Gear Making

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Gear

 A gear is a Mechanical component used to

transmit power and motion

 Key operations

 Reversing the direction of rotation

 Altering angular orientation of rotary motion

 Convert rotary motion into linear motion &

vice versa

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Characteristics

 In geometry gears are

 Toothed wheels

 Transmit motion & power from one shaft to

another when they are not too far apart

 Constant velocity ratio is desired

 In comparison with belt, chain & friction

devices

 More compact

 Operate at high speeds

 Precise timing

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Characteristics

 Gears can mesh with any component having

compatible teeth

 Gears of unequal (sizes) diameters can be

combined to produce mechanical advantage

 Rotational speed & torque of second gear is

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 In a gear set the smaller gear is called a

pinion

 The larger gear is called a wheel or simply

gear

 When two gears are meshed together, the

slippage or play between the teeth of the two gears is called backlash

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Classification

 Gears are classified according to the relative

location of the axes of mating gear shafts

 Gears with parallel axes

• Spur gears

• Helical gears

• Herringbone gears

• Rack and pinion gears

• Internal gears

 Shaft axes intersect if prolonged

• Straight bevel gears

• Spiral bevel gears

 Shaft axes neither parallel nor intersecting

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Gear Types

 Spur gears

 Most common type

 Transfer power between parallel shafts  Good mechanical efficiency

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Gear Types

 Helical gears

 Variation of spur gears

 Teeth are slanted at an angle

 Allowing more teeth coming in contact

with each other

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Gear Types

 Herringbone gears

 Double helical gears

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Gear Types

 Rack & pinion arrangement

 Convert rotational motion into

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Gear Types

 Bevel gears

 Connect two intersecting shafts  Making an angle with one another

 Slightly less efficient than spur gear  More expensive

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Gear Types

 Worm & worm gear

 Shafts are generally but not

necessarily at right angles in different planes

 Axes are orthogonal to each other but

not intersecting

 Expensive

 Efficiency drops off quickly as gear

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Gear Materials

 Certain non ferrous alloys

 Cast iron

 Plastics (Teflon)

 Steels (most common)

 High strength to weight ratio

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Nomenclature

 Pitch circle

It is the circumference on which gear teeth are developed (an imaginary circle)

 Addendum

of a tooth is the radial distance from the pitch circle to the outside diameter or

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Nomenclature

 Dedendum

of a teeth is the radial distance from the

pitch circle to the root or dedendum circle)

 Diameteral pitch (P)

referred to as pitch of a gear, is the ratio of the number of teeth (N) to the pitch

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Nomenclature

 Circular pitch (p)

is the distance from a point on one teeth to the corresponding point on an adjacent

tooth, measured on the pitch circle p = πD/N and P = π/p

 Gears & gear cutters are standardized

according to the diameteral pitch P

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Nomenclature

 Pressure angle (_ф)

If a common tangent is drawn to the pitch circles of two meshing gears then a line of action (angle of thrust) is drawn at an

angle called pressure angle to this line

 Usually the angle is 14.5° or 20° of the

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Nomenclature

 Module (M)

It is the metric standard for pitch

 The length in mm that each tooth will

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Nomenclature

 Centre distance

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Nomenclature

 Involute gear profile

In an involute gear the profiles of the teeth are involutes of a circle

where involute of a circle is the spiralling curve traced by the end of an imaginary taut string unwinding itself from that

stationary circle

 In involute gear design all contact between

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 Gear casting

 Gear forming

 Gear generating

 Gear shaping

 Gear cutting

 Gear Planning

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 Gear shaving

 Gear grinding

 Gear lapping

 Shot blasting

 Phosphate coating

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 Gear casting (low melting point metals)

 Sand casting

 Die casting

 Investment casting

 Centrifugal casting

 Injection moulding (plastics)

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Steps in investment casting:

(1) wax patterns are produced

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(3) the pattern tree is coated with a thin layer of refractory material

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(6) the mold is preheated to a high temperature, which

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(7) the mold is broken away from the finished casting

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 Gear forming

 Roll forming

 Stamping

 Powder metallurgy

 Extrusion

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 Gear generating

 Hobbing is the process of gear

generation by means of a cutter called a hob

 It is a continuous indexing process in

which both the cutting tool and work piece rotate in a constant relationship while the hob is being fed into the

work

 Each hob tooth cuts its own profile

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 Types of hobbing process

 Arial hobbing

 Radial hobbing

 Tangential hobbing

 Conventional hobbing or arial hobbing

(to generate spur gears)

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 Gear shaping

 Rack type cutter generating

process

 Pinion type cutter generating

process

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 Gear cutting by milling (two variations)

 Disc type cutter

 Gear blank moves under the rotating

cutter and a tooth space is cut

 The gear blank is then indexed to the

next position for cutting the second tooth space

 Procedure is repeated – until all

teeth milled

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 Gear cutting (two variations)

 End mill cutter

 The cutter rotates about an axis which is

set radially with respect to the blank and at the same time the cutter is traversed parallel to the axis of the blank

 The axial cross-section of the cutter

corresponds to the shape required for the space between two adjacent teeth on the finished gear job/work piece

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 Gear milling is a simple, economical and

flexible method for gear making

 Spur, bevel and rack gears can be

produced by this process

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 Gear planning

 Oldest gear making process

 Uses rack type cutters for

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Gear Speeds

 Gear speeds of two meshing gears vary

inversely with both the pitch diameters and number of teeth

 s/S = D/d = T/t

 D = Pitch diameter of gear  d = Pitch diameter of pinion  T = No of teeth of gear

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Machine Shop Safety

1. Never work alone

At least two workers must be in the shop when power tools are being used

2. Never work when you are impaired

This includes when you are too tired, stressed or hurried to work carefully

3. Safe conduct in the machine shop

Your conduct in the workshop should be safe and responsible and not hurtful or damaging to any worker/operator

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Machine Shop Safety

4. Always wear closed toe shoes in the shop

Tools, chips and fixtures are sharp and often hot. Shoes will help protect your feet from injury. Leather shoes are preferred when machining

5. Eye protection is essential

Always wear safety glasses when working or cleaning tools

6. Remove or secure anything that might get caught in moving machinery

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Machine Shop Safety

7. Keep your hands away from sharp tools

Make sure that nothing that you do will cause you to be cut. Use safety gloves to handle hot or sharp objects

8. Dust, chemicals and smoke can be dangerous

Work in well ventilated areas. Minimize contamination and use appropriate protective equipment

9. Ask for guidance and help

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Machine Shop Safety

10. Be careful while working

If you do not know how to do some thing, ask. Before starting, study the machine and know which parts are moving, which are stationary and which are sharp and may be potentially

dangerous. Listen to the machine and never leave the machine running unattended

11. Clean up after yourself