing mechanisms, where quiet, positive (no

slip) drive is required. The figure below shows a toothed belt and toothedpulleys used to drive a camshaft in a motor car engine.

Toothed belt and toothed pulleys


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The gear wheelThe gear wheel is a basic mechanism. Its purpose is to transmit rotary motion andforce. A gear is a wheel with accurately machined teeth round its edge. A shaftpasses through its center and the gear may be geared to the shaft. Gears are used ingroups of two or more. A group of gears is called a gear train. The gears in a train arearranged so that their teeth closely interlock or mesh. The teeth on meshing gears

are the same size so that they are of equal strength. Also, the spacing of the teeth isthe same on each gear. An example of a gear train is shown below.

Single gear gear train


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Rotation directionWhen two spur gears of different sizes mesh together, the larger gear is called awheel, and the smaller gear is called a pinion. In a simple gear train of two spurgears, the input motion and force are applied to the driver gear. The output motionand force are transmitted by the driven gear. The driver gear rotates the driven gearwithout slipping.

The wheel or the pinion can be the driver gear. It depends on the exact function thedesigner wishes the mechanism to fulfill. When two spur gears are meshed thegears rotate in opposite directions, as shown in the figure below.

Wheel and pinion


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Gear types

Bevel gearsThese gears have teeth cut on a cone instead of a cylinder blank. They are

used in pairs to transmit rotary motion and torque where the bevel gearshafts are at right angles (90 degrees) to each other. An example of twobevel gears are shown below.

Bevel gears


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Crossed helical gearsThese gears also transmit rotary motion and torque through a right angle. The teeth of a helicalgear are inclined at an angle to the axis of rotation of the gear.The diagram below shows how the axis of rotation of two helical gears are crossed at rightangles. Helical gears are smoother running than spur gears and are more suitable for rotationat high velocities. An example of two crossed helical gears are shown below.

Crossedhelical gears


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Worm and worm wheelA gear which has one tooth is called a worm. The tooth is in the form of a screw thread. A wormwheel meshes with the worm. The worm wheel is a helical gear with teeth inclined so that theycan engage with the thread like worm. Like the crossed helical gears, the worm and worm

wheel transmit torque and rotary motion through a right angle. The worm always drives theworm wheel and never the other way round. The mechanism locks if the worm wheel tries todrive the worm. Worm mechanisms are very quiet running. An example of a worm and worm

wheel is shown on the right hand side below. An application of the worm and worm wheel usedto open lock gates is shown on the left hand side below.

Worm and worm wheel


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application


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The helical gearThis gear is used for applications that require very quiet and smooth running, at high rotational

velocities.Parallel helical gears have their teeth inclined at a small angle to their axis of rotation. Eachtooth is part of a spiral or helix. The helical gears shown below have splines cut in their centerholes. The gears can move along a splined (grooved) shaft, although they rotate with the shaft.

An example of a helical gear is shown below.

Double helical gears give an efficient transfer of torque and smooth motion at very highrotational velocities. An example of a double helical gear is shown below.

Double helical gear


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Spiral bevel gearsWhen it is necessary to transmit quietly and smoothly a large torque through a right angle athigh velocities, spiral bevel gears can be used. Spiral bevel gears have teeth cut in a helix spiralform on the surface of a cone. They are quieter running than straight bevel gears and have alonger life. Spiral bevel gears are used in motorcar rear axle gearboxes. An example of spiralbevel gears are shown below.

Spiral bevel gears


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Face cut gearsIt is possible to cut gear teeth on the face of a gear wheel. Also, gear teeth can be cut on theinside of a gear ring an example of which is shown in the top figure below. Internal gears havebetter load carrying capacity than external spur gears. They are safer in use because the teethare guarded. An example of an external face cut gear is shown below.

Internal face cut gear


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External face cut gear


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Rack and pinionA rack and pinion mechanism is used to transform rotary motion into linear motion and visaversa. A round spur gear, the pinion, meshes with a spur gear which has teeth set in a straightline, the rack. The rack and pinion can transform rotary motion into linear motion and visa

versa in three ways:

a. Rotation of the pinion about a fixed center causes the rack to move in a straight line.

b. Movement of the rack in a straight line causes the pinion to rotate about a fixed center;c. If the rack is fixed and the pinion rotates, then the pinion's center moves in a straight linetaking the pinion with it.

rack and pinion


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application


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Spur gearsThe spur gear is the last gear we will look at and the most important as far as we are concerned.We will be looking at the gear terms and how to draw the gear teeth using Unwins construction.Firstly, we will discuss the spur gear itself.

A spur gear is one of the most important ways of transmitting a positive motion between twoshafts lying parallel to each other. A gear of this class may be likened to a cylindrical blank

which has a series of equally spaced grooves around its perimeter so that the projections on oneblank may mesh in the grooves of the second. As the design should be such that the teeth in therespective gears are always in mesh the revolutions made by each is definite, regular and in theinverse ratio to the numbers of teeth in the respective gears. This ability of a pair of well madespur gears to give a smooth, regular, and positive drive is of the greatest importance in manyengineering designs. An example of two spur gears in mesh are shown below.


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Spur gears


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Involute spur gear terms


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The spur gear terms:

y Thepitch circle is the circle representing the original cylinder which transmitted motion byfriction, and its diameter the pitch circle diameter.

The center distance of a pair of meshing spur gears is the sum of their pitch circle radii. Oneof the advantages of the involute system is that small variations in the center distance do notaffect the correct the correct working of the gears.

The addendum is the radial height of a tooth above the pitch circle.The dedendum is the radial depth below the pitch circle.

The clearance is the difference between the addendum and the dedendum.

Thewhole depth of a tooth is the sum of the addendum and the dedendum.

Theworking depth of a tooth is the maximum depth that the tooth extends into the toothspace of a mating gear. It is the sum of the addenda of the gear.

The addendum circle is that which contains the tops of the teeth and its diameter isthe outside or blank diameter.

The dedendum or root circle is that which contains the bottoms of the tooth spaces and itsdiameter is the root diameter.


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y Circular tooth thickness is measured on the tooth around the pitch circle, that is, it is the length of an arc.

Circular pitch is the distance from a point on one tooth to the corresponding point on the next tooth,measured around the pitch circle.

The module is the pitch circle diameter divided by the number of teeth.

TheD

iametrical pitch

is the number of teeth per inch of pitch circle diameter. This is a ratio.Thepitch point is the point of contact between the pitch circles of two gears in mesh.

The line of action. Contact between the teeth of meshing gears takes place along a line tangential to the twobase circles. This line passes through the pitch point and is called the line of action.

The pressure angle. The angle between the line of action and the common tangent to the pitch circles at thepitch point is the pressure angle.

The tooth face is the surface of a tooth above the pitch circle, parallel to the axis of the gear.

The tooth flankis the tooth surface below the pitch circle, parallel to the axis of the gear.If any part of theflank extends inside the base circle it cannot have involuteform. It may have ant other form, which does notinterfere with mating teeth, and is usually a straight radial line.


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The importance of clearanceClearance is the distance from the tip of a tooth to the circle passing through the bottom of thetooth space with the gears in mesh and measuring radially.The correct clearance is vital to the motion of gears. To view two spur gears rotating in meshand the necessity for clearance.

Two spur gear rotating in mesh


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Close up rotating gear to show clearance


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