Gears & Gear Making

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Gear

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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 Altering speed ratios

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Characteristics In geometry gears are

Toothed wheels Transmit motion & power from one shaft to

another when they are closer to each other (not too far apart)

Constant velocity ratio is desired In comparison with belt, chain & friction

devices are : More compact Operate at high speeds Precise timing Large power

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Characteristics (Contd) 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 may be different from that of the first

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In a set of gears, the smaller gear is called a pinion

The larger gear is called a wheel or simply gear

Characteristics(Contd)

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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• Worm gears

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Gear Types

Spur gears Most common type Transfer power between parallel shafts Good mechanical efficiency Cheapest

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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 Wide load distribution Less noise

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Gear Types

Herringbone gears Double helical gears Can absorb axial thrust within the gears

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Gear Types

Rack & pinion arrangement Convert rotational motion into

translational motion

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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 Noisy at high speed

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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 ratio

increases

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Gear Materials Certain non ferrous alloys Cast iron Plastics (Teflon) Steels (most common)

High strength to weight ratio Low cost

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Nomenclature Pitch circle

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

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

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Nomenclature

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Nomenclature(Contd) 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 diameter (D) or P = N/D

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Nomenclature(Contd) Circular pitch (p)

is the distance from a point on one teeth to the corresponding point on an adjacent tooth, measured on the pitch circlep = πD/N and P = π/p

Gears & gear cutters are standardized according to the diameteral pitch P

6-pitch (6P) gear has 6 teeth per inch of P

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Nomenclature(Contd) 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 gears

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Nomenclature(Contd) Module (M)

It is the metric standard for pitch The length in mm that each tooth will

occupy if the teeth in the gear were spaced along the pitch diameter

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Nomenclature(Contd) Centre distance

It is the distance between centres of two meshing gearsBacklashWhen two gears are meshed together, the slippage or play between the teeth of the two gears is called backlash

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Nomenclature(Contd) Involute gear profile

In an involute gear the profiles of the teeth are involutes of a circlewhere 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 two gears occur in the same fixed flat plane (the plane of action) even as their teeth mesh in and out

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Involute gear profile

Involute Curve

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Involute gear profile

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Involute Gear Tooth Profile

p = πD/N and P = π/p

Diametral Pitch

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Diametral pitch (P). This is the English (inch) method for Specifying pitch, and P is a number which represents the number of teeth for each inch of reference diameter. Thus If a gear has 60 teeth and its reference diameter is 6 in, There are 60/6= 10 teeth for each inch of reference diameter, signifying a diametral pitch of 10

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(Contd)

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Gear casting Gear forming Gear generating Gear shaping Gear cutting Gear Planning

Gear Making Processes

Gear Manufacturing Procedure

For making gear, diameter and number of teeth should be known. Dp number can be calculated as:

Dp number = teeth + 2 / Diameter of gear

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Procedure(Contd)Index plateFor indexing there are three methods1. Direct Indexing (24/n)2. Plane Indexing (40/n)3. Angular indexing (360/n)For index plate, we will use plane indexing and if we want to have 18 teeth (40/18 = 2-2/9). Hence there are two turn of shaft, 2 holes & index number is 9

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Gear shaving

Gear grinding

Gear lapping

Shot blasting

Sand Blasting

Phosphate coating

Gear Finishing Processes

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Gear casting (low melting point metals) Sand casting Die casting Investment casting Centrifugal casting Injection moulding (plastics)

Gear Making Processes

©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

Steps in the production sequence in sand casting

ASSIGNMENT-2

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Describe different steps in Investment casting and Die Casting

Date of submission; Next week same day Your Assignment should be hand written with

sketches